(f^orneU ItttnetBitg SItbracg ALEXANDER GRAY MEMORIAL LIBRARY ELECTRICAL ENGINEERING THE GIFT OF TK3231.KlT""'""""'"-"'"^ Ovj^rtiead transmission lines and distribu 3 1924 004 602 771 4«r>T' OVERHEAD TRANSMISSION LINES AND DISTRIBUTING CIRCUITS OVERHEAD TRANSMISSION LINES AND DISTRIBUTING CIRCUITS THEIR DESIGN AND CONSTRUCTION BY F. KAPPER TRANSLATED BY P. R. FRIEDLAENDER, M.I.E.E. MEMBER OF THE PACULTV OF ENGINEERINO, LONBON UNIVERSITY NEW YORK D. VAN NOSTRAND COMPANY 25 PARK PLAGE 1Q15 , , ., Printed in Great Britain PREFACE This book is intended to explain the fundamental principles and to give the data essential to the proper carrying out of the very varied operations which fall to the share of the present-day overhead line engineer. No attempt has been made to provide a complete text-book of structural engineering, so that the calculations have in all cases been carried only as far as seems necessary to cover the direct practical requirements of the designer and constructor of overhead lines. Especial importance has been given to simplicity of statement, and the numerous worked examples, mostly taken from practice, wUl prevent any doubt arising in the application of the various formulae. The book is based on a practice of many years in this particular branch, and will, it is believed, prove useful not only to the younger e.igineers but also to those of more experience. Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004602771 CONTENTS 1. Introduction . . . . 2. CONDUOTOE Materials . Copper, Aluminium, Bronze, Steel, Constants 'Monnot Metal." Tables of 3. Sag and Tension of the Line .... Supports at the same Level. .... Supports at different Levels .... Numerical Example ..... Effect of Temperature ..... Critical Span ...... Numerical Example ..... Determination of the Maximum Sag Sag and Stress in Insulated Lines .... Sag and Stress in Safety Network Constructors Sag and Stress in Steel Wire Eopes Supporting Cables Numerical Example . - . Tables and Curves of Sag and Stress in Stranded and Solid Copper Effect of the Wind ...... Numerical Examples and Tables .... 4. Design of the Supporting Structures Determination of the Stresses Numerical Examples and Tables The General Laws of Mechanics Table of Beam Sections Table of Moments of Inertia, etc. Special cases : — Independent Pole Stayed Pole . Strutted Pole Double Pole of A Form Double Pole of H Form Numerical Examples 5. Stability of Poles and Masts Symmetrical Ground Pressure Unsymmetrical Ground Pressure Active and Passive Earth Pressure Influence of the Forces below the Ground Level Numerical Example Physical PAGE 1 3—9 Wires 10 10 11 13 16 18 18 21 25 26 27 27 28—33 34 36 38 39 40—44 44 47 48 50 56 58 59 60 61—77 78 79 79 80 82 84—92 Vlll CONTENTS 6. The Forces on the Foundation Block ..... Numerical Example ....... 7. Fixing the Poles in the Ground ...... Anchoring and Staying AVooden Poles and Iron Masts with and without Foundation Block ....... 8. Concrete and Cement Foundation Work ..... PAGE 93 93 96 97 103 9. Pole Construction .... Wooden Poles .... Multiple Wooden Pole Structures (A and H Poles) Pins and Cross Arms Mast Feet ..... Iron Masts ..... Examples from Practice 10. Overhead Line Insulators 107 107 109 111 115 116 127 134 Low Tension and High Tension Insulators, Pin Insulators, and Suspension Insulators, Guard Rings, Tables .... 134 — 146 11. Attachment op the Insulators to their Pins With Impregnated Hemp and with Cement . 147 147, 148 12. Attachment of the Wire to the Insulator Top Groove ; Side Groove ; Terminals 13. Joints and Branch Connections on the Line Arld's Twisted Tube Joint . Hofmann's Riveted Joint . Hofmann's Conical Joint Soldering ..... 14. Arrangement of the Wires . Distance of the Wires from the Ground Distance between the Wires Grouping of the Wires Transposition of the Wires . 15. Eabthing ..... Earthing under Normal Running Conditions Protective Earthing Earth Plates .... Earthing Pipes .... Strip Iron Earthing Electrodes 149 149—152 153 153 155 156 1-58 160 160 160 162 164 166 166 166 167 167 168 CONTENTS IX 16. Crossings over Postal Wires, Railways, and Roads Earthing Cradles ..... Safety Couplings ..... Protective Nets Special Safety Suspensions .... Ulbrioht's Network Conductor Bridge Constructions for Crossings . Post Office Regulations for Crossings Special Safety Suspension Arrangements PAGE 169 169 170 171 173 174 174 177 195 17. Erection of Poles and Masts The Excavation for the Poles Transportation. and Erection of the Poles 197 197 198 18. Erecting the Wire Checking the Sag . 202 205 19. Rules and Hints for the Design and Erection of Overhead Lines 209 20. Instruments for Surveying and Laying Out the Route of a Line . 212 Measuring Rod ; Steel Measuring Tape ; Ranging Pole ; Levelling Staff . 212 Pedometer ; Optical Square ; Cross-Shaft ; Pocket Telescopic Level 213 Clinometer ; Theodolite ....... 216 Miner's Dial ; The simple Measurement of Angles . . . 216 Trigonometrical Height Estimates over Short Distances . 217 Measurement of Distances on Inclines .... 218 21. Surveying the Route or a Transmission Line Surveying in Open Country Surveying in Obstructed Country . Recording the Results Supports erected on Private Property Supports erected on Public Property 219 219 220 221 222 224 22. The Most Economical Length of Span Examples with Tables and Curves . 226 227—235 23. Comparison of the various Supporting Structures with Reference to THE Minimum Annual Charges ..... 236 Example with tables and curves . . 237 O.T.L. ^ CONTENTS 24. Local Overhead Distributing Systems General Consideration of the Problem Wooden Pole Supports .... Roof Supports ..... Wall Supports ..... Determination of the most Suitable Supportiag Points Constructional Material .... Working on Roofs ..... Distributing Points : Corner Points. Leading-in Wires ..... Branch and Junction Points and House Connections Street Lighting ..... Cost of Local Distributing Systems Curve showing the Average Cost of Local Distributing Systems PAOB 239 239 240 242 242 243 245 252 256 257 259 262 269 270 25. Agreements with Contractors .... Special Conditions ...... Schedule of Prices for High-Tension Transmission Line Work Schedule of Prices for Overhead Local Distributing Systems 272 275 277 279 26. Tools and Appliances. For High-Tension Installations For Low-Tension Installations 282 282 283 27. Regulations dealing with the Erection and Operation of Overhead Lines ........ 285 Board of Trade Regulations for Overhead Lines .... 285 Regulations of the Verband Deutscher Elektrotechniker for Overhead Transmission Schemes . . . . . . .287 1. INTRODUCTION Electricity, which httle more than a decade ago was chiefly used for lighting purposes, has been developed technically and economically until it is now one of the most important power transmission agents. Current is now transmitted over hundreds of miles of line at voltages which only a short time ago appeared impracticable. The use of these high voltages has enabled large amounts of energy to be conveyed economically to greater and greater distances from the generating stations. The recent important advances in electrical engineering, especially the introduction of metal filament lamps and cheaper and more adaptable motors, has ensured the spread of electricity to the country districts. The reduction in price of current has largely increased the consumption, and, together with the higher economy of modern generating plant and its lower cost, has improved the profit-earning powers of electricity stations, especially of those catering for widespread country districts. Although the possible applications of electricity in country districts are very numerous and varied, yet the consumption figures can naturally never attain to those of the towns, so that, whilst town supply systems may prove profitable in spite of heavy first cost, to country stations low first cost is essential. In towns underground cable systems are permissible, but in widespread country districts, with few exceptions, overhead wires alone can be considered. The modern movenient towards the production of electricity in bulk at the coal mine or in the water power station, and its distribution thence over wide areas to both large and small consumers, has given increased importance to over- head lines, and at the same time, with a view to the necessary continuity of supply, has greatly added to the demands made on the designers and builders of such lines. And rightly so. In the generating station stand-by machinery is available which can replace the running machinery, in case of accident, in a few minutes. A skilled personnel is constantly on the watch to prevent faults and interruptions from developing, or, at any rate, from affecting the consumers. Further, the machinery in the station is not subjected to the severe atmospheric conditions which affect the overhead line. It is true that every possible provision in case of accident to the line itself is also made. The switching over to a reserve section, however, occupies a certain amount of time. However carefully the inspection of the line is carried out, it stiU only amounts to a periodic examination of each section, and it is impossible to avoid danger from storms, snow, sleet, birds, etc., and accidents from such causes can only be prevented by the most careful design and mechanical construction. O.T.L. B 2 OVERHEAD TRANSMISSION LINES The cost of the Hne, as a rule, forms the greater part of the capital cost of a central supply system feeding a widespread country area, and economy in this item will favourably affect the future profits of the station. Of course this economy must not be carried too far so as to affect the continuity of supply and, to a certain extent, the sesthetic effect of the structures. The search for economy should not be in the direction of cheap but unsuitable constructions and materials, but rather towards the careful selection of proved and standardised arrange- ments. Low first cost is by no means synonymous with low running charges. The expenditure on the repair and maintenance of badly designed installations generally exceeds the interest and sinking fund charges on the more expensive but carefully designed line. The same point of view applies to the local distributing systems within the villages or small country towns. Here, however, the sesthetic considerations must be given more weight. In the streets, and especially in the main streets, the whole arrangement of the lines and their supports must, as far as possible, be brought into agreement with the spirit of the place. At the same time this idea cannot be carried so far as to make each supporting structure a special and independent design. The construction selected should be such that the separate parts can be employed under the most varied conditions. The size of the stock carried and the delay in procuring materials will then be greatly reduced, and instead of each part being an individual production, repetition work, with its reduced cost, will become available. The erection of the line, in view of the small margins worked to in modern structures, requires special care and experience. Only conscientious and skilled men, controlled by engineers and inspectors fully conversant with the theory and practice of line construction, should therefore be employed, as this alone will ensure the careful adherence to the rules throughout. 2. MATERIALS FOR THE CONDUCTOR ,^_ f A MATERIAL Suitable for the electric conductor should have a low price ; combined with a high specific conductivity, low specific gravity, and great ' mechanical strength. The conductor material must also be considered with regard to its chemical inertness against atmospheric effects and with regard to the ease with which it lends itself to suspension and erection. ^— The most suitable materials, and those which best satisfy these conditions, are copper and aluminium. For special cases bronze, steel, iron, and " Monnot metal," or copper-clad steel, already much used in America, have been introduced. Table 1. — (a) Copper. Copper Wire Eemarks. Soft. Medium. Hard. Specific gravity 8-9 8-95 8-96 Breaking stress in lbs. per 31,600— 43,000— 59,000— "1 The higher values are for wires of 1 -04 to -12 inch diameter. ITor '' larger sizes the lower values are sq. inch. 37,000 51,500 66,000 Elastic limit in lbs. per 17,000 30,000— 43,000— sq. inch. 38,500 51,500 J gradually approached. f The higher values are for the smaller Safe working stress in lbs. per sq. inch. 7,000 14,300— 18,500 /^OOO— \ V^__^26,000J wires (-04 to -12 inch). If the ■{ wires are stranded into a cable only 90 to 92 per cent, of the load [ should be allowed. Modulus of elasticity E in 14-3x106 18x106 19x106 1 For cables Ei = -G E and ^ ft ^ ^ J '^' El -GE lbs per sq. inch. Extension coefficient Thermal expansion co- ■7x10-7 •56x10-7 -53x10-7 1-68x10-5 1-68 X 10-5 1-68x10-5 efficient a. Specific conductivity = 1 1-48x10^ 1-46x106 1-46x106 Specific resistance in ohms per inch cube. The rules of the Verband Deutscher Electrotechniker allow a working stress of 7,000 lbs. per square inch for soft copper wire and 17,000 lbs. per square inch for hard drawn copper wire. If the breaking stress and the stress producing permanent set are actually determined by tests on samples of the material, a working stress amounting to half the elastic limit stress is allowed, and this latter must itself be 80 per cent, of the breaking stress. Hard drawn copper may only be used in the form of stranded cables made up of wires not exceeding b2 4 OVERHEAD TRANSMISSION LINES •12 inch in diameter, since the breaking stress diminishes with increasing dia- meter. The special advantage of stranded cables over solid wires, besides the greater ease in handling, lies in the fact that damage to the hard surface or a weak place in one of the wires of the cable does not endanger the whole cable. For hard drawn copper cables the makers wiU guarantee a breaking stress up to 60,000 lbs. per square inch, the elastic limit being 80 per cent, of this. The allowable working stress would therefore be 24,000 lbs. per square inch. In order to allow for unavoidable errors during erection, however, 21,000 to 23,000 lbs. per square inch should not be exceeded. Hard drawn copper cables are chiefly used for high-tension overhead lines with long spans, whilst for shorter span work, and especially for local overhead distributing circuits, copper of medium hardness is employed. This is used in the form of sohd wires up to areas of -04 square inch, whilst for larger sections stranded cable is preferable. For the thicker wires the guaranteed breaking stress is 43,000 lbs. per square inch, and for stranded cables 47,000 lbs. per square inch. The elastic limit again occurs at 80 per cent, of these figures, so that the allowable safe stress is 17,000 to 19,000 lbs. per square inch, but is usually reduced to 14,000 to 17,000 lbs. per square inch. Soft annealed copper wire is no longer employed for overhead lines, because the elastic limit is so low that moderate excess loads may lead to excessive exten- sions, which dangerously increase the sag and necessitate a tightening of the wire. In local distributing circuits where the supports cannot be unduly strained copper of medium hardness should be selected and worked at a comparatively low tension. Soft copper is still sometimes used as a binding wire for attaching cables to insulators. Table 2.- — (b) Aluminium. Si' Stress in lbs. iier sq. inch. Modulus of Elasticity E in lbs. per sq. inch. Extension Coefficient Thermal Expansion Coefficient a. Specific Conductivity per inch cube. Is Breaking. Elastic Limit. Working. 13,000 for cables 10,000 Remarks. 2-7- 26,000— 29,000 23,000— 26,000 20,000— 21,500 18,000— 19,500 10-5X10S 7-7x105 •95x10-' 1-3x10-' 2-3 XlO-"* 2-3x10-5 •88x108 •88 X 105 f The larger values are for wires of -04 to -12 inch diameter or cables made up ol such wires. For larger diameters the lower values are approached. The rules of the Verband Deutscher Eleotrotechniker allow a maximum working load for aluminium of 13,000 lbs. per square inch. The breaking stress is 26,000 to 28,500 lbs. per square inch, and the elastic limit 20,000 to 21,500 lbs. per square inch, whilst the corresponding figures guaranteed by the makers for stranded cables are 21,500 and 17,000 lbs. per square inch respectively. Thfe safety of erected aluminium lines working at 13,000 lbs. per square inch is OVERHEAD TRANSMISSION LINES 5 distinctly less than that of a copper line, so that it is advisable not to exceed a load of 10,000 lbs. per square inch, and even then the safety is not equivalent to that of copper lines. The cross-section of aluminium, for the same conductivity, must be 1"7 times that of copper, so that the stress on sections of equal conductivity in hard drawn copper and in aluminium are 23,000 lbs. per square inch and 10,000 X 1-7 = 17,000 lbs. per square inch. Consequently the loading of terminal and corner masts for aluminium lines are reduced in the ratio of 4 : 3 approximately. The reduced tensile strength and higher expansion coefficient of aluminium, however, necessitate greater sag, so that, especially with the longer spans, the aluminium line involves the use of considerably higher masts. The masts, therefore, generally turn out to be dearer rather than cheaper for the aluminium line. The greater cross-section of the aluminium wire increases the wind pressure, and this especially affects the intermediate masts between terminal and corner masts, so that these must be considerably strengthened, partly because of the increased force and partly because of the increased height of application of the force. When wooden poles are used and in the case of local distributing net- works the aluminium wire may effect economies equivalent to the actual difference in cost between the copper and the aluminium. In these cases the spans are not great, so that the sag, which is proportional to the square of the span, is not appreciably greater than for copper wires and the height of poles need not be increased, consequently the special advantage of aluminium (reduced weight) favourably affects the necessary strength of supports. One disadvantage of aluminium lies in its softness. The chief strength lies in the hard outer skin, and in order to avoid injuring this special care in erection is necessary. At the same time, owing to the reduced weight, the cost of erection as a whole need not be greater than for other materials. The low melting point of aluminium increases the danger of breakdown through defective insulators ; and another electrical disadvantage is the greater inductance of the line resulting from the greater sag and consequent greater distance between the wires. The use of solid aluminium wires is no longer recommended by the manufac- turers because of the difficulty of insuring sufficient homogeneity in thick wires. Examination of actual breakages showed the formation of glass-hard brittle material at the breaking point ; but such occurrences have disappeared since the use of stranded cables has become general. The price relation of copper and aluminium depends both on the relative conductivities and the relative specific weights. The cross-section of the aluminium must be 1-7 times that of the copper, whilst the two specific gravities are 2-7 and 8-95, consequently for equal total cost : — 2"7 X 1'7 Copper price = Aluminium price X — ^7^^ — = Aluminium price X "51 i.e., the cost will be alike in the two cases if the price of copper per lb. is 51 per cent, of that of aluminium per lb. The curve Fig. 1 shows the saving, or (when 6 OVEEHEAD TEANSMISSION LINES the copper price is less than 51 per cent, of the aluminium price) the loss, effected by the use of aluminium in place of copper. The difficulty of producing a reliable soldered joint with aluminium has not yet been overcome, so that it is preferable to make joints by means of clamps or sleaves of the same material. The atmospheric effects are no more troublesome for aluminium than for copper. Alkaline fluids affect it, and certain acids attack it easily with the ■f 70% E 3 c tec e c S -HO « S 3 10. c -20 ■30 O,* 0,5 o.e 0,7 Cj9 to Fig. 1. r,z >.3 Price of Copper is Price of Aluminium evolution of hydrogen. Therefore unprotected aluminium lines are to be avoided in the neighbourhood of chemical works. On the other hand, it has been found that brine-laden sea air affects aluminium less than copper owing to the strongly adherent oxide coating on the former. In the Soiith of France long overhead Table 3. — (c) Bronze. ?.>f Stress in lbs. per sq. inch. Modulus of Elasticity E iu lbs. per sq. inch. Extension Coefficient Thermal Expansion Coefficient a. Specifie Conductivity per inch cube. 11 t»0 Breaking. Elastic Limit. Working. Remarks, 8-92 8-8 71,000 100,000 49,000 64,000 26,000 36,000 17X10= 17x10= ■59x10-' ■59X10-' 1-8x10-^ 1-8x10-5 1-3x10= ■9x10= 1 For cables Ei = ■& E El *6 E lines of aluminium run along the sea coast and have shown no signs of deteriora- tion over a long period. OVEEHEAD TRANSMISSION LINES 7 For the longer spans, when the amount of sag is to be limited, bronze is sometimes used. This is an aUoy of copper and tin in various proportions to suit different requirements in the way of conductivity and strength. The marked reduction in conductivity with the compositions of greater strength, together with the sensitiveness of the hard material to bending and the consequent Table 4:.—{d) Steel. s& Stress in lbs. per square inch. Modulus of Elasticity E* in lbs. per sq. inch. Extension Coefficient ^4- Thermal Expansion Coefficient a. Specific Conductivity per inch cube. II Breaking. Elastic Limit. Working. Remarks. 7-95 • 100.000 130,000 185,000 71,000 100,000 185,000 36,000 50,000 71,000 27 X 10" 30x10" 30-7x10" ■37x10-' •333x10-' •325X10-' 1 |. 1-2x10-5- J -16X10" •145X10" •125x10" Steel. Cast-steel wire. Patent cast-steel wire. * For stranded cables ^i ■ •6.Bandft = i=:g^. increase in the care required for erection, has limited the use of the copper bronzes to special cases. For very special cases in which very small sags are required with excep- tionally long spans galvanised steel wire conductors are sometimes employed. The conductivity is very poor. The maximum permissible conductor area is limited by the strength of the supporting poles. However, the length of the Table 5. — -(e) " Monnot metal " {Copper-clad Steel). .Si Stress in lbs. per sq. inch. Modulus of Elasticity E in lbs. per sq. inch. Bxtensiou Coefficient. Thermal Expausion Coefficient a. Specific Conductivity per inch cube. Breaking. Elastic Limit. Working. Remarks. 8-45 8-3 83,000 128,000 67,000 107,000 34,500 64,000 27X10" 30x10" •37 X10-' •333x10-' 1^2 X 10-5 b2xl0-5 •74X10" •54x10" For cables ^1 =-6E r and Si - „^ „ ■6E steel wire portions of the line are generally so small compared with the whole length of line that the increased ohmic resistance and inductance of these portions are negligible. The desire to combine the high conductivity of copper with the strength of steel led at a very early stage to the use of steel ropes supporting copper con- ductors. This construction puts excessive stresses on the poles in the case of long spans, not only because of the increased weight to be carried, but also because of the increased effect of snow, ice, and wind pressure. OVEEHEAD TRANSMISSION LINES Table 6. — Bare Copj>er and Aluminium Conductors. Number of Single Wires. Diameter of the Single Wires. Outer Diameter of Strand. Weight in lbs. jier mile. Copper. Aluminium. Copper. Aluminium. Copper. Aluminium. Sq. Sq. Sq. Sq. Cop- Alumi- Copper. Alumi- mil.. incli. mm. incli. per. nium. nium. mm. inch. mm. inch. mm. inch. mm. inch. 6 ■0093 2^76 •109 2^76 ■109 190 __ 10 ■0155 17 ■0265 7 3-56 ■14 1^76 •069 3^56 ■14 5-28 ■207 320 164 16 •025 27^2 ■042 7 4^52 •178 2^23 •088 4^52 ■178 6^6 ■26 510 265 16 ■025 27^2 ■042 7 1^71 ■067 2^23 ■088 51 ■2 6^6 ■26 520 265 25 •039 42^5 ■066 19 564 ■222 1^7 ■067 564 ■222 8-35 ■33 800 410 26 ■039 42^5 ■066 19 213 ■084 \-n ■067 6^4 ■252 835 ■33 820 410 35 ■054 59^5 ■093 19 252 ■1 20 ■079 7^7 ■304 9^95 ■392 1,140 580 50 ■078 85 •132 19 19 h83 ■072 2^39 ■094 9^2 ■362 n-9 ■47 1,620 830 70 ■108 119 ■184 19 19 2^16 ■085 2^82 ■111 10-9 ■44 14^1 ■555 2,280 1,150 95 ■148 161^5 ■25 19 19 2^52 ■1 33 •13 12:7 ■5 16-3 ■642 3,100 1,570 120 ■186 204 ■316 19 37 2-84 ■112 2^65 ■104 143 ■562 18^7 ■74 3,900 1,990 Attempts were also made in connection with telegraph work to replace plain steel wires by wire of improved conductivity and protected against rust by using steel wires coated with copper. The earlier attempts failed, however, through the difficulty in getting the copper coating to adhere to the steel. Cracks used to occur in the coating, and the subsequent electro-chemical action soon caused the steel to rust through. Recently, however, the Americans have suc- Table 7. — Insulated Copper Conductors for use when Low-Tension Mains Cross Telegraph Wires. Cross- section. Single AVires. Outer Diameter of the Insulated Cible. Gross Weight in lbs. Diameter. Sq. inch. Number. , mm. inch. mm. inch. Per mile. Per foot. 6 •0093 1 2^76 •109 6 •24 285 •054 10 ■0155 1 3-56 •14 7 •28 410 •078 16 •025 1 4^52 •178 8 •32 625 •118 16 •025 7 1^71 •067 8^5 ■34 660 •125 25 ■039 1 5-64 •222 9 •36 930 •175 25 •039 7 213 •084 10 •4 980 •186 35 •054 7 252 ■1 11-5 •46 1,340 •255 50 •078 19 1^83 ■072 13 •51 1,870 •355 70 •108 19 2-16 •085 15 •6 2,57a •49 95 •148 19 2-52 •1 105 •65 3,400 •65 120 •186 19 2^84 •112 18 •71 4,300 •82 OVERHEAD TRANSMISSION LINES 9 ceeded in producing in " Monnot metal," or " copper-clad steel," a conductor in whicli the copper coating is metallurgically welded on to- the steel core. A steel block is dipped into molten copper untU the outer steel shell becomes alloyed with copper. In order to obtain the required thickness of copper coating the steel block then has a mass of copper cast round it, and the whole is then rolled out into wires of the diameter desired. These bi-metaUic wires have come into Table 8. — Wire Ropes of Galvanised Iron and Steel Diameter of Rope.* Wire Material. Number of Wires. Working load lbs. Weight per mile lbs. mm. inch. Iron . 84 3 ■118 220 90 Iron 49 3 •118 245 129 Steel 49 8 •118 610 129 Iron 96 5 •197 610 465 Iron 42 5 •197 890 320 Steel 42 5 •197 2,650 320 Iron 72 7 •275 1,330 560 Steel 77 7 •275 4,900 610 Iron 42 9 ■355 2,450 1,040 Steel 49 9 •355 6,700 1,160 Iron 42 12 •475 4,900 1,940 * With hemp core. very general use in America, both for telephone and signalling work and also for power purposes. It has not yet been much taken up on the Continent, although it is a most suitable material for long-span work, as it combines high conductivity with high tensile strength and the capacity to withstand all weather conditions. The fear that cracks would occur or that the copper coating would strip off owing to the unequal expansion coefficients of the two metals has proved to be unfounded. The intermediate alloyed layer appears to equalise the differences successfully. 3. TENSION AND SAG OF THE CONDUCTOR (1) With Sttfporting Points at the same Level A PERFECTLY flexible wire suspended between two fixed points A and B (Fig. 2) assumes, under the action of its own internal and external forces, the form of a catenary curve, the stress on which is at all points proportional to the length. The catenary curve is almost identical with a flat parabola, so that, for simplicity, the equation to the parabola will be used in the following pages. When the sag or dip d is very small com- pared with the span a, as is almost always the case, and when allowance is made for the fact that overhead lines are not perfectly flexible, the error introduced by this simplification is negligible. (For the exact treatment see Keil, " Elektrotechnischer Anzeiger," 1911, Parts 63, etc.) The forces being balanced as in Fig. 3, the following relations hold : s .d = ^ ag . I a Fig. 2. S = a^g d = a^g The tension s exists at the apex (or vertex) of the parabola. At other points it is greater and reaches its maximum value at the supporting point A. The smaller d is in relation to a, however, the less will the tension at the support differ from the minimiim stress s. In practice d is very small (the sag is only about 1| to 3 per cent, of the span), so that it may, with sufficient accuracy, be taken that the stress at the supporting points is equal to s. The equations 1 and 2 are independent of the temperature and hold for any condition of tension in the wire. The flat parabola (Fig. 4) has a radius of curvature r at the apex equal to the parameter of the parabola, so that the length of arc corresponding to chord a and the angle cj) is given by the expression : Fig. 3. OVEEHEAD TRANSMISSION LINES 11 2 ""'"'^ 180 r sm 2 r andsm _=_+_ (^-J + 27475 l2rj + " " ' ' ■"2 *" L2r"'~2.3 \2»7 +2.4.5 \2r/ + • • • • J From Fig. 4 : -j— Ird from which the parameter of the parabola r = ^. Inserting this in the above expression : :~ 8ti L 2 ■ a^ + 2. 3 I 2 ■ aV J '+51^) +¥(?)'+■■•■] Fig. 4. and since 2c? is very small compared with a, the higher powers may be neglected, giving : . ^ -=«['+§(¥r]-.-+ 8^ 3 ' a ) (2) With Supporting Points at Different Level.s. When the line is supported between two. points at unequal heights the apex of the parabola no longer falls midway between them, z.e., the parabola , is not symmetrical. The symmetrical form is arrived at if the span is altered from AB either to B'B (Fig. 6) or to AG (Fig. 6). In the former case an intermediate support B' , and span a! are assumed, and in the latter case an external support C and a span a" are assumed. The imaginary spans can be determined from the results of the survey work on the route, (a) Solution for Fig. 5 : — Fig. 5. The summation of the moments about A ov B gives s (^ + /) = (a ^ I) X 9' X a 2" =l«-| a a a "^ ~ 2"'^ "4 ~ ''' T 12 OVEEHEAD TRANSMISSION LINES subtracting one equation from the other — 2 s/ = ga^ — gaa' + 9 -^ 9 ^ or a = a 2jf ag 4 The sag d is found for the span a' in the same way as for -supports of equal height. (6) Solution for Fig. 6 :— The summation of the moments about j A and B gives ^ a a s{d-f)= ( a-y) » a — Fis. 6. Subtracting one of these equations from the other — 2sf = g 2sf = g 2\ 2 g (a - ga^ 2 2 gaa a" = a + ^ . 5 The distance of the lowest point (apex of the parabola) from the sup- porting point A is : 17 ^" n / I 2 sf ag y ^ 2 s/ + a^g 2ag Its distance from the other supporting point B is : W = a-^=a-^(a + ^ 2 ^ \ ^ ag W _ a^9-2 sf 2ag If this equation gives a negative value the lowest point of the parabola lies outside the span, as shown in Fig. 7. It is sometimes necessary, for instance in connection with undulating country, OVERPIEAD TRANSMISSION LINES IB to be able to determine the sag at any given distance Vp from the apex (Fig. 8). This can be done as follows : — If r is the; parameter of the parabola again : V^ = 2rd [v,.r 2r . x„ For x^ = d — dj, Y1 = AL^1_ d d — d„ d„ = d V^—V„ The position of the apex of the parabola with regard to the supports depends on the difference in level and on the distance between them. If the sag happens ^-<7^ 1 F 'f — ;> . * — - a J a'->- . - a"J Fix. 7. Fig. to be the same as the difference in level the apex will fall at the lower supporting point. With small spans and great differences of level the apex may even fall outside the span, as shown in Fig 7. (7 From the equation d = -„— (in which g stands for the weight of unit length 8 s of line of unit cross-section and s for the stress or tension per unit of cross-section) it is clear that the sag is independent of the cross-section of the wire and the fol- lowing rule holds good : The^sag^ of a conductor for a given span with the same stress {lbs. per sqvure inch) is the same for all areas of cross-section. This is of importance in practice, as it means that all the wires of the same material mounted on the same poles wiU have the same sag if tightened up to the same stress (lbs. per square inch). Numerical Example 1. Suypo rtsat the same Level. Two masts for a high-tension transmission line are to be placed 400 feet apart. The cross-section of each of the copper wires is -055 square inch. With 14 OVEEHEAD TEANSMISSION LINES a specific gravity of 9 the weight of 1 foot of wire of 1 squarejinchsectioiLis_3-9Jbs. The tension in each wire at + 20° C. is to be 490 lbs. = 9,000 lbs. per square inch. It is required to find the sag and the length of wire. By equation 2 : n,^ ^U/,^.^^^ '^ = ts= 8 X 9,000 = ^^^ ^'^'- V ^ I Iv- ^ By equation 3 : r , 8 ^^ .^^ , 8 X (8-65)2 .^n k f ^ ^ = « + ¥¥ = ^'^^ + 3 X 400 = *^^^ ^'^*- Example 2. Supports at different Levels. Distance between masts, 650 feet. Difference in level at the two ends, 16 feet. Copper^tggs of -055 square inch section. Weight of 1 foot of 1 square inch section =(3-9 IbsJ) Stress at + 10° C. = 8,800 lbs. per square inch (480 lbs. total tension pe^^Irej. It is required to find the sag and the distances of the apex of the parabola from both supports. By equation 2 : d = — -^ ; and by equation 4 : 2 s/ a = a ag If the calculation is worked out in reference to the higher of the two supports the results are as follows : — By equation 5 : a" = a + ?-^ ag _.- . 2 X 8,800 X 16 _.. . , = ^^^ + 650 X 3-9 = ^^^ ^""*- , , a"^g (761)2 x 3-9 „„ , . , ^^d ^ = ^ = 8 X 8,800 = ^^'^ *"^*- By equation 6 : 2sf + a^g 2ag 2 X 8,800 X 16 + (650)2 x 39 2 X 650 X 3-9 OVERHEAD TRANSMISSION LINES 15 By equation 7 : '2ag __ (650)2 X 3-9 - 2 X 8,800 X 16 2 X 650 X 3-9 Example 3. = 270 feet. Supports at different Levels. Distance between masts = 650 feet. Difference in level between the two supporting points =130 feet. Transmission line consisting of three copper conductors each of "055 square inch section. Weight of 1 foot length of copper 1 square inch in section =3-9 lbs. Tension in each wire at 0° C. = 490 lbs. = 9,000 lbs. per square inch. It is required to find the sag and the position of the lowest point of the span (apex or vertex of the parabola). By equation 5 : By equation 6 : y^ 2sf + a^g 2ag T-r _ 2 X 9,000 X 130 + (650)2 X 3-9 _ _„_ . , - - •^ - 2 X 650 X 3-9 - '^^ *^^*- By equation 7 : 2ag __ (650)2 X 39 - 2 X 9,000 X 130 . "^ - 2 X 650 X 3-9 - " ^-^^ *®®*- i.e., the apex of the parabola lies outside the supporting points. Example 4. Using the values given for example 2, find the sag at a point 163 feet from the high level supporting point. ' It has been found that d = 32-16 feet and F = 380 feet. Putting these values into equation 8 : dp = d ^^ "^3^^\ with Vp = 380 - 163 = 217 feet, .. = 32.16 '^%-//"'' = 21-, feet. 16 OVEEHEAD TRANSMISSION LINES The Effect of Temperattjee on the Tension and Sag of Conductors. From equation 1 it is seen that the product of stress and sag is a constant for a given span and material of conductor ; any diminution in sag results in an increase in stress, and vice versa. The fact that all materials used as con- ductors increase in length when subjected to increased tension complicates the problem. The sag, in fact, does not increase so much as would be expected from equation 1, for as soon as the tension falls the wire contracts and shortens in length and so diminishes the sag. When the line is erected between its supports it is given a certain tension and sag. These will not be permanently retained however, but will vary with a,ny additional loads on the wire and with temperature changes. The former are pro- duced by wind force or by snow or ice accumulations on the wire, which act as an addition to the tension due to the wire's own weight, and so lengthen out the wire and increase the sag. The ordinary atmospheric variations of temperature lengthen or shorten the wire more or less according to the value of the thermal expansion coefficient of the material. In the following calculations : Sg = maximum allowable stress in lbs. per square inch ; s^ = stress at temperature f C. ; i/„ = length of the conductor when subjected to the maximum stress ; ij = length of conductor at temperature f G. \ g = weight of 1 foot of the conductor material 1 square inch in section, in lbs. ; g, = additional load^in lbs. per 1 foot length of conductor 1 square inch in section ; a = length of span in feet ; d = sag in feet ; t = temperature in ° C. for which 5, and d are to be determined ; a = thermal expansion coefficient for the material ; 1 /3 = elastic extension coefficient (= -^). ' III In accordance with equation 3, the length of a conductor hanging in the shape of a flat parabola is : \L = a + ^ 8d^ , 3a' If the temperature is raised the wire expands in length by an amount equal to a feet for each foot length of wire and for each 1° C. rise. Each piece of wire which originally measured 1 foot will lengthen by an amount equal to « t when the temperature rises by t° C, so that the total additional length will he L at and the new total length will be i' ^ L -\- L^t. The elasticity of the material counteracts this extension, for under the influence of the increased length the tension falls and the material draws itself together. OVEEHEAD TRANSMISSION LINES 17 The ratio of the increase in length to the original length is called the strain, and this is proportional to the stress up to the elastic limit of stress for the material. The ratio — ; = /3 is called the extension coefficient, and its reciprocal stress -^ = ^ is called Young's modulus of elasticity, i.e., that imaginary load or stress at which a wire, say, 1 foot long would become extended by an equal length of 1 foot, assuming that the elastic limit were not previously reached, so that the initial rate of extension continued to hold good. If the stress on a wire of length L is reduced by s lbs. per square inch the wire win become shortened by an amount equal to Ls 0. Starting with the lowest temperature t„, at which the length of line is L„, and calling the length corresponding to a temperature t° L', this new length would be: L' = L, + LAt - Q= L,[\ + «(« - U], {t — ■ t^) being the temperature difference considered. Owing to the elastic contraction, however, the length of wire at tempera- ture t is not U but only i(, where L, = L,[l + u{t - Q] + LS{si - s„). Here 5„ is the maximum stress occurring at tha lowest temperature t„ and Sj is the stress to be expected at the higher temperature t. Further ^ L, = a + ||U. = a + |f , or a + ll" = a + 1^ + LAt - Q + LM^t - So). In the two last terms it is quite permissible to replace the length of wire L„ by the length of span a, so that "^ + 1 ¥ =^ "^ + 1 "I" + "^ " (^ ~ *"^ + ''^^'' ~ '''^■ 2„ «2^ / 0,(1 Q,n Substituting the values for dt and d„ in this (viz., cij = — ^ and d.„ =-^ the result is : t 2«2 1 «2„2 i8-t^=i8-t + «('-*^)+^(^*-^»)' and from this f iirt o ft aft a 24 5^2^ " " 24 s„^0 /3 ^ "' The minimum temperature t„ is, in Mid Europe, commonly taken as — 20° C. The rules of the Verband Deutscher Elektrotechniker specify that the stress conditions are to be calculated both for a minimum temperature of — 20° C. without additional load and also for a temperature of — 5° C. with an assumed ad d^t.innal ice a,nd snow load amounting to -015 kg. per metre length and per O.T.L. c 18 OVEEHEAD TKANSMISSION LINES square millimetre of cross-section ( 6'6 lbs, per foot length and per square inch of crQss::sectiaa.) For these two cases the equation then is : and s - "^'9^' - 9 _ ^'g^' _ « f/ + 5^ 11 If the numerical values given by these two equations are worked out for various spans it will be found that up to a certain length of span the — ^ 20° C. rule gives the greater stresses, whilst beyond this span the — 5° C. with snow and ice load works out as the higher. Schauer * has worked out the expression for this critical span and finds that its value is given by , , — ~~, 9'/ — g^ where a = span in feet ; s = allowable stress in lbs. per square inch ; g = weight of line material in lbs. per foot length and 1 square inch section ; g^ = weight of line loaded with snow and ice per foot length and I square inch cross-section ; a = thermal coefficient of expansion. This formula gives the following values for the critig al lengths of span : — lbs. per \| sq. inch. feet. Hard-drawn copper with a maximum working stress of 21,500 .. 166 Medium copper wire ,, ,, 15,800 . . 124 Soft copper wire „ „ 7,200 . . 56 Bronze wire „ „ 40,000 . . 326 " Monnot metal " (or copper-clad steel) wire with a maximum working stress of . . . . 57,000 . . 390 Aluminium wire with a maximum working stress of . 10,000 . . 117 Steel wire „ „ 72,000 . . 490 Iron wire „ „ 34,000 . . 235 Example 5. A H.T. line of hard drawn copper is erected on masts 390 feet apart. What sag must it be given at -j- 10° C. ? . ^ For hard drawn copper with a maximum working stress of(21,500 lbs) per square inch the above table shows that the critical span is 166 feetTconsequently the maximum tension when the span is 390 feet will occur at — 5° C. with addi- * Elektrische Kraftbetriebe und. Bahnen, 1910. h OVERHEAD TRANSMISSION LINES 19 tional load (and not at — 20° C. without additional load of ice and snow). Using equation 11, therefore, and the values in Table 1 : St — (3-9)2 X (390)2 = 21,500 - (6-6 + 3-9)2 X (390)2 24 X -53 X 10-' X 5(2 --=""" 24 X -53 X 10- ■^ X (21,500)2 1-68 X 10-5 (10 + 5) from which By substituting for Sf the expression x ~ •53 X 10-' S(3 + 11,800 5(2 - 1-82 X 10^2 = 0. 11,800 X — 3,933, the above is reduced to a? — 46-2 x 10^ a; - 1-7 X 10^2 ^ q. Solving this by means of Cardan's method, x = 13,200, or 3,933 = 13,200 - 3,933 = 9,267 lbs. per square inch. St = X The sag is then a^g _ (390)2 x 39 8 5, 8 X 9,267 8 feet. Example 6. A line passing over undulating country has to cross a cutting having a span of 1,250 feet and a depth of 66 feet as shown in Fig. 9. It is required to know whether this span can be bridged with hard drawn copper wire weighing 3-9 lbs. per foot of 1 square inch section, with- out exceeding a stress of 21,500 lbs. per square inch and without letting the line approach the ground by less than 20 feet. The height of the masts up to the lowest cross-arm is 30 feet. In this case the points to be con- sidered are the sag at the highest temperature (+ 40° C.) and the maximum tension, which will occur at the — 5° C. temperature with the additional ice and snow load. -1250 Ft '■ Copper ■ Bronze Fig. 9. By equation 11 : "'^' =5.-.^ -^(^ + 5) 24 5(2^ 24 5„2/3 /3 or St 1,2502 X 3-92 1,2502 X 10-52 24 X 5(2 X -53 X 10-' ~ ' 24 X 21,5002 X "53 X 10-' from which The sag then is 1-68 X 10-%^^ , .. - -53 X 10-' ('^ + ^)' 5( = 7,900 lbs. per square inch. oV ^ 1,2502 X 3-9 ^ 8 5( 8 X 7,900 c 2 20 OVERHEAD TRANSMISSION LINES Also the sag at — 5° C. with additional load is : , aV 1,2502 X 10-5 „ , . . , d = -H-^ = -5 „, -„„ = 94-5 feet. 8 s„ 8 X 21,500 The maximum sag, therefore, occurs at + 40° C. Since the bottom of the cutting is only 66 feet deep it will not be possible to effect this crossing in the way suggested, using masts 30 feet high. Three ways are available for getting over the difficulty : — (1) The height of mast may be increased from 30 to 50 feet. This involves heavy and expensive masts with correspondingly heavy foundations. (2) The line may be supported on a central mast at the bottom of the cutting at a sufficient height to give the required minimum clearance at all points. In this case the central mast may be taken as 40 feet high, so as to allow for the fact that the line will hang in the form of two parabolas, each having a certain amount of sag. The case is, therefore, that of a line having a difference of level between the two supports of 56 feet and a span of 625 feet. By equation 5 : a" = a + M = 625 + ' \'''''\\ '' = 990 feet. ag^ ' 625 X 10-5 Knowing this imaginary span a", the tension in the wire at 40° C. can be found by equation 11 : 9902 X 3-92 _ 2j gQQ _ 9902 ^ 1052 ' 24 X 5(2 X -53 X 10-' ' 24 X 21,5002 x "53 X 10 from which Sj = 7,900 lbs. per square inch. The sag at 40° G. = d = "f^ ^J^^ = 60 feet. The sag at - 5° C. with additional load = d = " ^^ _.^ = 59-7 feet. Since the difference in level between the supporting points is 56 feet, the lowest point of the line will lie about 4 feet lower than the bottom support, viz., at a height of about 36 feet above the ground. This lowest point is found by equa- tion 6 to be : „ 2 X 7,900 X 56 + 6252 x 3-9 ^__ , ^ ^ = 2 X 625 X 39 = ^^^ ^^^* away from the higher supporting point. Since the bottom of the cutting is not quite horizontal, but rises somewhat at the sides, the extra clearance available (36 feet in place of 20 feet) will be an advantage. (3) The third possible solution lies in choosing a conductor material which can safely be tightened so as to hang with less sag. If bronze is selected OVERHEAD TRANSMISSION LINES 21 with a safe working stress of 36,000 lbs. per square inch the stress at + 40° C. is obtained by equation 1 1 : _ (1,250)^ X (3-8)^ _ _ (1,250)^ X (10-4)^ ' 24 X St^ X -59 X 10-' ~ ' 24 X (36,000)2 x -59 X 10"' 1-8 X 10-s - -59 X 10-' (^0 + ^)' from which St = 15,400 lbs. per square inch, A ^-x, ■ J (1,250)2 X 3-8 ,o ^ . ^ and the sag is d = - o ./ i g ^ att = 485 feet. ° 8 X 15,400 At — 5° C. with additional ice and snow load the sag is : (l,250)2_x 10-4 _ "^ - 8 X 36,000^ - ^^ ^^^*- The lowest point of the line will therefore be 66 + 30 — 56 = 40 feet above the floor of the cutting. The sag could, therefore, be considerably increased even, and this would reduce the tension and enable less powerful masts to be used. The permissible maximum sag is 96 — ^ 20 = 76 feet. This maximum sag will occur at — 5° C. with additional load and will correspond to a stress (1,250)2 X 10-4 8 X 76 = 26,500 lbs. per square inch. The above example shows that the maximum sag does not always occur under the same circumstances ; it may, in fact, either occur at the highest tem- perature or at the low temperature when the additional load due to ice and snow is present. The two sags wUl be alike when the extension due to the additional load in the one case happens to be equal to the elongation due to the temperature rise in the other case. The temperature for which this holds good can be found as foUows : — The sag at — 5° C. with additional load is : ^"87; At the temperature t° C. the sag is : Equating these two values : d=^. 8 St 8 s, 8 St' s„ St' "g. Inserting this value in equation 1 1 gives : g a^g,^ _ _ a^gj' ^ ^ (f ^ n) or ^"^^^^ ~ J^^ ~^ ^^' and t = s,{l-j)^-5 . . . 13 iJz' 22 OVEEHEAD TEANSMISSION LINES i.e., the temperature at which the sag due to the line's own weight is equal to the sag at — 5° C. due to the line's weight together with additional load is independent of the length of span. Fig. 10. The stress /o which the line must be subjected to make the sags alike is s„ = ^ (« + 5) X —^^— ■ " ^ gz - g 14 OVEEHEAD TRANSMISSION LINES or, wit'i t = 40° C, s, = 45 I {—^^ \ . From this equation the following values are obtained :- 23 15 Stress in lbs per sq. inch. Metal. Single Stranded ■ Wire. Cable. Hard drawn copper ....... 22,700 13,800 Medium copper ........ 21,500 13,000 Soft copper . . ... . . 17.200 10,500 Aluminium ......... 12,700 9,600 Bronze with 71,000 lbs. per sq. inch breaking stress 12,700 9,600 „ 100,000 22,000 13,400 Copper-clad steel „ 83,000 23,200 13,800 „ 128,000 25,000 15,200 Steel „ 100,000 22,700 13,500 „ 130,000 24,500 14,800 „ 185,000 25,200 15,300 If these values are selected as the maximum working stresses the sag at — 5° C. with additional load will be the same as at + 40° C. With higher stresses the maximum sag will occur at — 5° with additional load, whilst with lower stresses the maximum sag will occur at + 40° C. The stresses which must be selected in order that the sag at — 5° C. with additional load shall be the same as at any other given temperature between — 5° C. and + 40° C. are shown in Fig. 10 for copper and aluminium wires and cables. Example 7. A H.T. line consisting of a stranded copper rope or cable of -078 square inch cross-section crosses a road at right angles. The maximum stress that is ever to occur is not to exceed 5,700 lbs. per square inch. At what height above the roadway must the lowest cross-arm be placed in order that the line shall never approach nearer than 23 feet to the road ? The general arrangement is shown in Fig. 11. Using equation 12, the critical length of span works out as : a = 6s V J^ "^ a = 6 X 5,700 V 10 X 1-68 X 10 = 46 feet, therefore the maximum stress will occur at load, and by equation 2 : 10-52 _ 3.92 5° C. with additional snow and ice aV _ 130^ X (3-9 + 6-6) _ "^-85- 8 X 5 JOO - '^ ^ *''®*' 24 OVEEHEAD TEANSMISSION LINES Since the selected stress is less than 13,800 lbs. per square inch the maximum sag win occur at + 40° C. and the stress Sj at this temperature is found by equa- tion 11 : 1302 X 3-92 „ „„„ 1302 X 10-52 24 s,2 X -89 X 10-'' = 5,700 — 24 X 5,700 X -89 X 10" 1-68 X 10-5 . .^ , .. - -89 X 10-^ (^Q + ') from which Sf = 1,680 lbs. per square inch. By equation 2 the sag at this temperature of 40° C. is, therefore, , a2ff 1302 X 3-9 ^^° = rfr 8 X 1,680 -'^'''- Should the line accidentally break on both sides beyond the roadway the suspension insulators would swing inwards and so shorten the effective Fig. 11. span and increase the sag. Assuming that the insulators swing out of the vertical by an angle of, say, 35°, and that the length of the suspension is 4 feet, the reduc- tion in span would amount to 2. X 2-35 = 4-7 feet (since an angle of 35° subtends a chord of 2-35 at a radius of 4 feet). The maximum length of wire in the span occurs at + 40° C. and is : When the outer wires break and the temperature is + 40° C, this length of wire wiU exist in a span of 130 — 4-7 = 125-3 feet. Using the equation L = a + 8(P 3a' OVBEHEAD TEANSMISSION LINES 25 the value of the new sag is found to be : d = Vl {L~a)a = V | (130-5 — 125-3) 125-3 = 15-6 feet. o o The height to the lowest cross-arm will therefore be made up as follows : — Feet. Minimum height above roadway . . . .23 Sag with outer wires broken . . . . .15-6 Length of insulator chain ...... 4 Height 42-6 Sag and Stress in Insulated Condxjctoes. The formulae developed above are also directly applicable to the case of insu- lated conductors. The weight of line is made up of the weight of the conductor and the uniformly distributed weight of insulation material. The thickness of the insulating covering is approximately the same for all the ordinary wire sections (•01 to -2 square inch). Since the specific gravity of the insulation is decidedly less than that of the conductor, it follows that the total weight per unit of length and per unit of cross-sectional area diminishes with increasing diameters. It is therefore necessary to use different stresses with insulated wires of different cross-sections if all are to hang with the same amount of sag. The additional load due to ice and snow depends on the gross cross-section q^ of the insulated wire instead of on the net cross-section q of the wire itself. The additional load per foot run and per squs.re inch section of conductor is, therefore, 6-6 x — in place of 6-6 lbs. If gi is the gross weight of the insulated conductor in lbs. per foot run, equation 2 for sag becomes : ^^g!x .9. + 6-6g, ^g q 8 s ^nd * - 7 ^ — O — ^^ The critical length of span, allowing for the variable weight per unit of gross cross-section, is different for each section. For the limiting cross-sections covered by Table 7 (-0093 and -186 square inch) and for a maximum stress of 14,300 lbs. per square inch of conductor, equation 12 becomes : — /lO X 1-68 jV -352 _ ■ 68 X 10- = 6 X 14,300V -352 — -0542 = £9-8 feet. (-0093) for the -0093 square inch wire. / 10 X 1-68 X 1 0"^ and a = 6 X 14,300V 3-422— -82^ = 63 feet. (-186)2 for the -186 square inch wire. 26 OVEEHEAD TKANSMISSION LINES As these spans are very short, the maximum stress will usually occur at — 5° C. with additional load rather than at — 20° C. without additional load. In order to determine at what temperature the sag of an insulated conductor without additional load will be the same as at — 5° C. with additional load, equation 13 can be used. For the -0093 square inch section wire : •56 X 10-' t = 14,300 1 •054\ ■35/ X 1-68 X 10- — 5 = 351° C. and for the -186 square inch section : •82' t = 14,300 \ 3-42 X •93 X 10-' 5 = 65-2° C. 1^68 X 10- The maximum sag will, therefore, usually occur at — 5° C. with additional load for ordinary copper wires of medium hardness worked with a maximum stress of 14,000 to 17,000 lbs. per square inch. Example 8. Owing to the neighbourhood of postal wires a certain overhead local dis- tribution scheme is to be carried out with insulated wires. The sections of the wires are -0093 square inch and ^054 square inch and the distance between the poles is 130 feet. What amount of sag must be given to the smaller wires in order to keep the maximum stress to 14,300 lbs. per square inch ? What stress will the larger wire be subjected to if it is to have the same sag as the smaller wire ? From Table 7 for insulated copper conductors it is seen that Lts. per foot run. Ui = -054 91 = -255 Inch. = -24 = -46 Square inch. Qi = -04:5 qi = -167 The sag of the -0093 square inch Avire at — 5° C. with additional load is, by equation 16 : 1302 .054 _|_ 6-6 X ^045 For a Fo: a For a 0093 square inch wire .... 054 „ „ .... 0093 square inch wire, the outside diameter 034 0093 square inch wire, the gross cross-section vo** ,, ,, ,, ,, d = X = 5-56 feet. •0093 '^ 8 X 14,300 For the same sag the maximum stress in the -054 square inch wire will be 1S02 ■054 X •255 + 6^6 X -167 8 X 5^56 = 9,600 lbs. per square inch. Sag and Stress in Safety Network Condtjctobs. In the special network conductors sometimes used at important crossings, the weight of the longitudinal wires is increased by that of the cross wires, which may be considered as uniformly distributed over the length. OVEEHEAD TEANSMISSION LINES 27 The sag and stress of the longitudinal wires can, therefore, be determined by equations 16 and 17 if for the weight g, the total weight of longitudinal and cross wires is used, and if the total additional load of ice and snow for the combined cross-sections q^ of the longitudinal and cross wires is worked out and reduced to the cross-section actually subjected to stress. The factor of safety should be made at least 4, because the additional load specification is rather low for small sections of wire and because the cross points in the network conductor encourage the collection of snow. Sag akd Stress of Steel Wire Ropes Supporting Cables. Telephone wires run underneath H.T. transmission lines are sometimes made up in the form of a two-core lead-covered cable, in order to ensure satisfactory speaking even in case of an earth fault in the H.T. lines. Such a cable is supported by means of a steel wire rope. The loading of this rope can be considered as uniformly distributed, and the effective weight on the rope can be taken as equal to the weight of the steel itself together with that of the conductor cable and the supporting links. The additional load due to ice and snow must be estimated from the total cross-section of the steel rope and the cable. Sometimes the space between the steel rope and the cable may become com- pletely fiUed with snow and ice, and in that case the usual allowance is far exceeded, so that an ample factor of safety should be employed in these constructions. Example 9. Underneath a H.T. -copper line of -055 square inch and having a span of 490 feet and a maximum sag (at — 5° C. and with additional load due to ice and snow) of 14-2 feet, a steel wire rope of -062 square inch section, 185,000 lbs. per square inch breaking stress, and a weight of -22 lb. per foot run is suspended. To this rope a telephone cable is attached having an outside diameter of -59 inch and a weight of -56 lb. per foot run. With what sag should the steel rope be erected if the maximum working stress in it is not to exceed 50,000 lbs. per square inch ? By equation 16 : rf = 490_^ X (-22 + -56) + 6-6 (-062+ -59^ X. I) ^ ^S-g feet. ■^^^ , 8 X 50,000 Since the sag of the steel rope is more than double that of the H.T. copper line, it will be advisable to increase the sag of the latter by reducing its stress to between 14,000 and 17,000 lbs. per square inch so as to relieve the strain towers and comer poles somewhat. The temperature at which the sag of the steel rope wUl be the same as at — 5° C. with additional load can be found by equation 13 : m+:^ ,A ^ /.gs X 10- t = 50,000 \^l - .22 ^ .gg ^ g.g (^.062 + -59^ x ^) j "^ If 2 X 10- = 163° C. -5° 28 OVERHEAD TEANSMISSION LINES In Tables 9 to 12 particulars as to sag and stress in bare copper cables with various spans are collected. The weight of the cable has been taken as 4-05 lbs. per foot run of 1 square inch cross -section, which corresponds approximately to a lay of seven times the diameter. For longer lays than this (up to fifteen times the diameter) the weight can be taken as 4 to 4 03 lbs. per foot run and 1 square inch section. OVEEHEAD TEANSMISSION LINES 29 In dealing with cables it must be remembered that their modulus of elasticity is considerably less than for solid wires (see Tables 1 to 5). lerSif.lnch 16000 \ Siresi in Hard drawn L ~opper overhead lines 15000 \, I400t \ \ K \ The maA'mum Stress /s 23. 000 /bi \ per D"3 t -S Cwjth extra load far \ all lengths of span. 13000 \ moo \ \ \ N \ N \ X ^. iinoo \^ ^^■v,^ ^ \^ ^<^ \ ~--.^,_^^ ■^~S^ ^>>v^ \^ 10006 ^ ^■-■-,^ "^-^^^ ig^ :;;\ ^^^ — _i^ ' 300L ■ _ *30'_ — — I'-SV Distanc s beCivt en pole ; In yard t Booa 1 1 no IIS 120 125 130 /40 150 160 Fig. 13. 180 The sag and stress for solid copper wires with a weight of 3-9 to 3-97 lbs. per foot run of 1 square inch section can be read off the curves in Figs. 12 and 13. 30 GVEEHEAD TEANSMISSION LINES m EH o •r-l N IB O t^ * — ■■ ":i bO rri OJ •ca CO =0 ,i3 <^ ■i-H X! 00 (N N o O o ooooooooo looocqasooomot- C0__ (N_^ (M__ rH^ 1-1 o_ o^ o_ <» cq (n" (n" in ci (n" m" ^^1 ^^1 ^^1 ^^1 ^M" ^^' ^^^ ^^^ ^M^ -* « 1— 1 N O ooooooooo >OOC0I>O00ffii(MO eo_^ eo__ (N ^^ i-H^ o_^ o_^ o_^ o_^ clOCOlOt~-* lO i-MCSicb-o>oi> cococococococococo o CO 00 G3 N O to I> o o >o" ooooooooo COOCOOO(N50050(M CO^ CO^ (N[^ i-f^ T-H^ 0_^ 0_^ 05^ I33_ (N" OCOfylI>-10 0 ■* CO^ (N r-J^ rH 0__ C» O 00__ eq N "9 °P l>dbd3Or-lrMCS!00cb r-lrtrt(N(M(M(N(NlN IN N o o CO o o t- OOOOOOOOO lOOt^lOCOOSOStNCO »o^ "*, ^ (N" (M" (M" CcT C__>OCOi— (00300t> ■* (N 00 lO 00 Tj4 l>dbd5d3OrHi-H(NC0 N o o l> ooooooooo OI>I>OC010>0C0tH 0^50CO^OJ1>«510tJ4 -* (N (M" (M" i-T r-T rt rH ,-^ P (33 i-Hi>-^r-icpeooocooi ■^-^lOOcot^t^dbdo 2 « tSJ o o o ooooooooo 000«30l>lMai03 C-OOOOOlO-^tNi— 1 CO" Co" in" Cq <-^ r^ i-T rt" rn" p r— 1 f-H CO rH(Ncs)cocb4(-^>oio 1 e i i o 1 00>00>00>00>00 g rtr-HcqeqcocoTh 1 ++++++++ o OVERHEAD TRANSMISSION LINES 81 CO 4^ o e I s e «} a e e t3 o o •-J3 . c3 1 f^ .s « OS ft CQ a o o -* u OS a a F— 1 N o o ooooooooo lffl_ Ttl lO lO ■*__ ■^__ Tj4_ 1* M CO CO ^i r^ ^31 ^31 ^1 ^1 ^3" ^3' ^S^ ^^ ^^ ft CO r-H ONCOlOOt^OSON l> 02 000000000000000305 o I— 1 CO N OOOOOOOOO >o oioooooiMomi-i t> ■* 0 1« ■* rj^^ ■^__ Ti|^ CO CO^ ^1 r^ T3I ^p ^1 ^1 ^1 ^1 '^ ^31 ^31 f— 1 fl CO 10 -*10COI>050CO,10CD 03 i-H OOOOO'-Hi-Hi-Hf— t § CD tSJ OOOOOOOOO in 0(NOinoeoi>0^ CO -^ coTj(-^-^-.#Ti(->#inio S O tsj g OOOOOOOOO oooocooooot- C0_^ TtJ^ 03^ 00_^ !>_ CD_ 10^ ■*_ Tj<^ CO^ (N^ 10 rt" TfT TjT ■^'" -^ tiT -^ TjT TjT TjT F-H fi CO 10 lO »o »o 00 ic >ot~dbop-HCsi-^«o^- I> 05 000000030303050505 § CD r— ( IS! 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In Mid-European countries it is not essential to make any allowance in the stress calculations for wind pressure, as the strongest winds do not generally occur in cold and frosty weather. In special cases where the wind is to be allowed for it is usually taken as exerting a minimum pressure of 30 lbs.* per square foot of effective surface in a horizontal direction. The weights of the line and any additional load act vertically downwards, so that the total force on the line is the resultant of two forces at right angles to one another as shown in Fig. 14, where g represents the effect of the weight, W the effect of the wind force, and R the resultant of the two. The force of the wind is not steady, but occurs in gusts, so that the line is set swinging. The deviation E from the vertical (Fig. 14) is given by the expression E = d sin y, and R = \/W^ + g"^ W d X W sm y = and E = VW^ + g^ --._ ] This is the result assuming steady conditions, but if anything approaching resonance exists between the frequency of the wind impulses and the natural frequency of the line greater deviations may be produced. The wind pressure W has the effect of an additional load on the supports of the Fig. 14. line and acts at the centre of gravity of the area on which it impinges. If the surface lies normal to the direction of the wind the effective area is equal to the actual area or, in complex building structures, to the sum of the separate areas. Experiments made by Grashof have shown that the wind pressure w in lbs. per square foot of normal surface is given by the expression : w = -0052 v^, where i' = wind velocity in miles per hour. The usual assumption of 30 lbs. per square foot for wind pressure would, therefore, correspond to a wind velocity : * The Board of Trade rules now allow 25 lbs. (previously 30 lbs.) per square foot of effec- tive area, tlie efleotive area for circular sections being taken as -6 x diameter x length. The General Post OflEice rules aUow 17 lbs. per square foot of effective area, the effective area being taken as f x diameter x length. The rules of the Verband Deutscher Elektroteohniker allow 125 kg. per square metre (25-6 lbs. per square foot) of effective area, the effective area being taken as -7 X diameter X length. In the following pages, in order to be on the safe side, the value of 30 lbs. per square foot of effective area will be used and the effective area for circular sections will be taken as -7 X diameter x length. OVEEHEAD TEANSMISSION LINES 35 '^ = V ^^-„ = 76 miles per hour, ■00o2 ^ an exceptionally high speed. When the wind strikes a surface F, not normally, but at an angle a, the pressure normal to the surface is, according to Newton, W = Wj^ X F = w X sin^a. X F. If the wind acts on a prism with a rectangular face of length L and breadth A, then, if the direction of the wind is normal to A, the total force on the surface is W = w X A X L 19 If the wind direction lies along the diagonal, W = wx2xAxLx sin* 45° = VF X. w X A X L . . 20 The wind pressure on a small element - cZa of a cylinder whose axis is normal to the wind direction is : dW = w sin *a -X da., from which : d [^ '-*• PF = i« .- sin *« (ia = ^ wd = -667 wd . . .21 ^ Jo " where d is the diameter of the cylinder in feet and W is the total force on a length of cylinder of one foot. The figure -667 is usually rounded off to -7. Using this and putting in the diameter d in inches instead of feet : Force onf 1 foor length of conductor ^='W=^wxd,X-r^^wxd,x '058. Putting in the usual maximum value for t<; = 30 lbs. per square foot, this becomes i~. /'^ ~~r ^^(/^. Tf = 30 X ci X -058 = /x 1-75 Ibs/zf . ^" .22 With cables the surface acted upon by the wind is somewhat greater than that corresponding to the circumscribed circle D. With 7 -strand cables the 4 effective diameter is approximately -,- Z), and with 19-strand ones approximately 7 ^ 5^- The wind pressure in lbs. per foot raip in these cases ^ there fore, bee r — ■ '"■"" " "*""'"7u« v^ Jwi^ "l *"■-■=/: omes .■ T.-a - T— — I Tf =-^X Z) X 1-75 = J5 X 2-35 J . . .23 and ff =|-X i* X 1-75 =Z) X 245 . . . .24 5 where D is the diameter of the circumscribed circle in inches (outer diameter of cable). D 2 36 OVERHEAD TRANSMISSION LINES Example 10. A 19-strand aluminium cable of -108 square inch section is suspended with a span of 260 feet. What is the value of the wind pressure on the cable, what is the resultant force due to wind pressure and weight of cable, and how far is the cable driven out of the vertical by the wind if it is assumed that the maximum wind pressure occurs at a temperature of + 10° C, at which temperature the wire has a sag of 37 inches ? The outer diameter of such a cable will be -44 inch, so that, by equations 18 and 24 : Total wind force = W = 245 X "44 x 260 = 280 lbs. Total weight of cable (by Table 6) : •108 260 = S' = 580X:^X5^ = 34 1bs. Resultant R = s/W^~+~f = */2802 + 34^ == 282 lbs. W 280 ®"^ ^ "" rS "^ 282 "" '^^^^ y = 82° 15' E = dsbxy = 37 X "9905 = 36-7 inches. If the same calculation is carried out for a 19-strand copper cable of '108 square inch section with a sag of 25 inches at 10° C. the following results are obtained : — W = 2-45 X -44 X 260 = 280 lbs. 260 9 = 2,280 ^ 5,28 ^ = 112 lbs. R = :± V2802 + 1122 = 302 lbs. sin y = W R ~~ 280 302 ~ •93 y = 68° E = d sin y = 25 X ^93 = 233 inches. It will be seen that the aluminium cable is driven out by the wind force almost twice as far as the copper one. The danger of two neighbouring wires swinging into contact if their synchronism of motion is disturbed is, in fact, considerably greater with aluminium than with copper owing to its lower weight and greater sag. Example 11. It is required to find the total wind pressure on a wooden pole 29 feet high having a diameter of 8 inches at the top and 12 inches at the ground level. At which height from the ground does this pressure act ? What is the wind pressure effective at the height of the cross-arm placed 27-5 feet above the ground ? OVERHEAD TRANSMISSION LINES 37 By equation 22 the total pressure on the pole is : W = 1-75 X d X L^ 1-75 X ^ + ^^ X 29 = 510 lbs. This force acts at the centre of gravity of the effective surface and, therefore, at a height of 12 + (2X8) ^29 ,oKt ^f ^u A — TO I Q — ^ T ~ irom the ground. The moment of the force is, therefore, M^ = 13-5 X 510 = 6,880 Ib.-feet. The effective pull at the cross-arm at a height of 27-5 feet is : 6,880 27-5 = 250 lbs. Table 13.; — Wind Pressure due to Three Lines, in lbs. per Mast* Cross- Wire Wire Wire Cable Wire Cable Cable Cable Cable section •0093 •0155 •0255 •0255 •039 •039 •054 •078 •108 of each square square square square square square square square square Distance line. inch. inch. inch. inch. inch. inch. inch. inch. inch. between Masts in Feet. Dia- meter of each •11" •14" •178" •2" •222" 252" •304" ■364" •44" line. 49 ■ 28 36 46 52 57 66 78 93 110 98 37 72 92 104 114 132 156 186 220 130 74 96 128 138 153 175 206 250 282 196 112 144 184 206 230 262 312 371 440 260 150 192 245 275 306 350 418 495 586 326 187 240 307 342 384 440 515 620 735 395 - - - 224 287 370 410 ~ 460 525 620 745 880 "^^ 490 " 280 360 460 512 570 680 780 930 1,100 ^ 560 315 410 520 585 650 740 880 1,060 1,250 660 370 480 610 685 765 875 1,040 1,240 1,470 980 560 720 920 1,030 1,150 1,320 1,560 1,860 2,200 1,300 745 960 1,220 1,380 1,530 1,750 2,060 2,480 2,950 * Not including the -wind pressure on the mast itself. 4. DESIGN OF THE SUP.PORTING STRUCTURES. The various forces acting on the line — tension, weight, and wind pressure — have to be withstood by the structures supporting the Hne. These three forces act simultaneously, but in different directions, on dead end (or terminal or strain) masts, angle masts, and masts at road, rail, or postal line crossings. Mere supporting masts (intermediate masts), which serve_ to keep theline off the ground onC^traight_stretches^ are not influenced by the tension in the line so long as the lengths of the various spans are equal. On these only the wind pressure on line and mast is effective. Consequently the greatest resisting moment for such masts is not required in the direction of the line, but at right angles to it. In fact, in the direction of the line, these masts should be as flexible as possible. The greater the elasticity in this direction the better and more safely will the one-sided stress, which results from a breakage of the line, be able to distribute itself over a large number of masts and so reduce the overloading of any one mast. The further off a mast is from the point of disturbance the less will its deflection be, and experiment has shown that all appreciable deflection has disappeared by the fourth or fifth mast. The supporting structures may be subjected to tension, compression, bending, crippling, and twisting for.es. The tensile and bending loads are due to the tension of the wires and the wind pressure on line and mast. Compression and crippling (or lateral bending) are due to the weight of the line, and as this is generally very small compared with the ^ther forces special tests in this Fig. 15. OVEEHEAD TEANSMISSION LINES 39 direction are not usually necessary. Twisting of a mast may occur when a wire breakage takes place, especially if the cross-arms are long. Elasticity in these masts, particularly in the case of intermediate supporting masts, is a great advantage in this connection again. Fig. 15 is a photograph of an intermediate mast which, in consequence of a breakage of the line, has become twisted through almost 90°. When all the lines were cut clear the mast returned to its original condition without damage. With regard to the allowable stresses to which overhead structures may be subjected, the rules of the Verband Deutscher Elektrotechniker are a useful guide. These state that for wooden poles the maximum stress is not to exceed 1,000 lbs. per square inch and for iron masts 21,500 lbs. per square inch. Struc- tures of special materials are not to be stressed to values exceeding one-third of the maker's guaranteed breaking stress. Cast-iron structures are not to be subjected to more than 4,300 lbs. per square inch. The Board of Trade regulations require wooden poles to be designed with a factor of safety of 10 and steel poles with a factor of safety of 6. Full particulars of wooden poles and their breaking stresses are given in the British Post Office Technical Instructions, XIII. of 1911. Determination of the Stresses. The line tension and the wind pressures on line and mast acting at various leverages form the bending moments tending to deflect the mast. The weight of the line together with the vertical com- ponents of any of the forces acting at an angle, s "Z^T subject the foundations to compression and / ^ — / ' crippling (or lateral bending) stresses. Twisting y ^^'■^^"'"'^ ' of the supporting structures results from un- /^'^y^"'^^ '' equal tensions in the lines or from one-sided "k/% ^ ^ line breakages. / Forces acting in the same plane but at Fig. 16. different angles can be geometrically added. This addition can be carried out either graphically or analytically. In many cases the graphical method, making use of the polygon of forces, is the simpler and quicker one to use. The simplest case is that involving the use of the parallelogram of forces. In order to find the resultant of two forces P^ and Pg (Fig. 16) acting at a point A, a parallelogram is drawn with the two force vectors as its sides. The diagonal B, of this figure then gives the resultant force. This single resultant force pro- duces exactly the same effect at the point A as the two component forces. In the same way any single force E may be split up into two component forces lying in the same plane as E. Analytically the resultant can be found by the expression : R = '^JP.^ -\-P^^ -2 Pi Pa cos (180 - «) or, since cos (180 — a) = ^ cos a, li ^ \/Pi2 + Pa^ + 2 Pi Pa cos « 25 40 OVERHEAD TRANSMISSION LINES And sin ;S : sin y : sin « = P2,: Pj : B. So that the angles between the resultant and the components are found from the expressions : P. P sin 6 = -^ sin a ; sin y = ^ sin a. R R For the case when « -- 90° : R = VPi^ + P2 26 and For the case when P^ ^ P^: cos /3 = -J^ ; cos y = -^; tan y = -pJ. J? = P V'2 (1 + cos a) ; J • /l + cos a '^ r, ^ r^ «■ and smce sJ — —^ = cos -5 .. i? = 2 P cos ^ . 27 In Fig. 18 When several forces acting at a point are to be added together graphically they are drawn so that the start of one vector coincides with the end of the previous vector. If, for instance, the sum- mation of the four forces P^ to P4, acting at the point A (Fig. 17) is to be carried out, a polygon is drawn starting from A and with each vector following the last as stated. Pi- 2 is the resultant of P^ and P^ ; Pi_3 is the resultant of P,, P, and P and R = Pj_^ is the resultant of all four forces. p-noQ , Fig. 19. Example 12. The roof pole (Fig. 19) of a local distributing circuit is loaded with a hori- zontal side pull of 1,100 lbs. At the same point, and at an angle of 45°, an anchoring stay is to be applied. What tension will exist in the stay iron ? OVEEHEAD TEANSMISSION LINES 41 (a) Graphic Solution. — Draw, to any convenient scale of lbs., the horizontal force 1,100 = AB. From A in the direction of the stay (45°) draw a line to re- present the direction of the force in the stay. From the end B of the horizontal force draw a line parallel to the stay direction, and from the point of intersection of this line with the pole draw a line parallel to AB (or P) meeting the stay line at D. Then AD is the force acting on the stay wire, and this measures up as 1,560 lbs. (6) Analytic solution. — From Fig. 19 it can be seen that „ P 1,100 - _„_„ Q = --■ — rs = L^„ = 1,560 lbs. ^ sm 45 -707 The forces P and Q form the resultant B, which acts vertically downwards on the pole. The value of R can either be measured off the diagram directly or can be found by the expression R = 1,100 tan 45 1,100 lbs. The resultant R is increased by the weight of the hne, additional load, and suspension gear, so that the total force compressing the pole is given by the algebraic sum of these items. Fig. 19 shows that as the angle y diminishes the resultant R increases. When y is less than 45° R is greater than P. It is advisable, therefore, to keep y greater than 25°, so that the vertical force shall not be excessive compared with the horizontal force. Example 13. A high-tension transmission line alters its direction through 130°. The maximum line tensions acting on the corner mast are 2,320 lbs. and 1,850 lbs. respectively. How great is the resultant of these forces, and at what angle does it act to the greater force ? (a) The graphical solution by means of the parallelogram of forces is shown in Fig. 20. The resultant R is found to be 1,820 lbs., and the angle /3 between R and P^ is 51° 35'. (6) By equation 25 : Fig. 20. and or R = y/Pi^ + Pz^ + 2 Pi Pa cos a = ^2,320''' + 1,850'-' + 2 X 2,320 X 1,850 X cos 130, cos 130° = - sin 40° .-. R = 1,820 lbs. . ^ Pa . 1,850 ^ ._.„ sm/3 = ^sma = j^g2oX -766, sin /3 = -7835, or yS = 51° 35', 42 OVERHEAD TRANSMISSION LINES or cos 13 2 ,320^ + 1,820^ - 1, 850^ 2 X 2,320 X 1,820 51° 35'. = -621 Example 14. For the purpose of crossing a river at right angles the direction of a section AB of a H.T. line (three wires of -054 square inch each) is changed as shown in Fig. 21. The maximum effect on the masts under the most unfavourable conditions is to be investigated. \ ^A -#-'—- 3eOyds. Fig. 21. The maximum stress in the wires is 23,000 lbs. per square inch. If aU the wires break in one of the neighbouring sections the total load falling on the mast (neglecting the effect of mast deflection) would be : Pj^ = 3 X -054 X 23,000 = 3,720 lbs. (1) Determination of the Forces at Mast A. The tension in the direction AG may be considered (Fig. 22) as made up of two components, P' and P", at right angles to one "» another and acting along the main axes of the mast : P' = P^ cos 17° = 3,720 X -9563 = 3,550 lbs. P" = P^ sin 17° = 3,720 X 2923 = 1,090 lbs. P" acts in the opposite direction from Pi, so that a resultant remains : Pi - P" = 3,720 - 1,090 = 2,630. OVEEHEAD TRANSMISSION LINES 43 />, There are therefore two forces, P' and Pi — P", acting at 90° to one another and whose algebraic sum is P' + Pi - P" = 3,550 + 2,630 = 6,180 lbs. Should all the wires in the crossing become severed the mast will be subjected to the sum of the two forces P' and P" acting at right angles. The algebraic sum of these forces is : P' + P" = 3,550 -f L090 = 4,640 lbs. (2) Determination of the Forces at Mast B. The force in the direction BD may be considered as composed of two forces at right angles to one another as shown in Fig. 23. These components are : P' = P^ cos 35° = 3,720 X '8192 = 3,040 lbs. and P" = Pi sin 35° = 3,720 X '5735 = 2,130 lbs. Pi - P" = 3,720 - 2,130 = 1,590 lbs. The sum of the two forces at right angles to one another is therefore : P' + Pi - P" = 3,040 + 1,590 = 4,630 lbs. If the wires become severed in the crossing the force on the mast will be that due to the two components P' and P" at 90° to one another. The algebraic sum of these is 3,040 + 2,130 lbs. = 5,170 lbs. Example 15. At a feeding point on an overhead distributing system five lines branch off as shown in Fig. 24. The five horizontal forces are : P^ = 1,400 lbs., P^ = 1,000 lbs. Pg = 400 lbs., P^ = 2,800 lbs., P^ = 1,400 lbs. How great is the resultant force ? Fig. 23. Pa • 2800 P3'*00 P, - 1400 Ps' 11-00 P* Ps 9 N. 1 v A P' ■iK. 2ia. Pa Fig. 24. By drawing the polygon of forces (Fig. 24a), to some such scale as 1,000 lbs. to the inch the resultant force is found to be P = 1,740 lbs. The supporting structure must, therefore, be designed to withstand this force. 44 OVEEHEAD TRANSMISSION LINES Table 14. — Total Pull on " Dead End " or Strain Masts. Maximum Stress in the Line Wire in lbs. per square 3 X 0155 3 X 025 3 X 039 3 X 054 3 X 078 3X-108 3 X 148 3 X 186 = 0465 = 075 = 119 = 162 = ■234 = ■324 = •444 = ■558 sq. inch. sq. inch. sq. inch. sq. inch. sq. inch. sq. inch. sq. inch. sq. inch. inch. lbs. lbs. , lbs. lbs. lbs. lbs. lbs. lbs. 8,600 400 640 1,000 1,400 2,000 2,800 8,800 4,800 11,400 530 850 1,840 1,860 2,670 3,730 5,100 6,400 14,300 670 1,070 1,660 2,340 3,330 4,650 6,300 8,000 17,200 800 1,280 2,000 2,800 4,000 5,600 7,600 9,600 20,000 980 1,500 2,350 3,250 4,650 6,500 8,800 11,200 23,000 1,070 1,700 2,670 3,720 5,800 7,450 10,000 12,800 The General Laws op Mechanics. (a) Tension and Compression. If P is the tensile or compressive force acting on a rod of cross section F then P = F X k 28 ■where k is the allo-wable tensile or compressive stress per unit of area for the material. The strength against tension and compression is approximately the same for many of the materials used in overhead line construction. The ratio of the maximum possible stress K -which the material can stand ■without breaking to the -working stress k is called the factor of safety s : k X s = K. (6) Compression combined with Bending. When a rod subjected to compression is very long compared -with its cross- section it will, after a certain force has been exceeded, begin to be deflected out of the straight line and -will then be acted upon not only by a compressive stress "I '/yy/:////yy//^^yyyf. T Fig. 2.5. — One end fixed, the other free. Fig. 26.— Both ends free, and guided in the ori- ginal axis. Fig. 27.— One end fixed and the other free, and guided in the original axis. Fig. •WMV/Z/f////. 28.— Both ends fixed. k^ but also by a bending stress k-^,. If the applied force P is further increased these t^wo stresses will eventually cause coUapse. OVEEHEAD TRANSMISSION LINES 45 Euler has deduced the following formulae for the four arrangements of the ends of the strut shown in Figs. 25 to 28 : — P=^y . . 29 ^ = -^¥ • • ^^ (Fig. 25.) (Fig. 26.) P = 2 772 y . .31 p = 4 ^2 ^ . .32 (Fig. 27.) (Fig. 28.) where P is the force which can be applied to the strut without coUapse. E is Young's modulus of elasticity, J is the moment of inertia of the section ; and I is the length of the strut. It wiU be seen that P is inversely proportional to the square of the length I and, therefore, falls rapidly as the length increases. There is a certain critical length of strut I' for which the resisting power against direct crushing is equal to that against collapse through lateral bending (crumpHng). If K^ is the maximum stress which the material can stand under compression and F is the cross-sectional area of the strut, then F X K^is the maximum total compressive force. Equating this to the rupturing force given by Euler's formula (29) : ^ IF - ^^'" from which the critical length is found to be '■-Wm ''-^ In the same way for the formulae 30, 31, and 32 : — r = TT ^/^ 30a FK, I' = 1-41 ^ \f f^ 31a V = 2^ J^^ 32a (c) Bending Stress. The maximum bending moment M must not exceed the product of the resisting moment W of the cross-section and the safe working value of the bending stress ki, for the material used. The bending moment is equal to the appUed force P multiplied by the length of lever arm L at which it acts, i.e., the perpendicular distance between the line along which the force acts and the fulcrum or turning point. The resisting moment, or modulus of section, of a cross-sectional area with reference to its neutral axis is equal to the moment of inertia with reference to this axis divided by the distance e of the outermost fibres from the same axis. 46 OVERHEAD TRANSMISSION LINES So that PL = M = W X h = -h ■ ■ ■ -33 The moment of inertia J„ of a section with reference to any given axis is equal to the moment of inertia with reference to a parallel axis drawn through the centre of gravity (centre of figure) increased by a quantity equal to the area F multiplied by the square of the distance a between the two axes : — J„ = J + F X a^ 34 The safe working bending -stress k^ must, as in the case of the tensile and compressive stresses, be kept sufficiently far below the elastic limit to avoid any danger of permanent set oc-curring. M P X L From the e^Luation M = W X k,,, the value oi k,, = ^ = — ^ — is found. If the resisting moment is alike for all the cross-sections the maximum stress produced at the various cross-sections will be different because the bending moment changes with the length of leverage. If the stress is to be the same at all cross-sections the resisting moments must change in the same way as the bending moments. Supports specially shaped in this way are called beams of constant bending stress, or beams of uniform strength (see Table 15 for examples). (d) Shearing Stress. The resistance of a body to shearing stress is proportional to its cross-section. If /^', is the maximum allowable shearing stress then P = F X K 35 The safe working shear stress ^, is dependent on the shape of the cross- section and should, in general, not exceed two-thirds to four-fifths of k^, where k. is the allowable tensile stress for the material. (e) Torsional Stress. The twisting moment P X L, where P is the force applied and L the leverage at which it acts, is equal to the polar resisting moment (or modulus of section with respect to torsion) W^i which the body offers to twisting, multiplied by the allowable stress A;,(. Ma = P X L = Wa X ka 36 The plane of action of the twisting moment Jf ^ is at right angles to the axis of the body, so that no bending component exists. For a circular section yyn jg For an annular section having an inner diameter d^ and an outer diameter d : TT7- '''" Ct (t-l OVEEHEAD TRANSMISSION LINES 47 0,(0 a nj «o 3) o u O W Wl 00 |2 00^ T-H | r«l 5- + 1 "*- "s. fc ^ i -e s? II I o" II o ■% ITK f I I i o t=l» fei-* s sir ^ I I ©) 00 C o *■ O (M + i-s I O.T.L. 50 OVERHEAD TRANSMISSION LINES Special Cases. (a) Independent Pole. By equation 33 : ~ M =W X K The resisting moment (or modulus of section) of a circular cross-section is so that Tf = f2 X d^ M = f^d^h = P XL, -where L is the length between the ground level and the point at which the force acts, and P is the force acting with the leverage L. The maximum diameter Z),^ at the ground level is then D^ = 2-15^^^ 37 The required outline- for the longitudinal cross-section of the beam to give a constant bending stress all the way up can be shown to be a cubic parabola. The approximate straight line outline corresponding to this is a beam having a 2 diameter Z)„ at the top equal to - D,„ or A. = 1^0 38 From this it follows that for wooden poles having considerable taper it is not the actual diameter of the ground level which should be used in the formula, 3 but rather that diameter which is given by D„ = - D^. With poles whose dia- 3 meter (Z>„) at the ground level is less than - Z)„ the maximum stress occurs slightly below the ground level. For all practical purposes, however, it is sufficiently accurate to deal with the diameter at the ground level. Besides the bending stress, due to the line tension and the wind pressure on line and mast, a vertical load also exists due to the weight of the line wire (together with any additional load), the weight of the insulators and fittings, the weight of the mast itself, and the vertical components of any forces or reactions applied at an angle to the horizontal. These forces acting in the direction of the mast axis subject it to crushing and lateral bending stresses. The critical length of mast can be found by the use of formula 29 (a). For a circular cross-section the 77 moment of inertia J = ^d*. Inserting this value in formula 29 (a) the critical length of pole is found as : Kd ■nd IE = ^ V 1 OVERHEAD TRANSMISSION LINES 51 For a pole diameter of 8| inches for instance, and a material having a maximum safe compressive stress of 4,600 lbs. per square inch = K,i, and for which Young's modulus is 1-5 X 10^ lbs. per square inch, the critical length „ 314 X 8f /lb X 10« .- . , I = —^-^ v/ 4,600 = ^2 mches. This shows that for all wooden masts likely to be used in overhead line construc- tion the resisting power against lateral bending is less than that against direct crushing. For an iron tubular mast the moment of inertia is where d is the outer and d^ the inner diameter. By equation 29 (a) the critical length : I', taking (Z = 3 inches, di = 2f inches, E = 30-7 X 10" lbs. per square inch and K^ = 47,000 lbs. per square inch, for wrought iron, is : „ _ TT / 30^ X 10« X 05 (3* - 2^75^) * ~ 2 V -785 X (32 - 2-752) X 47,000 ~ ^'^ ''^''*^®®- If a vertical compressive load P„ exists on the cross-section F at the ground level the total stress will be ^ = ^ + 1^" ^^(^) P being the effective sideway pull. Since the length of aU wooden poles exceeds the critical length the danger of lateral bending is more important than that of direct crushing. The allowable stress for this case is * M P ^ = w-9 ^^(^> Here ^ ,-^ M = — tan wL, and w ^ \l ^=\ in radians. w V EJ / The Verband Deutscher Elektroteckniker lays down the rule that, for /wooden poles used on straight stretches of line the diameter of the pole at the ( top (in centimetres) is to be ^ Z>„ = 1-2 VaL 39 where A is the sum of the diameters of all the wires attached to the mast in millimetres and L is the mean height of the wires above the earth in metres. (If A is in inches, L in feet, a nd D„ in inches the exp ression becomes D„ = 1-32 VaL). In order that the stress at "fKe~grOijnd leveFsiiall not exceed that at any "._— — — '^'* See Hiitte, Vol. I., p. 599. '^T /J E 2 62 OVERHEAD TEANSMISSION LINES other point on the mast the diameter at the ground must, in accordance with equation 38, be Poles at angle points, at road crossings, and, in general, where special safety is desirable, should be designed so that k^ in equations 37 and 38 does not exceed 1,000 lbs. per square inch. The application of these rules is indicated by the following examples : — {a) A line is to be erected on wooden poles 24 5 feet high with the mean height of the wire supports,23 feet above the earth. There are to be three copper lines of - 108 square inch sections and -44 inch outer diameter, and a specific gravity of 9-2 (4 lbs. per foot per square inch), three copper lines of -078 square inch section and -36 inch outer diameter and specific gravity of 9-2, and two copper lines of -025 square inch section and '175 inch diameter and a specific gravity of 8-9 (3-85 lbs. per foot per square inch). What should be the dimensions of the poles 1 ^ i ; j. O, (5; 1^^ -^ \~l By the rule of the V. D. E. J,^ D^ = 1-32 v'2-r5 X 23 = 10^ inches, 3 3 and Z)„ = — Z)„ = ^ X 10^ = 15f inches. The bending load is due to wind pressure on the line and the poles. Taking the span as 130 feet (the maximum allowed by the V. D. E. rules when the total cross-section of the wires exceeds 300 square millimetres or -47 square inch), the wind pressure is (by equation 22) : On the Unes = '2-75 X 130 X 175 = 625 lbs. 10-1 -I- 1 'iS. On the mast = ^ ^ ^ X 24-5 X 175 = 560 lbs. On the supports and insulators, say = 50 lbs. The wind pressure on the lines and insulators acts at a height of 23 feet above the earth, whilst the wind pressure on the poles acts at the centre of gravity of these, or at a height of 15| + 2 X 10| ^ 24;5 _ ^^.^ f^^^ 151+lOJ X 3 -^i^teet. The effective moment acting at a height of 23 feet is therefore {(625 + 50) X 23 + (560 X 11-4)} 12 = 263,000 Ib.-inches. By equation 37 the diameter of pole at the ground level, assuming a stress ij of 1,000 lbs. per square inch, should, therefore, be r, „ ,^ 3/263,000 ,„ „ . , Z)„ = 215 sj-Y^ = 13-8 mches, ^^ £»„ = I X Z>„ = I X 13-8 = 9-2 inches. OVERHEAD TRANSMISSION LINES 53 If the diameters found by the rule of the V. D. E. are used in place of these the stress k^ will be less than 1,000 lbs. per square inch, thus : ,_, „ ,_ 3/263,000 15f = 215 ^ — j^ , or k^ = 680 lbs. per square inch. Taking the maximum possible bending stress for pine to be 6,750 lbs. per square inch, the factor of safety therefore works out ^^=10 680 The force P„ acting vertically along the axis of the pole is made up as follows : lbs. Weight of line wire 130 (108 X 3 X 4 + 078 X 3 X 4 + 025 X 2 X 385) = 315 Additional load 66 X 130 (108 X 3 + 078 X 3 + 025 X 2) = 520 Weight of pole above ground level 900 „ „ insulators and fittings " . . 180 1,915 By equation 37 {a) the total stress is, therefore : ^ _ 263,000 jg^g y^ ~-X (15|)« + (15f)^' X -785 = ^^^ ^^^- P"' ^1"^^" ^^^^- *If the smaller diameter of 13-8 inches had been used the total stress would have worked out at _ {(625 + 50) 23 + (490 X 11-4)} 12 1,600 _ ^990 lbs. per * ^ ^ (13-8)3 "•" (13-8)2 X -785 — {square inch. By equation 37 (6) the stress, taking into account the load P^ tending to produce lateral bending, is found as follows : Using the larger pole of 15| inches diameter : jfc = 77 M_ 1,915 32 (15|)3 (15|)2 X -785 P I'P' where M = — tan wL, and w = yj j " w ' \ EJ ■■■ ^ = Vr ''''' = -00065, •5 X 10« X (-05 X (15|)4) .-. tan w X i = tan -00065 X 23 X 12 = tan -18 = -18 practically. 54 OVERHEAD TRANSMISSION LINES P, the effective sideway pull due to the wind, = (625 + 50) + 560 X ~ = 1,053 lbs. 1,053 X 18 ■•■ * = -00065 X ^, X (15f)3 - (151)^' X -785 = '^^<^ ^^^^ "^'^ ^'^"^^^ ^"«^- In the same way for the smaller mast of diameter 13' 8 inches : — Ic = 950 lbs. per square inch. Even in this case the factor of safety is 6,750 ^ 950 (6) Three copper lines, each of -054 square inch section, and outer diameter of -3 inch and specific gravity of 9-2 (4 lbs. per foot per square inch), are to be erected on wooden poles situated 260 feet apart. The insulators are mounted on angle pins screwed* into the poles. The poles are 27-5 feet high and the mean height of the lines is 26 feet. What diameter should the poles be made ? Using the V. D. E. rule the diameter at the top should be : D„ = 1-32 VaI = 1-32 \/-9 X 26 = 64 inches, 3 and Z>„ = 2 ^0 = ^'^ inches. Wind pressure on the lines = 9 X 260 X 175 = 410 lbs. „ pole = ^"^ ^ ^'^ X 27-5 X 175 = 385 lbs. Leverage for the wind pressure on the line = 26 feet. „ pole = 13 feet. Bending moment Jf = {410 X 26 + 385 x 13} 12 = 186,000 Ib.-inches. Using equation 37 the diameter at the ground level would work out at : „ „ , ^ ,/ 186, 000 , „ „ . , Du = 2- 15 ^ ^^^^^ = 12-2 mches, 2 and D„ =-^ D,^ =81 inches. If the diameter found by the V. D. E. rule is used the working stress h would be found as follows : 9-6 = 215 3/1 86.000 from which k = 1,920 lbs. per square inch. The factor of safety would be 6>750 _ 1.920 - •* ^- OVERHEAD TRANSMISSION LINES 55 lbs. The weight of the hnes = "054 x 3 X 4 x 260 = 168 Additional load =66 x 054 X 3 X 260 = 276 Weight of pole above ground . . . . = 376 „ ,, insulators and fittings . . . = 20 P^ = 840 By equation 37 (a) : 186,000 186,000 ,840 „,oniu • u = — ~ — + -ff^ = 2,180 lbs. per square inch. 840 ^ J X 9-63 + (9-6)2 X -785 86 72 If the larger diameter of 12 '2 inches were used , {410 X 26 + 470 X 13} 12 , 970 , ,,^„ * = ym r 116 ~ I'l^O lbs. per square inch. Appljong equation 37 (6) the results for the smaller pole are ^ ^ _ 840 __ / '" 840 '^ " 86 72 ' "^ ~ V 1-5 X 106 X 440 00115 tan m; X i = tan -00115 x 26 X 12 = -36 practically. (410 + 385 X i|) + -36 840 „-^.„ » = V 26/ — = 2,170 lbs. per square mch. -00115 X 86 For the thicker pole h = M 970 /" 970 •00076 176 116' ■" V 65 X 10* X 1,120 tan w X i = tan -00076 x 26 X 12 = -237 practically. ^ ^410 + 490 X II) X -237 _ 970 ^ ^ -00076 X 176 ^^^ Summary of Results. 145 lbs. per square inch. Calculated Values of Stress in lbs. per square iuch. Bending by Equation 37. Bending and Compression- by Equation 37 (o). Lateral Bending by Equation 37 (6). Using the Dimensions given by the V. D. E. Rule. Example {a) Example (6) 1,000 1,000 990 1,140 950 1,145 695 2,180 56 OVEEHEAD TEANSMISSION LINES The above summary shows that in all those cases for which wooden poles would be hkely to be used the calculation by means of the simple bending equation (37) gives sufficient accuracy. The vertical loading is, with the factors of safety usually employed, quite negligible both as regards plain compression and as regards the lateral bending effect. The actual stress in example (a) is a little lower and in example (6) a little higher than the standard figure assumed (1,000 lbs. per square inch), but the differences are unimportant. The rule of the V. D. E., on the other hand, shows large deviations, but in connection with this it may be pointed out that usually the taper of wooden poles is not so great as that indicated by the rule. An average taper is 1 inch in about 8 feet. Thus in example (a) the pole having a top diameter of 10^ inches would have a butt diameter of 10J+(ix-q-)= 13-4 inches instead of 15| inches, whilst the correct diameter for a stress of 1,000 lbs. per square inch is, by equation 37, 13-8 inches — a fairly close agreement. In example (6) the V. D. E. rule gives a smaller pole than equation 37, and at the same time the taper is quite normal ; consequently in this case the pole would actually have the excess load. A line such as this with spans of 260 feet would probably not be worked at less than 1 ,000 volts, and in that case the V. D. E. rules specify a minimum top diameter of 7-1 inches, and this would partly get over the difficulty. At the same time there is no serious objection to working with a stress of 1,300 to 1,500 lbs. per square inch, as there would still be a factor of safety of 4| or 5, which is ample, even allowing for the unavoidable deterioration of wooden poles with time. A single unsupported wooden pole is only suitable for comparatively small loads. At angle points and in windy districts, or for heavy lines, the stability must be increased by means of stays or struts, or double masts of A form (Fig. 31 ) or H form (Fig. 32) can be used. \(b) Stayed Poles. Using the lettering shown on Fig. 29, the couple with regard to the fulcrum E is : P'c = P {e + c) P' P 1 The relation is only accurate if the pole is free to swivel on the point E. As, however, the pole is let into the earth this does not hold and an additional resisting moment M^ for the ground effect must be taken into account. The above equation will not serve for the determination of the unknown M^, so that the value of P' must be obtained from P by another pro- cedure. P' is to have such an amount and direction as to prevent the point E' Fi?. 29. OVERHEAD TRANSMISSION LINES 57 on the pole deviating from the vertical. Consequently the bending tendencies due to P and P' must be equal and opposite at the point E' . For simplicity the cross-section of the pole will be assumed to be uniform. The bending deflection produced by the force P at the point E' wiU then be* _ PL^ (^ 1 ^\ ^ ^ 2EJ \L~ S W P' produces the deflection P' c* f^EJ^ "3 Equating the two deflections and putting L = e -\- c : p'=p(i+i-:)- The tension P^ in the stay wire is found from the equation P' = P, sin a or P, = ^ = ^ (l + l^)=F Xh . ■ .40 ' sin a sm a \ ' 2 c/ For a stay of circular iron wire or rod the diameter is : d = 1-13 Vrr^ (l + 1-^) • • • -41 k^ sm. a. \ '2 c/ Taking Jc^ as 21 ,500 lbs. per square inch for wrought iron this becomes : d = -0077 V^ (l + i-^) • • • 41 (a) sm «. \ 2 c/ If iron wire with an allowable working stress of 28,500 lbs. per square inch is used the total cross-sectional' area of these stay wires must be : F = -000033 X -^ (l + 1^) . . . 41 (6) sm a \ ' 2 c/ ^ ' The pole will experience its maximum stress at E' where the three stresses P X e due to (1) the line tension = — rr, — , (2) the vertical component of the stay wire P' COS cc tension = s, — , and (3) the weight of the line with additional load, Pv insulators and fittings = -pr all add up. The total stress at this dangerous section is thus : * See Hiitte, 21st edition. Part I., p. 565. 58 OVERHEAD TRANSMISSION LINES For wooden poles of circular section this becomes : 10 X P X e + ^ {P (l + 1^) cos a + P,,} . 42 (a) The vertical forces subject the part of the pole E E' oi length c to a tendency to lateral bending (crumpling) if c exceeds the critical length. Since the foot of the pole is practically fixed whilst the point E' is free, the third of Euler's equations (31) applies : P = 2 77^ — vg — = maximum allowable compressive force. Putting c for L and the whole vertical load for P in this equation, the necessary moment of inertia in the above case is, therefore, found to be : and for a pole of circular section : 1 + 2^ ) cos a + P^ 43 D = / -^ V ° ^ 2 X cos «■ This force combines with the lateral bending force to give a total of p , _ P X i , P„ J^k — A r IT A ' 2 cos a P,.' is usually so small compared with P^. that it can safely be neglected. 51 (e) Double Pole of H Form. This arrangement is shown in Fig. 32. The total horizontal force is divided P equally between the two poles, each being acted upon by-„. The tie bolt V, with the strut or truss-rod s arranged as shown, is subjected to a tensile force of P' = 1' + 3j 2 c The force tending to bend the diagonal strut s laterally is, by equation 45, P 1 + ^-^ 2 sm a sin a 1 + 3e 2c 52 Fig. 32. If the diagonal truss-rod were placed as shown dotted in Fig. 32 it would be subjected to a tensile stress whilst the cross-bolt V would receive a compressive stress. If both truss-rods are fitted the cross-bolt V remains unstressed and can be omitted without weakening the structure. OVERHEAD TRANSMISSION LINES 61 The stress on the dangerous section of the pole at E^ depends on the arrangement adopted. If the crossed truss-rods are used the stress, according to equations 42 and 47, will be : . = 9^ + i,{^±p(i + |i cos a 53 If only one truss-rod and the cross-bar V are used the unsupported mast remains unaffected by the vertical component of the lateral force on the truss-rod, so that k = P X e + 54 Example 16. The terminal stretch of an overhead distributing line consisting of two wires of -025 square inch each is to be strained to a pole 23 feet high. The position of the pole prevents the use of stays or struts. What diameter at the top and at the base should the pole have ? The maximum tension in the line is 11,500 lbs. per square inch. The force acting on the pole 23 feet above the ground is P = 2 X -025 X 11,500 = 575 lbs. By equation 37, for a maximum bending stress of 1,000 lbs. per square inch, the diameter at the base of the pole is D„ = 2- 15 D,= r^D = \ c, iTT-A n , . ;.„„ = •263 inch. V 2 X 3-14 X -7 X 14,500 A I inch diameter bolt could therefore be used and would allow for the additional stress due to the tightening of the collar. This additional stress should always be allowed for by making the cross-section of bolt 1-2 or 1-3 times that calculated : The dangerous section in the collar-band occurs at ff. The cross-section here, if flat iron l^g x i'e inch is used, and the hole is made ^^ inch diameter, will be 2 (l-S- . 7_1 V -3- ^ V-^ie 16^ ^ 16' and the stress kg = f, ,, 3 ' ^ , 3 = 3,900 lbs. per square inch. ^ Ut6 "^ 1^) 16 When a stay tightener or stretching screw is used the thread diameter of its bolts must be at least equal to that of the stay-rod itself. This has been Fig. 37. found above to be -31 inch, so that a J inch thread, having a diameter of -39 inch would be required. This would be suf3ficient if the bolts had plain closed eyes at the ends, but if open hooks are used a greater dia,meter is necessary. The cross- section b (Fig. 37) is subjected to tension by the force P and also to bending by the moment P X a. The diameter d should, therefore, be made | inch and the distance a = d. The stress at the section b is then found as , P , P X a * = ^ + F^ W where W = ^d^, k = 1,090 , 1,090 X i oo ,^^1, . , i ' .no^ + o.\a V (A^ = 22,400 lbs. per square mch. {If X -785 ■ 304 X (I) 32 In order to save material the section at other points than b can be some- what reduced. O.T.L. F 6G OVERHEAD TEANSMISSION LINES Example 21. A certain long-span line is to be carried on channel-iron masts of the dimen- sions shown in Fig. 38. The wind pressure acting on the wire (useful load on the masts) amounts to 930 lbs. This force is applied at a mean height of 475 inches (39 feet 7 inches) above the ground. The weight of the lines and addi- tional load on the span of 525 feet is found to be 375 lbs. The design of the masts is to be carried out. The following iron sections will be employed in the calcu- lations : — Vti 930 Us ■ I. Main Stays. Channel-iron of German standard profile No. 10 (= British Standard Section No. 3 approx.) having a weight of g = 21Jlbs. per yard and a cross-section /j, = 2-1 square inches. Thickness of flange = -24 inch. The cross-section weakened by a |-inch rivet amounts to // = 2-1 — (1 X -24) = 1-98 square inch. Moment of inertia about the a;-axis = J = 5 (inches)*. ,, „ „ y-a,xis = J^ = -1 (inches)*. Resisting moment or modulus (of section) about the a;-axis = W^ = 2-5 (inch)l Distance of centre of figure = < f inch. II. Diagonals. Bar iron I^q x t6 inch with a cross-section /, of -88 square inch and a weight g oi 9 lbs. per yard. Cross-section weakened by a | inch rivet to/,; = -88 — -5 X _9_ 16 " — '6 square inch. ^3^ (-S-\S V 1-9- Moment of inertia Jnim.= j^ = ^"'' .o ''' = "023 (inch)* 12 12 III. Bivets. Half inch diameter with a cross-section of -196 square inch. I. Main Stays at the Loaded Section E. These are subjected to both vertical and horizontal forces. The former are — weight of mast, fittings, and lines (including additional load) ; and the latter are — ^wind pressure on the lines (useful load on the mast) and wind pressure on the mast surface normal to the line direction. OVEEHEAD TEANSMISSION LINES 67 (1) Vertical Forces. Weight of the whole mast (47 feet 6 inches long) 940 lbs. part of the mast above the ground level Q = 940 41 X lbs. 47-5 lineG = 810 = 375 Total weight P^ 1,185 Ib.-inohes. 440,000 (2) Horizontal Forces. Wind pressure on the lines Wi = 930 lbs. acting with a leverage of 475 inches gives a bending moment Jfj = . Wind pressure on the mast Wa = 41 X -33 X 30 = 410 lbs. acting at a leverage equal to the height of the centre of figure of the mast (mast centre) and giving a bending moment 41 V 12 M^ = 410 X ^ = 101,000 2 Total bending moment . Distance of centre of figure from the neutral axis (Fig. 39) : 20 f. M,= 541,000 10 — 1= 9-375 ins. These loads produce in the ■*

X cos ^ = 2,080 X s ^ = 1,620 lbs. ^ X oi Assuming that the whole force is taken up by one rivet only, the factor of safety against shearing will be I X 57,000 . _ s„ = 3 =4-6, l,620/,,3 where 57,000 lbs. per square inch is taken as the breaking tensile Fig. 10. stress and two-thirds of this as the shearing stress. IV. DeUection of the Pole. The deflection produced at the end of a free support is * ^ ~ E X J ^ 3' For the ground level section the distance of the centre of section of the channel iron from the neutral axis has been found to be Z = 9-375 inches. If the width of the top of the mast is 6 inches the distance of the centre of section there will be 6 2 f = 3 — I = 2f inches. The average distance of centre of section from the neutral axis is, therefore, 9-375 + 2-375 2 = 5-87 mches, = e,„ * See Hntte, Part I., p. 565. OVEEHEAD TRANSMISSION LINES 69 and the mean moment of inertia is J^ = 2 (J, +/, x^ej) = 2 (-7 + 2-1 X 5-872) ^ 147 (inches)*. The force effective at the top of the mast is made up of the wind force on the wires together with the wind force on the mast area reduced to the leverage of the top of the mast. The bending moment of this latter force has been worked out above as M^ = 101,000 Ib.-inches. The effective force at the top of the mast due to this is therefore W,' = iH^ = 213 lbs. The total force at the top is therefore 213 + 930 = 1,143 lbs., and the deflection at the top is 1,143 X 4753 3 800 Lbs / = 30-7 X 106 X 147 X 3 = 9 inches. Example 22. A " dead end " or strain mast is to be designed for the line dealt with in example 21. The one-sided line pull amounts to 3,800 lbs. and acts at a point 475 inches (39 feet 7 inches) from the ground. The dimensions are shown in Fig. 41, and the following material will be used : I. Main Stays. Angle iron 2^5 X 2x% X -35 inches (= British Standard Section No. 7 approx.) having a weight g of 17| lbs. per yard and a cross-section/^ =1-7 square inch. The cross-section where weakened hj a, ^ inch rivet is fg' = 1-7 — (J X -35) = 1-52 square inch. Fig. 41. Moment of inertia about the ^-axis = J^ = 1 (inch)^. aj-axis = J„i„.= -42 (inch)*. Distance of the centre of figure from the neutral axis = ^ = -76 inch. II. Diagonals. Angle iron lx% X li% X A inches (= B. S. E. A. 3 approx.) weighing g = 5 lbs. per yard and having a cross-section of f^ — -48 square inch. The cross- section where weakened by a J inch rivet is f/ = -48 — (J x ^^) = -4 square inch. Moment of inertia about the x axis = J,„j„ = -045 (inch)*. 70 OVEKHEAD TEANSMISSION LINES III. Rivets. Half inch diameter with a cross-section of •196 square inch. Wind Pressure on the Mast in the Direction of the Line Pull. The front surface of the mast exposed to the wind is made up of : square feet (1) The surface of the angle irons of a length of 495 inches each and a width of 2^-^ inches = ^ ^,ft ^o^'"^ • ■ = 17-6 (2) Surface of the diagonals = ^^ ^,^^" ?1 ^^"^ ■ ■ . = 5-6 (3) 65 per cent, addition for the back surface of the mast, the cross-arms, and the insulators . . . . . = 15 Total surface . . = 38-2 Allowing 30 lbs. per square foot the wind pressure = 30 X 38-2 = 1,150 lbs. This force wiU act approximately at the centre of figure of the mast area which occurs 33i + (2 X 10) ^ 495 _ 33i + 10 X 3 - ^03 inches above the ground level. The effective force, reduced to the height at which the line pull occurs is therefore 1,150 X 7^ = 490 lbs. 470 This added to the Une pull of 3,800 lbs. gives a total pull of 4,290 lbs. I. Stresses in the Angle Irons at the Ground Level E. Vertical load : — 495 1^^- Weight of mast above ground Q = 1,800 x -^wj = 1,555 Weight of hnes G = 375 Total weight . . . . P„ = 1,930 This force produces a compressive stress in the angle iron of k^ = -. — ^ = :, — '- — z-^ = 285 lbs. per square inch. " 4 X /„ 4 X 1-7 r u Horizontal load : — The bending moment due to the line tension and wind pressure is Ml, = 4,290 X 475 = 2,040,000 Ib.-in^hes. OVERHEAD TRANSMISSION LINES The distance of the centre of figure from the neutral axis (Fig. 42) is : e 71 2 — £ = -^ 1 = 16 inche.3. The bending moment M^ together with the total vertical force P^ produces in each of the four angle ^ 1 irons a tensile or compressive force of 1 1 N.A. = 31,000 lbs. N.A N.A. P,= l(^+P.)=i(^J'^»+U30) 1^ = 32.000 lbs. These forces produce tensile or compressive stresses in the angle irons at the ground level of P. Fig. 42. _ _ 31,000 /;~ 1-52 Pg ^ 32,000 /; 1-52 = 20,500 lbs. per square inch. = 21,000 lbs. per square inch. With a gross length between diagonals of 55 inches and a ices length of 51 inches the factor of safety against lateral bending is ■n^ X J X E 3-142 X 1 X 30-7 X 10' Pa X P 32,000 X 5P = 3-62. II. Diagonals. Since the angle iron mainstays are practically parallel, the greatest tension or compression in a diagonal (Fig. 43) will be Ki?. 43. 1 4,290 D =-^ X 2 cos a and cos a = 31 \/3l2 + 23^2 D = 1 X 4,290 X ^^M^+^ML = 2,680 lbs. The stress is then _ I> _ 2,680 ■48 5,600 lbs. per square inch. 72 OVERHEAD TRANSMISSION LINES D 2,680 Tc. = 6,700 lbs. per square inch. fd'- -4 The factor of safety against lateral bending is then D xV ifi X -045 X 30-7 X 10 "^ 2,680 X 392 (St = = 3-32. III. Rivets. The force acting on the rivets is D. Taking the safe shearing stress hs 2 = s ^2, where h^ = safe tensile stress, and the breaking stress under tension as 57,000 lbs. per square inch, the factor of safety for the rivets is I X 57,000 X -196 „ - Sn = 3 = / 8. 2680 OjO NPIO N910 J u //Jl -ZO- o]o -zo- Fig. 44. IV. Deflection of Top of Mast. The distance between the centre of figure and the neutral axis is 16 inches at the ground level, and at the top of the mast it is ^' — f = ~2 - I = ^i inches. The mean value is therefore, e™ = — y-^ = 10-1 inches. The mean moment of inertia is therefore J^ = 4 (Jf + /, X ej) = 4 (1 + 1-7 X 10-P) = 700 (inch)*, and the deflection at the top of the pole will be / = P X P 4,290 X 4753 E X J X 3 700 X 30-7 X lO'^ X 3 7 inches. OVEEHEAD TRANSMISSION LINES 73 T -P- 1240 Lbs. Example 23. The mast of example 22 is fitted with cross-arms as shown in Fig. 44 con- sisting of two channel irons (German standard section No. 10 = B. S. C. 3 approx.) connected by bar iron 2| inches X -4 inch section. The one-sided puU P is 1,240 lbs. What stress are the channel irons sub- jected to 1 No. 10 channel iron has a cross-section of 2'1 square inches and a moment of inertia J^, = -7 (inch)*. The distance of the centre of figure from the neutral axis is f inch and the resisting moment (or modulus of section) W^ is 2-5 (inch)^. The force P produces a tensile or compressive force in the two channel irons of -^S- -^2'w- 10' — ;«-- I Figf. 44a. From Figs. 44 and 44a it can be seen that Z = 31 inches and e == llj inches. k' 1,240 X 31 Hi ■ In addition to this the channel irons are also subjected to a twisting couple due to the pull P of the wires on the insulators at a leverage of 10 inches. This produces a bending moment in the channel irons with the leverage Z = 31 inches and equal to : ilfrf X I 1,240 X 10X31 M„ Hi The total stress on one channel iron is therefore P'l240lbs-mr 1,240 X 31 1,240 X 10 X 31 lU X 21 Hi X 2-5 = 15,200 lbs. per square inch. Example 24. Fiof. 45. The force of 1,240 lbs. used in the last example acts on an insulator whose pin support is shown in Fig. 45. What stress is set up in the material ? The pin is subjected to bending stress at the section y — y and the amount of this stress is , _M _ P X I X 32 ^ 1.240 X 3| X 32 '' ~ If 7t X d^ 77 X (1|)» = 8,800 lbs. per square inch. 74 OVEEHEAD TEANSMISSION LINES At the section b — h the collar H is subjected to compression and the pin itself to tension. The compression at 6 — b is H = P (I - 2 -\- -4) pin): Distance of centre of figure. The distance of the centre of figure from the neutral axis (central axis of e = 3 X X i?2 _ r2 X (1*)« 3 X 3-14 ^ (1J)2 _ (-72) (•^2)^ = -6 inch. "6 t ■^P-Z-1-5 ^d — H 15,500 Fh i 77 (112 _ -722) The compressive stress in the collar is then = 13.200 lbs. per square inch. The tensile stress in the pin is H 15,500 kJ F, Fig. 46. 12 X -785 = 20,000 lbs. per square inch. and in the thread (-84 inch minimum diameter) the stress is 28,000 lbs. per square inch. This would be too high for a wrought-iron bolt, so that the bolt must either be made of steel or its dimensions must be increased. Example 25. The stresses in the insulator pin shown in Fig. 46 are to be investigated, the force on the insulator being 245 lbs. Bending moment at a — a : — If = 245 X 1 T^e = 383 Ib.-in^hes. if _ 383 X 32 = 7,600. Pressure on b — 6 : — H = 245 X 4| and e = ■.H = 3 X 314 245 X 4| •20 •53 - -313 . X .g2 .. .3^ = '26 inch. = 4,500 lbs. OVEEHEAD TRANSMISSION LINES 75 The compressive stress at 6 — b is, therefore, , H 4,500 ,„„„„„ ft'ci = p = .24 = 18,800 lbs. per square inch. (P = I TT (•5'2 - -312) = -24 square inoh.) Fig. 17. The tensile stress in the f inch bolt, with a minimum diameter of -51 inch and a cross-sectional area of -203 square inch, is 4 500 k = rK(Zo~ — 22,200 lbs. per square inch. Fit. 48. Example 26. Bent iron brackets for insulators can be carried out in square-se3tione(l iron either as shown in Fig. 47 or as in Fig. 48. The resisting moment (modulus of section) W of Fig. 47 is -1178 X h^ and of Fig. 48 it is - • 76 OVEEHEAD TEANSMISSION LINES What maximum horizontal force can the brackets stand without the stress exceeding 21,500 lbs. per square inch ? The bracket is subjected to a bending moment P X Z, so that for Fig. 47 :— h X IF 21,500 X -83 X -1178 P = I 6| = 190 lbs. For Fig. 48 p^kxW^ 21 500 X -83 ^ ^^^ ^^^ I Ot X t> The arrangement of Fig. 48 will therefore permit of a considerable saving in material. Example 27. Design of a cross-arm of the form shown in Fig. 49. C-. anr^el iron of German Standard section No. 10 (= B. S. C. 3 approx.). Cross-section = 2-1 square inch. Moment of inertia J^ = 4-95 (inch)''. „ J, = -71 {inch)^ Distance of centre of figure = ^ = | inch. p.l330lbs. ~'''^m P -1330 Fi?. 49, The force of 1,330 lbs. tends to twist the system with a leverage of 17| inches : M„ = W, X k,. The polar resisting moment W^, is equal to the polar moment of inertia J^, divided by the greatest distance of any point of the section from the centre of section s. This distance is 3f inches. The polar moment of inertia of a symmetrical cross-section is equal to the sum of two equatorial moments of inertia taken with reference to two axes through the centre of figure and at right angles to one another. OVEEHEAD TEANSMISSION LINES 77 In this case : J, = 2 X 4-95 . . =9-9 (inch)*. J, = 2 {-71 + 2-1 X (11-1)2} = 15-22 (inch)*. Sum = 25-12 25-1 •■■ ^^^ = ^3-^ = 6-7 (inch)3. and t, = LMLxni Q3 4, ; — -^j * = 3,503 lbs. per square inch. Security against lateral bending : — The force P on one of the channel iron beams : P = 1'330 X_AV ^ 4^800 lbs. 104- Factor of safety : ^^ X J^ta X E 3-142 X -71 X 30-7 X lO^ P X P 4,800 X 572 = 13-8. The pin used with this cross-arm would be fitted with a hexagon head above the bedding face for use whilst the nut at the bottom is being tightened. Smaller pins are, for the sake of cheapness, often simply provided with two parallel flats for this purpose. 5. STABILITY OF POLES AND MASTS. Poles and masts used for line supports must be embedded to a depth depending on the height of the mast, and the soil must then be well punned or rammed down, and in the case of soft ground additional special precautions must be taken. General rules cannot be laid down, and each case must be con- sidered on its merits. The foundation arrangements depend chiefly on the kind of mast used and on the length of span. Wooden poles on straight stretches, considering the comparatively short spans for which they are used, generally need no special precautions in good firm ground. In sandy or loose soil, or, in the case of corner poles, even in firm ground, a special construction is often necessary in order to ensure sufficient stability under the most unfavourable conditions. Iron masts, used as strain or terminal masts at dead ends, and so subjected to heavy line tensions, or masts subjected to great wind pressure, are often mounted on concrete foundations, which give the necessary stability and also protect the bottom of the mast from rust. The allowable compressive stress (stress at the edges of the base) depends on the nature of the ground. For rock foundations pressures of 100 to 200 lbs. per square inch are allowable. For rehable building foundation (stony ground, gravel and coarse sand, dry layers of fine sand, loam and clay) values of 55 to 85 lbs. per square inch should not be exceeded. Still smaller values must be kept to for unsafe ground (wet clay, loam, and sand). Drifted sand, bog-land, turf and made ground give very poor support, and the pressure should not exceed 15 lbs. per square inch. The size of the foundations depends on the forces acting on them, the nature of the ground, and the degree of stability desired. The degree of stability must be chosen to match the factor of safety on the rest of the line construc- tion. Mere supporting (or intermediate) masts are cut as fine as possible in order to economise cost, as they are normally only required to withstand the wind pressure on the line acting at right angles to the direction of the line. This force also is the chief one to be considered in designing the foundation. Such masts are not expected to withstand the additional forces thrown on them when a breakage of the lines occurs on one side. If the masts are made of channel iron they will generally be able to give sufficiently in the direction of the line to take up the additional load without permanent damage to themselves, especially as this deflection reduces the tension decidedly. For larger line cross-sections and for long spans such elastic structures do not usually offer any advantage. The cost of providing intermediate masts of sufficient strength to withstand the heavy OVEEHEAD TRANSMISSION LINES 79 additional load due to wire breakage would be prohibitive. It is usual, therefore, to design these masts simply to withstand the wind pressure, and in case of a breakage in the wires on one side the masts are subjected to excessive stresses and more or less damaged. If, however, the fixing in the ground is of such a nature that the masts are free to set themselves at a moderate slant when the excessive sideway force occurs, this will relieve the tension without damaging the mast itself. The deviation from the vertical wUl, in any case, be small, so that little risk of the mast falling exists. The labour and expense of righting such slanting masts is decidedly less than that of replacing or straightening bent iron masts. Symmetrical Ground Pressure. The sum of the weights of the foundations, the mast and fittings, and half the wire (including additional load) in the two neighbouring spans constitutes the vertical load acting on the mast foundation and producing a symmetrical compressive force on the ground. If o-j = the compressive stress, 2 G = sum of aU vertical forces, F = area of foundation surface, then o-g = -^ ...... 55 Unsymmetrical Ground Pressure. If a force Q (Fig. 50) acts at a point, which is not the centre point, on a surface of breadth b and length of 1 inch, an unsymmetrical distribution of the stress on the ground occurs. The compressive stress on one half wiU be greater the further the point of action is from the centre point. If two equal and opposite forces Q are assumed to be applied at the centre point ^ and in a direction parallel to the original force Q, Fig. 50. no alteration in the conditions wUl result. The force acting downwards at produces a uniformly distributed compressive stress 0-1 on the ground. The remaining two forces constitute a couple with the twisting moment Q X x which produces a bending stress o-g. In this case a compressive stress + o-^ is produced at m whilst a tensile stress — 6 ' Q X Q X X 1 X &2 ■ At the points m and n respectively the following forces exist on the ground ; / &X x \ "'" = b V + —r-J' Q / 6 X x \ If ^.. = «=!(- 6 X a;' b b this case : 2Q 56 57 For X > - o-,i becomes negative, and for a; < - it becomes positive. If the stress at n is : b b b b , ^' • . =^ .... 58 '-„. 3 2/ ■ • ■ If the compressed surface is of rectangular form with a depth = b then •"'Ijt, '' Lateral Earth Pressure (Active and Passive Earth Pressure). If a mass of earth is sifted on to a horizontal surface it will form a conical mound whose side forms a definite angle <^ (the " angle of repose ") with the horizontal, depending on the nature of soil used (Fig. 51). If this cone is cut anywhere at an angle steeper than its natural angle the soil at this point will no longer be in stable equi- librium and must be supported. The force acting on such a support is called the active (effective) Fig. 51. earth pressure. A passive (static) force also exists when a supporting wall is pushed outwards against the earth behind it by outside forces, as, for instance, in the case of a mast foundation forced against the OVEEHEAD TEANSMISSION LINES 81 surrounding earth. The passive earth pressure is decidedly greater than the active earth pressure. For hard punned soil or virgin soil the angle up to which no sliding takes place- is considerably greater than for new soft ground. Under certain conditions,, however, such as a rising of the surface water, etc., the cohesion of the earth can be seriously reduced or even abolished. The direction of the earth pressure depends on the form and inclination of the waU surface and the friction coefficient between waU. and earth. In order to be on the safe side loose earth and no appreciable friction between foundation and earth should be assumed, i.e., the earth pressure should be taken to be at right angles to the waU surface. On these assumptions and for the simple case shown in Fig. 52, viz., wall surface vertical and base horizontal, the active earth pressure is given by the expression Fig. o2. ^a = -| X y X P X tan2 (45° - -|) . 60 The passive earth pressure acts on the wall at an angle (f> below the normal to the waU surface : E, = lx y Xh^ tan^ (45° + | 61 If the specific earth pressure is o" : ^ = o- X 1 X 2» so that oiQ 137 X 27 ^ 20 ^ ^''^ ^^'- OVERHEAD TRANSMISSION LINES 89 (/) Rectangular base (110 x 83 inches) with vertical section as shown in Fig. 62. The lateral earth pressure and the weight of the earth lying over the pro- jecting ledge not to be taken into account : — Weight of foundation : 2,330,000 = (47,300 + 1,830 + 375) x = 49,505 X x, X = 46-8 inches, y = 8-2 inches, 4 X 49 505 o"m = g-o X 83 ^ '^^'^ ^^^' P^^ square inch, 49,505 .... . , = 110 y S^ ~ P®^ square inch. Total pressure = 48-5 + 5-4 = 53"9 lbs. per square inch. Saving as compared with (e) : 51,000 - 47,300 137 X 27 15 X 20 = 1^^- -83x83 -/f0x83 iiMiiiMliMiiiirpnr ^Ji tj.ujUlii lu liiil ill Hiin:nni!i2 -f. Fig. 62. Fig. 63. In dimensioning foundation blocks in this way it must be remembered that the pressure on the surface a subjects the section b (Fig. 63) to a bending moment. The stress set up by this should not exceed 57 lbs. per square inch. a 1 For floor pressures of 40 to 70 lbs. per square inch it is advisable to make 7- = ^ {i.e., angle a. = 45°) or = -. With higher earth pressures the ratio should be 1 * kept to -, or a foundation having the shape of a truncated regular pyramid, as shown in Fig. 64, should be selected. The production of this form of foundation with slanting sides is somewhat more difficult, as the concrete cannot easily be forced into the corners unless the mould is gradually built up as the concrete is poured in and rammed home. 90 OVERHEAD TRANSMISSION LINES (9) Foundation as shown in Fig. 64, consisting of a truncated regular pyramid with square base : The general expression for the cubic content of a truncated pyramid is ^-h(F+f + vfjj: where F and / are the bottom and top areas and h is the height. The weight in this case is, therefore, 1 137 i X 79(982 + 872 + V982 >< 372) ^ p^^ = 54,000 lbs. Fig. 64. 2,330,000 = (54,000 + 1,830 + 375) X x = 56,205 x. X = 41-2 inches. y = 7'8 inches. _ I X 54,000 7-8 X 98 54,000 = 46-5 lbs. per square inch. 5-6 lbs. per square inch. Total pressure 98 X 98 46'5 + 5'6 = 52-1 lbs. per square inch. Example 29. To determine the depth to which the pole used in example 16 must be let into the ground. Here the height above the ground = Z = 23 feet (276 inches) and at this height a force P of 575 lbs. acts. The diameter of pole at the ground level was found to be 11 -6 inches, so that the diameter in the ground may be taken as 12 inches. Taking the effective width of compressed surface as -7 X Z) it will be •7 X 12 = 8-4 inches in this case. The maximum pressure on the surrounding earth is not to exceed 43 lbs. per square inch. By equation 65 : 1 2 v M '^ ^ x>n2- • Assuming the depth h of pole underground to be 6 feet 6 inches (78 inches), then, since Xg = ^ha,ndM = P (I + M = 575 ('276 + 1 X 78 and 12 X 575(276 + 5 X 78^ V "^ / _ AO.K •7 X 12 X 782 as required. 43' 5 lbs. per square inch. OVERHEAD TRANSMISSION LINES 91 Example 30. The supporting mast of a long-span overhead line is to be fixed in the ground without concrete foundation. In order to obtain the necessary stability the foot is arranged as shown in Fig. 65. The total tension acting at the top of the pole, a height of 42 feet above the ground, is 1,100 lbs. The holding down force on the mast foot is made up of the weight of the earth resting on the rectangular .79 Fig. 65. surface 79 X 51 inches, together with the weight of the earth forming the two side wedges. In order to be on the safe side only the " active " earth pressure will be considered, the " passive " pressure due to friction being neglected. The weight of the mast and lines is 1,800 lbs. Assuming dry loam (see Table 17) the weight per cubic inch 7 = -055 lbs. andtan^ (45° •189. 92 OVEEHEAD TEANSMISSION LINES Weight of earth : 79 X 79 X 51 X -055 + 2 X ^ X "055 X 79^ X 51 X "189 = 17,400 + 3,300 = 20,700 lbs. Moment of forces acting on the base plate : 1,100 X (42 X 12 + 79) = 640,000 Ib.-inches. and 640,000 = (1,800 + 20,700) x = 22,500 X x. a; = 28'3 inches. y = ~ — X = 39-5 - 28-3 = 11-2 inches, and the floor pressure is % (1,800 + 20,700) „« ,, . . = '^ ^ ' , ^ ' ^ = 26 lbs. per square inch. 11-2 X cl v ^ = 5-6 X 1,800 + 20,700 79 X 51 Total pressure . . .. = 31'6 ,, „ Since dry loam can stand 60 to 85 lbs. per square inch safely the dimensions of this foot could be reduced if desired. 6. THE FORCES SET UP IN THE FOUNDATION BLOCK. The forces acting within the body of the foundation block are indicated by Fig. 66. The stresses o-^ at the top and bottom are alike and oppositely directed. The change of direction occurs at the centre of the block. The distributed forces indicated by the two shaded triangles may be replaced by a pair of forces Q acting at the centres of figure of the triangles, and, therefore, with a leverage 2 oi - h'. The couple thus set up counterbalances the moment acting externally on the mast : and and M = ~h'Q; ■ n 1 ^' smce V = 2 '^e 2" M = T. X A'2 6M Fig. 67. Fig. 66. If b is the breadth of the supporting surface and h' its height then "^^ bx h'^ ^^ Example 31. The mast dealt with in example 21 is to be embedded in a concrete foundation block of the form shown in Fig. 67. What is the maximum stress at the edge of the concrete ? The moment of the forces acting is M = P(l + ^) =930 (475 + '^\ = 480,000 Ib.-inches. 6 M 6 X 480,000 115 lbs. per square inch. ■ ■ ' b X h'^ 4 X 792 This stress is far below that allowable for concrete of even poor composition. 94 OVEEHEAD TKANSMISSION LINES For foundations for poles a poor quality of concrete is allowable — such as is obtained for instance with a composition made up in the ratio 1:5: 10 — as even this withstands a maximum crushing stress of about 1,700 lbs. per square inch. If the mast is put into the ground without any concrete foundation the conditions would be as follows : — The bottom of the mast would rest on a sheet-iron base-plate say 27 X 7 inches. The weight of the mast is 1,000 lbs. and that of the line with additional load is 880 lbs. The symmetrical earth pressure is therefore 1,000 -f 880 27 X 7 10 lbs. per square inch. If a maximum pressure of, say, 70 lbs. per square inch is allowed, then the moment which can be taken up by pressure on the base is 70 in 27 i!f;j = 27 X 7 X S-— X -X =■ 25,500 Ib.-inches. ^ o The moment effective two-thirds of the way down the embedded portion of the mast is M = 930 (475 + I X 79) = 490,000 Ib.-inches. The moment to be taken up by means of lateral earth pressure is therefore Mr = 490,000 - 25,500 = 464,500 Ib.-inches. The maximum (edge) pressure is, therefore, 12 X M, 12 X 464,500 cr fc^2 = 4 y^ 792 — = 223 lbs. per square The counteracting passive earth pressure is 0-^, = ^ X A X tan^ (45° + I) , a,nd, taking tan^ (45° + 2j as 4-56 and 7 as -063 for dry soil,-^ inch. 2y o-^ = -063 X 79 X 4-56 = 22-7 lbs. per square inch. The mast is therefore unable to support itself with this arrangement unless an enlarged foundation is provided. The surface over which the pressure is distributed could be increased by fitting strong iron sheets or girders or by using old railway sleepers. OVEEHEAD TRANSMISSION LINES 95 Example 32. The condition of the strain mast of example 22 with the concrete foundation of example 28 is to be investigated as to the stress set up in the concrete. A concrete mixture in the ratio 1:4:8 will be assumed, having a crushing stress of 2,000 lbs. per square inch. The safe working stress will be taken as one-fifth of this = 400 lbs. per square inch. The dimensions of the foundation are shown in Fig. 68. The compression surface is that offered by the four angle iron stays each 2j^ x 2j% inches, the remaining members not being taken into account. The moment acting on the foundation : = i»f = P(^ + | 75 4,200 475 + '■ If, I ?6 J A 1 s — = 2,150,000 Ib.-inches. A/ \i \/\ .2'. 3" -e'./i- Fig. 81. way that a mass of concrete does. The first method only effects a small saving, as the additions required to the mast foot in the form of strong corrugated iron are large and expensive. 102 OVEEHEAD TRANSMISSION LINES When the forces on the mast are not large the required addition to the com- pressed surface can be obtained by the use of old railway sleepers in place of the corrugated iron. In these cases the uppermost sleepers should only be fixed after the earth below has been rammed hard. Fig. 82. Fig. 79 shows a construction using corrugated sheet or sheet iron stiffened with angle iron, the angle iron also serving to increase the strength of the mast foot. Figs. 80 and 81 show methods of increasing the area of the mast base. Instead of wooden beams some arrangements employ Z-iron clamps in which reinforced concrete beams can be placed (Fig. 82). 8. CONCRETE AND CEMENT FOUNDATION WORK. The cement used for concrete work must be fresh and of the best quality, and must be quick or slow setting, according to the nature of the work. The former is only required in the presence of water, whilst for all other work slow- setting cement is preferable. A cement is called slow if it does not begin to set in less than one hour after mixing. The actual hardening of the cement begins after the setting, and proceeds more quickly with slow-setting cement than with the quick-setting variety ; a greater degree of strength is also attained by the former in a shorter time. It is, therefore, not at aU necessary to employ quick-setting cement for work that is required for use quickly. Whilst hardening is proceeding the work should be shielded from sun and wind, which tend to deprive the cement too quickly of the water which is necessary for hardening. In cold weather both the setting and the hardening processes are delayed, and a longer time should be allowed to elapse before loading the work. If cement work has to be carried out in frosty weather care should be taken to use as little water as possible and to mix thoroughly. If warm water is used the rate of setting can be increased. Cement mixed with sand alone forms cement mortar, whilst cement with sand, broken stone, and gravel forms concrete. According to the nature of the work the proportion of the different constituents is varied ; for greater strength the proportion of cement must be increased. The materials used must be selected with care, as their quality has great influence on the resulting concrete. The sand (quartz sand) must be sharp, not too fine-grained, and, above every- thing, clean. If it contains loamy, clayey, or vegetable matter it should be well washed before use. Sand which is not clean reduces the strength of the concrete and may even spoil it entirely. ^The gravel also should contain no foreign matter. Stones should generally be washed. The water used in mixing should be clean and free from scum. Porous, water-absorbing stone (brick, sandstone, etc.) should be soaked in water before use. Soft or brittle stones should not be used for concrete, as their strength would not equal that of the cement mortar forming the rest of the concrete. For cement mortar cement is mixed with from one to four parts of sand. For the smooth coating of foundations mixtures in the ratio 1 : 3 or 1 : 4 are sufficient. In preparing cement mortar the measured amount of cement is sifted over the required quantity of sand, and the mixture is then stirred together until a 104 OVEEHEAD TEANSMISSION LINES mass of uniform colour results. Only then should water be added, the mixing being continued all the time. The following table (Table 18) shows the quantities required for the various qualities of mortar : — Table 18. Proportions of Mixture. One Cubic Yard of Mortar requires : Cement. Sand. Cement (lbs.). Cement stated in No. of 1 Cwt. Sacks. Sand (Cubic Yards). Water (GaUons). 1 1 1 1 1 2 3 4 1,600 1,060 790 625 14-3 9-5 7-1 5-6 •67 •89 1^0 1^05 78 72 69-5 68^5 Concrete consists of a mixture of cement, gravel or sand and broken stone. These materials are mixed in various proportions, according to the strength required. Gravel, which should consist of all sizes of pebbles between J inch and 2^ inches in diameter, is the most economical constituent, as it provides the smallest proportion of gaps to be filled by the cement mortar. Experiment has shown that such gravel contains about 35 per cent, of gaps and that the densest and firmest concrete is obtained when twice as much gravel as sand is used. If, owing to shortage of gravel, broken stone (road metal) has to be used instead, more cement mortar will be required, as the gap spaces in this case amount to about 50 per cent. It is advisable not to use broken stone of uniform size like road material, but to make it include all sized pieces up to 2 inches in diameter. Often a natural mixture of gravel and sand can be obtained and used with advantage. Its proportion of gravel can be determined by sifting through a sieve of about J inch mesh. More gravel or more sand can then be added, accord- ing to the proportions desired. It must be remembered that concrete consisting only of gravel or stones and cement is far weaker than when sand is present as well. When preparing concrete the sand is spread out on wooden boards or on an iron sheet ; cement is then sifted over it as uniformly as possible and the whole is well mixed whilst dry ; then water is added, mixing all the time, until the con- sistency of moist earth is attained ; finally the previously prepared gravel or broken stone is added and the whole well mixed agam. In making the mixture accurate box measures should be used. The ramming home of the concrete is a point of much importance. Only well-rammed concrete can attain the required density and the consequent strength. OVEEHBAD TEANSMISSION LINES 105 The ramming should be carried out on each layer of 6 inches or 8 inches in thick- ness and should be continued until a layer of water appears on the surface. It is sometimes necessary to add a layer of new concrete to an existing con- crete structure. In such a case the old surface should be well damped and then covered with a thin layer of pure cement. An intimate combination of the two surfaces will then take place. The following table (Table 19) shows the usual proportions employed and the quantities required when usiag gravel and broken stone of graduated coarse- ness and containing about 35 per cent, of gap spaces : — Table 19. Proportions. Breaking Stress, after about 30 Days, in lbs. per sq. incb. One Cubic Yard of Hard Rammed Concrete requires : Cement. Sand. Gravel. Cement (lbs.).* Sand (Cubic Yards). Gravel (Cubic Yards). Cement in 1 Cwt. Sacks. 2 3 4 5 6 4 6 8 10 12 2,950 2,350 2,000 1,700 1,350 530 340 255 212 160 •45 •45 •45 •45 •45 •9 •9 •9 •9 •9 4^75 305 2-25 1-9 143 * Specific gravity of the cement = 1 'i. When using coarse broken stone of uniform size and showing about 50 per cent, of gap spaces the proportions are as shown in Table 20 : — Table 20. Proportions. Breaking Stress, after about 30 Days, in lbs. per sq. incb. One Cubic Yard of Hard Rammed Concrete requires : Cement. Sand. Broken Stone. Cement (lbs.).* Cement in 1 Cwt. Sacks. Sand (Cubic Yards). Broken Stone in Cubic Yards. 2 3 4 5 6 3 4^5 6 n 9 2,550 2,000 1,700 1,350 1,150 680 475 375 340 240 6-1 4^25 3-35 305 215 •6 •6 •6 •6 •6 •9 •9 •9 •9 •9 * specific gravity of the cement = r4. If concrete is to be laid under water it must contain equal quantities of sand and gravel. Even then the strength is never equal to that of rammed concrete 106 OVEKHEAD TEANSMISSION LINES with twice as much gravel, because the mortar and gravel cannot get into close intimacy without ramming. In agitated or flowing water concrete will not set, as the motion prevents a uniform distribution of the cement. The water must, therefore, either be diverted or brought to rest. Only as much concrete should be prepared as will be wanted at the time. Before it dries the surface of the concrete should be given a smooth finish. Concrete work can be carried out in frosty weather at temperatures down to — 3° C, but the work should be done very quickly, and very little water should be used in mixing the concrete. The finished work must be covered up, as in the case of hot weather, to prevent too rapid drying. Concrete structures must be carefully superintended, as much bad work is carried out for want of knowledge or through carelessness. The materials — sand, cement, gravel, and clean water — have often to be transported from a distance, and it can easily be understood that the workmen through hurry or to save trouble will be tempted to make up any shortage by employing the nearest substitute, such as the excavated soil, etc. Many such cases have occurred, as well as cases where large unbroken stones, that happen to have been dug up in the excavation work, have been thrown into the mass of concrete to take the place of some of the proper gravel or broken stone. Some- times also the dry materials are placed in the hole without a drop of water and damping is subsequently carried out by pouring water on the surface. The ram- ming home is often also omitted in order to save trouble or to economise material, for the greater the amount of ramming the greater the amount of material required to fill a given space. It is a common plan to attempt to economise on the expensive cement, and this fraud is difficult to discover after mixing has taken place. Sometimes only the outer and upper portions of the structure to a depth of 10 or 12 inches are of concrete whilst the rest has been filled in with heavy stones, earth, etc. In other cases the quality of the concrete used for the lower portions is not up to the specification. Such irregularities can only be avoided by thorough supervision. 9. POLE CONSTRUCTION. (a) Wooden Poles. Red fir is the most commonly used wood, but American pitch-pine, larch, and Scotch fir are also used. The poles should be straight stems of sound growth, without knots or splits, and not spongy or showing cracks across the end sections. Stems showing a particularly twisted or spiral growth should be rejected, as they tend to twist still further as they dry, and so disturb the hang of the line. The diameter of the ground level should, in accordance with equation 38, be 1-5 times the top diameter. The taper for ordinary lengths should, therefore, amount to about 1 inch in diameter in 8 feet. Variations from the above diameters amounting to J inch are permissible if the average for all the masts in a consignment tallies with the specified figures. When the mast section is not quite circular the smallest diameter should be measured. In the length a variation of + 1 per cent, should be allowed. The top of the mast is usually cut conical in order to let the water run off better, and the surface is painted over with hot tar or asphalt. This offers sufficient protection usually and no additional cover is required. For supervision and statistical purposes each pole should be provided, at a height of about 5 feet, with a brandei table giving the length, diameter at the top, year of erection, and name of ths firm supplying it. Impregnation of wooden masts is resorted to in order to lengthen their life. The best known preservative processes are impregnation with copper sulphate, with zince chloride, with creosote and kyanising with sublimate of mercury. The German Post Office statistics dealing with six and a half million poles between the years 1852 and 1909 show that 83 per cent, of these were treated with copper sulphate, -2 per cent, with zinc chloride, 4-4 per cent, with creosote, •1 per cent, with various other compounds, 11-9 per cent, with sublimate of mercury, and -4 per cent, received no preservative treatment. The same statistics show that the average life of the poles treated with copper sulphate was 11-7 years. Figures published by the Bavarian telegraph authorities show that kyanised poles have a life of 17-5 years. The British Post Office results for creo- soted poles show that an average life exceeding thirty years can be counted on with these when the process is carefully carried out. If these lengths of life are compared with that of an untreated pole (seven to eight years) it will be seen that the impregnation, which only adds 12 per cent. or 15 per cent, to the cost of the pole, is of decided value. The widespread use of the copper sulphate process on the Continent is partly due to the fact that it was the first and, for a long time, the only process known. 108 OVEEHEAD TEANSMISSION LINES and partly because the better (kyanisation) process involves a long period of drying, thus entailing the carrying of a large stock. In recent years, however, the kyanis- ing process has spread rapidly. Thus, in the German Post Office between 1903 and 1909 the increase amounted to 6-5 per cent, of all the poles used. In the same time interval the number of masts treated by the creosote process increased to 1-4 per cent. In England most of the overhead transmission work has been done with poles creosoted by the Bethell or the Riiping processes. The antiseptic action of the copper sulphate solution is comparatively weak. Experiment has shown that pieces of wood treated with a 2| per cent, solution of copper sulphate were no longer free from fungoid growths after two months, whilst pieces soaked for one hour in a 1 per cent, solution of sublimate showed no sign of fungus even after six months. The copper salts ;n copper sulphate do not possess the power to form insoluble or nearly insoluble compounds with the constituents of the wood. The impregna- tion is, therefore, very liable to be drawn out of the wood. This is especially objectionable in chalky ground, where the carbonic acid in the water rots the wood. Impregnation iviih Copper Sulphate. This process was discovered in 1841 by Boucherie. A 1-5 per cent, solution of copper sulphate is driven into the stem from the root end under its own hydro- static pressure (a head of 10 or 12 yards). The sulphate forces the sap out at the other end of the pole. The process is continued until in place of the sap the blue impregnating solution flows out. As the impregnating liquid must find the sap in a fluid and unchanged state, only newly-felled wood can be treated in this way. In the spring, when the sap is thin and fluid, not more than ten days should intervene between the felling and the impregnation, and in summer, when the sap is viscous, not more than eight days. A period of ten or twelve days is required for the impregnation of a 30-foot pole. Impregnation with Creosote. The well-dried or seasoned pole is placed in a vacuum chamber for one hour and the previously heated creosote is then forced in under a pressure of 90 — 120 lbs. per square inch. In this way the cells are completely fiUed with oil, and this means a considerable outlay on creosote. In the Riiping process the creosote is withdrawn later, so that only the walls of the cells remain coated with the oil. This is effected by raising the pressure in the chamber to about 220 lbs. per square inch, and after a sufficient time has elapsed the oil is drawn out of the chamber, and then the compressed air in the wood drives the excess oil out of the cells. The Sublimate of Mercury Process. The powerful antiseptic action of chloride of mercury makes it possible to impregnate wood sufficiently by simply laying it in a | per cent, solution of it OVEEHEAD TEANSMISSION LINES 109 placed in wooden troughs free from iron supports. Kyanising (discovered in 1832 by Kyan) takes eight to ten days. Only fuUy-dried wood, in which all the crack formation resulting from drying is completed, can be subjected to this pro- cess. The dangerous section as regards mechanical stress is that at the surface of the ground, and here the alternations of dryness and damp are most severe and damage to the wood occurs most quickly. It has, therefore, always been this point to which protective treatment has been most applied. Besides many expensive and more or less effective suggestions two simple devices have proved satis- factory. One is due to Wingenfeld, and consists in applying several coatings of a hot mixture of asphalt and tar to the dangerous portion of the pole. The layers are held in position by flat jute bands wound round the pole. The cost of treating a pole in this way at the point of erection is only about one shilling. The process is specially applicable to existing pole lines. The soil around the pole is removed to a depth of 15 or 20 inches, and after the pole has been left exposed in this way for a few days to dry the protective covering is applied. The second method (due to Messrs. Himmelsbach, of Freiburg) consists in applying a layer of special protective material called " Stockschutz " to the portion of the pole on both sides of the ground level, as shown in Fig. 83. Wooden poles are treated in this way at the following prices : — ;^mp,. Fig. 83. Table 21. Length of Pole (feet) : — Protected height . Lengths under ground Free length . Price in shillings . 26 29-5 32-5 36 39 42-5 46 49 52 56 59 62 5 5 6 5 5 5 5 6 6 6 6 6 5 5 5 5-25 5-5 6-25 6-5 6-8 7-5 7-8 8-2 8-8 2-5 2-.'5 2-5 2-75 31 3-75 4-1 4-25 4-6 4-9 5-2 5-8 1-6 1-8 2 2-2 2-4 2-6 2-8 3 3-2 3-4 3-6 3-8 65-82 6 9 6-2 4 The following table (22), on page 110, shows the cost of impregnated wooden poles using Kyan's (sublimate of mercury) process and Riiping's (creosote) process : — (6) A AND H Poles. Single wooden poles are not suitable for long-span lines owing to the heavy wind stresses. In such cases double masts in A form (Fig. 31) or in H form (Fig. 32) can be used. The latter is less often used in power transmission work than the A pole, which is fairly commonly used for lengths of span up to 500 feet. The strength of an A pole at right angles to the line direction is four or five times as great as that of a single pole of the same diameter if the spread of the feet no OVERHEAD TRANSMISSION LINES H CO 7—8 20-8 21-25 « -* 7—8 32-55 33-2 » o MM IM CO to >> I-H to OS OS MM I-* rH 50 61—7 25-85 26-35 "1"* >^ «, to ^r '^ to '^'^ MM «5 to o -f to 1-H rH j;^ CO CO O MN1 ^ ^ 00 "5 » CO rntN «5 in MM IM in ihM Oi o to r-l (M «5 '=°® MM ^_ ^ i, CO b- db to ^ '^ J. J Diameter at top (inches) Price : Kyan (shillings) Rtiping (shillings) .1 1 ac Diameter at top (inches) Price : Kyan (shillings) Riiping (shillings) MM 00 i-H (M OO 05 •-IM 00 i> 00 to ■* t- o 1-1 mM to to CO b- CO CO CO rtM to «5 to CO CO MM 00 rH (M M5 00 CO CO CO ■*! (M to J> CO CO b- mM to (M (M CO CO 05 05 IM IM MM 00 00 00 CO CO >n K5 CO CO 0) o o ■M CS . CO pf ■I-! -i-i :53 3s« c.S .. cS ft 0) >-i;3 PPM OVEEHEAD TEANSMISSION LINES 111 is made equal to about one-eighth of the length of the pole and if the two poles are firmly bound together top and bottom. The strength can be still further increased by fixing a cross-bar a third or half the way up the pole. The wood used for this cross-bar, as well as that usfed for connecting the feet, must be hollowed out to fit the round poles. The connection is made with f or | inch bolts. The tops of the poles must be bevelled off to suit their slope, and can be fixed together by two bolts or by a hard wood key block held by screws. The bedding surface of the bolt head and nut must be made ample by the use of thick washers. 7' to -L, cr J y 3'-3-i 4^-y- "3 "3 V: ipHi -f !l Iran N?ef id -1^- ^1 I I Ulran N? Gi Fig. 84. The usual relation of length of pole to spread of feet, 1 : 8, is a compromise between the requirements of economy and those of strength. Increased spread adds to the stability of the pole, as is shown by formulae 49 and 50. The use of wooden poles at corners or as strain masts or the combining of several A poles is not advisable. The appearance is unsatisfactory and the saving as compared with an iron mast is small, whilst the latter is decidedly safer and costs less to maintain. Pins And Cross-Arms. The shape and size of the insulator pins and the cross-arms on which they are carried is determined by the distances to be maintained from wire to wire and from wire to pole, and also by the forces which have to be withstood. 112 OVEEHEAD TEANSMISSION LINES The simplest form is that of the bent or swan-neck pin shown in Fig. 47, provided with a wood-screw end and screwed direct into the pole. The insulator should be so placed that the line is attached at the same height as the screw end of the pin so that the tension of the Kne shall not tend to twist the pin. Bandlron Band Iron i^yi'iG Fig. 85. Eor fixing in walls the wood-screw is replaced by an end shaped as shown in Fig. 48. These hooked pin supports can be employed for voltages up to 10,000. With higher voltages the necessary distance between the rim of insulator and the back of the pin can only be attained safely by the use of excessively thick iron sections. In such cases, therefore, straight pins mounted on cross-arms are preferable. Fig. 84 shows one such arrangement. Here a U-shaped structure of channel iron or bar iron is placed on the top of the mast. This arrangement can also be used for straining the wires to. The length of mast can be reduced by about 1 yard as compared with that required for the swan neck bracket. The price is from 2s. Qd. to 3s. Qd. each, excluding insulators and pins. Other common arrangements are shown in Figs. 85 and 174. The former shows the construction suitable for two insulators to which a line is to be strained. The same method can be used on A poles when an earthing wire is to be carried at the top of the pole. A cap carrying the earthing wire is placed over the top of the double poles where they are screwed together. A single insulator can be fixed OVEEHEAD TEANSMISSION LINES lis to a wooden pole as shown in Fig. 86. This consists of a couple of bent bar-iron clamps separated, by a length of pipe as a distance piece at the crossing point. They are fixed to the pole by coachscrews or bolts or by a collar Band Iron "Band Iron Iz'i^ Fig. 86. For small tensions straight pins made up of ordinary bolts, such as are obtainable in aU lengths and diameters, can be used. The pin diameter should be yL to J inch smaller than the in- sulator thread. When the shank of the straight bolt is insufficient for the forces involved a conical bolt as shown in Fig. 46 is employed. For large insu- lators and heavy pulls the form shown in Fig. 45 can be used. The insulator pin rests on a cast-iron mantle sub- jected to compression. The short pin above the mantle takes up the bending moment, whilst the longer pin inside the mantle is only subjected to tensile stress, so that great loads can be safely carried with comparatively small dimen- sions. In order to reduce the brush discharges the mantle has sometimes been made of porcelain. In order to determine the strength of this type of pin load tests have been O.T.L. Fig. 87. 114 OVEEHEAD TEANSMISSION LINES carried out as shown in Fig. 87. The load was applied at a distance of 8 inches irom the cross-arm, corresponding to the normal distance at which the tension on the insulator acts. The de- flection A produced at the end of the pin and the amount B by which the cast-iron mantle was raised from its seating were measured and found to increase as follows : — Load. A B Lbs. Inch. Inch. 1.770 . •07 . 2,220 . •138 . 2,650 . •212 3,100 •31 .059 3,550 •55 •118 3,780 . •98 .236 Fig. 88. - --^ . ■ - .-U It will be seen that the j bending of the pin begins at about 1,500 lbs. and increases almost proportionately up to 3,100 lbs. Above this load A increases very quickly. The measured deflections -07 inch, •138 inch, and -212 inch cor- respond to calculated stresses of 28,000, 54,000, and 83,000 lbs. per square inch. The bending of the bolt within the mantle only commences at a load of 3,100 lbs. Present-day insulator pins are only made of best wrought iron or steel. At one time in America, and also in some European installations, wooden (oak) pins soaked in paraffin were used, but experience has shown that such pins become charred by discharges and are soon destroyed. Cross-arms are generally made of channel iron or are built up from bar iron. Channel iron with wide flanges is the most satisfactory, as, besides offering a high resisting moment to . i- -/0-2- Fig. 89. OVEEHEAD TRANSMISSION LINES 115 bending, the wide flanges enable the pins show different methods of construction. In the case of Fig. 90 the distance between the line and the cross-arm has been increased by bending up the ends of the cross-arm at right angles. This has been done in order to reduce the risk of short circuits or earth connections being brought about by birds settling on the structure. In some districts birds, especially rooks, have caused considerable trouble in this way, and this trouble has been practically removed by bending up the cross-arms in the way shown. In the case of wooden poles the cross-arms are attached by means of iron bands as shown in Fig. 91, or as in Fig. 88 when a single size of band is to serve for poles of varied diameter. For the latter purpose the band is made in two parts bolted together, and the cross-arms are pro- vided with long slots. When the cross-arm has to carry heavy lines it is advisable to shape the channel iron to suit the curvature of the mast and to roughen the inner surface with chisel cuts. Cross-arms of considerable length are stiffened as shown in Figs. 88 and 89. to be securely mounted. Figs. 88 — 90 Fig. 90. Fig. 91. -A I (c) Mast Feet. However well the mast is pro- tected, it will be almost impossible to prevent the embedded portion of a wooden pole from deteriorating quicker than the post above the ground. It has, therefore, been sug- gested that wood should only be used for the upper portion and that the foot should be made of some more permanent material. In this way reinforced concrete feet with iron rails for gripping the mast or, more commonly, feet made entirely of iron in very varied shapes have come into prominence. The longer life thus attained by the upper i2 116 OVEEHBAD TEANSMISSION LINES wooden pole (twenty to twenty-two years for kyanised poles) partly compensates for the additional cost involved in providing and maintaining the iron foot. 'Foot ^oF Ye-inforceii concrete '+- t->~/ronfJ'?IO Fig. 92. Mast Feet of Reinforced Concrete (Fig. 92). The two channel irons, which serve to support the pole, he in longitudinal slots in the concrete column in the ground and project about 1 yard above the latter. Two coUars attached to the channel irons embrace the pole. Wedges are driven between these collars and the pole, varying in size according to the diameter of the pole. In order that the bottom of the pole shall not touch the ground a small boss is provided at the top of the concrete column. A special mast foot of this type has been patented by the Weserhiitte of Bad Oeynhausen and is shown in Figs. 93 and 94. The mast foot is. let into the ground sufficiently far to let the bottom, of the pole clear the ground level by 4 or 5 inches. The channel iron type is used on straight stretches of line, but for corner masts or heavily-loaded masts the angle iron form is preferable. The latter is generally concreted into the ground, and in loose soil this has also to be done for the channel iron type. The considerable additional cost of the concrete foundation, however, generally makes it preferable to employ the ordinary wooden pole in a concrete foundation block in such cases. Poles which have begun to rot can be given a new lease of life by fitting iron shoes as shown in Fig. 95. The soil around the mast is removed to a depth of about 20 inches and a strip of channel iron is driven into the ground hard against the pole and is temporarily fixed to the pole with coach screws. The pole is then shored up by means of struts and a notch is cut into it reaching about to the centre. A second channel iron is then driven into the ground opposite the first, and the two are securely bolted together through the pole and the remaining half of the pole is cut clear. [d) Iron Masts. Iron masts are practically always used for the higher voltages and for large power schemes, since in these cases their higher first cost is compensated for by OVERHEAD TRANSMISSION LINES 117 Figs. 93 and 94. 118 OVEEHEAD TEANSMISSION LINES their longer life and the greater reliability of supply which they ensure, masts are of very varied design. The most important types are : (1) Simple tubular masts. (2) Multiple tubular masts. (3) Lattice-work masts. (4) Lattice-work structures or towers. Such -Bp .rJ-.-. B^U.4r-^.. - - U Iron N9I2-I& ;J Fig. 96. Simple tubular masts (Fig. 236) and multiple tubular masts (e.g., masts made up of three simple tubular poles arranged at the corners of a triangle) are not very commonly used. Owing to their good appearance they are sometimes used in villages and the smaller towns, but for long-distance transmission schemes they are too expensive. These tubes are made in lengths up to 26 feet of wrought iron with a breaking stress of 55,000 to 57,000 lbs. per square inch or in Siemens- Martin steel with a breaking stress of 80,000 to 85,000 lbs. per square inch. The Mannesmann tubes are made of the latter material. The prices vary between 17s. and 30s. per cwt. The smaller tubular masts are made in one piece, tapering towards the top and consisting of two or three lengths welded together. Larger masts are made up in sections shrunk together or held together by shrink rings. Table 23 gives the dimensions and the allowable loads for Mannesmann tube masts of the forms I., II., and III. (Fig. 96). OVERHEAD TRANSMISSION LINES 119 CO n Weight of Mast in lbs. ®01>(N«5000000 OSlOiHC000!O>Oi-H50C0tO 1 o 1 a o M o ■S a:) OS §3 ft OOOOOOOOOOO ooooooooooo »> r^^ i-H i> o^ » >o »n >o_ o^ x^ irT !o" w" i> of oo" o" ih" oT r^ in" ooooooooooo ooooooooooo ®^ =0 -^^^ i> ©_^ 0__ 0_ « CO IN o__ Qo" oT of rn" ■*" eo" 1© «f ^" to" a" ooooooooooo ooooooooooo *i ^^ °*. "l °°„ "l *. °°„ *„ °°„ ®„ so" ■*" ^" os" oo" rn" ef co" o eis" oo" i-IMi-lrHi-IININlNlNININ o| l5 J 1 >nCp(N>pNrH»pt>INOp>p r 1 lNC0-*»f5W00OOOOO (NCO-^lCtOOOO-^WSOOO (N CO ■* «5 ® 00 rH C0_^ "5^ 1> IN i-T i-h" i-T i-T in" 1 I— ( .9 1 1 s >a lo >n COrHCOCOtOtOtOCOtOCOCO eb-*"*-*-*ii5«5>ni»ii50 •ca «5 -« »n ©■^qsosaiMrHipcocptp ■*»n>o«5>o»>!>l>dbdbcib a «5 lO »0 W5 b- IN »5 IN >0 >n «5>0«b6l©b.l>C0050505 •^ ;p50rHrHin>n«51C>0>OK5 «5tbi>b.i>cbob6>666 r^ r^ i-i -a t01>i>l>00010SOrHr-lrH rH rH I— i rH o ^1 ^1 ^1 T^ ^1 ^^ ^^ ^1 ^1 ^1 T^ rO 0!0«OCDiffl50COt-i>t-l> 55 OOOT}irjiooo»n>o»n>r5 rHrHrHINlNCOCO-*-*-*-* (NINiNlNININWiN»5ININ - lNOlO>n«5«5 l>l>i>O0ODODO00DO0O0O0 (NIN(N(NININ(NlNlNlNlN 1^ t>l>i>QOO0i»-*INlNlNlN IN(NINCOCO-*«5CO»!OCO COCOCOCOCOCOCOCOCOCOCO 1— I 120 OVEEHEAD TRANSMISSION LINES .k. y--t- h ^ t~- c^ - 9 -^ (^ f^ 4- d mw: — - -f - wm - s -.- d -^ 1 3 - Ir-V ^^, m :' U //-Off MM mm Fig;. 97. OVERHEAD TRANSMISSION LINES 121 Sijuare Iron %>i% Multiple tubular masts have the advantage that they can be built up on the spot, so that the cost of transport and erection is less than for lattice-work masts, although the first cost is considerably greater. Their light and attractive appear- ance is their special advantage, however. Lattice masts are made up of sectional wrought iron, usually channel or angle iron. In many cases the iron section to be used is determined by the object for which the mast is intended. For instance, elastic supporting masts (Fig. 97) are preferably made up of channel-iron sections, whilst strain masts and masts for heavy loads generally should be made up of angle-iron sections. The diagonals of angle- iron lattice masts should be so arranged that, unless crossed diagonals are used, only two bars meet at any one junction point (Fig. 98). In the interests of good appearance the diagonals should be fixed on the inside of the angle irons, although the out- sidearrangement(Fig. 180) offers the advantage of easier and more reliable riveting work. Tail masts and those intended for heavy loads can be ap- preciably cheapened by reducing the sections of the tubes, diagonals, etc., to- wards the top. The correct distribution of the material can be determined by calcula- tion. The rules of the V. D. E. limit the stress in iron structures to 21,500 lbs. per square inch. If Thomas wrought iron with a breaking stress of 57,000 lbs. per square inch is used, the factor of safety is about 2-5, and this figure should be maintained throughout the structure in normal cases. The deflection produced with the above maximum stress is not allowed to exceed 2 per cent, of the free length. This assumes that the drilling and riveting has been carefuUy carried out and that the rivets fit the holes closely. The rivet holes should not be punched but drilled. Besides the holes for the rivets holes must also be provided for fixing the description plate and the earth wire, and these holes should not be drilled in places where they wiU weaken the structure unduly. Unauthorised climbing of masts can be prevented by fixing spiked collars (Fig. 99) round the mast at a height of about four yards. The height of these collars or chevaux defrise, must be kept the same on all the masts so that the width of opening inside the collar should be adjustable. Fig. 98. Ffe. 99. 122 OVERHEAD TRANSMISSION LINES Tables 24 and 25 show the dimensions of some of the standard flat lattice- work masts and quadrangular lattice-work masts supplied by the Weserhiitte Iron Company of Bad Oe5nihausen. These masts are designed for a four-fold factor of safety, corresponding to a working stress of 14,300 lbs. per square inch. If greater or less stress is desired the load applied can be increased or reduced proportionately from the value given. In these tables : — L = gross length of mast in feet. I = length above the ground level in feet. Zj = length underground in feet. 6 = breadth at the top of the mast in inches. Bo= „ „ ground level in inches. / = the theoretical deflection in inches. O = approximate weight in lbs. Table 24.- -Flat Lattice-Work Masti of Channel Iron. AJlow- able 440 lbs. 880 lbs. 1,100 lbs 1,350 lbs. Load. I 16-3 26 39 16-3 26 39 16-3 26 39 16-3 26 39 h 3-9 4-9 5-2 5-2 6-5 6-5 5-2 6-5 6-5 5-2 6-5 6-5 L 20-2 30-9 34-2 21-5 32-5 45-5 21-5 32-5 45-5 21-5 32-5 45-5 h 3-75 3-85 5-1 4-6 4-6 5-1 4-75 4-75 4-9 4-8 4-8 5-6 Bo 8 11 18 11-5 15-5 21-5 11-6 15-5 21 11-5 15-5 21-8 f 205 4-7 5-1 1-77 4-7 5-5 2 4-7 5-8 1-8 41 6 G 305 465 850 355 530 1,100 430 640 1,320 510 760 1,540 Table 25.— Quadrangul ar Lattice- Work Masts of Angle Iron. 3 . < 8£0 lbs. 1,350 lbs. 2,200 lbs. 3,350 lbs. 4,500 lbs. I 16-3 26 39 16-3 26 39 16-3 26 39 16-3 26 39 16-3 26 39 h 3-9 4-6 5-6 4-6 4-9 5-9 5-2 5-6 7-2 5-6 6-2 8-2 6-2 6-6 8-8 L 20-2 30-6 44-6 20-9 30-9 44-9 21-5 31-6 46-2 21-9 32-2 47-2 22-5 32-6 47-8 6 5 5-6 6 5-6 6 6-8 6-9 6-9 8 8-1 9-3 10-2 8-5 9-4 9-8 B„ 11-1 15-2 20-5 11-6 15-7 21-2 13-5 18-2 24-4 15-8 21-5 28 16-2 21-8 28 f 2 3 55 5-5 1-62 315 5-8 1-46 3-6 5-8 1-26 2-9 4-8 1 i 2-75 5-2 G 475 790 1,530 650 1,000 1,920 890 1,400 2,550 1,200 1,800 3,260 1,430 : 2,270 4,300 Such masts are supplied at from lis. to 15s. a cwt., according to the weight of the individual masts and the number ordered at one time. For the longer spans, for river crossings and for very heavy lines, complete ironwork structures and towers are used (see Figs. 101 and 102). OVERHEAD TRANSMISSION LINES 123 - ~*if./fc-_ _ Frequently it is considered desirable to reduce the maximum sag of a line by loading it permanently by means of a weight sufficient to produce a con- stant tension equal to the maximum allowable tension. This maximum tension would occur, in the case of the long spans now under consideration, at — 6° C, with the additional load due to ice or snow. This is, however, also the condition for maximum sag if the line has been erected with a maximum stress of 23,000 lbs. per square inch in hard drawn copper wire (13,800 lbs. per square inch in stranded copper, cable.) The method of reducing the sag by means of weights would, therefore, be inapphcable unless the line happens to have been erected with lower stresses than the above — an unlikely occurrence ia long-span lines. It has, however, already been pointed out that the allowance made for additional ice and snow load in the rules of the V. D. E. is not sufficient to cover abnormal cases, and loads of as much as ten or twenty times those values have been measured on occasion. These excessive loads are especially liable to occur at river crossings ; consequently these are sometimes provided with a tension arrangement combined with a switch in such a way that as soon as the tension exceeds the allowable value the circuit is opened at the crossing and the line is made "dead." This switch is also actuated if a line breaks — the falling of the tension weight opening the switch. Provision against excessive ice load can be more simply made , by straining the wires, which should preferably be of copper-clad steel (monnot metal), so that at — 5° C. with normal ice load the stress is only one-haK or two-thirds of the allowable value and by raising the height of the masts somewhat so as to permit increased sag. If special straining weights are to be employed the arrangement must be such that the weight, the rope, and the pulleys are easily accessible, and not alive, so that they can be adjusted without interfering with the operation of the line. The insertion of a strain insulator between the line and the tension rope (see Fig. 100) enables this to be done. Iron structures should be painted over with liquid cement at those portions which are to be let into concrete. At other parts they should receive one coating of rust-proof paint (bitum nous paint) or red-lead. This painting should be repeated every two or three years, and as this cannot generally be done with the line alive it is necessary to switch it off, at any rate whilst the top of the mast is being painted. In order to avoid this difficulty it is common practice to galvanise the upper portion of the mast down to about 1 yard below the lowest cross-arm. The lower part of the mast can then be painted whilst the line is in use, if reasonable care is taken. Fig. 100. 124 OVEEHEAD TEANSMISSION LINES OVERHEAD TRANSMISSION LINES 125 Strain Towers at the Crossing oF the Ems. Fig. 102. 126 OVERHEAD TEANSMISSION LINES 2.9 -s^ IF? -i Fig. 103. OVERHEAD TRANSMISSION LINES vn Examples from Practice. The structures described below are some carried out by the Weserhiitte of Bad Oeynhausen. Pig. 101 shows a tower 206 feet high to the underside of the cap, and 2 16' 5 feet high overall, used by the Siemens-Schuckertwerke of Berlin for a high-tension line crossing of the river Trave at Herrenwyck. Besides wind pressure it has to withstand two pulls at right angles to one another — one being 16,600 lbs. and the other 3,500 lbs. The weight, including that of the anchoring arrangements, amounts to about 19 tons. It was erected in mid-winter during violent storms and without any scaf- folding, working upwards from the base. Ordinary ladders are provided for mounting this tower for inspection purposes. Platforms are inserted at intervals to avoid risk of fatigue when climbing. I "^ Solid earth Foot of Tower in Fig. 103. 128 OVEEHEAD TRANSMISSION LINES Labkice WorkMasb For a Pull oF 6200 Lbs Scale I: 225 ^i-i}, L2^*2f '-^" LZ*'2t^'^ . 16' xie. Labtice Work Mast T i ^i l i- Fora Pull oF 3500 Lbs. Scale i:l30 Centre oF Gravity ortsrbh Pressure Fig. 104, Fig. 105, OVERHEAD TRANSMISSION LINES 129 O.T.L. 130 OVEEHEAD TEANSMISSION LINES Fig. 102 shows a similar structure used by the Siemens-Schuckertwerke for a crossing of the river Ems. The total height of the tower itself is 235 feet 9 inches. Owing to the frequent occurrence of floods and floating ice the foundations project 1 3 feet above the ground level, so that the top of the tower is 248 feet 9 inches above the ground. The space between the four foundation blocks is filled in with gravel covered with a concrete cap so as to prevent the accumulation of ice. Railway Crossing. Scale ims OVEEHEAD TEANSMISSION LINES 131 'irr^ o.p d;s 81 ' // 1 ■ ..9:ie ?3;y ] K 2 L32 OVEEHEAD TRANSMISSION LINES 50'.0"to Mast Centre ■3'r Fig. 109. OVERHEAD TRANSMISSION LINES 183 The mast shown in Fig. 103 is provided with a circular platform to give a secure foothold for workmen inspecting or repairing lines or insulators. The mast is mounted by means of a staggered arrangement of iron projections. Similar structures suppHed to the AUgemeine Elektrizitats Gesellschaft of Berlin are shown in Figs. 104 and 105. One of these is used at a crossing of the Oder at Schiffsmtihle and since the ground was not firm enough it was mounted on piles as shown in Fig. 104. These masts are provided with hinged feet, so that the mast can be completed on the ground and can then be raised bodily into its vertical position. Fig. 106 shows a H.T. line crossing a road with telegraph wires by means of a special safety bridge structure. The free spans are kept very short and the mechanical stresses very low. The tensions of the line spans on both sides of the crossing are taken up by the towers. Fig. 107 shows a railway crossing arrangement. Here the masts are provided with an inspection platform and the various circuits are separated from one another by safety screens. Fig. 108 shows an application of UUbrichts' network conductor for crossing a railway line. In this case double masts are used at each side of the crossing. Another double mast arrangement is shown in Fig. 109. The methods adopted for securing masts to their foundatio .s by means of foundation bolts are indicated in Figs. 110 and 111. The safety screens used for separating the various circuits on a mast are also shown. These make it possible for repair work to be carried out without danger on one of the circuits (after making it dead) without interfering with the second high-tension circuit. The round iron rings shown at the tops of these masts are intended as perches for the larger birds to settle on in preference to the wires. Figs. 112 and 113 show two forms of mast and cross-arms as used Avith suspension insulators. The large space occupied by the bases of the towers in these cases make them only suitable for use on land of low value. . ^^ TappinQ for earthing \ dia Fit;. 110. 10. OVERHEAD LINE INSULATORS. Insulatoks are practically always made of " hard " porcelain, whose chief constituent is kaohn, to which are added small quantities of quartz and felspar. The glazing consists of a mixture of kaolin and silica. A freshly-broken porcelain insulator should show a light speckly fracture and must not be porous. Glass insulators were at one time commonly used in France and, for low voltages, in America, but their brittleness and inability to withstand the weather conditions have practically led to their disappearance from transmission work. An attempt has also been made to com- bine the high breakdown voltage of glass with the weather-resisting powers of por- celain by making up combination insula- tors whose outer part is of porcelain and inner portion of glass. At the present time, however, the porcelain insulator is practically without competitor. A good insulator should, in the first place, show a high piercing voltage. The thickness of porcelain necessary for mechanical safety makes this condition easily attainable. The actual piercing of an insulator is likely to cause far more serious damage than a mere surface dis- charge, as the latter will generally only cause the opening of a circuit breaker. The second condition to be fulfilled is that of a high surface discharge voltage with the insulator damp or wet. The vol- tage required to cause a surface discharge must be high enough to prevent breaks down occurring under ordinary conditions OVERHEAD TRANSMISSION LINES 135 Fig. 111. 136 OVERHEAD TRANSMISSION LINES of voltage rise, such as may take place through switching operations, atmospheric effects, earthing of one of the wires (thus throwing the full Hne-to-line voltage on the insulatoi), etc. All insulators should have a high surface insulation resistance so as to minimise the leakage losses and to avoid trouble with neighbouring postal lines. This can be attained by suitably dimensioning the insulator, and a margin should be allowed so that in case of slight damage sufficient insulating power remains to avoid a complete breakdown before the insulator can be replaced. Besides these electrical properties the insulator must also be able to withstand the vertical and horizontal mechanical forces due to the line. The vertical downward pressure due to the weight of the line is small compared with the hori- zontal force due to the line tension. The insulator must have at least as great a Formulation to Fig. 111. OVERHEAD TRANSMISSION LINES 1B7 Supporting Hast for Suspension Insulators Section a. b Scale I- 110 Fig. 112. 138 OVERHEAD TRANSMISSION LINES Mast ror Suspension Insulators, For use on Curves > Pull' 3500 lbs. Sc3/ei:/2S Fig. 113. OVEEHEAD TKANSMISSION LINES 139 factor of safety, under the most extreme temperature conditions, as the rest of the structure. Often mechanical breakdown does not occur directly through failure of the insulator itself, but indirectly through bending of the pin. This results in the pin bearing hard against the lower portion of the porcelain thread and eventually causing fracture. It is therefore advisable to use pins of ample dimensions. Experience has shown that trouble with insulators, especially in industrial districts, has most often been caused by smoke or soot deposits and sometimes, in the case of coast lines, by salt deposits. As regular cleaning of the insulators is generally out of the question, their shape must be such that the rain itself serves, at least partially, to keep the sur- faces clear. Fig. 114. Fio-. 115. Fig. 116. The surface of the insulator must be entirely unaffected by the acids which are present in the atmosphere. In the case of glass, at least, deterioration has been found to occur from this cause. Further, the surface of the insulator must not be damaged by brush discharges or their accompanying phenomena. Ebonite is unsuitable on this account because it is slowly attacked by ozone. Ambroin insulators also have to be protected against brush discharges by means of porcelain caps. Glass, owing to its brittleness, is easily fractured when a temporary discharge occurs. For economy and ease in erection it is essential that the whole design of the insulator shall be as robust as possible. Thin rims are easily broken in transport. Very flat tops are objectionable, as the line wire is liable to be bent into contact with them. The insulator should, further, be more or less seK-protective against faUing boughs, stone throwing, etc. All these points imply a massively-built insulator, bub, on the other hand, considerations of cost and charges for transport and for erection make it desirable to keep the insulator as light as possible. These latter considerations are especially important in connection with the very large insulators used for the highest voltage transmissions. The impossibility of building high-voltage insulators sufficiently light has, in fact, largely extended the use of the suspension type of insulator described below. Finally it should be pointed out that the insulators should have as open 140 OVEEHEAD TEANSMISSION LINES a shape as possible, for narrow, dark openings are liable to be taken possession of by insects, which will in time completely fill them with their webs and excretions. The earliest power circuit insulators were developed from the existing tele- graph insulators, of which the first double-petticoat type (Fig. 114) was introduced by Chauvin in 1858. This type is still much used for low-voltage installations and occasionally also for high-voltage Schemes up to 3,000 volts. Fig. 115 shows an insulator especially suitable for low-voltage Mork. Fig. 116 shows the same type carried out as an inverted (or suspension) insulator. Fig. 117. Fig. 118. KiK. 119. Fig. 120. Fio-. 121. Fig. 122. For voltages over 3,000 the double-petticoat type was enlarged into the triple- petticoat type, which ten or fifteen years ago was a great favourite for voltages up to 6,000. As far as mechanical considerations are concerned this type meets all requirements, but it does not offer much resistance to surface or rim discharges because of the short distance between pin and outer rim. Some improvement was effected by increasing the outer diameter through the addition of a cover outside the existing cylindrical mantle. The limit was soon reached, however, through the excessive weight and size required and the difi&culty of getting sound porcelain, free from cracks and blow-holes, in these large sizes. A fundamental change was introduced in 1897, in the form of the Delta insulator, by the Hermsdorf Porcelain Company. This tj^e gave greatly improved security against surface (brush) discharges. Fig. 117 shows the original form, in which it was widely used. Figs. 118 to 121 show the way in which experiment and experience led to its gradual development into its present form. The net result has been the attainment of an increased flash-over voltage without increased weight of porcelain. Table 26 contains some particulars of the most commonly used Delta insulators of the Hermsdorf Company. The type with wide pin opening is intended for use on strain masts and corner masts, where the pin has to be of large diameter to withstand the heavy pulls. Fig. 122 shows an insulator made by Ph. Rosenthal & Co., of Selb, in which the middle petticoat is replaced by a number of shorter mantles or ribs. Fig. 123 shows the same makers' " Kammerisolator." Table 27 gives particulars Fig. 12,1 of these two types. OVERHEAD TRANSMISSION LINES Table 26. 141 Type J, Type 3. Height of Line Working with Net weight with Net weight Wire above Voltage. narrow in lbs. ■KiAe Pin in lbs. the Crcssarm Pin opening. opening. (inches). 8,000 1,381 1-45 _ 5 10,000 1,382 2 — — 7-5 15,000 1,383 2-85 1,402 2-9 5-75 20,000 1,384 3-75 1,403 4-2 6-75 23,000 1,385 5-1 1,404 5-3 7-5 27,000 1,386 6-2 1,405 6-7 8-75 30,000 1,387 7-5 1,406 8-2 9-5 33,000 1,388 9-7 1,407 10 11 37,000 1,389 11-2 1,408 12 12 40,000 1,390 12 1,409 14 13-5 43,000 1,391 14-5 1,410 16-5 14-5 48,000 1,392 16-5 1,411 19 15-5 50,000 1,393 20 1,412 22-2 17 53,000 1,394 22-2 1,413 24-5 18 56,000 1,395 26-5 1,414 30 19 60,000 1,396 29 1,415 32-5 20 It was explained above that the use of large masses of porcelain led to the presence of cracks, etc. This trouble can be partly overcome by careful design, but for voltages above about 20,000 it becomes necessary to make up the insulator in two parts in order to ensure sound results. The two parts may be either Table 27. Working Ribbed Insulator. Kammeri :olator. Test Voltage of the whole Insulator Voltage. No. Weight (lbs.). No. Weight (lbs.). and of the Component Parts. 5,000 8,000 10,000 12,000 15,000 20,000 25,000 30,000 32,000 37,000 40,000 45,000 50,000 901 902 903 904 905 906 907 908 909 910 911 •67 •84 1-42 195 3 8-9 4^9 7 8^4 9-7 127 1,001 1,002 1,003 1,004 1,005 1,006 1,007 1.008 1,009- 1,010 1,011 1,012 1,013 •74 •95 1^38 2 3-2 4^4 55 7^8 9-6 124 13^8 17 20-5 25,000 32,000 40,000 45,000 56,000 66,000 80,000 85.000 45,000 and 45,000 50,000 „ 50,000 50,000 „ 50,000 55,000 „ 55,000 60,000 „ 60,000 142 OVERHEAD TRANSMISSION LINES furnaced together, i.e., combined into one piece in the oven, or they can be finished off quite separately and be subsequently cemented together. In the case of three-part insulators a combination of both processes is often used. All the insulators dealt with up to the present, although differing widely in form, have one thing in common, viz., the rapid increase in weight with increasing voltage. Thus the insulator No. Jl,384, for 20,000 volts, weighs onty 3-75 lbs., whilst Jl,396, for 60,000 volts, weighs 29 lbs. Trebling the voltage has increased the weight nearly eight-fold. This fact has made it necessary to develop another form of construction altogether, especially for the higher voltages over 60,000 or 70,000. It was for this purpose that the so-called " suspension insulators " were introduced, being probably developments of the old strain insulators. The characteristic point about them is that the line wire is carried below the suspension point and that this enables a number of independent insulators to be used in series in place of a single large pin insulator. Owing to the improved arrangement of the surface the leakage from suspen- sion insulators is often only half as great as for the corresponding pin insulator. Also the piercing voltage of a group of suspension insulators is at least as great as that of a multi-part pin insulator, and it can be increased at will by adding more units to the string of suspension insulators. The greatest improvement, however, is the reduced tendency to brush discharges, which appear at compara- tively low voltages with pin insulators because of the surface creepage. Another important advantage of suspension insulators is the comparatively small stock of separate designs which is required to meet all cases. With pin insulators, on the other hand, several types and sizes are generally wanted on each scheme. With the suspension type it is practically only necessary to vary the length of the chain of insulators. The separate units are comparatively small and easy to manufacture with great uniformity. This results in economy with regard to stock carried, time of delivery, reliabUity, etc. The possibility of raising the voltage of an installation without scrapping the existing insulators is another advantage possessed by suspension insulators. It is only necessary to add one or two units to each chain to satisfy all the require- ments of the higher voltage. If pin insulators were used it would be necessary to employ specially large and expensive insulators from the first if a complete change is not to be required when the voltage is raised at a later date. Even where an increase in voltage is not contemplated, it is often a great convenience to be able to increase the margin of safety of the insulation at certain parts of a line (e.g., near the sea coast, in industrial districts or across fen or marsh land) as experience accumulates. The ease in mounting and in replacing sus- pension insulators as compared with pin insulators is a further advantage not to be underestimated. In case of damage, instead of having to replace a whole expensive pin insulator it is only necessary to replace the particular unit of a suspension insulator which is the cause of the trouble. The risk of serious damage, for instance through stone-throwing, is reduced to a minimum, as the probability is that only one unit will be damaged and the remainder of the chain of suspension insulators will usually be able to prevent an interruption of the supply. OVERHEAD TRANSMISSION LINES 143 The actual shape of the suspension insulators is as varied as with pin insulators. Some shapes are extremely simple, consisting merely of a single flat or corrugated saucer ; others are of complicated shape, and sometimes are made up of two or more parts. Two fundamental types, however, can be distinguished. In the one the iron connections are linked together in the form of a chain only separated by the body of the insulator (Fig. 124), and in tbe other the upper and lower metal parts of an insulator embrace one another cylindrically with the Fig. 124. Fig. 12.5. porcelain between, but are not actually Mnked through one another (Fig. 125). In the former arrangement the porcelain is chiefly in compression, and should a break occur in the porcelain the iron parts still remain linked and mechanically useful in preventing the line from falling. In the second type, if carefully shaped, the porcelain can also be subjected to compressive stress, or at least shearing stress, but, should a breakage occur, the risk of the line falling is considerable. The great Fig. 126. Fig. 127. Fig. 128. mechanical strength of this type, however, makes breakage unlikely, whilst at the same time it possesses certain marked advantages over the linked type from the electrical point of view. Of the linked patterns, the best known and the one most commonly used in America is that shown in Fig. 126, due to Hewlett. When used for straining purposes, i.e., in a horizontal position, the shape is somewhat modified, as shown in Fig. 127, so as to make both sides symmetrical. Recently the channels in the chain form of suspension insulator have been 144 OVERHEAD TRANSMISSION LINES given a square liore in place of a circular one and have been sliarpljr curved at the top. This allo^^'s metal strip in place of wire roj^e to be used for the connections. These metal bands are much more conveniently joined than wire rope by means of screws, etc. Rope, in fact, rec^uires the use of special clips unless thin rope is used and is wound through the insulator time after time, in which case the resulting friction gives sufficient grip. Fig, 128 shows a suspension insulator of the Delta type made by the Por- zellanfabrik Hermsdorf, which has been used on a number of power schemes. Its size is such that each unit can stand about 25,000 volts. For 50,000-volt circuits, therefore, two units are used in series and for 70,000 volts three. Fig. 129 shows a suspension insulator made by the Porzellanfabrik Rosenthal. The same firm's strain insulator as used on the 1 10, 000-volt power scheme of the Lauchhammerhlitte is shown in Fig. 130. Fig. 131 shows a strain mast fitted with these insulators. On straight stretches the suspen.sion insulator meets all the mechanical demands, but at corner points, where a sideway pull has to be taken up, their flexible nature makes somewhat special arrangements necessary. These side pulls some- times subject the insulator to heavy stresses, and to meet these such strain insulators are often given an additional -l or | inch in depth. The breaking load is in this way raised to about 14,000 lbs. For specially heavy loads still greater depths are sometimes employed. The mechanical safety of the installation is increased ^^'hen susj^ension insulators are used, as their flexibility enables ecpialisation of stresses to take place both on straight stretches and at corners. Should a line breakage occur on one side of a mast the latter will not be suljjected to much stress, as the chain of insulators \\'ill set itself in a slanting direc- tion and virtually lengthen the line, thus relieving the tension in it. The mechanical and electrical protection against damage through lightning is enhanced when suspension insulators are used because the earthed mast and cross- arms project well above the lines. This fact goes a long way towards paying for the some^vhat increased height of mast recpiired with suspension insulators, AMth pin insulators the same protection has to be sought by running an earthed Avire rojje over the line. Another mode of jjrotection against lightning or other discharges to the insulator supports is oft'ered by the use of the guard ring introduced by the Por- Ficr, 12!i. OVERHEAD TRANSMISSION LINES 14J zellanfabrik Hentschel and Miiller (Figs. 132—134). This metal protective ring is intended to take the discharge direct from the line to earth without its touching the insulator or, at any rate, only touching the upper surface. Should a ■x: i L Fig. 130. discharge occur over the surface of the insulator the well-known outward tendency of the arc will soon cause it to jump across to the guard-ring. Such a ring reduces the breakdown voltage of the insulator in the dry state, but does not affect the wet state breakdown voltage. The Niagara, Lockport and Ontario Power Company has carried out extensive experiments with these guard-rings on a H.T. line. Half of a 37.5 mile line was fitted with guard-ring insulators and the other half with ordinary ones. The result was that, in a given period, of the pro- tected insulators only one was entirely destroyed and 13 others were somewhat damaged, but not enough to put them out of action. The corresponding figures for the unprotected insu- lators were 54 and 36 respectively. It was found that four times as many of the insulators at the top of the masts broke down as of insulators mounted on the cross- arms. The protective ring can be either made of Avire netting or of band iron. If made of narrow mesh netting or of perforated sheet it also serves as a partial protection against stone-throwing. O.T.L. L Fi.L'. LSI. ^^5" Fig. 132. 146 OVERHEAD TEANSMISSION LINES Fig. 133 shows clearly how the arc in the case of a ring-protected Delta insulator passes direct from the line to tlie ring without touching the inner mantles. Fig. 134 shows the action wJien jjrotective rings are fitted both at the top and the Fie;. 133. Fig. 134. -a»j| WM ^^^^' IbMHi^ '^^^^^^^^^H ^m^ _..«iiH ^mP>wM"M»i-.' -mtSl^^^^M IP -^-jp-''"' ht. (.-.:;.;■.. at^i-i^B K -- 'iii IHHHHHMIHHHb^H ■l^'fli^ ^^^B^^^^^^^H »▼ H^^H ^^^V^IV^ i^m^^^^m^i^^^^^^^g ^K^ "i ^^^^^^^^^1 ^^k* ^^^^^B^^ 4 ^M Fi?. LS.i. iDOttom. The air gap between the two rings is bridged without the arc touching the msulator. Fig. 135 shows the breakdown of the same insulator without pro- tective rings. U. ATTACHMENT OF THE INSULATORS TO THEIR PINS. Instjlators are attached to their pins in a great variety of ways. All sorts of cements for the purpose have been put on the market, some of which are unsatis- factory because they attack the pin chemically and others because their co- efficient of expansion differs from that of porcelain. A good method consists in the employment of hemp soaked in linseed oil or in red-lead. The mounting however, requires training, skiU, and reliability, and it is not advisable, therefore, to leave it to unskilled labourers to fill in their time on. The safest plan is to obtain the insulators ready mounted on their pins from the porcelain factory. The PorzeUanfabrik Hermsdorf state that a great many insulator defects which they have investigated have turned out to be due to faulty mounting and the use of unsuitable materials for it. They have therefore circu- lated the following instructions. In the first place mark the point on the pin to which the lowest mantle reaches when the pin is screwed home (Fig. 136). Next fix the pin in a vice and wrap the thread round with selected long-stranded hemp, covering the end of the pin as well. The prepared end should be of approximately the same diameter and length as the threaded hole in the insulator. The hemp is then to be smoothed over by hand or by a special tool and to be painted with linseed oil or red-lead. Before screwing the pin in a small pad of felt, leather, or asbestos should be placed on the floor of the hole. The pin can then be screwed home. The mark previously made on the pin will serve to prevent excessive tightening of the screw and cracking of the insulator, but, in order to make quite sure that no undue stresses have been set up, it is advisable to unscrew the pin slightly again after it has been screwed home. Special machines for ensuring rapidity and uniformity in mounting insulators have been devised. One such arrangement, for instance, is used to compress and smooth the hemp pad on the pin. The hemp-covered pin is turned by hand whilst being at the same time pressed into a trough-shaped recess. When ready the pin is held in a vice and the insulator is screwed on to it by means of a second tool on the principle of a band brake, which enables a secure hold to be got of the insulator, and is especially useful for large sizes. The screwing on of large insulators is, however, always a delicate matter, and greatly dependent for good results on the conscientiousness of the workman. On this account the use of cement for fixing the pin has been largely resorted to in America with very satisfactory results . With the best cement carefully prepared l2 Fig. 136. 148 OVEEHEAD TRANSMISSION LINES there is little danger of cracking, but it is always advisable to introduce a layer of compressible material between the pin and the porcelain so as to admit of the cement expanding and contracting. A thin felt sheath is suitable for this purpose, and experiments carried out under the authors' direction on insulators prepared in this way have shown quite satisfactory results over long periods and with the widest temperature variations. It has often been stated that one advantage of the hemp fixing is that the insulator can be replaced in case of accident without removing the pin from the cross-arm. This is a mistake, however, as the conditions existing when repair work is going on (storms, want of time, awkward position on the mast, etc.) prevent the remounting being properly carried out. It is always preferable to take the new, ready-mounted insulator to the mast and to replace the complete pin and insulator. All pins should, therefore, be provided with hexagon or flatted grips to facilitate unbolting with a spanner. Suspension insulators of the bolt and cap type (Fig. 125) must be so arranged that the expansive force of the Portland cement used to fix the bolt and the cap cannot damage the porcelain. This can be effected by making the cemented bolt he within the tempered steel outer cap, which then takes up the expansive force and prevents damage. 12. ATTACHMENT OF THE WIRE TO THE INSULATOR. In the case of pin insulators the wire can be carried either in a recess at the top of the insulator or in the neck groove. At bends and corners the wire should be laid in the neck groove in such a way that the line is forced against the side of the porcelain by the line tension, the insulator lying within the angle formed by the line. On straight stretches of line the use of the top groove, or of a recess in a special cap carried at the top of the insulator, is preferable to the side groove. The binding or tying-in arrangement between line and insulator is subjected to — (1) the weight of the line, (2) the wind pressure on it, and (3) any difference in Mne tension which may exist on the two sides, and consequently must be very carefully carried out. Objectionable interruptions of supply through falling wires have often been caused by insufficient binding. By using the top groove to carry the wire the effect of the line weight and the wind pressure is removed from the binding wire, and the latter has only to counteract the net longi- tudinal tension. The line weight falls directly on the insulator, and the wind pressure is taken up by the sides of the groove. By suitably , shaping the groove all necessary security against blowing down can be ensured. The top groove support has one disadvantage in that the wire is liable to rub against the edges of the recess. Trouble from . this cause can, however, be avoided by slipping a split metal tube over the wire or by winding a spiral of metal ribbon round it at the insulator (see Fig. 141). The line wire must be fastened to, the, insulator in order to prevent longitudinal motion, which would in time damage, both the wire and the insulator. Unbalanced longitudinal tensions can be set up, in a line as a consequence of unequal span lengths (temperature changes) oh, the two sides of an insulator, unequal snow or ice deposits, the settling of birds (swallows, starlings, etc.), or through unequal wind pressures, and are, sometimes quite considerable. Large line wires are, therefore, now generally securely attached to the insulators by means of special caps and clamps. When making a tie care must be taken to avoid kinking the main wire by means of the binding wire. The old, simple arrangement shown in Fig. 137 is now seldom used because of the way in which it bends the line and tends to damage it. Care must also be taken not to nick either the line wire or the binding wire with the pliers when making the tie. The material used for the binding wire can be soft copper for short spans and local distributing networks generally. In other cases Fig. 137. 150 OVERHEAD TRANSMISSION LINES Fi,L'. 138a. Fie. 138b. Fi;;. 138c. medium hard drawn copper should be used. For hne wh'es between -14 and -22 inch in diameter a binding wire of from -09 to -11 inch in diameter should be used, whilst for larger lines the binding wire should be -11 to -14 inch in diameter. Copper-clad steel forms a good binding wire and, owing to its strength, a some- what smaller diameter than the above can be used. This is also suitable for tying aluminium lines, the soft metal being protected by means of a tube or a spiral band at the tying point. The binding should be done by hand without the use of pliers. In order to avoid waste it is advisable to supply the wiremen with the binding wire in ready-cut lengths, the correct length being first determined by trial in the following way : — When the Top Groove is used. — Two binding wires are wound round the insulator in opposite directions and in such a way that their two ends are of unequal length (Fig. 138a). The two ends of each wire are then twisted together until the OVERHEAD TRANSMISSION LINES 151 twisted portion reaches to the bottom of the groove when bent upwards (Fig. 138b). Of the four wire ends thus left the two short ones are now given four or five twists round the line conductor on opposite sides of the insulator (Fig. 138c). Each of Fig. 138D. Fig. 138b. the long ends is brought right over the msulator and then also twisted four or five times round the line wire (Figs. 138d, 138e). ' When the Neck Groove is used. — In this case a single binding wire is required, which is carried round the neck of the insulator, starting with the middle of the 1 Fig. 1S9. Fig. 140. wire, and its two ends are then twisted round the line wire — one from above downwards and the other from below upwards. The ends are then brought forward and both are carried once more round the insulator and line wire, crossing the starting point and finishing off with six or eight twists round the line wire on both sides of the insulator (Fig. 139). 152 OVEEHEAD TRANSMISSION LINES Another method of tying a line into the top groove is shown in Fig. 140. This is the method used on the 60,000-volt line of the Ontario Power Company. The long twisted portions are here intended to add to the section of the conductor (aluminium) so as to delay its melting through in case an arc should occur. StUl another arrangement is shown in Fig. 141, where the binding wire is replaced by terminal clamps and the line is protected by a metal sheath slipped over it. Figs. 142 and 143 show a terminal clamp arrangement suitable for cases Fig. 141. Fig. 142. Fig. 143. in which the line wire slants relatively to the insulator owing to considerable differences in level between the supporting points. The odd bits of wire cut off during binding operations must be carefuUy collected as they form a serious danger to cattle by becoming mixed with their fodder or by running into their feet. The longer pieces of wire also are favourite missiles for boys to sling over the line and are then liable to cause short circuits. In the case of suspension insulators the line wire is carried in a clamp cemented into the first member of the insulator chain as shown in Fig. 132. 13. JOINTS AND BRANCH CONNECTIONS ON THE LINE. In all cases in which the presence of a joint weakens the wire mechanically the branch wire must be relieved of all tension. The joint should, whenever possible, have the same strength as the wire, but, in any case, it should have a factor of safety of 2J considering the actual stress existing on the wire. The making of the joint must not weaken the main conductor, must not produce sharp bends in it, it must be light and simple to make, and must not be affected by variations in tension or by the vibrations of the line. The electrical resistance of the joint should remain permanently at least as low as that of an equal length of the original wire. Soldered joints are allowable on soft copper lines and also, when mechanical stress is removed from the joint, on bronze and medium and hard drawn copper conductors. The heating reduces the strength of hard materials some 25 to 35 per cent. Since soft copper is no longer used for transmission lines, the joints usually required, where mechanical stress cannot be avoided, have to be carried out with- out soldering, at the same time satisfying the above-stated conditions as nearly as possible. Solid wires up to about '22 inch in diameter (-04 square inch area) can be connected by means of spiraled metal sheaths (Ar!d couplings) (Fig. 144). When employed for connecting stranded cables over about -025 square inch in section these couplings become very long and expensive. They are largely used on stranded aluminium cables and have proved eminently satisfactory. The length of sheath required for various copper and aluminium conductors is given in the following table : Table 28. Cross -section Length of Sheath in Inches. Cross-section of Aluminium in sq. inches. Length of Sheath for Stranded of Copper in sq. inches. Solid Copper Wire. Stranded Copper Cable. Aluminium Cables in inches. •0155 8 •039 18 •025 10 18 •054 20 •039 12 23| •078 22} •054 14 31| •108 27} •078 16 35| •148 33} •108 — 40 •186 40 •148 — 43} •235 43} •186 — 47 •29 47 — — — •31 49 154 OVEEHEAD TRANSMISSION LINES In making the joint the procedure is as follows :— After baring and cleaning the wire ends they are placed in a sheath of the correct size (Fig. 144) with a projection of about i inch at each side. The ends Fig. 144. of the sheath are then gripped by means of special spanners (Fig. 145) and twisted spirally : five or six complete twists are required. The spanners must be placed about an inch from the ends of the sheath in order to avoid splitting the latter. In the case of stranded cables the twisting must be carried out in the same direction as that in which the stranding has been done. The wires twisted together should be free for at least 25 or 30 yards so that the twist can be suffi- ciently distributed. Fig. 146 shows a finished joint. The twistiag is found to affect the strength of the wire deleteri- ously, as can be seen from the following experiments carried out on hard drawn copper wire cables having a break- ing stress of 57,000 lbs. per square inch : — Fig. 145. Table 29. No. of the Sample. No. of Wires. Diameter of the Wires (inches). Total Cross- section of Cable (square inches). Breaking Load (lbs.). Breaking Stress in lbs. per sq. inch of Cable Cross- section. In the Cable. In the Central Strand. Remarks. 25 35 50 70 7 19 19 19 1 7 7 7 •083 •06 •073 •085 •0884 •054 •0785 •109 2,030 2,150 3,830 4,830 52,800 39,800 49,000 44,500 In all cases the (^ break occurred in the cable directly against the joint. OVERHEAD TRANSMISSION LINES 155 Another satisfactory form of joint, especially suitable for solid wires, is Hofmann's riveted joint (Fig. 147). The bared and cleaned wire ends are placed in the sheath and a conical punch is knocked through the holes in the sheath and forces the wires into the side recesses in the sheath (Fig. 148). Into the open- ings thus left (Fig. 149) rivets are inserted which keep the wires in position and ^^^^^^'^^^i^j^^&i^^^'^^^S Fig. 146. prevent them being pulled straight again. The subsequent riveting gives a sound and intimate contact between the wires and the inner surface of the sheath. It is essential that the rivet shall force the sheath material flat on to the wire, and this end is best attained by hammering the rivet squarely, and not in such a way as to form the usual head. The inventor of this joint has determined the Fig. 147. Fig. 148. Fig. 149. exact size of the sheath for each section of wire by experiment, and only this size should be used. This is important, as sometimes the wiremen, finding a difl&culty in inserting the wire in the correct sheath owing to burring of the wire ends, have used a larger size of sheath and obtained a poor contact in consequence. Hof mann also supplies similar junctions for use with stranded cables (Fig. 150) . Fig. 150. In these the rivets are replaced by screws. The making of the joint with these is simple and can be left to unskilled labourers. The material of the conductor, especially if stranded, is somewhat weakened by the driving in of the punch, and by the riveting process and experiment has shown that a copper cable of •054 square inch section, for instance, having a normal breaking load of 3,100 lbs. breaks at about 2,650 lbs. when it contains one of these riveted junctions. A preferable arrangement for strands of many wires is the conical junction (Figs. 156 OVEEHBAD TEANSMISSION LINES 151a and 151b) supplied by the same firm. The component parts of this are shown by Fig. 152. The nuts a and e are first slipped over the cable ends ; the outer layer of wires is then spread out somewhat with a screw-driver and the conical pieces b and d are slipped over the middle wires of the cable (Fig. 153). The nuts a and e are then pushed towards the ends of the cable and screwed by hand into the cover c as far as possible. Then, by means of a special spanner and an ordinary spanner (Fig. 154), the nuts are tightened up without allowing the Fig. 151 A. Fig. 151b. Fig. 153. Fig. 152. Fig. 151. cable to turn. The following tables (Tables 30 and 31) show some experimental results on riveted and conical junctions respectively, obtained by the Dresden State Testing Bureaux : Table 30. No. of Test. Diameter of the Hard Drawn Copper Wire. (inches). No. of Rivets in the Joint. Breaking Load in lbs. Breaking Stress in lbs. per square inch of Wire Section. Remarks. 1 2 3 4 5 •265 •222 •176 ■142 •109 3 3 2 2 2 2,960 2,200 1,410 880 490 54,000 56,800 57,000 55,800 52,500 Break occurred in the j wire at the joint. 1 Break occurred in the i^ wir ^ inside the sheath j at the first rivet. OVBEHEAD TRANSMISSION LINES Table 31. 157 No. of Wires. Diameter of the Wire (inches). Total Cross- section of Cable (sq. inches). Breaking Load (lbs.). Breaking Stress in lbs. per sq. inch of Cable Section. No. of Test. In the Cable. In the Central Strand. Remarks. 25 35 50 70 7 19 19 19 1 7 7 7 •083 •06 •078 •085 •0384 \ ■054 1 •0785 •109 2,200 2,250 2,930 3,000 4,000 5,400 57,200 58,400 54,000 55,200 51,300 54,000 Break occurred in wire at the lower clamp. Break occurred in wire at the upper clamp. Ditto. Ditto. 7 outer wires broke and then the whole slipped out of clamp. Break occurred in wire at the upper clamp. Some interesting comparative results for a solid wire, a soldered joint in the same wire, and the same wire joined by one of the riveted sleaves are given in Table 32 :— Table 32. Dia- Area of No. of meter Wire Test. of Wire (square (inches). inches). 1 •222 ■039 2 •222 •039 3 •222 ■039 Nature of Test Piece. Hard drawn solid copper. Ditto with sol- dered joint. Ditto with riveted sleave joint. Breaking Stress. In lbs. per sq. inch of Wire Section. 60,300 38,000 57,000 As a Percent- age of that of the Solid Wire. 100 631 941 Remarks. Break occurred at the upper clamp. Break occurred in the wire close up to the soldered joint. Break occurred in the wire close up to the riveted sleave. By these results the riveted and conical grip joints are shown to be practically as strong as the original wire. It must be remembered, however, that in these cases the joints were made with great care and by skilled workmen. In practice it is not possible to count on the conscientious workmanship which is essential if 158 OVERHEAD TRANSMISSION LINES such reliable joints are to be obtained. Consequently it is advisable to relieve these points of as much stress as possible and to design the line with this end in view. The " dead ending " of a line at a strain tower or pole or at a corner pole can be carried out with any of these shackles, terminal clamps, or riveted con- nections. The first two methods are now seldom used, as they are expensive and not too safe. Riveted clamps are quite satisfactory for the purpose provided the tension on the line is not very great. The conical grip clamp is also suitable. The cable end is attached in the way described above for conical grip clamps and the loop on the other side of the clamp is slipped over the insulator. The loop is of galvanised drawn steel wire rope, which is sufficiently flexible to bed well against the insulator. A separate screw is often fixed at the top of the junction piece for the purpose of making electrical contact with the outgoing wire, and is a specially handy addition. The line can also be attached to a strain insulator by means of a special metal cap cemented to its top and fitted with a terminal as shown in Fig. 155. The rules of the V. D. E. only permit joints in overhead lines to be made by soldering, screwing or equivalent methods. Merely twisting together or the use of a twisted sheath joint is not permissible because of the high resistance introduced by any existing oxide coating or by an oxide coating forming in the course of time. On the other hand, soldering is only applicable to hard drawn copper wire if the joint is relieved of mechanical stress. For soldering copper a soft solder composed of 55 parts of tin and 45 parts of lead and melting at about 200° C. is used. A solder flowing more readily than this is obtained by using equal quantities of tin and lead and pouring off the half-solidified crust. A great many soldering compounds have been placed on the market for the purpose of simplifying the process by providing an easily running solder with the flux, or deoxidising agent, ready mixed with it. They are especially suitable for overhead line work, as they tend to improve the quality of the soldering done. The flux must be free from acid, as acids attack the soldered joint and its surroundings imless carefully removed after the joint has been completed. Most commonly resin and soldering tallow are used. The latter consists of 1 lb. of tallow, 1 lb. vegetable oil, | lb. powdered resin, and ^ pint of salammoniac solution. This mixture is entirely free from acid and a very good flux. The soldering of aluminium presents great difficulties in practice, partly because of the rapid reoxidisation of the cleaned parts and partly because of the easy disintegration of the alloys used as solder. There is no need, therefore, to describe the different methods that have been suggested. Joints on the line and branch connections are best made by some such mechanical device as the Arid twisted sheath joint or by means of screw clamps. The strong electro-positive action of aluminium is liable to set up electrolytic action when it is in contact with other metals if moisture is present. Spliced OVEKHEAD TRANSMISSION LINES ]59 or twisted joints must therefore be covered with a protective coating and weather- proof cover to keep out moisture. In spite of every precaution, however, the electrical resistance of such a joint is sure to be higher than that of the unjointed wire, and it will gradually increase with time. A good joint can be effected by means of previously prepared terminals made of two metals, e.g., copper and aluminium, soldered together. In this way incomplete contact between the aluminium line wire and any different metal can be avoided. 14. ARRANGEMENT OF THE WIRES. Distance from the Ground. The Board of Trade regulations specify that the minimum height of a line from the ground shall be 20 feet, and 25 feet when crossing a public road, canal or railway (the German rules give 19-5 and 22-8 feet respectively). These figures apply to the line itself, and not to guard wires or nets. In most cases the actual height of the line above roads is determined by the requirements of the postal authorities with regard to telegraph lines running under or close to the trans- mission line (the German rules specify a minimum distance of 6' 5 feet under the most severe conditions — even including a breakage of all the line wires ;'n one of the neighbouring spans). Distance between the Wires. The distance apart of direct current lines of opposite polarity depends on the physical conditions, length of span, sag, and arrangement of the wires. Lines arranged in a staggered manner can be run closer together than lines run hori- zontally side by side. The deviation produced by wind pressure is, for a given cross-section, dependent on the specific gravity of the line material and on the sag. With a given length of span, therefore, heavy lines under considerable mechanical tension require less separation than lines of lower specific gravity or with less mechanical stress. Although, in general, the various lines will swing in syn- chronism, yet occasionally, owing to whirlwinds or to the settling of large numbers of birds on the lines, this cannot be relied on, and contact occurs unless ample separation is allowed. Whilst with direct current lines the maximum separation is only limited by mechanical considerations and the question of cost, alternate current lines have to be run as closely together as possible in order to reduce their self induction or inductance. The limit in these cases is decided by safety of operation and, in the case of the higher voltages, by the question of brush dis- charges. For very long lines the question of electrostatic capacity also comes in. In schemes up to about 50,000 volts the separation of the lines is generally determined by the safety of operation (avoidance of contact between wires, the certain extinction of any accidental arc, etc.). The horizontal distance apart in the case of 150 to 200 foot spans should be at least 12 inches and the vertical distance at least 8 inches. The approxmite horizontal distance apart A in inches can be determined in terms of the span a in feet and the voltage E in kUovolts by the following empirical formula : — A = -048 a + 4 VI 67* * This applies to copper conductors. For aluminium the distance must be increased about 25 per cent. OVERHEAD TEANSMISSION LINES 161 Another rule due to Hafner, and applicable to spans of 150 — 165 feet, is A = 1 X J E 68 Whilst Uppenborn gives the formula A = ley E 69 This last gives results which are too small for the higher voltages. Formulae 68 and 69 take no direct notice of the length of span. The results obtained by formula 67 for copper wire with various voltages and spans are collected in Table 33. For aluminium these values have to be multiplied by 1-25. Table 33. I jengtli of S )an in Feet Working Voltage. 165 ■230 295 326 360 3!)0 425 Appr 460 oxim 490 atese 625 par at 6S5 ion in 590 inch 620 es. 655 980 1,300 1,630 110 12 14 16 18 20 20 22 24 26 28 28 30 32 34 49 65 81 220 12 14 16 18 20 20 22 24 26 28 28 30 32 34 49 65 81 380 12 14 16 18 20 22 24 24 26 28 30 32 32 34 49 65 81 500 12 14 18 20 22 22 24 26 28 28 30 32 34 36 51 67 83 1,000 — 16 20 20 22 24 26 26 28 30 32 34 34 36 51 67 83 3,000 — 18 22 24 24 26 28 30 32 32 34 36 38 40 55 71 87 5,000 . — . 20 24 26 28 28 30 32 34 34 36 38 40 42 55 71 87 10,000 — 24 28 30 30 32 34 36 36 38 40 42 44 44 59 75 91 15,000 _ 26 30 32 34 34 36 38 40 42 42 44 46 48 63 79 95 20,000 — — . 32 34 36 38 40 42 42 44 44 46 48 50 65 81 97 30,000 — . — — . 38 40 41 44 46 46 48 48 SO 52 54 69 85 100 40,000 — — . — — — 44 46 48 50 52 52 54 56 58 73 89 104 50,000 — ■ — — — — — — ■ ' — ■ 52 54 56 58 60 60 75 91 106 The necessary separation was determined by practical experiment in the case of the Rjukanfos transmission scheme.* A section of line in the most unfavour- able spot was erected for this purpose with a span of 700 feet. Three copper cables each of -23 square inch section were suspended at distances of 23^ and 31 J inches respectively in a normal manner, and were provided with indicating devices to show when the lines came into contact. The experiment lasted from the beginning of February to the beginning of May, and showed that with the 23|-inch distance the lines swung into contact whenever a strong wind occurred, whilst the 31^-inch distance only permitted contact on one occasion, when a hurricane was blowing. When the separation was later increased to 39^ inches no contact occurred. The well-known fact that the various lines generally swing synchronously was also observed to hold in the case of these experiments. The deflection of the lines was often, during a strong wind, several times as great as the distance apart of the wires without in any way reducing this separation as far * See Elektrische Kraftbetriete und Bahnen, 1912. O.T.L. M 162 OVEEHEAD TEANSMISSION LINES as could be seen. The contacts, when they occurred, were never initiated by the large swinging motions of the cables, but by sharp local blasts or whirlwinds which started vibrations of short wave length. Grouping of the Wires. When several parallel connected lines are mounted on the same poles the arrangement in the case of direct current circuits is simply determined by con- jr Jf n 2 Fig. 156. Fig. 157. Fig. 158. siderations of convenience and care in supervision (minimum number of crossings at distributing and feeding points, convenient arrangement of branch circuits, and house services) and by the question of the best distribution of the mechanical stresses on the poles (heavy wires at the bottom, earthed wire at the top, etc.). I I Y t 1 Z T \ / / / / / / / f \ \ \ \ \ \ \ \ ^\ 1 / h^ ^^-^ ^ -' ^1 ' Fig. 159. Fig. 160. Fig. 161. In the case of alternate current lines the grouping has to be considered also from a further point of view, viz., that of keeping the resulting self-induction down to the lowest possible Value. A given number of wires can be arranged on the poles in a variety of ways, but there is always one way which gives a minimum of self-induction. The best arrangement from this point of view is not always applicable in practice, because the simplicity of the branch connection and the general accessibility of the lines are usually of more importance than a slight increase in inductive resistance. OVEEHEAD TEANSMISSION LINES 163 A few of the usual groupings will be discussed here : — (1) Two Single-phase Circuits connected in Parallel. — Here the best arrange- ment is that shown in Fig. 156. Then follow in order Figs. 157, 158, 159. (2) A Single Three-phase Circuit. — The wires should be placed at the angles of an equilateral triangle (Figs. 160 and 161). (3) Two Three-phase Circuits in Parallel. — If the six wires are placed at the angles of an equilateral hexagon in such a way that the parallel pairs are diametrically opposite (Fig. 162), the mutual induction of the I ,''' "--_ I ' ' U, I— —J' two circuits wiU diminish the self- induction of the various individual ^ ^"•~- ^ ' i_^i3^ circuits and thereby the effective "1-^^ ,'f' ^"^ self-induction of the whole. Almost I ""i'' I i ^>- the same result is attained by the arrangement shown in Fig. 163, where the three wires forming one circuit are situated at the angles of similar triangles. The nearer these triangles are to equilateral ones the Kig. 162. better the result. The groupings most commonly adopted for transmission lines are those shown in Figs. 164 and 165. Electrically the two are identical, as the equilateral triangles are simply reversed. Owing to the different distances between wires of ^^- ■'T^ ,-' 1- ' ^'''' V ^^-L ^< 1 1 ^> ^t^^ 1 ^' 1 ,.'' -> ! i;- S/ I I I . I J' Fig. 163. / \ / \ nf- ^-<^m — / \ n lit -J- Fig. 164. 't JJl Fig. 165. one circuit and wires of the other the voltages induced by lines I., II., and III. in the three phases of the other circuit are unequal. With moderate currents and moderate lengths of line the mutual inductive action can be neglected in comparison with the self-induction effect if one of the circuits is given one complete transposition. The same holds for the arrangements in Figs. 166 and 167. Fig. 166 is pre- ferable, but in both groupings the inequalities between the separate phases M 2 164 OVERHEAD TRANSMISSION LINES "L m* J 3 of each circuit are considerable. Although the arrangement of Figs. 162 and 163 give minimum inductance they are not to be I^ recommended, because, even when one circuit is disconnected from the supply, live lines exist on both sides of the pole — J and so prevent repair work being carried out safely on the dead circuit. The arrange- ment chosen should be such as to keep the two circuits entirely distinct, and in many cases they are separated by earthed Fig. 166. safety screens of wide-mesh wirework. I T t HT T Ht r Fig. 167. Transposition of the Wires. The voltage drop in a three-phase circuit is a minimum and equal in all three phases when the wires are situated at the angles of an equilateral triangle (Figs. 160 and 161). If the wires are placed in one plane (Fig. 168) the inductive drop in the outer wires is greater than in the centre one. In order to avoid unequal phase voltages at the far end of the line from this cause it is usual to transpose the wires once at suitable points between the beginning and end of the line (Fig. 169). When operating telephone wires are run on the same poles under a H.T. line, disturbances are produced in the former by electrostatic and electromagnetic induction from the latter, which, owing to the great sensitiveness of the telephone, may render speech difficult or even impossible. HI J I I in X X ni I IT Fig. 168. Fig. 169. In order to reduce this effect the two telephone wires should be kept as close together as possible and as far away from the H.T. line as possible (not less than 4 feet), and at the same time they should be transposed (or crossed) fre- quently. The transposition points should be so selected that one occurs at every branching point of the H.T. circuit ; and the distance between transposition points should be 400 to 600 yards. In order still further to improve the speech conditions the H.T. line itseH can also be transposed at intervals. This becomes essential when postal lines run parallel and close to the power line. The separate line sections conveying equal load are divided into three, or in very long sections into six, equal lengths, and the lines are so transposed that each phase occupies a certain position (for instance the bottom position) for the whole of one of these lengths (see Fig. 170). OVEEHEAD TEANSMISSION LINES 165 K? 3 yz yc yc >c >c Fig. 170. The transposition may advantageously be carried out at masts with double insulators. In the case of wooden pole lines two separate poles about 3 to 5 yards apart are often provided at the transposition points. If transposition is carried out on long spans accidental contacts are likely to occur owing to the reduced distances between wires at the crossing points. 15. EARTHING. The earthing of a body means its connection to the earth in such a way that it cannot attain a potential dangerous to anyone touching it when standing uninsulated on the ground. A distinction must be made between earthing for the purposes of operating the scheme and earthing for protective purposes. The chief cases of earthing for operative purposes are those of the earthing of the middle wire of three-wire systems and the earthing of the neutral points of multi-phase circuits. By this earthing the maximum voltage to earth is halved in three-wire systems and is reduced in the ratio of Vs : 1 in three-phase systems, so that systems having voltages of 2 X 250 = 500 or Vs X 250 = 435 can still be classed as low-voltage ones. The laying of the earthed wire must be carried out just as carefully as that of the other wires, since a break in it may permit dangerous voltage rises on the outers to occur. An earthed wire must not be replaced at intervals by the earth itseK or by portions of buildings, though metallic building structures may advantageously be used to augment the effective section of the specially laid earthed wire. Protective earthing has to be carried out when risk exists of parts of struc- tures not intended for current-carrying purposes becoming charged to dangerous voltages either through accidental contact with H.T. circuits or through current creeping across from the H.T. circuit. It operates by providing a good conducting path to earth for any such leakage currents. Consequently, when a human body comes into contact with the structure, only an infinitesimal current will flow to earth through it whilst the greater part passes through the metallic con- nection to earth. The relative values of these two currents is dependent on the existing condition of the earth connection, and its resistance should therefore be kept permanently at the lowest possible jfigure. The important role of this protective earthing, viz., the prevention of danger to persons, makes careful workmanship and avoidance of all risk of mechanical or chemical damage imperative. The cross-section of the earthing wire and the surface of the earthing plate must be chosen to suit the expected volume of current flowing to earth. In overhead line systems it is advisable to make the cross-section of copper earthing wires at least -04 square inch and of iron earthing wires at least -16 square inch. The copper should be tinned and the iron galvanised. Earth wires should be as open to inspection as possible and, where laid in the earth, should be of solid wire not of stranded cable. Good metallic contact at the connection points is essential. When connecting the earth wire to iron intermediate lead washers OVERHEAD TRANSMISSION LINES 167 should be inserted ; and the connection should be covered with a waterproof covering or paint. At places where damage is especially likely to occur the wire should be carried in thin earthenware pipes, and these should be filled in with asphalt. The earth wire on iron masts should, preferably, be laid on the inside of the mast and be clamped tightly to it. Should it be necessary to protect an earth wire against -mechanical damage because of its being laid on the outside of a mast or building or against chemical damage through the action of the soil, this can be done by running the wire in iron tubing and filling this up solid with compound. All earth wires should be run as straight as possible and without any sharp corners. Iron structures, e.g., lines of piping (gas or water pipes), can be used as aux- iliary electrodes by being connected to the earth wire. These connections must be carefully soldered whatever other means of making contact may be employed in addition. Earth plates should only be employed when a permanently damp stratum exists at not too great a depth. The galvanised or lead-covered iron plates, having at least 1 square yard of surface and a thickness of at least | inch, should be embedded in the soil straight (not rolled up). The connec- tion to the earth wire can be carried out as shown in Fig. 171. The wire is bent through slots cut in the plate in a loop around the plate and back on itself again. The portion of the wire lying between the slots is then soldered to the plate. The wire must be so placed in the earth that the ramming home of the earth cannot damage the connection. A better earth contact is obtained if, in place of one large plate, several smaller ones are used buried at distances of a few yards from one another. Wire netting, with wire of not too small section, also is preferable to the single solid plate. Suspension of the earth plate in flowing or stagnant water does not give satisfactory results because of the high specific resistance of water. In stony, dry soil the contact resistance of the plate can be diminished by surrounding it with fine ground coke (3 or 4 cwt.). When the surface water can only be reached at depths of 2 or 3 yards it is advisable to use tubular electrodes consisting of 2 or 2 J inch diameter gas piping 2^ to 3 yards long and provided with a ramming point at the end. At the upper end collars for connecting the earth wire should be provided. The junction should also be soldered and then painted over with hot tar. In order to improve the Fig. 171. 168 OVERHEAD TRANSMISSION LINES contact the pipe and the upper part of the soil around it can be sprinkled with salt. In this case the pipe should have holes drilled in it all along its length. The pipes are driven vertically into the ground as shown in Fig. 172. In dry soil several such pipes should he used. The best mode of earthing individual masts is by the employment of band iron electrodes radiating away from the mast (Fig. 173) at a depth of 20 to' 30 inches and each having a length of 6 to 10 yards. By combining the strips into a sort of rough network and connect- ing this to the mast and foundation a large contact surface is obtained and the surrounding earth is brought to the same potential at all points, thus lessening the possible danger when making contact with the mast. \ . Gas Pioe 4 Gas Pipe _ 2" to ZSi'diameter. '4 Fig. 172. Fig. 173. Bad earth connections have repeatedly been the cause of accidents. If the earth electrodes do not lie in the surface water the current is liable to evaporate the moisture near the earth plate, and an arc even may be started between the plate and earth, which vitrifies the sand and clay, and this may insulate the plate and necessitate the earth currents finding another way back. Under these cir- cumstances a high potential gradient will be set up in the neighbourhood of the mast, and contact with the mast, and even approach to it, becomes dangerous. In such cases the best solution is found in laying a through-running earth cable, this cable being earthed by one of the above-described methods at intervals of 500 to 700 yards. In this way a number of return paths are made available for the earth current. 16. CROSSINGS OVER POSTAL WIRES, RAILWAYS AND ROADS. Power lines carried over or approaching postal lines, railways, or roads must either be so designed that in case of the breakage of a power line no danger results, or else they must be so conservatively dimensioned in aU their details that the falling of a line or the overturning of a mast or tower is practically impossible. Fig. 174. A great many methods of satisfying these requirements have been introduced. (1) Earthing Bows (Figs. 174 and 175). — These are so arranged that a falling wire is bound to come into contact ■with them and thereby becomes earthed. Their reliability de- pends chiefly on the good con- dition of the earthing of the bow. The distance of the wire from the bow should be kept as small as possible to make Band Iron ^/i'x !%2 I 170 OVERHEAD TRANSMISSION LINES quite certain of contact occurring before the wire end can reach the ground or objects on the ground. A certain minimum distance, however, is essential in Kit;. 175. ordei- to cope \vith the perching of birds on the structure and witlr the unavoidable swinging of the wiie. For voltages over 500 the distance should not ))e less than 8 inches. The ear-thing bows are made of iron, and the contact face should be thickly galvanised unless the copper earthing wire is carried across the contact face. Earthing eyes oi' short-circuit rings (Fig. 176) can only be used for low-tension circuits, because the risk of contact bj^ birds is even greater than with earthing bows. The eyes are made of hard drawn copper wire -2 to -25 inch in diameter, as this is the safest material for ensuring contact with the falling wire. (2) Safely Couplings are arranged to switch off the wire from the supply as soon as a break occurs. Two designs are in use — the Gould coupling (Fig. 177) and the Hesse coupling. The hooked conducting pieces are fixed to the insulator and grip the terminal pieces fixed to the end of the line, and the line tension keeps the two in contact. When the tension ceases owing to a wire breaking, the ends of the wire are free 17(!. OVERHEAD TRANSMISSION LINES 171 to drop out. The use of these couplings involves the staggering of the various lines in a vertical direction, and adds appreciably to the line resistance. Further, when a M'ire snaps suddenly it may be jerked upwards, and in falling may come into contact with other uninjured H.T. wires, and the hanging ends then become a danger. (3) Protective Nets, though sometimes used, are a constant source of trouble to those in charge of the line. In the first place, their appearance is not pleasing ; then, if designed on ample lines they add enormousl}^ to the stresses on the masts, whilst if the design is cut fine they are liable to be damaged by snow and ice loads, or through the flue gases of locomotives, or through rust. Large mesh netting is not permissible, as the broken wires may slip tiuough ; small mesh netting, on the other hand, encourages the collection of snow and is subjected to heavy vi;^ i; Fi.L'. ITS. wind stresses. Both have the common disadvantages that, if the distance between wire and net is not ample, they are liable to swing into contact in a strong wind : swaying boughs or objects thrown at the line from the ground are lial^le to get 172 OVEEHEAD TEANSMISSION LINES caught, and, finally, a falling line may cause the thin wires of the netting to be melted by the heavy current to earth. The general safety of operation of the line is, in fact, diminished when safety nets are employed, and they are no infallible protection even against falling wires They should, therefore, only be employed in cases where no other form of protec- tion is possible. This is the case, for instance, when postal wires are run above a H.T. power line. The protective nets are sometimes closed on all sides, sometimes open at the top (Fig. 178), and sometimes trough-shaped (Fig. 179). The type closed on all four sides is essential when postal wires are carried over the power line and when at the same time the power line has to be prevented from falling. The commonest ! I Liron Zx'i'ufi' 1 — _ -J Llr^n Z'xZ'xH. ■ Fig. 179. arrangement is, naturally, the one with the open top ; the sides being raised 8 to 12 inches above the highest line wire. The trough-shaped type only becomes effective as a safeguard against falling wires if the sideway projection is at least equal to haK the height of the highest wire above the net. The arrangement shown in Fig. 180 is quite ineffective. This is a case of a 50,000-volt three-phase line, and the span at the road crossing is about 130 feet. A falling wire here would only be caught by the net under unusually favourable circumstances. Such a high voltage as this also increases the danger of burning out the wire of the netting through the heavy earth current. Trough-shaped nets placed over the power line must have a narrower or more open mesh, according to the direction taken by the postal wires requiring pro- tection. Box-shaped nets should, with a maximum length of span of 130 feet, have at least 16 inches separation between the line wires and the longitudinal wires of the net. The distance between the lowest line wire and the cross wires of the net should be such that even with the maximum sag a clearance of at least 12 OVERHEAD TRANSMISSION LINES 173 inches exists. The longitudinal wires should be of -16 to -2 inch diameter galvanised steel and the cross wires of -08 to -1 inch diameter galvanised iron. These cross wires should link the longitudinal wires together at intervals of 30 to 40 inches. For the purpose of adjusting the tension the longitudinal wires should be finished off with stretching screws at one end connected to the angle iron end frame (Fig. 181). Protec- tive nets must be well earthed. (4) Special Safety Suspen- sions. — According to the regu- lations of the V. D. E., protec- tive nets or other safety devices may be omitted if the trans- mission line installation is carried out in a specially secure manner. This additional secu- rity is assumed to be present if the line tension is reduced to only half the values usually permissible, if the masts are specially amply dimensioned, and if the insulators and their supports are so arranged that in case an insulator or a wire breaks the latter cannot fall or, at any rate, is earthed before it falls. These requirements are completely fulfilled by the use of the special safety suspension (Fig. 182) due to the Allgemeine Elektrizitats Gesellschaft. In this the line wire is held by three distinct insulators, so that if one breaks the line will still be supported without any appreciable change by the other two. It will be seen that the two auxiliary supports are continued backwards. This is done because, without it, when a voltage rise occurs, through internal switching operations or through atmospheric effects, the endmost insulators, being at the end of open- circuited lines, become points of maximum voltage rise and are liable to breakdown. This P'in;. ISO. ^/ledii Fi,-. 181. 174 OVEEHEAD TEANSMISSION LINES actually occurred on a number of the earlier examples, but the continuation of the auxiliary wires has greatly diminished the trouble and the resulting interruptions of supply. Professor Dr. Ulbricht * has devised the arrangement shown in Fig. 183 for the State Railways of Saxony. Each line conductor consists of two cables supported on separate insulators and connected together by a zig-zag arrangement of cross wires. In this way each conductor is in the form of a narrow network suspended in a horizontal plane and appearing little thicker than the ordinary line wire. If one of the longitudinal cables breaks the net sets itself on the slant, and the fact that something is wrong at once becomes noticeable from a distance. The connection of the cross wires to the longitudinal wires is carried out with Hofmann's riveted joints. The construction is not simple and is not easily carried out on the spot. The cross wires have to be carefully dealt with if the riveting of the joints is not to damage the longitudinal wires. Also, unless great care is taken in erection, large additional stresses may be set up in the longitudinal Fig. 182. wires, thus greatly reducing the factor of safety. The supporting structures also become considerably heavier and more expensive owing to the increased weight of line and the additional load of ice or snow to be provided for. The safety suspension of the A. E. G. is considerably simpler and, conse- quently, at least as reliable in practice as this network conductor. A photograph of some network conductors in position is shown in Fig. 184. (5) Bridge Construction fm- Crossings (Figs. 106 and 185). — These were at one time commonly specified by the authorities. Although they, of course, prevent the falling of wires, they are very expensive, so much so that some schemes have had to be abandoned owing to this additional expense. At the present time these bridges are only used in exceptional cases. Finally, one of the simplest possible safety arrangements in principle is to prevent the wire ends, when a break occurs, from approaching dangerously near to the ground (the rules of the V. D. E. specify a minimum distance of 3 metres). Of course this method is seldom applicable, as it involves either very short spans or very tall masts. * Elektrische Kraftbetriebe und Bahnen, 1910, p. 303. OVERHEAD TRANSMISSION LINES 175 176 OVERHEAD TRANSMISSION LINES Fig. 184. Fig. 185. OVERHEAD TRANSMISSION LINES 177 Elaborate rules have been drawn up by the German postal authorities with regard to the safe crossing of postal wires by overhead power lines. Accompanying these rules is a detailed sample calculation for an imaginary case showing exactly how the various stresses and factors of safety are to be arrived at. This calcu- lation is reproduced below as a useful guide in similar cases. The following data are employed : — Fig. 187 shows the relation between sag and length of span for hard drawn copper wire based on the fact that the maximum stress in the line shall not exceed 5,700 lbs. per square inch either at — 5° C. with additional ice load or at — 20° C. without additional load (giving a factor of safety of 10 in normal copper wire with a breaking stress of 57,000 lbs. per square inch). The weight of 1 foot of copper wire, 1 square inch in section, is taken as 3'9 lbs. or S = 3'9 X q = weight in lbs. per foot run {q being the sectional area of the conductor in square inches). Young's modulus of elasticity = E = 18-5 X 10'' lbs. per square inch. Thermal expansion co-efficient = a = 1-7 X 10~'\ ^$2 = 6'6 X g' = ice load in lbs. per foot run. Bg = 8 ~{- 82 = 10-5 X q (= 2-67 times B) = total load per foot run due to weight of wire and ice. The arrangement of the crossing is shown in Fig. 186, and the type of suspension in Fig. 192. Static Calculation. For the safety suspension of the overhead lines of the Power Company at the points where they cross Post Office lines. I. General. (1) Working voltage : 10,000 volt. (2) Nature of supply : 3-phase. (3) Position of the [5] crossings (JFig. 186).* (4) Description of the safety suspension. At all [5] crossings the triple insu- lator type of safety suspension will be employed. The line is strained to the middle one of three pin insulators mounted side by side on the same cross-arm, and at the same time it is connected by auxiliary wire ropes not more than 1 yard long to the two outer insulators. The auxiliary wire ropes are to be attached by means of screw clips (type ) (see Fig. 192). Earthing. — Every mast and the guard wire will be earthed by means of a galvanised iron plate 1 1 square feet in area lying below the level of the surface water. * The calculation is only carried out here for one of the crossings. O.T.L. N 178 OVERHEAD TRANSMISSION LINES Pole Nf93. i Fig. 1S6 OVERHEAD TRANSMISSION LINES 179 Materials used for the Safety Suspension. Breaking Part of the Structure. Material used and its Dimensions. Stress in lbs. per sq. inch. Remarks. H.T. transmission line Hard drawn copper -054 sq. inch section. 57,000 Auxiliary wire ropes. Hard drawn copper -054 sq. inch section. 57,000 Lightning conductor . Steel wire rope -054 sq. inch section. 57,000 Guard wire Galvanised iron wire -2 inch in diameter. 57,000 Masts with cross-arms Wrought iron 57,000 Three different types and insulator pins. of mast (see sketches Nos attached). Insulators Porcelain . Tested up to 40,000 volts. Saff oF ffard dratvn Copper Overhtad Lines, fMixim urn Stress S 700 lbs per sj: ineh.j HO- - S'nith ice 30 40 50 60 70 80 30 Span in Feet . 100 no IZO 130 Flcr. 187. N 2 180 OVEEHEAD TEANSMISSION LINES II. Calculations for Crossing No. 1. A. Line Sag. 1. IN THE CROSSING SPAN (FIG. 186). Span of the poM-er line (s) Sag of the power line (read off the curve of Fig. 187) (/) at - 20° C. „ - 10° C. „ - 5°C. 0° C. » + 10° C. „ + 20° C. „ + S0° C. „ + 40° C. (/;). - 5°C. (w.th additional ice load) Sag of the power line (/,„ax.i) • at + 40° C. when the main line wire and one of the auxiliary wire ropes fail at one of the suspension points, assuming that the length of the catenary is thereby increased by a = 6 inches * /ill ■.i=7(/ + iof + o " V 17-82+^ X 6 X 822 8 Sag of the power line (/max.a) • at — 5° C. with additional ice load, when the wires break in one of the neighbouring spans and the mast is deflected in the direction of the crossing span by an amount b, inches f /max.2 = -y/ // + |& X 5 = sj 12-82 + | X l"? X 822 Consequently /^axi is the sag to be dealt with (Fig. 186). With the maximum sag /^axi the sag at the crossing point (C = 120 inches from the mast) between the power line and the postal line will be : f^=^ ^Xf^ qXC X {s — C) _ 4 X 46-5 X 120 (822 — 120) Inches. 822 9-. 10-2 10-8 11-4 13-2 14-6 15-7 17-8 12-8 46-5 (3 feet IDJ inches) 26-3. g2 (822)2 23 * If this allowance seems open to question it should be confirmed by adding the necessary sketches and calculations. t For the determination of 6 see below. OVERHEAD TRANSMISSION LINES 181 2. IN THE KEIGHBOURING SPANS.* Span of the power line ....... Sag of the power line, with a maximum stress of at — 20° C. . - 10° C. . - 5°C. . 0° c. . + 10° c. . + 20-^ C. . + 30° C. . + 40° C. . - 5°C. . (with additional ice load) . 360 feet (4,320 inches) 17,200 lbs. per sq. inch Inches- LOO 105 108 111 115 119 124 129 124 From the ground to the upper- most telegraph wire From uppermost telegraph line to guard wire f • . , Sag (/„) of the guard wire (as for the power line) B. Masts (Figs, 186 and 189). 1. HEIGHT OF MAST, ft. in. 30 6 3 1 11 Height of guard wire suspension point above ground From guard wire to lowest power line Height to lowest power line sus- pension point Height of topmost power line insulator from ground . Length of mast in the ground . Gross length of mast to the uppermost insulator H 35 3 H 3i 39 say. 39 2^ (12 metres) 42 6 6 6| 49 0^ Fig. 188. * It is assumed that the lengths and the tensions in the two neighbouring spans are alike. t The regulations specify that an earthed guard wire or equivalent device is to be placed at least 1 metre (3 feet 3 J inches) below the lowest power line to avoid risk of postal wires being jerked up into contact with the H.T. lines when repairs are being carried out or when a postal wire snaps. 182 OVEEHEAD TEANSMISSION LINES 2. STRESSES IN THE MAST. (a) Due to the Tension of the Power Lines and the Guard Line.* J Forces due tot- In the Crossing Span. In tlie Neighbouring Spans. Point of Action (see Pig. 188). No. Cross- section (square inches). Stress in lbs. per square inch. Pull in lbs. Bending M oment in Ib.-inches. No. Cross- section in square inches. Stress in lbs. per square inch. Pull in lbs. Bending Moment in Ib.-inclics. a b c Power line Power lines Guard wire 1 2 1 •054 ■054 ■031 5,700 5,700 5,700 310 620 176 157,000 288,000 76,000 1 2 1 ■039 •039 ■031 17,200 17,200 17,200 660 1,350 530 340,000 630,000 227,000 1,106 521,000 2,540 1,197,000 (b) Due to Wind Pressure. Point of Action (see Fig. 188). Effective Mast Siirface in square feet. "Wind t Pressure allowing 125 kg. per square met-er (25'G lbs. per square foot). Bending Moment in Ib.-inches. d Corner stays . . 39-2 x 2 x -3 = 23-5 Diagonals . . . 78-4 x -75 x -13 = 7-6 50 per cent, addition for the leeward side = 15-5 Cross-arms and insulators . . . = 6 Total .... 52-6 sq. feet 1 y 1,380 lbs. J 355,000 From the above tables : — Total line tension in the crossing span „ neighbouring spans Wind pressure on mast ..... Line tension reduced to the top of the pole, in the crossing span Line tension reduced to the top of the pole, in neighbouring spans Bending moment in the crossing span Bending moment in the neighbouring spans Difference moment J Moment due to wind pressure .... Maximum moment§ * It is assumed that the line is straight. For the calculation of the forces on masts at angle points see note on p. 191. I The German regulations allow for a normal wind pressure of 125 kg. per square metre (256 lbs. per square foot approx.), but it is stated that on the sea coast or in specially windy districts a larger allowance may be necessary and should be employed. X The greatest net pull is determined from the difference between the pulls in the crossing span and in the neighbouring span, but this should be at least equal to the pull in the crossing span itself. In many cases it pays to increase the pull in the crossing span artificially by using thicker wires, two guard wires, etc., so as to reduce the difference in pull in the two spans. § Even when JIfj. is greater than M,, — M/,, and has, therefore, to be used in the calcTilation, the full wind-pressure on the mast must be allowed for. lbs. H,= 1,106 H„ = 2.540 H„ = 1,380 z,= 1,030 Zn = 2,320 Ib.-inches. M,= 521,000 M,=- 1,197,000 M^-M,= 676,000 M^ = 355,000 „-M, + M^ = 1,031,000 OVEEHEAD TRANSMISSION LINES 183 Fig. 189. Fig. 192. 184 OVERHEAD TRANSMISSION LINES 3. STRENGTH OF MAST. (a) At the lowest point on the angle iron stays (see Fig. 190) :- Section of angle iron Distance of centre of gravity from neutral axis . «/ Ynm. .......... Gross cross-section Section weakened by two rivets (f " diameter) Weight of mast Compressive force S^ 'M \ I -^"- max. 4 ■ + »)-i(i^% 2.860) Tensile foree S, _ i (% - j) = J {^J^ - 2,860 Factor of safety as regards compression : kxQ 57,000 X 2-9 H inches. X 3| X -425 1-38 inches.* sq. inches. Q = 2-Q q = 2-35 lbs. 9- = 2,860 = 18,000 = 16,600 factor. ^rf s. 18,000 = 9-25* Factor of safety as regards tension : ^ X ff n, = -^-^ : 57,000 X 2-36 16,600 Factor of safety as regards lateral bending * (collapse) : E X J„,„ 30-7 X 10« X 1-38 n,= S,i X P 18,000 X 53^ I being the free length between diagonals in inches (see Fig. 189). (6) At the lowest diagonal : — = 8-3 Section of diagonal angle iron .... ^ min. •■■...... Gross cross-section ...... Net section weakened by one | inch rivet . Load on one diagonal inclined at an angle of 45° : = D = i,(H„-H, + HJxV2 Factor of safety as regards tension : k xq _ 57^0 X -62 D ^ 1,980 Factor of safety as regards lateral bending f (collapse) : J^i, X E -088 X 30-7 X 10« % ■ inches. = 2| X lx% X -2 = -088 inches.* sq. inch. = -75 = -62 = 1,980 n, = ■ Dxl^ 1,980 X (391)2 factor. = 18 = 8-8 * The structure is considered sufficiently safe if the separate loads show a factor of safety of at least 5. t Even when the diagonals are staggered the whole distance between neighbouring diagonals in one plane should be employed and the minimum moment of inertia should be used. OVEEHEAD TRANSMISSION LINES 185 Factor of safety of the rivet against shearing : n, = . ^ 4 57,000 X "305 -= 8-8 D 1,980 (c) Deflection of the mast when the wires break in one of the neighbouring spans :— (For dimensions of the angle iron, see above under B, 3 (a). „ 3H+13I Distance of centre of gravity from neutral axis 2x2 inches. (See Fig. 191) = 10-25 Moment of inertia half-way up the mast . . = J == 4 (J^^ + Q X e^) = 4 (1-38 + 2-9 X (10-25)2) = 1,230 inch.* (Note. — ^■'min. must be used rather than J( as the latter gives too small a deflection.) Zf, X L3 _ 1,805* X (470|-)« J""^^^- 3 X ^^ X J~3 X 30-7 X 10« X 1,230 "~ -1 b^ f- /■■•3" H I — 350 lbs. — , ^ C'^60-310 , oec under B.Z.(aJ ) ^350 Lbs Fig. 193. J''i<<. 194. (d) Stability of the mast (Fig. 189) : — Weight of concrete foundation block f _ (59 X 59 X 55) + (86 X 86 X 23^) 1,728 Weight of earth resting on the ledges = (862 _ 592) 55 X 96 Weight of mast X 128 lbs. = 27,200 = 11,800 = 2,860 Total -weight = (? = 41,860 * This figure of 1,805 lbs. is obtained as follows : — When the -wires break in one of the neighbouring spans the mast is deflected towards the crossing span by a force equal to Zn — Zi: plus the (diminished) tension in the wires of the crossing span. This crossing span tension may be considered as having been halved due to the deflection and consequent increase in sag. The total deflecting force is .-. = Z,, = Z„ - Z, + ^ = Z„-^ = 2,320- 515 =1,&05. t The weight of concrete is to be taken as not more than 128 lbs. per cubic foot and that of earth as not more than 102 lbs. per cubic foot. 186 OVERHEAD TRANSMISSION LINES Distance of point of action of earth pressure from the edge of the foundation block 49 ft. 0| in. M V -'"max. A ^2 ft. 6 in. = y = half -width of foundation block p= (the ratio -r^ii-~w- being used in order to obtain the moment with regard to 42 ft. 6 m. ° the base of the foundation from the moment with regard to the ground level) 1,031,000 X ^^^ .„ 42 ft. 6in. 1^ o- 1, ••• 2/ = 43 ^^-^ = 14-8mches. Pressure at outer edge of foundation block ^' = * 14-8^X 86 = 3 14^8x86 = ^^ ^^'- ?"" '1"^"" ^'^- C. Cross-Arms and Insulator Pins. (a) Gross-arms (Figs. 192 and 193) : — ■ Cross-section of the channel iron — q = 1"4 square inch. Moment of resistance = TF^. = 1'07 inch.^ The tension Hi of one wire produces a tensile or compressive force on the two channel irons, respectively, of k^ H,x; ^ 350X3U _^,Q^^„ Distance of centre of gravity 15 Besides this a twisting moment if ^ = 350 X lOj (see Fig. 193) acting at the end of an arm Z = 31 J inches has to be withstood by the two channel irons at the section a — a (Fig. 192). This produces a bending moment in each channel iron : ^ j^ ^ J/rfX I ^ 350 X 10^ X 31^ ^ ^ g^^ ^^ ^^^^^ ' Distance of centre of gravity 15 ' ' The stress on one channel iron is therefore ,^ k M, 730 , 7,500 ^ ^„^ „ . ^ A = - + |=r = -—- + Yly7~ ^ 7,520 lbs. per square mch. (6) Insulator Pins (Fig. 194) : — Maximum pull* = 350 lbs. Moment M at the section a — a = 350 X 2f = 830 Ib.-inches. The 1-inch diameter pin has a resisting moment W of "105 inch.^ Id 830 .•. Bending stress = iC = ^ = 7-r^ = 7,900 lbs. per square inch. The compressive and tensile force at the section 6 — 6 (IJ inch in diameter) is + S, where e is the distance of the centre of gravity of half the bedding surface from the centre of the bolt, or * This assumes that the wires in both the crossing span and the neighbouring span are strained to the same insulator. If they are connected to separate insulators as shown in Pig. 192 it is sufficient to consider only the pull in the crossing span. OVEEHEAD TRANSMISSION LINES 187 _ 4 P + (lxA) + (tk)^ = '51 inch. lbs. .-. Compressive (or tensile) force = + /. = 6,000 ■51 The cross-section of half the annular contact face at & — 6 is - = ? (22 - 112) X 1 = 1-07 square inch. . .-. Compressive stress at 6 — 6 = j-— = 5,600 lbs. per square inch. ^ \ \ \ \ z„ cos a = 3800 Lbs. VJ-^ '1 L. ^ •i \ \ \ N o \ . o S. \S \, ^\^ k s \-*» •v*'«» Ij ^, ^ \ c \ s. N _-s^ Fig.. 195. The maximum tensile stress in the 1| inch diameter bolt having a cross- 6,000 section at the bottom of the thread of -7 square inch is = 8,600 lbs. per 57,000 square mch, givmg a factor of safety of ^"";t^ = 6-65. In order to simplify the calculations under II., A and B, the following tabu- lated results can be used : — A. Sag of the Line. Meaning of the Symbols. s = Length of span of the power line in inches. /_ 20 = Sag of the power line at — 20° C. /max.i — W (/+ 4(,)2 + ^ a X 5 = Sag of the power line in inches at + 40° C. when the main wire and one of the auxiliary supporting wires give way and thus lengthen the catenary by as = 6 inches. /^ = Sag of the power line at — 5° C. with additional ice load. 188 OVERHEAD TRANSMISSION LINES Table 34.* — Values of the Sag at various Temperatures and for various lengths of Span. s /.-.o /-,0 /-5 /-o /+ 10 /+20 /+30 /+ 40 J max 1 fe 400 inches. 1-6 2 2-35 2-95 4 4-95 5-9 6-7 30-5 3-55 and so on. 1,375 31-6 33 34-5 35 37 39 41 42-5 70 36-5 1,425 33-5 35-5 36-2 37 39-5 41 43 45 72 38-5 and so on. This table should be filled in completely so that the sag at any temperature can be read off at once. For the neighbouring spans a separate calculation can be made in each ease or a curve can be used. * The numbers inserted in this and the following tables are only intended as a guide and do not refer to the example just Worked through- OVERHEAD TRANSMISSION LINES 18i) B. Mast Height and Mast Stresses. Table 35. Meaning of the Symbols. /max 2 = V (f,.)^ -(- |6 X « = sag of the line in inches at —5° C. with additional ice lojad when the wires in one of the neighbouring spans break and mast deflects towards the crossing span by an amount b inches. /J luMx 1 X C ' S — C) . ... . . ■ • "ii (. = — 2 — ^ = maximum sag in mches at the crossnig ponit situated c inches from the mast. /* = the necessary minimum height of mast in inches measured up to the sup- porting point of the highest power line. This is determined from the height of the uppermost postal wire, the allowance of 2 metres between that and the lowest power line, the sag /„ the separation of the various power lines from one another, the height of the line supports above the top of the mast, and any difference in level which may exist between the foot of the mast and the foot of the telegraph pole. q = cross-section of power line in square inches. p = greatest tensile stress in the line in lbs. per square inch. Hg = pull of the lines acting in any one plane in lbs. = sum of all the pulls H^ in lbs. I = distance of the average point of action of the forces H^ from the top of the mast in inches (+ if above the top of the mast and — if below it). ^k ! = total force in the crossing span or neighbouring span reduced to the height Z \ of the top of the mast, in lbs. ^n-k = difference of the top pulls in lbs. R = resultant of Z,j and Z,^ in lbs. for masts at corner points. •^max. = the pull in lbs. used in the calculation of the mast. L = height of mast measured to the top of the main corner stays in inches. Z * X U b = — - — =; i = deflection of the mast in inches when all the wires in one of 3 X E X J the neighbouring spans break. * Note. — See note on p. 185 for explanation of Z^. 190 OVEEHEAD TEANSMISSION LINES Table 6 be d 'A .S CO J s ''max2 c h A Crossing Span. 1 No. of Lines. 1 P ^e ^k Z z^ 1 I. and II. 1,375 56 236 40 275 + 78 + 40 = 393 2 H.T. lines 5 „ 2 „ 1 Guard „ ■054 •054 ■054 •031 5.700 5,700 5,700 5,700 620 620 620 180 2,040 -35 1,880 2 III. 1,425 57-5 196 35 255 + 78 + 35 = 368 >J J> 2;^ COS OC OVERHEAD TRANSMISSION LINES 19] 36. Neighbouring Span. \-\ B* max. Mast. L No. of Lines. 2 P H, Hn I K h 2H.T. 2H.T. 2H.T. •039 ■039 •039 20 000 20,000 20,000 1,560 1,560 1,560 4,6S0 -23 4,400 2,520 — 2,520 1,300 12 470 3^4 it »J » •» it Left-hand Mast 4 200 Right-hand "Mast 2,650 Left-hand Mast 4.200 Right-hand "Mast 2,605 2,C00 12 1,300 12 470 470 1-S5 3 4 * Determination of iJ ; — , In order to determine the effective pull at the top of the mast in such a case as Fig. 195 the actual pull {Z^) in the neighbouring span is resolved into two components at right angles to one another and lying in the directions of the main horizontal axes of the mast. The net pull on the top of the mast is then 2^„ sin a + .Z„ cos a — Z,c= 2,300 + 3,800 — 1,900 = 4,200 lbs. In general the pull to be used is the greatest of the three values : Zk or Zn sin a.-\- Z^, — Z^ cos a. or Z„ sin a — Z* + 2„ cos «. That forces at right angles are directly added together is due to the quadrangular form of the mast. If desired, however, the actual resultant force B can be used, but the mast must then be so turned that one of its main axes coincides with the direction of i?, and if E is then smaller than Zu cos a + Z^sin a (Fig. 196) the latter value must be used. 192 OVEKHEAD TEANSMISSION LINES CO :3 G 3 .h o „^ o3 oi O oi c s ^ II + -M CO m =5 03 t! cq + T3 G O be O 03 -G Cm O ;-( G 03 >i ao3 G tS 3 oj o ,V -G o o o .2 a G o o S 03 B^c«- 0;ii G^;2 5 u 0) o be i« G o •-' oj G? 11^ ° o o ,oi 03 03 w G ^CT u =4-1 j3 o o +^ 0) +-> o T3 03 ■^3 2 2PS 3 r< G 2h OS G eS 4) o3 w ^ ^ ^ C.2 o o G 10 4-1 O C -^ S •SPbc JG "Is "2 3 53 ^ o ^ o '2 S i-g -G cS o <" a a B3 o CD t/3 4J H =4-1 O r^ ^ ?^ ^ G.S l-sg MO 03 G O bJO 03 bo G ,J3 ■2 „" G I— H 03 O 03 G § G be •2E ,sp a CS CJ S ;h =6 ri s . 42 S C6 UD -—4 CO bes 03 .« o X G cS ■'-' G C D o be & o ^^ aj Xi ■4J o =2 +j 03 a «4-l o be G G O ,2 £ T3 G G "^ 3 o *-l .« be4J a ?j 2 "*< a« ^ a II II 03 "a G o3 -e ^ OVERHEAD TRANSMISSION LINES 193 ^« g ■* r-4 IQIIO s N t^ «1 2 IS vo N » t^ o o !< l> I— 1 o O c n Q o o to o w" 00 + + (N :o ^ o 1— 1 ■ _, t> a ^ 1> [■^ ^H « -* CO b IN II <£) g ~«e 1S3 =3 :^ X • 1^ a. ^=" ;.^ ^ §■ n !;i tn ^ '^, e + II -g + +II § le lOlO.S o >0 10'? H •£> Si^i ^ lo-ii; s 1 X'-'S X«- 1— ( ^H i-H 1— ( o o fcv iO t' lO »o ^ - ffo CO o >^ a^-s g< M O ^ s t— I i-H iSl 03 1^ A^ B J- Fig. 197. O.T.L. O 194 OVEEHEAD TRANSMISSION LINES T3 (U 3 fl •rH a o o El CO W o o CO ;.^ 3 D o > =« o oil ve ngnc _5 -Si -P 3 ^1 s o Ci^ o .S ^3. o ;." ^ ^ 4J CO 4J <^-l A o -^ £ ^ O aaeS bc^ hr s •s.=. T) q (1) is ^ ^ nJ « ,3 o ai 3 01 ^1^ i;5 C5 Cb ^ !i SiN S2 s X to cS 3 o -p ft 3 "3 bo 3 O bo ai oj 3 o bo oj o o 3 o pC oj 3 >< 3 t3 O CM o ■p OJ T3 ^ 0) 3 03 3 A .3 ■p t: c ,3 " ^ oj O 3 -^ S Ji H -^^ O Xi oj 3 oj 3 O be 3 . 03 -g ■ ■ 4J 3 3 3 -T • « ■t-J -fl oj ^ •r-4 -3 o ■P 3 T3 -3 <^ c- :SN,| + ^ ft a^ ^ •S^QAia JO U0IlDas-KS0j;3 •S!J8AIH JO aac^amuja ■stBUoSBrg; JO uoi^oag •HSBK 10 8dXx o (N \n oc CO « 00 II N 05 ■S> c: 3 CO ?!, O ^ Ol a !.^ 1— ( < CO § ^ 5*0 00 (N lO o iO CO »o X • = X o II ^ &1 cS ^ OVERHEAD TRANSMISSION LINES 19^ Special Safety Suspension Arrangements. — Overhead power lines which approach roads carrj'ing heavy traffic so closely as to be a source of danger to 1 Fig. 198. "qh]"* ^Band Iron ifi'i' Fi?. 199. persons when a wire falls should be specially amply designed in every part so as to give a greater factor of safety than usual. These requirements will be complied with if the stresses are reduced bv about half, if the distance between masts is Fig. 2(10. diminished, and if factors of safety of 3 J to 4 are arranged for in all the component parts. In addition the risk of wires falling should be further reduced by the use o 2 196 OVERHEAD TRANSMISSION LINES of safety loops at the insulators (Fig. 198) and hoop guards (Figs. 199 and 200). The former hold the wire together even after a burn-out or weakening has occurred at an insulator. Hoop guards are also used at corner points, even when no special safety is demanded, so as to prevent wires falling when an insulator is destroyed. A safer plan, however, is to make all corner masts into strain masts and to use duplicate insulators. As in the case of earthing bows, the hoop guards must also be dimensioned so that the perching of birds on them cannot cause short circuits. The hoops should be fixed with two screws so as to prevent them twisting and causing contact to earth. Increased safety with suspension insulators need only be ensured for road, raU, and postal wire crossings. The best, but also the most expensive arrangement, consists in cariying the wires across in an ironwork bridge structure. In practice, however, sufficient security can be more simply attained by employing duplicate chains of insulators for each end of each line. The duplicate insulators are connected at the lower end by a cross-piece which carries a terminal at its centre, to which the line is attached in such a way that even if the screws should loosen the wire could not fall. The cross-piece is connected to the insulators by ball and socket joints so that it can set itself in any position under lateral tension. 17. ERECTION OF POLES AND MASTS. Excavation Work. — It is false economy to make the pole excavation too small. The workman must have ample room to use shovel and pick if the work is to proceed rapidly and smoothly. For wooden poles the hole is usually made 5 to 7 feet deep and 18 or 20 inches wide. The longitudinal section is stepped as shown in Fig. 201 . The lowest step serves for the workman to stand on whilst excavating the lower part of the hole. In ordinary soil the bottom of a 7 -foot hole cannot be made much less than 27 x 27 inches. If the earth is easy to work but not firm it is usual to make several steps on that side of the hole from which the pole is going to be put in (Fig. 202). The -*-y y/y/y//////; W///////M Fig. 201. Fig. 202. number of steps and the expansion of the opening towards the top must be in- creased with increasing instability of soil. In particularly bad cases the stepping process will have to be carried out on all sides. When water is present the hole must be lined or be carried out with the aid of the caisson shown in Fig. 203. This is made of 1 J or 2 inch boards, preferably placed horizontally, bound together by vertical angle pieces. The earth, generally loose sand, is drawn up by means of a bucket and rope. Holes in wet soil are only partially prepared beforehand and then finished off at the time the pole is actually erected. When concrete work has to be carried out the water is drawn off by means of a pump whose suction pipe reaches into a special sump arranged near the mast hole. In ground free from stones and not too dry (loamy agricultural land) the holes for the smaller poles can conveniently be drilled out. This is rather quicker 198 OVEEHEAD TEANSMISSION LINES than hand labour, and at the same time the stability of the poles is increased, as they are surrounded on all sides with undisturbed soil. The method is only applicable to small, hght masts which can be lifted bodily and let into the hole vertically. In rocky ground the holes are made with pick and crowbar or else they are blasted out. For the latter process a hole is bored with a rockdrill about 10 or 12 inches deep and into this blasting powder or a cartridge is placed and covered with dry sand. The charge is exploded by means of a pair of wires and a shot-firing set. The wires must be long enough to place the operator outside the danger zone, and the space between the operator and the charge must be clear and open to observation. Carriage and Erection of the Masts. — Wooden poles and light masts can be unloaded from the trucks by rolling or sliding them along a platform connecting the truck and the cart. The rolling is controlled by guide ropes slung round the upper and lower ends of the pole. The space to be traversed by the pole should be kept clear of people so as to avoid accidents in case a rope breaks and the platform shifts. Heavy strain masts and corner masts should be shifted by means of chain and pulley if no crane is available. When a number of masts have to be dealt with it pays to erect a derrick hand crane. Most railway companies possess travelling cranes for hand operation. These can be hired at a small charge and greatly facilitate the unloading. Masts should never be thrown off the trucks, as damage almost always results. The masts should be labelled to correspond with the route-marking pegs, and they should be delivered in the proper order in accordance with these mark- ings. The route map should also contain full particulars as to the position of the various masts. In the transportation of lattice-work towers or masts care must be taken not to bend or damage the diagonals or stays. Square lattice masts intended as intermediate or supporting masts only, and therefore of light cross-section, require special care. The ends must not be left free to swing about, as this would easily cause damage to the light angle iron stays. Wooden poles of ordinary length can be carried to the erection spot by the erecting gang, consisting in this case of four or five men. Iron masts are usually carried on a two-wheel trolley, which should have broad-rimmed wheels, as it will often be used on bad field paths or agricultural land. The raising of iron masts is carried out by means of ladders or by lifting struts — i.e., two poles coupled together near the top with rope (see Fig. 204). i Fig. 203. OVEKHEAD TRANSMISSION LINES 199 Fia. 20-t. Fiis. 205 200 OVERHEAD TRANSMISSION LINES The mast is brought up on the trolley to the foundation hole in such a position that the foot rests against the sliding boards or rails placed against the back of the hole (see Fig. 201). The head of the pole is then tipped up until a short pair of lifting struts or a ladder can l)e introduced. This enables the pole to be still furtlier raised, and a longer ladder can be brought into use and so on until the mast has been set vertical. The formation of a tipjiing slope (see Figs. 201 and 202) at the mouth of the hole is a decided help. Heavy masts are best erected by the aid of a windlass. The latter must be attached at a point above the centre of gravitv of the mast and on the mast axis. The bottom of the Vi^. 21 It;. mast (with a board in Ijetween so as to distribute the force over a number of diagonals) rests against a strong beam braced to the windlass at its other end. On soft ground the windlass must be suitably packed up. The use of a windlass does not do away with the need for lifting struts, as these are still wanted for guiding and supplementary work. Wooden poles can be erected either with lifting struts or with single struts fitted with forked ends (pole lifters) each operated by one man. At least three such forks are required, of which each pair in turn holds the mast in equilibrium. The difficulty of erecting a mast increases much more rapidly with its length than with its weight. Masts more than about 50 feet high cannot be erected in the OVERHEAD TRANSMISSION LINES 201 above-described manner. For these a derrick or similar erection with hand crane is necessary (see Figs. 205 and 206). This will serve to raise them to a certain height, and the final erection can be carried out by means of other cranes or pulley blocks attached to trees or anchored by means of stays at suitable points. When the masts are to be erected on a finished foundation block and held by foundation bolts the difficulty is somewhat greater (see Fig. 206). In order to pass the foundation bolts more easily the mast is first set on wooden blocks somewhat higher than the tops of the bolts, and whilst the adjustment is being completed the mast is held by guy ropes previously attached. In the case of the largest masts it is advisable to provide them with hinged feet (see Fig. 113). Strain masts or corner masts which have to stand one-sided loads are given a rake or set, i.e., their vertical axes are deflected a certain amount in the direction opposite to that of the puU. The deflection of a mast usually amounts to about 2 per cent, of the free length, and the rake is made equal to about half this amount. It is not advisable to compensate by rake for the maximum deflection, because the latter only occurs occasionally and for short intervals. If concrete foundations are to be used to improve the stability of a mast the ground should be covered with a layer of w^ll-rammed concrete, 6 or 8 inches deep, before placing the masts in position. Cross-arms and insulators are usually mounted before erecting the mast, unless their weight is likely to add appreciably to the trouble of erection 18. ERECTING THE WIRE. Iisr view of their great importance at the present time, overhead transmission lines must possess a far greater degree of reliability than was necessary in the early days of electrical supply, and consequently much more exact and careful con- structional work is demanded. The old-time erector only knew the dynamo- meter by name, the sag obtained was a matter of indifference, and the straining- up of the wire ceased when it appeared taut. The manufactured length of bare solid copper wire depends on the cross- section. Usually a copper ingot weighing about 175 lbs. is employed, so that a single coil of wire -14 inch in diameter (-0155 square inch in area) would have a length of 900 to 1,000 yards. Stranded cables made up of wires of smaller section than this would have a correspondingly greater length. Thus a seven-strand cable having a total cross-section of -054 square inch could be delivered in lengths of 1,900 to 2,000 yards. The weight of such a coil would be about 1,250 lbs., or, say, 12 cwt. with the drum. Greater weights than this are not convenient for transport and handling. If greater weights are insisted upon either ingots of greater size must be used — and this is not possible in all factories — or else several normal lengths will have to be joined together. This is effected by soldering and subsequent hammering of the joint to harden it. The individual soldered joints are distributed over several yards of cable, so that the strength is hardly affected at all. Joints in one or more wires also become necessary if a break occurs on the stranding machine. Faults must be looked for as the cable is unwound from the drum. These may take the form of a single joint of the whole of the wires at one point, meaning a weakening of the cable, or single wires inside the cable may here and there be simply twisted together instead of being soldered. In a particular case, for instance, a nineteen-st^and cable of -08 square inch cross-section was found to have the inner wire twisted together at several points, and in order to make room for the twist the second layer of six wires had been cut away for a short distance. Wire coils should be placed on a turntable (Fig. 207) and be unwound by drawing at the outside end. A simple brake arrangement prevents over-running. If no turntable is available care must be taken to unwind in the manner shown in Fig. 208, and never as in Fig. 209, as the latter permits loops and kinks to OVERHEAD TRANSMISSION LINES 203 form. Any kinks or other faulty points which may occur accidentally should be cut out. When cable has to be unwound from a drum the latter is provided with a spindle supported horizontally on V blocks. Small drums are sometimes mounted on an arrangement like a turntable with a vertical spindle. Over-running is prevented by a brake. The unwinding is carried out by a number of labourers Fig. 208. Fig. 209. or, in the case of cables of greater section and length, by horses. Wires must not be drawn along roads or stony ground, as .the hard outer skin may be damaged and the tensile strength diminished. At road crossings the wire should be carried on rollers or pulleys (Figs. 210 and 211) placed at such a height as to clear the traffic. Rollers should also be used, mounted on the cross-arms of the masts, whenever the masts are stiff enough to withstand the tension involved in drawing the wire along. This greatly reduces the damage done to the land through the tramping Fig. 210. of men and horses to and fro. If the masts are not strong enough for the purpose the rollers should only be raised above the earth sufficiently to keep the wire from rubbing on the ground. The cable drum should be placed so far from the first mast that the wire approaches the mast at an angle of at least 45°. A rope is first passed over the mast with the aid of a pole, and to the end of this rope the wire is attached and so drawn over the mast. The rollers prevent the wire from faUing off as well as protecting it from damage and easing the motion. 204 OVERHEAD TRANSMISSION LINES With aluminium wire this is the only allowable method. The softness of the metal makes it unsafe to drag it over the surface of even soft soil. When suspension insulators are employed the difficulties of erection are increased because of the greater overhang of the cross-arms and the large gap between the line and the cross-arm. The work is simplified if it is possible to shift the chains of the insulators on the cross-arms. In this case the line is drawn over rollers fixed to the mast (see Fig. 212) and is given the correct tension. Then the insulator chain is brought immed'ately over the line and clamped to it, and finally the insulator with the line is moved to its proper position at the end of the arm. Lines of small section and small span can be laid out flat along the ground and raised on to the masts by means of poles or by climbing up the mast. Larger sizes, especially if the spans are long, require pulley blocks to raise them into position (a 500-foot span of -054 square inch copper cable weighs about 1 cwt.). When gripping the wire to raise it care must be taken not to nick it. Aluminium lines are easier to handle as far as weight is concerned, but they require more care than copper ones. In straining-up wires pulley blocks are used or, in the case of long spans, windlasses with large drums to accommo- date a considerable length of rope are employed, fixed at the foot of a mast. The wire must be gripped by a clamp (Fig. 213) whose jaws are of the same material as the wire and which grips a length of wire equal to 12 or 15 diameters. Stretching screws (turn-buckles) or ratchets fitted with a spring balance (dynamometer) are used for adjusting the tension in the wire exactly. These are attached to the insulator pin or to the cross-arm. Draw tongs, giving a parallel grip for all sections of wire, are a useful additional tool. Having drawn up the wire with the pulley block, the draw tongs, with the spring balance attached, are applied to the wire and the tension is tried. If this is found to be approximately correct the spring balance is left in position and the exact adjustment is carried out by screwing the stretching screw in or out. When the supporting points of the line are at different levels the dynamometer should be inserted at the higher level. During the adjustment the wire should be lying on the rollers, which will be clamped to the cross-arm close to the insulator pin. Sometimes the rollers are provided with pins corresponding to those of the insulators so that they can temporarily take the place of the latter. Z';7!7V777^7777^77777777777. Fig. 212. -^^fe Figr. 213. OVERHEAD TRANSMISSION LINES 205 Owing to the small friction at the rollers the tension in a whole set of neigh- bouring spans will automatically adjust itself to the same value, so that the checking of the tension by spring balance need not be carried out at each span, but only once between each pair of strain masts. When a twin-core telephone cable has to be run underneath a H.T. line it is carried by a stretched steel rope. The cable is hung to the rope at intervals of 2 or 2^ feet by open hooks of galvanised iron strip attached to the cable by small collars or clips. After the steel rope has been strained up to the right degree the open hooks are slipped over it and the telephone cable is carefully drawn along by means of a rope. As a hook comes to a supporting point it is lifted off by hand and placed on the steel rope again beyond the obstruction. The cable must be carried by the rope all the time, even whilst being unwound from the drum. Sharp bends and excessive pulling must also be avoided if the thin lead covering is not to be damaged. The exact tension to which a line must be set depends on the temperature Fig. 214. existing at the time. A good thermometer is, therefore, essential in line erection work. The bulb should be wrapped round with wire of the same material as that which is being erected, because it is found that, in sunshine, the wire tempera- ture may differ appreciably from the temperature of the surrounding air. As a check the sag of the line should be noted after the erection has been carried out with the dynamometer, because the latter is not infallible. One way of doing this is to mark off a distance equal to the correct sag downwards along each of the two masts, starting from the insulator, and then to note whether the lowest point of the wire falls in the line of vision joining the two marks. With the pocket level described on p. 213 this determination can be carried out from one mast. The instrument should preferably be fitted with a clip for fastening it on a level with the insulator top. After setting it up level the telescope is turned on to the corresponding insulator on the next mast. Now lowering the instru- ment by an amount equal to the correct sag, the line of sight of the telescope should include the lowest point of the wire if the tension is correct. This method is also applicable when the suspension points are at different levels. The distance 206 OVEEHEAD TRANSMISSION LINES d^ between the line joining the two supporting points and the parallel tangent to the curve of the wire is the correct sag for the span a (see Fig. 214). The same instrument can be used to determine the sag for the auxiliary spans a' and a" dealt with in equations 4 and 5. The correct sag is set out downwards from one of the supporting points ; the instrument is fixed at the point so found and the telescope is there set level. If the wire is correctly erected the line of sight will coincide with the horizontal tangent to the line {i.e., will include the lowest point of the line or vertex of the parabola). An entirely different method of checking the sag* consists in determining the natural time of swing of the line considered as a pendulum. An overhead wire can easily be set swinging in a regular manner by taking hold of it near one of the supporting points and moving it to and fro laterally with the proper frequency. The natural frequency of such a pendulum is : -u g X M X h K where n = No. of single (half) swings per second ; g = acceleration due to gravity ; M = mass of the wire ; h = distance of the centre of gravity of the system from the axis of swing) ; K = moment of inertia of the pendulum. The moment of inertia of a parabola is K = -^.M X d^ 15 where d is the distance of the vertex of the parabola from the axis of swing, i.e., 2 the sag ; the distance of the centre of gravity from the axis of swing is h ^ ~ d, o Putting these values in the above expression and considering the number of (haK-) swings per minute = w^ = 60 w : . _60 /5X bO / id OT d = ( 1 = mches . . . .70. This formula holds for all materials and for all lengths of span, as neither the mass nor the length enter into it. It also holds for wires hanging between points at different levels. Table 38 gives the values of the frequency (in half-swings per minute) for the sags usually met with. The method is specially useful as a check on the sag both after and during erection. It is, however, only appUcable with safety if the air is still and if the line has not more than two points of attach- ment. It is carried out as follows : — * Dreisbach, E. T. Z., 1909, p. 1218. OVERHEAD TRANSMISSION LINES 207 A workman standing on one of the masts grasps the line loosely between finger and thumb about a foot away from the insulator and gradually sets it swinging by means of slight side way pressure. It is an easy matter to get hold of the right frequency for these pressures by looking along the line and following its motion. The deflection sideways produced in the line should not be greater than is necessary to follow it easily by eye — in any case not more than 10° out of the vertical. As soon as the wire is swinging regularly the swings in one minute are counted (one left-hand swing and one right-hand swing being counted as two swings). From the number so found and the table the value of the sag can be determined at once. In the case of short spans with small sags it is more con- venient to count the complete swings rather than the haK-swings. Lines lying between supports at different levels should be set in motion from the lower support. Table 38. — Determination of the Sag of a Line from its Natural Frequency. 1-2 MB 1^ p< K ft i 1— 1 .g bD 6 I— t a i° n Si K p. 1 .g K p. g bO W^^i^i^ -^^^ - i« 1^ - T w. 6 Sa9 In r Di/f*rener ff 8 BUtai,, fr An^U g/. o 10 Max. pull Th, J II 1\ipe of meet 12 ^aifhrabo^ Ft -' Total hei^i Ft K tounaaHon 15 Vatr - 1(> c ■0 R § ir Remarks [To face 2>. 221. OVEEHEAD TEANSMISSION LINES 221 In the case of long lines this method. is somewhat difficult and clumsy, and it is better to proceed as follows : — (1) Set up the theodolite at A (Fig. 226) and mark a point C" quite close to the line. Measure the angle BAG' = a as accurately as possible and determine the distance CC = a from a measurement of d by means of the double cross webs : a = d sin a.. Fig. 226. (2) Move the theodolite to C" and with it measure the angle AC'B = 180° — y (Fig. 227). The area of the triangle AC'B is ia .AB = ^dd' sin (180 - 7). C Fig. 227. The distance AB can,, with sufficient accuracy, be taken as equal to d -\- d', so that I a{d + d') = ^ dd' sin y from which dd' . Recording the Results. The observations made in the field for each section of the line are entered on a form such as that shown in Plate I., and this then serves as the basis of all future work. This form must contain full particulars as to the position of the several supporting points, type, size, and arrangement of the masts and founda- tions, road and railway crossings, angle points, differences in ground level, and details of the whole overhead gear. From it the necessary quantities can then be estimated and provided for by the order and stores departments. The careful collection and distribution of all the necessary material is essential if an uninterrupted and economical progress of the line erection is to be ensured. All material should be marked with the reference number of the mast to which it belongs. The list of materials (Table 39, Plate II.) is a constituent part of the route map, and copies of it should be given not only to the engineers in charge, 222 OVERHEAD TRANSMISSION LINES but also to the workmen, so that no doubt shall exist as to the correct use of the ma,terial delivered. At the same time as the hst of quantities is issued by the technical staff to the ordering department fuU forwarding instructions must also be given, so that each store receives its correct supply and that the unloading of the heavy iron masts may be carried out under the most favourable conditions. Supports Erected on Private Property. As soon as the general route of the line has been determined and surveyed the names of the owners of the various pieces of ground on which supports are to be erected should be discovered and entered up. As the exact position of the masts is not yet known, and as some of the owners may refuse wayleaves, owners of property to the right and left of the desired route should also be noted. When a mast is to be erected on a boundary line permission from the owners on both sides has to be obtained. The names are entered in a special list together with particulars as to the kind of mast to be erected in each case, and this is put in charge of some member of the staff specially qualified as regards tact, knowledge of the district, and of the local dialect for negotiating with the owners. All permits must be obtained in writing. For every mast, stay or strut, roof standard or bracket, a signed form giving fuU particulars of the arrangement should be pre- pared. A great deal of trouble, time and money, not to speak of legal proceedings, will be obviated if this rule is strictly adhered to. No mast should be erected or other work commenced until the whole of the permits and forms for the section are in the hands of the engineer in charge. The crossing of land or houses by wires without the owners' permission is, in spite of opinions to the contrary, illegal.* The signed form is the written agreement of the owner, for a certain considera- tion, to allow a definite portion of a transmission installation to exist on his land. * The state of tlie English law on the subject of wayleaves, etc., has been summarised as follows by Mr. B. Welbourn in a paper read before the Institution of Electrical Engineers in January, 1914 : — " (1) The Postmaster-General possesses powers for erecting telegraph and ielephone lines (under section 2 of the Telegraph Act, 1892, and section 4 of the Telegraph Act, 1908), "but the procedure to be followed is so cumbersome as to be almost useless. Even these powers are denied to electric supply authorities. " (2) Under the Electric Lighting Acts the consent of the local authority must be obtained by a statutory undertaker previous to the erection of overhead wires, whether these are in the public street or on private land. " (3) Non-statutory undertakers {such as iron and steel manufacturers, collieries, etc.) can dispense with the consent of the local authority, and both erect wires on private land and cross public roads, so long as the wires cause no obstruction above the roadway. " (4) Non-statutory undertakers may erect overhead lines without compliance with the Board of Trade regulations, but the Board has power, if it thinks fit, to order such com- pliance." The German law on the subject states : — ■ " The right of the ground owner extends also into the space above the ground surface and into the body of the earth under the surface. The owner, however, cannot interfere with work carried out at such a height that it cannot afiect his interests." (An owner's interests are affected if the possibility of risk to life or property through overhead lines is present.) OVERHEAD TRANSMISSION LINES 223 The permission can only be withdrawn under certain conditions which must be c (early stated in the form. The form of agreement should be worded somewhat as follows : — I AGREE that or their successors, as constructors of , (a) may attach a line support and the necessary stays, etc., to my house > (b) may erect a mast with its necessary supports, etc., on my land ; (c) may run a transmission line over my land I FURTHER AGREE that shall have access at all times to the buildings, grounds and annexes, and shall be free to carry out any work or alterations necessary for the proper maintenance of the supply. I undertake not to require the removal of the installation unless permanent damage to my buildings or property is being done and cannot be prevented, or unless, owing to structural alterations, the removal or modification of the installation becomes essential. The , on the other hand, agrees to maintain the installation in good working condition and to compensate me for damage caused by any part of it. This Agreement supersedes all verbal agreements. Dated Permission to make use of the ground is seldom given without payment. A small charge only is generally made, varying between 3s. and 10s. for an ordinary wooden pole. For masts occupying a considerable floor space the charges are, of course, a good deal greater and in some cases the ground has even to be bought outright. The question of the payment of an annual rent should not be enter- tained, as the owner thereby acquires the right of giving notice, and this may easily lead to future trouble. A form of payment advantageous to the electric ' supply company is the free installation of one or more lamps, but this should not include free supply of current, as a permanent charge is thus imposed on the station, and also a free supply of current generally leads to waste. The erection of masts and wires should, as far as possible, take place after harvest time. Even so it is difl&cult to avoid some damage to the property during erection, and some arrangement should therefore be come to at the commencement in regard to the estimation of such damage. A useful plan is to form, with the aid of the local mayor, a commission of three in each district. Each owner should agree to this arrangement -by signing some such form as the following : — I AGREE that all damage caused by — (a) the erection of the supporting masts for the line ; {&) the erection of the line ; (c) the cutting or clipping of trees shall be paid for in accordance with the decision of the assessment commission of three members, and this decision shall be final and binding on me to the exclusion of all legal proceedings. Payment is to take place within eight days of the commission's decision being arrived at. Dated (Signed) 224 OVEEHEAD TRANSMISSION LINES Supports erected on Public Property. Permission to use public property in the form of roads, lands, and buildings is easily obtained in cases where an agreement for the supply of electrical energy to the community has been come to. When this is not the case the local autho- rities have the right, just like private owners, to refuse permission for the erection of masts on their property or the carrying of wires over it. A draft agreement suitable for such a case is given below : — Agreement. The Local Authority of and the Electrical Company have this day come to the following agreement : § 1- The Authority grants to the Electrical Company the free use above and below ground of the roads, places and common lands within their boundaries, for the erection of masts, etc., for an electric supply scheme and for transmitting electrical power over and through the district, provided that all the police regulations and bye-laws are complied with. § 2. The erection of the line is to be carried out in conjunction with the Authority. A plan of the proposed line, to a sufficiently large scale, is to be deposited with the Authority before any work is commenced. For lines laid underground cross and longitudinal sections as well as the plan are to be submitted. The proposed positions of the masts are in the first place to be marked with pegs in the ground, and only after inspection and the carrying out of any necessary alterations in position is the work of erection to be proceeded with. § 3. No obstruction of thoroughfares or prevention of free access to property must occur during the erection. § 4. ,The Electrical Company is to carry out all its work in accordance with the Board of Trade Regulations (or Regulations of the Verband Deutscher Elektrotechniker) and to maintain it in good condition. The Authority retains the right to have the >vork inspected and tested by an expert. § 5. The Electrical Company is responsible for all damage done during the erection or alteration work, and will satisfy all claims whether made by the Authority or by third persons. After carrying out any work the Electrical Company must make good all road- ways, etc., and leave them in their original condition or in the altered condition demanded by the Local Authority. The Electrical Company is to bear all the costs of such making good. S 6. The Authority grants the permission stated in § 1 for the term of years beginning on 19 . The Electrical Company is to carry out such alterations to its lines as may from time to time be required in the public interest or as are necessi- tated by other underground work or building operations. OVEKHEAD TRANSMISSION LINES 225 § 7. The Electrical Company, without a permit from the Authority, may not sell or deliver electrical energy within the district, even if no public property is passed over by the service wires. § 8. Two copies of this agreement are to be signed by both parties to it, and each party is to retain one of the copies. Date (Signed) Date (Signed) The use of public roads and railway property is generally only permitted if a withdrawal clause is inserted in the agreement. Alterations or removals of line are, however, only demanded when they are unavoidable in the public interest (diversion or widening of roadway or railway), or if the undertakers have not complied with the terms of the agreement. The legal position with regard to the State telegraph and telephone service is, in Germany, laid down in the Telegraph BiU of December 18th, 1899. This states that public telegraph lines may be erected on all public roads, places, waterways, etc., but that they must be so carried out as not to interfere with existing electrical installations. Any protective arrangements which may be necessary for the working of the telegraph line must be carried out at the expense of the postal authorities. If, on the other hand, a power scheme has to cross an existing postal line the cost of any protective arrangements must be borne by the power company. Section 6 of the Bill obliges the builders of any power line to use every possible care to avoid interference with any existing telegraph lines. In accordance with section 6, sub-section 2, the postal authorities must comply with any offer to divert or alter a telegraph line if without such alteration the carrying out of a transmission scheme required in the public interest would become impossible or decidedly more difficult. The charges for such alteratic^ris fall on the postal authorities, provided that the owners or principal shareholders in the power scheme are the local public authority. In England very wide powers have been conferred on the Post Office by sec- tion 2 of the Telegraph Act of 1892 and by section 4 of the Telegraph Act of 1908, which often give its requirements undue weight when in conflict with electric supply companies.* * See paper by Mr. C. Vernier, " Journal of the Institution of Electrical Engineers," JSTo. 223, Vol. 52, 1913, p. 17. See also p. 222, above. •O.T.L. 22. THE MOST ECONOMICAL LENGTH OF SPAN. It has already been pointed out repeatedly that the use of long spans adds to the security of the installation because the number of insulators and, therefore, of possible faults is reduced. The distance between masts cannot, however, be determined from this consideration alone, because the cost of installation also varies with the length of span. The minimum total cost is obtained with a certain length of span which may be called the " economical span." This is that span which makes the saving due to reduced number of supporting points just balance the increased charges for the taller and stronger masts necessitated. The sag of a line does not increase linearly with the length of span but as the square of the latter, so that the required height of masts and the load on these increase rapidly for the longer spans. The economical span is not of the same length for aU lines. In level country it depends chiefly on the number, the cross-section, and the material of the lines. For given Mnes it is dependent in the first place on the kind of supporting struc- tures chosen, in the second place on the way in which they are carried out. In hUly, undulating country much longer spans may be possible than on level ground by making use of suitable hillocks to mount the masts on and to give the necessary clearance. A knowledge of the economical span does not necessarily mean that it should be used. If the cost of installation only increases slowly with increasing length of span beyond the economical one, it may pay to use a longer span in order to obtain the advantages which are bound up with a reduced number of supporting points. In the first of the following examples it will be seen that the economical span works out at 140 metres (460 feet), but since the cost has only increased by £5 when a 160-metre span is used the latter would certainly be preferable. The example fuUy detailed in Table 40 and Fig. 228 and the further examples whose results are shown in the curves of Figs. 229 — 232 are only intended to emphasise this point. The actual cost figures used are only approximate. They vary with the changing state of the market and also from place to place, so that no generally applicable figures could be given. In these examples 75 per cent, of the masts are assumed to be mere supporting masts, 15 per cent, strain masts, and 10 per cent, corner or angle masts. Fig. 229 refers to a 20, 000- volt three-phase line 10 kilometres long with three copper wires each of -055 square inch section ; Fig. 230 to a 20,000-volt three-phase line 10 kilometres long with three copper wires each of -078 square inch section ; Fig. 231 to a 20,000-volt three-phase line 10 kilometres long with three copper wires each of -108 square inch section, with a steel earthing wire -044 OVERHEAD TRANSMISSION LINES 227 square inch in section and a telephone cable supported by a steel wire rope -062 square inch in section ; and Fig. 232 to the same case as Fig. 231, except that there are two sets of three- phase lines (six wires each "108 square inch in section) instead of one set. w s s -t% 1 n 2s 's s \ S \ \ c \ \ \ ^ ^ \ ^ \ \ \ i ^ifc i ^ I '\ s \ S V \ A \ ^ ^vt X ^ '^ ^^* 31 ^I * ^r 1 2 t ^ A - _5 ^^ 4- ^ t i -2 -- ^ 1 - c: J C ^ .. ^ .. « 3 -^^ -5 ■ lS -^1 J ^ ' 2 3 -,3 t ^ 7 c n -T ^ t 2 J-T ■•^ r -1 2 -J ^ in ^- ^ ^ /% /X. ■~y / " / / / ^ / r^ y "J ?1 y S! 7^ §- ^ / <\ f s ^ ^ § s § 48§4|^|-| CO ^ s - _J _ _1 1 Q2 228 OVERHEAD TRANSMISSION LINES Table 40. — Summary oj the first cost ofaH.T. (20,000-toZ<) three-phase line 10 kilo- metres {61 miles) long and consisting of 3 X -039 square inch copper mires. Factor of safety 2-5 for supporting and corner masts and 4 for strain masts. {For Curve see Pig. 228.) Pull on Mast (lbs.). Cost in Shillings of: Type of Mast. Mast with Cross- arms Insula- tors, etc. Foun- dations and Earth Ex. cava- tion. Carri- age, Erec- tion, and Making Good. Paint- ing. Total per Mast. No. of Masts. Total Cost of Masts for the 10 Km. Copper Wire. Erec- tion of Wire and Earth- ing. Total for the 10 Km. 1. Spa n = 100 metres (326 feet) req uiring 100 masts 9 metres- high. Supporting Corner Strain 375 1,380 2,650 59 109 214 19 44 103 13 21 39 6-5 11 15 97-5 185 371 75 10 15 1 ;- 14,727-5 J 12,150 2,880 29,757-5 2. Spa n=120 metres (395 fe et) req uiring 84 m asts 10 metres high. Supporting Corner Strain 450 1,380 2,650 69 125 242 22 53 115 15 24 43 7 12-5 17 113 214-5 417 62 9 13 1 y 14,357-5 J 12,150 2,820 29,327-5 3. Spa n = 140 metres (460 fe et) req uiring 72 m asts 11 metres high. Supporting Corner Strain 530 1,380 2,650 82 150 290 26 57 126 17 27 49 8-5 15 20 133-5 249 485 54 7 11 1 1^14,287 J 12,150 2,760 29,197 4. Spa n = 160 metres (525 fe et) req uiring 63 m astsl2-2 metres high. Supporting Corner Strain 600 1,380 2,650 97 165 335 32 63 137 21 30 53 10 16-5 23 160 274-5 548 47 6 10 1 14,447 J 12,150 2,700 29,297 5. Spa n=lSO metres (590 fe et) req uiring 56 m asts 13 -5 metres high. Supporting Corner Strain 670 1,380 2,650 120 190 370 36 65 148 26 33 57 12 19 25 194 307 600 42 6 8 1 y 14,790 12,150 2,640 29,580 6. Spa n = 200 metres (652 fe et) req uiring 50 m asts 15 metres high. Supporting Corner Strain 760 1,380 2,650 135 235 405 41-5 70 160 30 37 61 13-5 23-5 27 220 365-5 653 37 5 8 j- 15,191-5 12,150 2,580 29,921-5 OVEEHEAD TRANSMISSION LINES 229 ■ !^ SS S g- 1 & i t 1 \ V :! ^ V t ^ A C ^ \ rj \ S V-.^ \ ~_ ^3t \i: 5' ^? ? \ ^ ^. \ S ^v \ \^ \^ S ^ \ ^ vi^ v^ ^p^ ^f- 5 \ S V l^v- ^ », 1 S -g ^ 1 V -2 X- 1 N 1 3lt - ^ 1 \ - 1 % J 5 - Z ^ I ^ 1 « r 1 t 1 L I 2 i 1 1 f -S t s / -S J- ^ / ?x 1 / V -7 7 « ^4^ ^ -7^'v- ^- i» y^ z. .^ z ^^ y ^ ^ ^ « -^^ 1 ^- c k^ tz X i ■" •~ J -§ 5 -ill J -i -1 -1 J 1" fi «a ^ " ^ ^ ^ 8 i^ in 230 OVERHEAD TEANSMISSION LINES [— r — [ — 1 —I — r — 1 1 1 1 1 1 1 1 1 1 "~ "~ " I "^r to E ' i Of. Qi \ % ~ s \ — " A -— -N \ w \ ^, 1 \ 5i \ _\ ^ ^ ^ ^1 2- ^T r ^ \ — s s \ \ 5.^ V \ N S ^ ^ i^ \p \£ o 1 ^ ^ ^ ^ X V 1 -^ ^ 4 ^ i- ^ ^ r "i "a '" i? tS ■- 1 ^ -1 ._ e- ^ J £ t 1 I t ^ jL i A ^ it ^ /^ J 'I t 7 ^J- 1 V ^ ^ -/ / ^ -f- -4 e- ^/^ -X % 7% -fir / / / / /' / / "=> ^ / ■s. /- ^ o ?, / Y r I" o ^ Q _s i !^ 3i3 II 111 49 <1 ^ 3 1 1 ■ OVERHEAD TRANSMISSION LINES 231 31 ^^ J' ^I |u '1l ^ T ^ 4 "1 ^ u \c cj^ y -r t \- ^yi \ ^^ t ^ T ^ -^ ^^ ^^ X^ rr ^"1 il ? > 5 s f a ~"*~ 5 i< , -5 1 ^t i^ 1 I x» II It 1 ^ ^ r ^ . 1 . _. » / ij j- it &t %t 1 ^ ' ^ ^ ^ 1 ^ ^^^ 71 7 ^i / /' ^ qi z 1 / a / |^Jl5S§|§J_§8§| 1 s^_s,^ j^^^ t^sr a^^^^^^^_g^_k^| 10 i i + 5 232 OVEKHEAD TRANSMISSION LINES h "? V ,e « % R: ^\ ■^ \ \ <0 X \ ^ N \, "^v \ N \ "^3 \' , r> 1 \ !« \ V \ \ \ \ p^ \ > \ V. \ ll_ ) r- " Jf o ffl Q) ■D -C 3 , if / ^ X o X t .1. / Z t Tl I ^ -^ ~^5- ? y ■? it t / ^ -/ ' r. J J 2 / to /J I z s / • t z V \/ / _y / ^ -^^^ ? ' y^ t / % y - / V y / y / a_ y 'r, , '- " )!- ^ t Z. il^^T %^ i 1 S -S _§ _§ _| o F> o p s s § ^ ? ^ 3 _s ^ 3 3 3 ^ ^ ^ p S =^3 "« 3 1 ;* ? "3~ a « ? 5 2 !j e 9 9 ? bo OVEEHEAD TEANSMISSION LINES 233 ~~ — — — — — — — — ~" ~ " ~ : u_ '! ^ 1* *- 2 ~ _ — 1 ~ ' ^ S \ lb - N \ , i \ S _. ^ s 1 V - s 1 \ !^ nj \ ' N S? ^ ( \ N -1! \ \ k ■o \ ^ ^ f ^ i \" ti- '3 o \ p c •^ to « ' o _o \ ^ to \ o lE m 3 z i D >i; o 'i: ■^ 01 ?! ? .c « < y dj O !; -» Ui R '• -^ § s c (o / io « / t o ' / 3 / -s < Di C k ^ o n S :t ti^ t 5 s h2 ^^ U ,P o « ^ ■t (n V o « lU u S s n c i' V F ) 3 F ^ a .2 i f L*? 03 i / o s / « c a M / d -u . w c£ / c O i' ■o /^ > - y n ^ /*' 1 ! S" D ^ SI ^ <; :z *=■ 1 ^ g § § § ^ ^ rf 5 1 5 s 5i J _ bp 234 OVEEHEAD TEANSMISSION LINES As a further example of the effect of the length of the span on the first cost of the line the case of a 20,000-volt three-phase line, whose plan is shown in Fig. 234, is worked out in Table 41. There are three copper wires each of -055 square inch section. The strain masts and corner masts are of lattice-work, and the road and rail crossings are carried out as special safety suspensions (factor of safety of 5). Three types of supporting masts are considered, viz., (1) Plain wooden poles, 7 inches diameter at the top ; (2) Wooden poles of A form ; (3) Iron masts with concrete foundations. The maximum stress on the copper wires is taken as 23,000 lbs. per square inch. The loads falling on the various masts are as follows : — - Load per Hast in lbs. Length of Span in Metres. Supporting Masts. Angle Masts for Angle Masts for Strain Masts. Angles of 120°. Angles of 135°. 40 178 3,720 2,850 3,720 60 265 3,720 2,850 3,720 80 356 3,720 2,850 3,720 100 445 3,720 2,850 3,720 120 530 3,720 2,850 3,720 140 620 3,720 2,850 3,720 160 710 3,720 2,850 3,720 180 800 3,720 2,850 3,720 OVEEHEAD TRANSMISSION LINES 235 Table 41. — First Cost of the H.T. line shown in Fig. 234, consisting of d X '055 square inch copper wires for 20,000 volts. Cost in Shillings of : Supporting masts com- plete with Cross-arms and Insulators. Angle Masts com- plete with Cross-arms and Insulators. Strain Masts com- plete with Cross-arms and Insulators. Railway Crossings complete with Cross- arms and Insulators. Road Crossings complete with Cross- arms and Insulators. Copper Wires. Erection. Total for the 10 Km. Span = 40 metres (130 feet). Sup porting mast s plain wood poles 7 metr es high abov e ground. 8,507 783 819 3,790 6,517 17,400 3,300 41,116 Span = 60 metres (196 feet). Sup porting mast s plain wood poles 7-5 me tres high abo ve ground. 5,806 853 894 3,790 6,517 17,400 3,240 38,500 Span = 80 metres (260 feet). Sup porting mast s plain wood poles 8-2 me tres high abo ve ground. 4,675 913 944 3,790 6,517 17,400 3,180 87,419 Span = 100 metres (326 feet). Sup porting mast s of A form (wood) 9 me tres high abo ve ground. 6,749 1,025 1,080 3,790 6,517 17,400 3,120 39,681 Span = 100 metres (326 feet). Supporting masts of iro n 9 metres hi gh above gro und. ' 9,384 1,025 1,080 3,790 6,517 17,400 3,120 42,316 Span = 120 metres (395 feet). Sup porting mast s of A form (wood) 10 m etres high ab ove ground. 6,134 1,206 1,251 3,790 6,517 17,400 3,060 39,358 Span = 120 metres (395 feet). Supporting masts of iro n 10 metres h igh above gr ound. 8,798 1,206 1,251 3,790 6,517 17,400 3,060 42,022 Span = 140 metres (460 feet). Sup porting mast s of A form (wood) 11m etres high ab ove ground. 5,523 1,351 1,401 3,790 6,778 17,400 3,000 39,243 Span = 140 metres (460 feet). Supporting masts of iro n 11 metres h igh above gr ound. 8,607 1,351 1,401 3,790 6,778 17,400 3,000 42,327 Span = 160 metres (525 feet). Supp orting masts of A form (w ood) 12-2 me tres high abo ve ground. 5,516 1,525 1,590 3,790 7,166 17,400 2,940 39,927 Span = 160 metres (S25 feet). Supporting masts of iro n 12'2 metres high above ground. 8,592 1,525 1,590 3,790 7,166 17,400 2,940 43,003 Spin = 180 metres (590 feet). Supp orting masts of A form (w ood) 13-5 me tres high abo ve ground. 5,453 1,790 1,870 4,020 8,040 17,400 2,880 41,453 Span = 180 metres (590 feet). Supporting masts of iro n 13 5 metres high above ground. 9,061 1,790 1,870 4,020 8,040 17,400 2,880 45,061 The results of Table 41 are shown in the form of Curves in Fig. 233. 23. COMPARISON OF THE VARIOUS SUPPORTING STRUCTURES WITH REFERENCE TO MINIMUM ANNUAL CHARGES. For a given type of support the problem of minimum first cost is solved when the " economical span " has been discovered as described above, but the question as to minimum annual outlay has not yet been dealt with. This annual outlay is made up of interest on the capital laid out, sinking fund charges, deprecia- tion, maintenance and repair allowance, and any rents paid out to owners for wayleaves. When the sum of these charges is a minimum the most economical arrangement has been arrived at. As an example, the annual charges for the case dealt with in Table 41 and Fig. 233 will now be worked out. Interest will be allowed for at the rate of 5 per cent. The whole of the capital cost is to be paid off in thirty years, which means an amortisation allowance of 1-79 per cent, if interest at the rate of 4 per cent, is allowed on the written-off amounts. The average life of Kyanised wooden poles may be taken as fifteen years. A double coat of paint wiU have to be given to all iron parts at intervals of three years. Replacements owing to lightning strokes and mechanical effects will amount to -5 per cent, annually in the case of wooden poles and to 2 per cent, annually for insulators. The values estimated on this basis are collected in Table 42 and Fig. 235. The curves show that the variations in annual charges change with the length of span in much the same way as the variation in first cost. This is due to the fact that the allowances for interest and sinking fund are generally much more important than the other annual charges, so that the latter hardly enter into the question of minimum annual cost if the work is reason- ably weU carried out. Although reduced first cost affects the dividend -earning power of the line favourably, it is a mistake to obtain the reduction by the use of poor materials or workmanship, as the increased attention and repair work and the shorter life involve annual outlays which may well exceed the saving in interest. It has already been pointed out that aesthetic considerations also often make it necessary to exceed the absolute minimum cost of installation and even the minimum demanded by good workmanship. This is confirmed by the above example. It wiU be seen that the minimum annual charges if iron lattice masts are used occurs with a span of 120 metres, but the increase for a span of 160 metres is quite small, so that the longer span should certainly be used as often as prac- ticable. Not only will the electrical and mechanical security be improved, but the trouble for wayleaves, owing to the reduced number of supporting points, will also be diminished, a straighter run of line will be possible, and the long spans and slight, flexible masts will aesthetically improve the general effect and be less likely to be called in question by lovers of the countryside. OVEEHEAD TEANSMISSION LINES 237 f— ~ ~ " V- ? c 'i; < S SJ ^ -: S s. \ £ S \ :! \ \ 1 ,C s 5 ^n ^ ?J V V ) C^ c 3 \ i I w \ « V h * ir Lii 1? c 'T :? « ( i .C 1 « « fe ¥ ■S ? o )j s » ^ % ^ _ -c f u CD y C / 1 _ c r J ^_ -4 01 ? ! ~ c -e -c s JN , ~ iS- c to dJ J _J a; (h o> r • _ ' 0) H -s ( 1- _i » — 1 — — ' X- o .Q ^ y* ^ _ ~~ " <^ o o a ^ y _ _ _ " Q) « y y — — t -o o £ -^ y __ _ _ _ "~ "■£ :^ u ^ X , _ _ ~ O c y y ^ ,_ _ — ' ~ TV "1 o -■? ^ ^ _ ~ -c te m ^ y^ — ^ ^ X _ ^ ^ ~ — 10 " ~ * ( > ,^ ^ _ — — -e c r * Ld ^ c t ~ — ~~ ""■ ^ "~ r^ - - — — 1^ ^ !? s p o !S S ^ % s _g Jg -s -^ — VJ "~ "S ^ ^ si ^ ^ 1*1 ^ •1 !^ % _!\ J J^ _ ._ __ _ _ J _ — — — _ — — — LI 288 OVERHEAD TRANSMISSION LINES CO CO O CO (N i •o o CO ■* eo (N 00 CO rH »0_ O § N N rH 05 ■* CO O CO ® O CS i> CO i-H rH rH Co" in" co" O 05 1> i> CO o o CD r— * >0 CO rH ■* 00 rH t- CO rH CO 00 o CO l> O rH "*( O 1 1 est- CO rH 1-H 5 .2 rH «5 rH «5 CO rn" w" rH t- CO rH CO N co" «n 01 -* d 1 rH IN 00 »« CO O rH (M M O W5 O >0 rH o o p— ( -# O «5 05 ob rH__i> CO rH CO 00 l-H -* J> i-H IN co" rot- CO rH (M i-T co" »n l~ O 05 rH o rH »0 IN K3 «5 rH t- CO rH CO o I— 1 «5 M5 (N co" o 05 Op ip (N 6 OS rH «5 6 00 i> CO i> i> (N M^ CO (N rH 00__ IN 05 05 rH ^ CO o 1 1 J> 'Ji IN CO O 0_i> CO rH C0__ in" 05" «5 «5 fM w g »H rH A J5 Tj »f5 (35 >n O 03 m t- CO C35 Oq (33 o CO o 1 CO -* ■* W5 6 OS rH CO CO rH o »n ITS rH CO «5 l35__J> CO rH IN ■^ >n CO (N rH (M r-T co" ©^ t- ■* % «,-■ .1 Fig. 252. to pass through a roof they are preferably made of tube, and the ends are either hammered flat or are provided with proper supporting end pieces. The cross-arms are commonly made of bar iron or channel iron. Bar iron arms, as shown in Figs. 258 to 260, are somewhat Hghter than those of channel iron (Fig. 261). The type shown in Figs. 259 and 260 has the aesthetic advantage that the centre of the insulators lies in the same plane as the centre of the pole. Sometimes collars to which bent (swan-neck) insulator pins are attached 252 OVEEHEAD TEANSMISSION LINES are used. This, however, is a more expensive arrangement and, owing to the weakness of the bent pins, is not suitable for angle points. Bar iron or channel iron cross-arms or tubular wall-arms are especially convenient if the insulators "^ -<3E nJ have to be used in an inverted position (Fig. 277). The insulator is mounted on its pin with hemp as usual, and the mouth is then closed with a lead disc held in position by a nut so as to prevent water penetrating. The different arrange- ments are shown in Figs. 240, 261, 277, and 280. ,-■ 1.1,1 ^ygg^"g ^x • ' arm WL 3 Cross bar bolted to3or4 beams J Projecting Arm Fig. 254. Fig. 255. If, as is sometimes the case, it is necessary to place an insulator at the top of a tubular standard an adapter having a f or | inch tapped hole for the pin is screwed on to the top of the standard. A method of fixing an insulator at the top of a wooden pole is shown in Fig. 86. Working on Roofs. When work is about to begin warning placards should be put up in the neigh- bouring streets, yards, etc., or other means should be adopted for notifying OVEEHEAD TEANSMISSION LINES 253 passengers of danger from falling objects, etc. Where a considerable amount of work has to be carried out protecting sheets should be erected. Any police regu- lations or local bye-laws applying to such work should be strictly complied with. Fig. 256. T=^ I I -i-i-r =i=p: -3f£- Fig. 257. Men working on roofs or masts should be provided with safety belts. When soldering work has to be carried out in or on houses, great care must be exercised. Blow-lamps must not be lighted in lofts or on roofs. The amount of opening- Sandlron Ifie'^ Fig. 258. < — 1^1 ^h 'Band Iron 72' ^ lOcD ^ rfk Band Iron Zfe'^x Fig. 259. up of the roof should be reduced to a minimum, and any openiags made should be carefully protected from rain when work ceases and during the night. Material must only be hauled up on the outside of a house after all necessary steps to protect the walls and roof from damage have been taken. 254 OVEEHEAD TEANSMISSION LINES The wires should only be put up after all stays, struts, etc., have been fitted. At corner points and distributing points, where the staying has to be done in a direction counteracting the resultant pull, temporary additional stays are put up Fig. 260. with ropes and puUey blocks until after all the wires have been erected, when the pull ■WUl fall in the correct direction. In order to raise the wires into position a rope is first thrown over the standard and the first wire, together with the start of a second rope, are attached to the end of the first rope. This is then drawn up until the first wire reaches the standard. Then the second rope is used to draw up the second wire, together with a thu'd rope, and so on, until all the wires have been got up. If a large number of wires have to be dealt with it will save time to fix up an endless rope pass- ing over a pulley on the groimd and another on the roof. Pulleys or rollers (Figs. 210 and 211) should be provided close to the insulators for drawing the wires over without damage. After the wire has been raised it is fixed to the first standard by means of a conical or riveted coupling, and the straining up of the lines and fixing them to the insulators can then be carried out as previously described. Fig. 261. OVEEHEAD TEANSMISSION LINES 255 .^^:^ «0 ^ 7 i>a . JT^ "^ ll II. I 4^ Yff •-^Casft/ie rr^ ' j-i i ' ^ — ' — ■ -EHI Tf^ i --"t-- ■* 'TP i T Ff — W -ti-^-| ■Tf^t-l titV ' ' A B^- .Vs" '15". . a'l5 ,. M* u. BandlronZ'^i^ Fig. 262. 256 OVEEHEAD TEANSMISSION LINES Distributing Points : Corner Points. At those points in a local distributing network at which a number of wires branch off in various directions special precautions have to be taken in order to attain a neat appearance. For this purpose distributing rings are sometimes employed consisting of a pair of concentric flat iron rings as shown in Fig. 262. The insulator pins are clamped in the space between the two rings and can be arranged at any point of the circle. If fuses are to be introduced at the distri- buting point the arrangement shown in Fig. 263, due to Weckmar, is very con- venient. The fuse can be replaced without making the line dead. For this pur- pose the fuse is carried in a holder with knife contacts fitting into spring clips under an ebonite cover. If the insulator is provided with two grooves the incoming and outgoing wires can both be fastened to a single insulator and the fuse inserted between them. Fig. 263. Fiff. 264. The same construction can be used for introducing a section switch as shown in Fig. 264. The switch is opened by a pull exerted on the slot at one side. Whatever arrangement is adopted, however, the control and replacement of fuses when mounted on the masts is objectionable, especially when a fuse has to be replaced quickly on a dark and stormy winter's night. The delay is the greater because it is work which can only safely be left to experienced men. The mini- mum use of fuses on local overhead distributing networks should, therefore, be aimed at, but in the larger networks their introduction at certain points is unavoid- able. In these cases it is a great improvement if the fuses are fixed in switch- boxes conveniently placed, instead of on the masts. The additional cost for the extra lead and return wires is small compared with the total cost of the instal- lation. The switch-boxes then also serve as convenient places for resistance and current measurements in the various circuits. The overhead structure at distributing and corner points should be specially carefuUy carried out, as they are usually prominent. Cross-arms placed one above the other should not be used unless they can be set exactly at right angles. In other cases distributing rings are preferable. OVERHEAD TEANSMISSION LINES 257 Lbading-in Wires. The point at which connection is made between the overhead hne and the house wiring requires careful treatment in order to avoid the possibility of water draining in and to maintain good insulation. The simplest arrangement consists in passing ebonite tubes through the wall and providing them on the outside with inverted funnel or bell mouth-pieces (see Fig. 26.5. Fig. 266. Fig. 267. Fig. 265). For smaU wires twin or triple mouth-pieces in porcelain opening into a tube are sometimes used. A good arrangement is that shown in Fig. 266. This is of porcelain and has a projecting flange for bedding against the wall and large enough to cover up the ragged edges of the hole in the waU. A corresponding flanged pipe (Fig. 267) is pushed into the hole from the inside. Fig. 268. Fig. 269. Fig. 270. Houses carrying a tubular roof standard or wall arm can be conveniently supplied by wires running through the inside of the pipe. An upright pipe is fitted with a leading-in cover of porcelain as shown in Fig. 268. This consists of three parts — a porcelain bush, a porcelain star-piece to keep the wires separated, and a porcelain cap to keep out the rain. Figs. 269 and 270 show the same thing when provided with an additional insulator at the top of the leading-in arrangement. The additional insulator can be replaced by an earth wire terminal or a lightning conductor point if required. Another modification is shown O.T.L. ■'^ 258 OVERHEAD TRANSMISSION LINES in Figs. 271 to 273. The special point about this type is that the underpart of the inlet is provided with lugs to which the wires can be tied with binding wire and so made more secure against damage by storms. If leading-in wires of smaU section have to be carried through narrow pipes the porcelain fitting shown in Figs. 274, 275, and 276 is suitable. This is made in Fig. 271. Fig. 272. Fig. 273. two pieces, and the upper one is fixed over the lower one, after the wires have been passed through, by means of wire pins. Wires can be led into horizontal tubular wall brackets in the ways shown in Fig. 277 and Fig. 278. In the former the end is provided with a two or three-way inverted funnel cemented in, and in the latter the wires are carried through holes in the underside of the pipe, which are protected with insulating bushes. Fig. 274. Fig. 275. Fig. 276. When a large number of wires have to be led in or out, as, for instance, in the case of a distributing point where the fuses are mounted on a separate switch- board, the standard can better be made in the form of a double pipe to give suffi- cient room, or, if the single pipe is of sufficient size, the leading-in arrangement shown in Fig. 279 can be used. When the same thing has to be done on a wall surface a separate standard carrying the various insulators is sometimes fixed to the wall as shown in Fig. 280, which shows the connecting point between a transformer sub-station and the overhead line. OVEEHEAD TEANSMISSION LINES 259 Branch and Junction Points and House Sebvicb Connections. Soft copper cannot be used for overhead lines because of its low breaking stress and elastic limit. On the other hand, soldered joints on hard drawn or 412 ■f^^-^ i; %^~W \= % ■-"-_L -"-"-. ■^—^ Fig. 277. medium copper weaken the wire and are, therefore, not permissible ; consequently the joints at distributing points and for branch lines are generally made by means m -jiSsciiE ^^T^W ^3- Fig. 278. of terminals. These also cheapen the jointing process. Examples of the terminals used are shown in Figs. 281^ 284. The connecting wires should be kept as straight as possible both in the vertical and horizontal planes (see Figs. 277 and 278), and all corners should either be sharp and square or neatly and regularly curved. When leading wires into fittings, etc., spirals, much employed by some wiremen, should be avoided, as they soon lose their shape and look very untidy, besides wasting wire. All unearthed leading-in wires must be kept out of reach. Fig. 279. S 2 260 OVEEHEAD TRANSMISSION LINES When carrying out the house connections the wires crossing the roads should be kept horizontal, as far as possible, by choosing houses of similar height on both sides of the street. Also as few crossings as possible should be used and these t-Tli^Sffil^i—r- should be at right angles to the road and to the lines. For instance, if three houses hke a, b, and c (Fig. 285) are to be supplied from one distributing point it is quite unnecessary to run three sets of wires across the road as shown. A single crossing and a continuation wire as shown at d, e, / are sufficient. The wkes must be kept at least 16 feet above the roadway in order to avoid interference with traffic (in many localities 20 feet is insisted upon). OVERHEAD TRANSMISSION LINES 261 As has been explained above, the fusing of house connections at the distri- buting point is objectionable and troublesome. The service wires need not, however, be kept of the same section as the feeders. The smallest allowable copper section for low-tension wire is -01 square inch (rules of the V. D. E.), and this can carry a maximum current of 31 amperes when used inside buildings. When used overhead in the open the only limit to the current is that which causes a dangerous temperature rise (dangerous either to surrounding objects or to the wire itself). It is clear, then, that even the smallest section can carry such heavy currents in case of short circuits that damage to the line is unlikely. In many country districts on the Continent plugs for supplying current to portable motors for threshing machines, wood-cuttiug machines, etc., are plenti- fully provided. These should be distributed so as to allow the motor (generally Fig. 281. Fig. 282. Fig. 283. Fig. 284. fitted with a cable drum and 80 to 100 yards of cable) to be used in practically any position. It is preferable to increase the number of sockets for plugs rather than to extend the easily damaged cable unduly. The plug contacts should be strongly and amply designed and so arranged that the plug can only be inserted the right way round. Short circuits have often occurred through careless handling of the flexible cable or heavy overloading of the motor, so that it is advisable to fit fuses in front of the plug and in such a way that they can be replaced by unskilled work- men. When these plug sockets are to be mounted on wooden poles the attachment flange must be suitably curved. For wall plugs the flange is straight. The leads down to the socket should be run in piping as shown in Fig. 286. The plug and socket shown here is not provided with fuses. These should be arranged in a closed iron case above the plug socket, 262 OVEEHEAD TRANSMISSION LINES Stebet Lighting. W/. w ^ r-- m In most districts the street lighting is carried out with the majority of the lamps only alight till midnight and all-night lamps are only placed at street corners and other important points. If each lamp is arranged with its own switch it can, of course, be used at will either as a haK-night lamp or as an aU-night one. Although this gives the simplest circuit arrangements and the lowest first cost it is seldom used because it nullifies one of the chief advantages of electric street lighting, viz., the power to switch on all the lamps from the one central station. The two chief groups — half-night and aU-night lamps — can be sub-divided into smaller groups of, say, not more than twenty lamps each. For this two additional wires would have to be run along the poles for each kind of lamp, or four additional wires in aU. This is the best arrangement, as it enables the lamps to be entirely discon- nected from the supply when switching off, but it is too expensive on extensive schemes. ^ ^ M m 1 1 r: I I ^ / / / / A < Fig 286 OVEEHEAD TRANSMISSION LINES 263 A cheaper arrangement is attained by using a single wire as the common return for both the half -night and the all-night groups (Fig. 287, top). In this e .-5 t. loco to » SJSg sng S Jill ojio i»to I to fa sjBg sng sjeg sng arrangement also the lamps will be dead on both poles during the daytime ii the time switches used are double-pole ones. Single-pole time switches are generally provided with a hand switch so that both lines can be disconnected from them when repairs are proceeding (Fig. 287). 264 OVERHEAD TRANSMISSION LINES In the case of three-phase suppHes without neutral wire the three distri- buting mains and the three lamp wires should be symmetrically arranged on the poles either in groups of two and four wires or in three groups of two wires. If a fourth (or neutral) wire is provided this should be placed by itself at the pinnacle of the pole. In order to reduce the cost still further it is common to connect each group of lamps between one switching wire and one of the distributing mains (Fig. 287, middle). In this way each street with part-time and whole-time lamps only requires two additional wires, The lamps, however, are alive even when switched off. Any further group of lamps will require one extra wire as shown in Fig. 287 (bottom). In aU these cases the switching is best carried out by automatic time switches, which are now obtainable for the purpose, and which can be set to operate at any desired time. Some of these are provided with an astronomic adjustment which automatically varies the time of switching in or out by a few minutes every day in accordance with the variation in the times of sunrise or sunset. Time switches ensure the switching on and off of lamp at definite predetermined times, but sometimes it becomes necessary to be able to switch on or off at other times owing to fire, floods or similar causes. This can be accom- a p™> T Fig. 289. Fig. 290. 266 OVEEHEAD TRANSMISSION LINES plished if hand-operated switches are introduced as shown in Fig. 287. These switches are also useful in case the time switch is out of order. The separate lamp circuits are provided with fuses at the switching stations. Fig. 291. Besides these main fuses it is advisable to give each lamp a fuse at least on one pole, so that when a fault occurs the blowing of these individual fuses shall indicate the faulty point quickly. The fuses should be placed as close to the distributing Fig. 292. main as possible. Sometimes the fuse is inserted close to the lamp (Fig. 288), but it then only protects the lamp itseK and not much of the lead to it. A better arrangement is to add a somewhat stronger fuse at the junction of the lamp lead with the distributing main so that in case of a lamp fault the lamp fuse OVEEHEAD TRANSMISSION LINES 267 which is easily replaced on the spot, blows first, but, at the same time, the lamp lead is also protected. The leads to the individual street lamps should be run in steel conduit carried down the mast or down the wall (Fig. 236). If, owing to the use of roof standards, it is necessary to carry the lamp leads down the standards and through the interior of a house the wiring must be such that no unauthorised current can be tapped off. For this purpose the wire should be run in tubing either without joints or with joints which can be lead sealed. The lamp leads should not be run, exposed, down masts or house walls, as they are too liable to be damaged during repair work. Lamps can be fixed either to masts or houses along the roads. They are generally of 32 or 50 candle power, and are placed from 40 to 100 yards apart and, as far as possible, alternately on each side of the street. The lamps can be mounted on wall brackets (Fig. 288) or singly or in pairs on poles (Figs. 289 and 290), and the fittings used vary from the simplest to the most ornamental. The best hght distribution is obtained by hanging the lamp over the centre of the street at a height of about 5 yards. This can be accomplished in a number of ways. Figs. 291 and 292 show two such suspensions. If the system works with one pole earthed the uninsulated supporting wire can act as one supply lead. If the second suspension wire is to act as the second supply lead a strain insulator must be inserted at one point (Fig. 293). The arrangement of Fig. 292 is 268 OVEEHEAD TEANSMISSION LINES /^^^^^^^^^^^ s\^^^^^^ Fig. 29i. adopted when the lamp has to be placed at a spot where two houses do not face one another. In this case a ladder is needed when attending to the lamp. This inconvenience can be avoided if the lamp is arranged for lowering. Fig. 294 shows a lowering gear due to C. A. Schafer, of Hanover. The simple lock fitted on this is shown in Pigs. 295 and 296. No ratchet is used with this OVEEHEAD TRANSMISSION LINES 269 type, as the lowering is done by means of a rope carried by the attendant and hooked into the lock as shown. Cost of Local Distributing Systems. Local distribution schemes carried out and run by a private company only become practicable when some reasonable return on the capital expended is likely to be received. In such cases appearance is naturally little considered. The wish of every community to keep its roads and squares free from overhead wires, Fig. 295. Fig. 296. or at least to allow only pleasing structures and those of the best workmanship to be erected, often compels them to take up the matter themselves. A public autho- rity is able to borrow money on more favourable terms than a private firm. Also with pubhc ownership, where all the inhabitants reap the benefit of the concern, it is not so important to ensure the payment of a dividend in its earlier years. A private company cannot generally afford to use its net profits, after paymg a reasonable dividend, to reduce the price of the current, because of possible losses in other ventures. Any profits which a pubhc authority makes, on the other hand, can be used for municipal purposes even if they are not returned to the community directly in the form of lower charges for current. A private undertaker, however, is likely to push the business more quickly 270 OVEEHEAD TEANSMISSION LINES than a public authority, and will be ready to deal with large consumers earlier. It may, therefore, be advantageous in some cases for the pubhc authority, after S ; 2/ SO / / / 25 oa / A /e ra <;< C 0! t of i )C 1/ ■}y er ^e 3d / a is ■ri 6t ti "7 s /s 'e ns ^ / / 22 sc / / / o< V 20 oo ,? ^> ♦ ^^ i^) / 17 ;o <; ,^' / t^V ^ 1 1 ,e, / 1 ^ y IS oo ,«• y ■y ^y ^^ /2 50 b^: V ^^' / ,1 y .c / 10 oa ^ I/ ^ ■^ ,-,0 y ^ '' •S' »> ' s'; ^ ^ 7 50 % / ■" •^ / it* ni ^ ^ ^ / t' is --' / "U iti ,x ^ S 30 / ^0' ■!^ ^ ^ k- / in l' nnt e>- >• ■^ / K bu 1 >^ ^ ^ — — 2 SO M ^ ^ >" io ns ^ — 1 — " _, ' ^ Hi ui e — ^ — ■ ^ ^ ' _- — — " n — — ' /V! r nh \ih. ta -7^ - A » ). 'O •fC nn « JO •fT V <7 50 ?i m 22 50 ?/! 1(7 2? 50 ,7r on JJ 50 H OO 3/ so U vo u iO -f^ 00 _ _ _ Fig. 297. carrying out the installation, to hand it over to a private company to work, for some years at least. Since the cost of a local distribution scheme depends on circumstances OVERHEAD TRANSMISSION LINES 271 which cannot be reduced to a uniform and generally applicable basis, it is only possible to give approximate average values as a guide in estimating. Such values obtained from over 100 estimates are collected in the curves of Fig. 297. The price of copper has been taken as £85 per ton. The cost of house connections and meters has been based on the assumption that every two inhabi- tants will require one incandescent lamp and that each service will supply five lamps. The allowance for street lamps has been determined from actual town plans. 25. AGREEMENTS WITH CONTRACTORS. Optbn the work of mast carriage-and erection, and in some cases also the run- ning of the wires, is handed over to contractors who make a speciahty of it. In order to avoid subsequent misunderstandings and disputes it is essential to draw up a detailed specification and agreement giving full particulars as to the commencement, progress and end of the work, method of carrying out the work, taking over, guarantees, penalties and conditions of payment. In order that this agreement may have general application it is usual to add a schedule of prices for the various separate items and giving details as to quahty and type of material. A model agreement for such a purpose is given below : — Agreement. Between Messrs. (hereinafter called the " Undertakers ") and (hereinafter called the " Electrical Company "). § 1, General. The Undertakers agree to construct the overhead electrical supply system in accordance with the drawings, instructions, and schedule of prices attached to this Agreement. The Electrical Company will supply the Undertakers, free of charge, with com- plete drawings, on which all necessary information will be given. Deviations from the drawings are only to be carried out on written instructions. Deviation from the drawings without such instructions will be made at the Undertakers' risk. Any additional material involved by such alterations may be deducted for by the Electrical Company from the Undertakers' account or the alterations may be put right at the Undertakers' expense. The Electrical Company shall have the right to change the order of the work during its progress, and the Undertakers shall not make any additional charge, provided that they are able to keep their staff employed. If this is not the case, suitable compensation is to be determined and credited to them. § 2. Surveying the Route of the Line. The positions of the masts and other important points on the line will be fixed by the Electrical Company which will also undertake to obtain all wayleaves and other necessary permits from the landowners and local authorities. The Under- takers are to check the positions marked on the drawings before commencing work, and after that they are to be held fully responsible for the correct placing of the masts, etc. They may demand copies of the permits obtained by the Electrical Company from the various owners and of the drawings submitted to the local authorities. The mast positions will be marked by the Electrical Company with particulars OVEEHEAD TEANSMISSION LINES 273 as to type, height, and foundations. The general arrangement drawing will give particulars as to cross-arms, distance between wires, cross-section of the wires, guard nets, mast switches, earth wires, etc. The numbers of the masts on the drawing will correspond with those marked along the route. In the case of the local distributing network the masts and other supporting points will be indicated on sketches. § 3, Malerial. All material required for the erection of the line, unless otherwise stated, will be supplied by the Electrical Company and will be available either at a stores within the district covered or will be delivered at the nearest railway station. The Undertakers shall, as soon as possible after receiving the contract, supply to the Electrical Company a schedule of all material required to comply with the drawings and shall send for the material as shown on this schedule. Masts must be called for in truck-loads, and if a whole truck-load is not used in any particular case the Undertakers must bear the difference in freight involved. The Undertakers are responsible for the correct delivery of the masts as to time and place. Any delays produced through mistakes in these points are to be made up. The Undertakers must certify for all goods received by them. Invoices will be supplied for all goods sent by rail. Any complaints as to quality or quantity must be notified to the stores depart- ment at once, and, if dealing with a. case of rail delivery, within twenty-four hours. Later notifications cannot be considered. After leaving the stores all goods are carried at the Undertakers' risk, and they will also be responsible for charges resulting for delay in emptying trucks. Excess material, so far as it is in good condition, will, on written notification, be taken back by the Electrical Company and allowed for. If more material has been drawn from the stores than the drawings show to be necessary, the excess may be deducted for from the Undertakers' account. S 4, Commencement and Progress of the Work. The work is to be commenced within fourteen days of the signing of the contract, and is to be proceeded with at the rate of yards a day. If the Undertakers are late in completing the work, a deduction of 1 per cent, on the contract price will be made for each completed week's delay, without any definite damage to the Electrical Company having to be proved. If the agreed date for completion is exceeded by four weeks the Electrical Company may hold the Undertakers responsible for all loss resulting from the delay. If the Undertakers do not commence work at the date agreed upon, the Electrical Company is at liberty, without reimbursing the Under- takers for any expenses they may have been put to, to take the work in hand them- selves or to let it to a third party. The Undertakers may refuse to commence work if more than 3 per cent, of the necessary permits are not yet in their hands. If, in spite of these omissions, the Electrical Company requires the work to proceed, the Undertakers may claim for all extra expense involved. Applications fdr extensions of time may only be made by the Undertakers if there has been delay in supplying them with material, or if local authorities have stopped the work, or if changes in the route have caused delays. The permission for extension of time must be received in writing. O.T.L. T 274 OVERHEAD TRANSMISSION LINES § 5. Contractors'' Staff and their Supervision. The Undertakers are to carry out the work with their own staff. They naay only sub-let with the express permission of the Electrical Company. Supervision of the work, if not done by themselves personally, must be carried out by a responsible person whose name shall be handed in to the Electrical Company. On the part of the Electrical Company a clerk of the works will be in charge, and his requirements must be exactly complied with by the Undertakers. Grievances, suggested improvements, or alterations are to be brought before the Electrical Com- pany in writing, and the reply in connection with them must be received in writing. § 6. Standard of Work. The work is to be carried out in accordance with the standard regulations of the and any special rules laid down by the Electrical Company. The Undertakers further agree to comply with any additional instructions issued by the Electrical Company during the progress of the work. Any modifications necessitated by non- compliance with the rules are to be carried out immediately on written notification being given to the Undertakers at their own expense. Otherwise the Electrical Company may undertake the work or hand it over to a third party and charge the Undertakers with the cost. § 7. Tools and Appliances. The Undertakers must supply all necessary tools and appliances for the work and for the transport of the materials without extra charge. They are also to provide the necessary materials for mast foundations such as stone, gravel, sand, cement, etc. The Electrical Company is to have the right to test any materials at any time and to reject any not up to the standard quality. § 8. Work lying outside the Contract. Work not included in this contract and for which no price has been specified must not be carried out without written permission. If this rule is ignored the Elec- trical Company may cause such work to be dismantled at the cost and risk of the Undertakers. No payments will be made to the Undertakers for such unauthorised work. § 9. Responsibility of the Undertakers. The Undertakers take full responsibility for compliance with all laws and bye- laws dealing with the safety and insurance of workmen or other persons. They guarantee the Electrical Company against all charges for damage due to their negli- gence. An exception is to be made in the case of damage to cultivated land as a result of carrying the line across it, such damage being made good by the Electrical Company. § 10. Guarantees. The Undertakers agree to employ only the best material, and guarantee the quality of the workmanship for a period of one year. All expenses due to defective material or workmanship during that period are to be made good at their expense within fourteen days of written notice-being given. Otherwise the Electrical Company may undertake the work and charge the Undertakers for it. OVERHEAD TRANSMISSION LINES 275 § 11. Taking over the IVork. The Undertakers are to notify the Electrical Company in writing of the comple- tion of the work. Within fourteen days of this notification a joint examination and inspection of the whole work shall be made, and the material used shall be checked over. The number of parts used shall be counted and the length of wire shall be determined by measuring the straight distance between the masts. An allowance in length for the sag and waste and in the number of insulators through breakage shall be made amounting to 3 per cent. If this inspection should show that the work does not comply with the specification in any point, the Undertakers shall be allowed a period of days to put the matter right. If at the end of this period the work is still not in order, the Electrical Company shall be free to undertake the work them- selves or to engage others to do it and to charge the Undertakers with the expenses thus incurred. The Undertakers shall be responsible for the whole of the work until it has been taken over by the Electrical Company. § 12. Penalties and Payment. The Undertakers agree to pay in the sum of £ to cover calls upon them in the way of penalties, etc. Payment will be made within four weeks of the taking over of the work provided the account is sent in within fourteen days of that date. The Undertakers, during the carrying out of the work, are entitled to demand payments on account up to 90 per cent, of the value of the work completed at the time. Proof of the value of work completed must be produced by the Undertakers. § 13. Arbitration. In the case of any dispute or question with regard to this contract arising, it is to be submitted to arbitration. Each party may propose an arbitrator within four- teen days, and these will then, within one month, select a referee. If the arbitrators are unable to agree as to a referee the latter is to be selected by . If either party fails to select an arbitrator within the time, the other party shall be free to select the second arbitrator as well witlim another period of fourteen days. The arbitrators and referee after hearing both sides are to hand in a written report within one month, and this shall be binding on both parties. This report shall also contain provisions for the payment of the cost of the arbitration. § 14. Signatures. The contract is to be prepared in duplicate and each party is to sign both copies and to retain one of them. Special Regulations for the carrying out of Overhead Line Installations. The undertakers must take every precaution so as to reduce damage to property to a minimum. Damage due to faulty workmanship or malice must be made good by the undertakers. When excavation work has to be carried out on agricultural land the surface soil must be separated and used again for covering the spot when the work is T 2 27G OVERHEAD TRANSMISSION LINES completed. Turf must be replaced where necessary. The ground surface must be put back into its original state within eight days. Any superfluous material is to be removed within four days. The foundations used must be such that the ground pressure does not nor- mally exceed lbs. per square inch. If the undertakers consider that the stratum of soil met with at the given depth is unable to support that pressure they must notify the clerk of the works at once verbally and the electrical company in writing. The electrical company must then decide the matter. Any addi- tional weight of concrete that may be required shall be credited to the undertakers at the rate agreed upon. The masts must be transported and unloaded carefully. Bent or damaged parts must be replaced at the undertakers' expense. The erection must not be delayed for want of lifting tackle, scaffolding, etc. Masts in straight stretches must be set vertically so as to cover one another exactly in the line of sight. Masts at corner points are to be given a set or rake according to instructions from the electrical company. All iron parts must be delivered ready painted. If several coats have been specified these must be of sufficiently different shades to make the difference easily noticeable. The colour of the last coat is to be specified by the electrical company. The undertakers may use any suitable material obtained in the course of the excavation work in making the concrete. The sand used must be sharp. Sand and gravel must contain no loamy or clayey admixture. If necessary the materials must be washed before use. Broken stone must be reduced to 2| inches diameter at most. Soft stone must not be used. The water used in making the concrete must be quite clean and free from scum. The concrete, of the consistency of moist earth, is to be put in in layers 6 or 8 inches deep and well punned. If either a frost occurs or there is great heat the newly -laid concrete must be well covered up. The earthing of the masts, etc., must be specially carefully carried out to the instructions of the electrical company. Barbed wire protection is to be fitted to the masts after the wires have been erected. Danger notices are to be attached to the masts after they have received their last coat of paint. Aluminium wire must not be drawn along the ground, and copper wire must not be drawn over stony land or roads. Faults in the wire, if discovered during erection, must be cut out and notified to the electrical company at once. The wires are to be tested with a dynamometer for tension and sag in accord- ance with the tables supplied by the electrical company. In straight stretches butt joints covered by thin split copper tubing only are to be used. Any openings which have had to be made in roofs are to be covered during rain and after working hours. Bad and unreliable roof structures must be suitably strengthened. The undertakers are to instruct their employees to take special care in connection with roof work, so that the owners are disturbed as little as possible. OVERHEAD TRANSMISSION LINES 277 The staff employed on local distribution systems is to be carefuUy chosen, and the electrical company shall have the right ta discharge men whose conduct is open to censure. Schedule of Prices for High-Tension Transmission Line Work. Carriage (from the stores or railway siding) and erection of wooden poles, excavating holes for them 6 feet 6 inches deep, punning in the soil again, leaving the surface in its original state, and removing all superfluous, material : Up to 36 feet long, per pole ....... ,, oy ,, ,,..,..., 2. Wedging the poles with two layers of broken stone (including the supply of 7 cubic feet of hard broken stone) : Extra per pole ......... 3. Carriage and erection of struts, excavating hole 5 feet deep, fastening strut to pole with f inch bolt, setting the strut in position, supplying and setting a base plate of stone or hard wood, filling in and making good : Per strut .......... 4. Carriage and erection of double masts of A form as under (1), fitting the two poles together, supplying two old railway sleepers for the foot (8 feet 6 inches long, 5| inches deep, 9 inches wide) and two beams impregnated with creosote (about 36 X 5 X 9 inches) for the central cross-piece and supplying five f inch bolts : 36 feet long 39 „ . . 43 „ . . 46 „ . . 5. Wedging these poles as under (2) 6. Carriage and erection of mast feet of channel iron, excavating 5-foot hole as under (1) : Per mast foot ......... 7. Carriage and erection of wooden poles in mast feet, including wedging and supplying the wedges of hard wood impregnated with linseed oil: Up to 29 feet long, per pole ....... ,, 36 ,, ,, . . .... 8. Fitting a stay of round iron rod, including excavating a 5-foot hole for an anchor plate, supplying a stretching screw and strain insulator, and making good ......... 9. Carriage and erection of an iron mast 36 to 46 feet long : Up to a weight of 10 cwt., per mast ..... 20 .... „ 30 „ „ ... s. d. 5 3 6 6 7 2 6 7 25 6 27 29 31 5 3 6 4 4 5 4 8 4 7 16 24 29 278 OVERHEAD TRANSMISSION LINES 10. Excavating for concrete foundations, per cubic yard 11. Carrying out the concrete foundation work with a concrete mixture in the ratio 1:3:6, smoothing the outer surface of the foundation block where it projects above ground, removing superfluous material and making good : Per cubic yard ......... 12. Additional cost where a caisson has to be used to exchide water : Up to 3 feet depths, per mast ...... ,, «^ ,, ,, ...... 13. Additional cost when water has to be drawn off : Up to 3 feet of water ........ Greater depth, or rock work ...... 14. Fitting a plain wooden pole or wooden A pole with 3 insulators on pins screwed into the wood ....... 15. Fitting a plain wood pole or wooden A pole with a cross-arm with two insulators and with a top cap for one insulator 16. Fitting a plain wood pole or a wooden A pole with three brackets for one insulator each, a top cap to carry an earth wire and connecting the earth wire to the brackets ....... 17. Fitting an iron mast with a cross-arm for two insulators and a top cap for one other insulator. ....... 18. Fitting an iron strain mast with a double cross-arm with four insulators and two more insulators on a top cap ...... 19. Fitting an iron mast for a special safety crossing for three wires 20. Fitting an iron mast with three cross-arms for one insulator, each 21. Fixing an insulator on its pin, including supply of the necessary hemp, red-lead, and felt washers ........ 22. Fitting an earthing bow ..... 23. Fitting a hoop guard . 24. Fitting a safety loop ....... 25. Providing an earth connection, including excavation to a depth of 6 feet 6 inches, sinking an earth plate, or driving in an earthing pipe, connecting it to the earth wire, connecting the earth wire to the mast, and providing a safety covering above the ground . 26. Erection of a mast switch for operation from the ground, including provision of enclosing casing for the operating ropes 27. Fitting a " Danger " notice ........ 28. Running wires ready for use, including laying out, tightening and fixing the three wires, making the joint and branch connections, assuming an average distance between masts of 120 to 170 yards : Wires -0155 square inch each, per yard of single wire ,, -025 ,, ,, ,, ,, *Uoy ,, ,, ,, ,, 'Uoo ,, ,, ,, ,, -078 ,, „ ,, s. d. 1 6 15 6 14 6 22 9 14 1 6 1 8 2 7 1 2 2 10 3 6 1 7 2 1 6 5 5 11 21 2 . 0-6 . 0-7 . 0-85 . 0-95 . 1-05 OVERHEAD TRANSMISSION LINES 279 29. 30. 31, Erection ready for use of a double telephone line of bronze or steel wire up to a wire section of -0155 square inch, making joints, branch connections and crossings : Per yard of double line ....... Erection of an earthing wire or guard wire up to -055 square inch section, laying, tightening, and attaching to mast cap or terminals : Per yard run ......... Giving iron masts a double coat of paint, including supply of the rust- proof paint : Per 1 cwt. weight of mast ....... 32. Numbering the masts : Per mast . 0-7 0-8 1 2 21 33. 34 35 36 SCHEDXJLB OF PbICES FOR OVERHEAD LoCAL DISTRIBUTING SYSTEMS. Carriage and erection of wooden poles of diameters up to 7 inches (as detailed under (1) ) : Up to 30 feet long, per pole 33 36 39 43 46 4 8 4 11 . 5 1 5 5 5 10 Addition when the soil is under the water level : Per pole .......... 5 Additional cost of plaster work : Per pole .......... 1 Wedging in a pole with two layers of broken stone, including the supply of 7 cubic feet of hard broken stone : Per pole .......... 37. Carriage and erection of strut as detailed under (3) without plaster work ........... 38. Erection of a stay wire of round iron rod or wire rope as under (8), excluding the insertion of a strain insulator and plaster work 39. Erection of a stay wire without earth anchoring .... 40. Carriage and erection of iron masts up to 40 feet long : Weight up to 6 cwt. ........ 10 „ . . . .... 20 „ 41. Excavation work for concrete foundations : Per cubic yard of excavated soil ...... 42. Carrying out the concrete foundation work as under (11) : Per cubic yard ......... 43. Extra when the bottom of the foundation is under water level up to a depth of 3 feet . . . . • ■ • 44. Extra for plaster work : Per mast .......... 2 6 2 3 8 3 12 14 21 14 10 3 7 280 OVEEHEAD TEANSMISSION LINES s. 45. Erection of a bracket or wall arm : With two fixing points ....... „ three „ ....... „ four „ ....... 46. Erection of a tubular roof standard, including the necessary roof strengthening work and fitting the necessary rain-proof joint, but excluding the making good of the roof ..... 47. Erection of a double tubular roof standard as in (46) 48. Fixing an insulator on its pin, including the supply of the necessary hemp, red-lead and felt washers ....... 49. Fixing a cross-arm with two insulators on either a wooden or iron pole 50. Ditto, with four insulators ........ 51. Erecting a ring-shaped insulator support for distributing points on wooden or iron poles ......... 52. Fitting an insulator by screwing into wooden pole .... 53. Fitting a straight pin insulator to cross-arm, etc. .... 54. Fitting an insulator with cement ....... 55. Fitting a mast cap to take an earthed wire or earthing cable 56. Sinking an earth plate or driving in an earthing pipe, connecting it with the earthing wire and to the neutral wire, fastening the earthing wire to the mast and giving it a protecting covering above the ground 57. Erection of a single pole lightning conductor and connecting it to the earth wire or earth plate ........ 58. Erection of an overhead fuse or section switch and connecting it with the lines ........... 59. Running the line wire complete and ready for use, including laying out, tightening and tying in, making joints and branch connections : (a) For bare conductors of -01 square inch section, per 100 yards ,, ,, 'OlSo ,, ,, )) ;> "0^5 ,, ,, )) !j *uoy ,, ,, ;; J! '078 ,, ,, !S !J *1U0 ,, ,, (b) Insulated conductors of -01 square inch section, per 100 yards ,, ,, '0155 ,, ,, )> J) 'VZo ,, ,, )) J) *uoy ,, ,, !5 >) "055 ,, ,, )) ?j '^ * ^ J, ,, !! ^; '108 ,, 5, 60. Making joints between bare and insulated conductors : Per pair .......... 61. Provision and erection of flat safety screens of galvanised wire, con- sisting of three longitudinal wires and cross-wires at 1 yard intervals, erecting the necessary cross-arms, and attaching the screen to the 4 2 5 6 2 7 6 14 If 7 10 3 2 4 3 8 1 2 9 6 2 2 1 7 4 2 4 7 5 6 6 6 10 7 10 9 2 4 7 5 1 6 8 6 10 8 9 2 10 7 1 7 Per yard 1 2 OVERHEAD TRANSMISSION LINES 281 62. Provision and erection of box safety nets with seven longitudinal wires, otherwise as in (61) : Per yard ........... 2 5 63. Erection of a wall arm or bracket and connecting it to the overhead lines ...........60 64. Carrying out a house service connection up to 10 yards in length, including the fixing of the insulators and the making of the joints : Two wire connection ........ Three „ Four „ ........ 65. Extra for length above 10 yards, per yard of single wire . 66. Giving two coats of rust-proof paint to a cross-arm with two insulators 67. Ditto, with four insulators ........ 68. Ditto, for a simple roof standard or wall bracket .... 69. Ditto, for a double roof standard ....... 3 10 6 2 9 6 1 7 1 1 1 4 2 e 26. TOOLS AND APPLIANCES. (a) Foe the Erection op High-Teksion Installations. 1. 1 tool box 4 feet X 2 feet 3 inches X 2 feet 3 inches. 2. 12 screw or conical terminals for wires from f to 1 inch in diameter. 3. 12 ditto for wires -04 to -4 inch in diameter. 4. 12 pairs of draw tongs for wires from -04 to -5 inch in diameter, each fitted with 1 yard of chain and a hook. 5. 10 stretching screws, each with two screws. 6. 2 bolt cutters, 2 feet long. 7. 1 dynamometer for a pull of 1 ton. 8. 1 ditto for a pull of -| ton. 9. 1 hack saw with six blades. 10. 4 2-lb. hammers. 11. 2 IJ-lb. hammers. 12. 1 riveting block. 13. 4 8-inch flat chisels. 14. 4 cross-cut chisels. 15. 5 punches (various). 10. 1 adjustable square. 17. 1 thermometer. 18. 1 spirit level. 19. 1 plumb line. 20. 2 flat files with handles. 21. 2 round files with handles. 22. 3 three-square files. 23. 12 f-inch bolts with hooks and eyes, 16 inches4ong. 24. 2 shifting spanners, 10 inches. 25. 15 different spanners. 26. 1 pair of pincers. 27. 2 pairs of gas pliers. 28. 12 combination pliers. 29. 2 universal pipe grips. 30. 4 screw-drivers. 31. 1 mallet. 32. 1 hand-vice. 33. 4 wood-borers, f inch. 34. 1 measuring tape, 60 feet long. 35. 4 long scales. 36. 2 complete sets of riveting tools for making riveted clamp joints, 37. 1 spoke shave. 38. 1 quart-size soldering blow-lamp. 39. 12 safety belts with tool pockets. 40. 7 pulley blocks with 25 yards of J inch rope to each. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. OVERHEAD TRANSMISSION LINES 283 41. 8 pairs of climbing irons of 10-inch width. 42. 10 sets of rollers, each set containing two rollers for fixing on cross-arms and one roller for fixing at the top of a mast. hatchet. .saw. axe. spade. shovel. vice. 2 sets of V blocks for mounting cable drums on. turntable. hand-cart. 1 small trolley with two wide wheels. (b) For the Erection of Low-Tension Installations. 1. 1 tool box 4 feet x 2 feet 3 inches x 2 feet 3 inches. 2. 12 screw or conical terminals for wires f to 1 inch in diameter. 3. 12 ditto for wires from -04 to -4 inch in diameter. 4. 12 pairs of draw tongs for wires from -04 to -5 inch in diameter. 5. 10 stretching screws, each with two screws. 6. 2 bolt cutters, 2 feet long. 7. 1 dynamometer for a pull of 12 cwt. 8. 1 ditto for a pull of 4 cwt. 9. 1 hack saw with six spare blades. 10. 4 2-lb. hammers. 11. 2 Ij-lb. hammers. 12. 1 riveting block. 13. 4 flat chisels. 14. 4 cross-cut chisels. 15. 5 punches (various). 16. 1 adjustable square. 17. 1 thermometer. 18. 1 spirit level. 19. 1 plumb line. HO. 2 flat files with handles. 21. 4 round files with handles. 22. 3 three-square files with handles. US. 12 |-inch bolts with hooks and eyes, 16 inches long. 24. 2 shifting spanners, 10 inches. 25. 15 different spanners. 26. 1 pair of pincers. 27. 2 pairs of gas pliers. 28. 12 pairs of combination pliers. 29. 2 universal pipe grips. 30. 4 screw-drivers. 31. 1 mallet. 32. 1 hand-vice. 33. 4 wood-borers, | inch. 34. 1 measuring tape, 60 feet long. 35. 2 complete sets of riveting tools for making riveted clamp joints. 284 OVERHEAD TRANSMISSION LINES 36. 1 hollow auger. 37. 1 spoke-shave. 38. 1 quart-size soldering blow lamp. 39. 12 safety belts with tool pockets. 40. 7 pulley blocks with pulleys 2 inches in diameter and 20 yards of rope to each 41. 8 pairs of climbing irons 10 inches wide. 42. 20 small rollers for fixing on cross-arms. 43. 1 hatchet. 44. 1 saw. 45. 1 axe. 46. 1 spade. 47. 1 shovel. 48. 3 ladders 20 to 83 feet long. 49. 1 vice. 50. 1 handcart. 27. REGULATIONS DEALING WITH THE ERECTION AND OPERATION OF OVERHEAD LINES. (1) BoAKD OF Trade Regulations eor Overhead Lines.* Under articles 2 and 22 of the regulations prescribed by the Board under section 4 of the Electric Lighting Act, 1888 : — Conductors. (1) The conductors shall be hard drawn copper wire or aluminium. (2) Hard drawn copper wire conductors shall have a breaking load of 24 tons per square inch and on breaking the elongation shall not be less than 2 per cent, in a length of 10 inches. Aluminium conductors shall have a breaking load of not less than 12 tons per square inch and on breaking the elongation shall be not less than 3 per cent, in a length of 10 inches. (3) The minimum sag of the conductors shall be regulated to give a stress due to its weight and to wind (but excluding its elasticity) of not more than one-fifth of the breaking load, at a temperature of 22° F. Wind pressure shall be taken at 25 lbs. per square foot, and the effective area of the conductor shall be taken as -^ of the diameter multiplied into the length. (4) The minimum height of any part of any conductor from the ground shall not be less than 20 feet except with the consent of the Board of Trade. (5) Conductors shall not cross any building other than a sub-station or be accessible to any person from any building or tree without the use of a ladder or other special appliance. Where the conductors are so placed that a tree, if uprooted, could come into contact with a conductor an earthed cradle enclosing them or some other precaution, approved by the Board of Trade, shall be provided to prevent all danger of any shock. (6) Conductors shall not be carried by undertakers across the premises of a consumer except with the consent of the Board of Trade and subject to such conditions as the Board may prescribe. Poles. (7) The conductor shall be carried on poles either (a) wooden poles, or (&) poles or structures of iron or steel, hereinafter called steel poles. (8) Each pole shall be clearly and permanently marked with a number. (9) Danger notices shall be fixed on at least one pole in five and on each pole at the crossing of a road. (10) Provision shall be made to prevent cUmbing by barbed wire being coiled round the pole in one or more coils of an aggregate length not less than 2 feet, the lowest coil being at least 8 feet from the ground. (11) Where guys or stays are used they shall be securely anchored and earthed. (12) A continuous earth wire shall be carried from pole to pole, and shall * For a collection of the various Board of Trade regulations dealing with electrical supply, see Electrician, " Electrical Trades Directory." 286 OVEEHEAD TRANSMISSION LINES be well connected to substantial earth plates at intervals of not more than five spans, or the ironwork on each pole shall be connected to a substantial earth plate. Wooden Poles. (13) The poles shall be soimd winter-felled red fir, free from large knots or other defects, with the natiiral butts, and shall be well injected with creosote, or they shall be of a description approved by the Board of Trade. (14) Single poles or A poles shall be used for the ordinary run of the hne. Stouter poles, H poles, or built-up or strutted poles, provided, if necessary, with stays, shall be used for terminals, for intermediate anchor poles, for important differences of span, and for corner poles where there is considerable change of direction. Ordinary poles provided with stays shall be used where the direction makes a small change. (15) Poles of ordinary lengths, unless in rock foundation, shall be set in the ground to a depth of 6 feet. The earth shall be well punned into the holes. Where necessary they shall be set in concrete. (16) The factor oi safety for the poles shall be calculated at 10 for a wind pressure of 25 lbs. per square foot, the effective area of a round pole being taken at '6 of the mean diameter of the exposed part into the length of that part. Steel Poles. (17) Poles of tubular type shall be painted with oil paint not less than once every five years, and poles of lattice type not less than once every three years. (18) Each pole shall be set in concrete. (19) The concrete below the pole shall be dropped on to a substantial cast- iron earth plate bonded to the pole by a wire or rod. (20) The factor of safety of the pole shall be not less than 6 taking the maximum wind pressure at 25 lbs. per square foot. In the case of lattice poles the pressure on the lee side will be taken as one half of the pressure on the wind- ward side. Arms. (21) The conductors shall be carried on insulators mounted singly, or in pairs on steel channel arms, or singly upon iron brackets fastened to the poles ; or, if wooden arms are used, an earthing strip or stout wire shall be fastened on the upper side of each arm. Road Crossings. (22) Where the line crosses over a public road, canal, or railway, the angle between the Hne and the direction of the road, canal, 'or railway at the place of crossing shall not be less than 60° and the height of the line not less than 25 feet. (23) Where the Hne crosses over a public road, canal or railway, or runs parallel to it at a distance less than one and a half times the height of the highest Avire from the ground, it shall be erected in a manner approved by the Board of Trade. Where for the protection of his electric lines or works the Postmaster-General makes requirements which, in addition to protecting these Hnes or works, afford ample provision for securing the safety of the public, further protection need not be provided. (24) Provision shall be made by earthing brackets or wires, or other device, to ensure that in the event of a failure of a conductor or of a pole, the line will be put to earth. OVEEHEAD TEANSMISSION LINES 287 General. (25) Galvanised iron wire used for stays, cradles, or other mechanical purpose, galvanised iron binding wire, arm bolts, nuts and washers, stay swivels, truss and brace rods and truss ties, tie and brace bolts, stay rod tighteners, and test pieces shall conform with the British standard specification for such material (British standard specification of telegraph material) so far as that specification is applicable. (26) The work shall be carried out, so far as circumstances permit, in accord- ance with the Post Office Technical Instructions for the Construction of Aerial Lines. (27) Where the line crosses, or is in proximity to, any other line or metal, precautions shall be taken by the undertakers against the possibility of a con- ductor coming into contact with the other wire or metal or the other wire or metal coming into contact with the line by breakage or otherwise. (28) Every Mne, including its supports and all the structural parts and electrical appliances and devices belonging to or connected with the line shall be duly and efficiently supervised and maintained as regards both electrical and mechanical conditions. (29) Every line, including its supports, will be removed on ceasing to be used for the supply of energy unless the Board of Trade are satisfied that it is to be again brought into use for such supply within a reasonable time. (2) Regulations of the Verband Detjtscher Elektkotechniker FOR Overhead Transmission Schemes. § 22. Overhead Lines : General. (a) Unearthed overhead lines must only be run on porcelain insulators or equally good insulators. (6) Overhead lines and apparatus connected with them must be so arranged that they cannot be reached from the ground, roofs, outhouses, windows, or other places open to the pubhc, without special appliances ; at road crossings, especially, they must be kept sufficiently far from the ground or be otherwise protected against contact. (1) Unprotected overhead lines for high-tension currents must, as a rule, not approach nearer than 6 metres to the ground, and in the case of public roads not nearer than 7 metres. (c) Supports and protective coverings for overhead fines at more than 750 volts to earth must be marked by a red zig-zag arrow. (d) Lines, protective screens and their supports must be sufficiently strong to resist wind pressure and snow loads. (2) Overhead fines may carry greater currents than indoor ones as long as their mechanical strength is not thereby affected. (3) For dimensions, etc. of overhead fine structures see the " Standards for Overhe.ad Lines," below. (e) According to the nature of the districts traversed, overhead lines, and also the generators, motors, and transformers connected to them, must be suitably protected against lightning. The fightning arresters used must remain operative even after repeated discharges. (4) When different phases or poles are protected by neighbouring lightning 288 OVEEHEAD TEANSMISSION LINES arresters care must be taken that dangerous voltages are not set up in any ground exposed to traffic because of the position of the earth plates. (/) Overhead high-tension lines must be bare, but where they are liable to be attacked chemically a suitable paint may be used. (g) In the case of overhead lines for over 10,000 volts all iron masts, stay "wires, etc., must be well earthed, if necessary by means of an earth wire run parallel to the supply line. Stay wires on wooden poles must either be earthed or provided with reliable strain insulators at points out of reach from the ground. (h) When overhead lines rim parallel to other lines or cross them suitable steps must be taken to prevent mutual contact, even in case of a wire breakage, or the contact must be made harmless, or else the whole construction must be carried out with an increased degree of security. (i) When a telephone line is run on the same poles as an overhead high- tension line it must either be arranged so that a dangerous voltage cannot be induced in it or else it must be itself treated as a high-tension Une. Telephone exchanges or call offices must be so arranged that even in the event of contact occurring no danger can result to the speaker or operator. (k) When a high-tension line passes over inhabited districts or when it approaches a public road sufficiently closely to involve danger to the passengers should a wire break, the line must either be carried at such a height that if a wire breaks the ends cannot reach within 3 metres of the ground, or else devices must be introduced to prevent the broken wire from falHng or to make it dead in the act of falling, or, finally, the whole structure may .be carried out with a suitable degree of increased security. (5) If safety nets are employed with a high-tension line they must be so shaped and situated that contact between them and a wire still intact is impossible and that a broken wire wUl be caught even if a strong wind is blowing. If the net cannot be definitely earthed it must be well insulated. (6) At corner points in a high-tension line guard hoops must be employed, which will prevent the faUing of the wire if an insulator breaks. (l) It must be possible to make any section of a high-tension line within inhabited districts " dead " if desired. STANDARDS FOR OVERHEAD LINES. I. Conductors. (a) Materials. Soft annealed copper wire may only be used for overhead lines if it is not subjected to a greater stress (see (b) ) than 5 kg. per square millimetre (7,200 lbs. per square inch). For hard drawn copper wire the maximum stress (see (6) ) must not exceed 12 kg. per square millimetre (17,200 lbs. per square inch) unless special tests are carried out on the material, in which case a stress amounting to half the elastic lim it may be employed. Copper may only be considered as hard drawn if the stress at the elastic limit is at least "8 of the breaking stress and if the elongation on a length of thirty-five times the diameter amounts to at least 2 per cent. In the case of stranded cables these figures apply to the separate wires. OVERHEAD TRANSMISSION LINES 289 Hard drawn copper wires may only be soldered at points where they are relieved of all tensile stress. For aluminium wire a stress of 9 kg. per square millimetre (12,900 lbs. per square inch) is permissible. When other materials are used the allowable stress must be determined by finding the ratio of elastic limit to breaking stress by tests, and in any case, the working stress must not be allowed to exceed half the elastic limit stress. (b) Stress Calculations. The stress in the wire is to be calculated first for a temperature of — 20° C. without additional load and then for a temperature of — -5° C. with an additional load due to ice. The weight of ice is to be taken as equal to -015 x q kg. per metre run of line, q being the cross-sectional area of the line wire in square milli- metres. In neither of these calculations must the stress work out at more than the limiting values given under (a). II. Poles and Masts. (a) Wooden Poles. The distances between poles must not exceed the following values : — Total Cross-Section of all the Lines and Guard Wires. Length of Span . 105 square milUmetres (-164 square inch) . . . .80 metres (260 feet). 105 to 210 square millimetres (-164 to -328 square inch) . 60 metres (196 feet). 210 to 300 square millimetres (-328 to -465 square inch) . 50 metres (163 feet). Over 300 square millimetres (over -465 square inch) . . 40 metres (130 feet). For these spans the diameter at the top of the poles Z should comply with the following rule : — Diameter of top in cms. = Z = 1-2 V D X H, where D = sum of the diameters of aU the wires on the pole in millimetres, and H = mean height of the wires above the ground in metres. Poles of less than 13 centimetres in diameter are not permissible. For high voltages up to 1,000 the minimum allowable diameter is 15 centimetres and for still higher voltages 18 centimetres. With greater distances between poles than in the above table either the pole diameters must be increased or coupled poles or wooden structures must be used. At curves ani at crossings with other electric lines, railways or roads the length of span must be specially shortened to suit the circumstances. In designing the pole structures in such cases a maximum stress of 70 kg. per square centimetre (1,000 lbs. per square inch) must be allowed. The maximum wind pressure is to be taken as 125 kg. per square metre of normal effective surface (25-6 lbs. per square foot). In the case of cylindrical bodies the effective surface is to be taken as -7 times the diameter multiplied by the length. O.T.L. u 290 OVEEHEAD TRANSMISSION LINES (6) Wrought-iron Masts. The maximum working stress in iron structures is not to exceed 1,500 kg. per square centimetre (21,500 lbs. per square inch). The most unfavourable case is that occurring with a wind pressure of 125 kg. per square metre of normal effective surface. For cylindrical surfaces the effective surface area is to be taken as -7 times the diameter multiplied by the length. (c) Masts of Special Materials. Masts of special material may be loaded up to one-third of the breaking stress guaranteed by the makers. In the case of cast-iron structures the maximum stress is, however, not to exceed 300 kg. per square centimetre (4,300 lbs. per square inch). (d) Erection of the Masts. Poles and masts must be let into the ground to depths depending on their length and on the nature of the soil (in average soil the depth is usually 1-5 to 2-5 metres). The soil must be well punned (in soft soil special precautions may be necessary) and the mast must be specially anchored or strutted at corner points. In the case of road crossings with high-tension lines a mast must be placed close to the road on each side and falling of the masts must be prevented as far as possible either by their construction or by means of suitable stays or struts. When both sides of a road are available for mast erection the east side should preferably be chosen, because then the prevalent westerly storms will not be able to blow down the masts across the roadway. When wooden poles are used in especially stormy districts every fifth mast should, even on straight stretches, be either specially strengthened or provided with stays so as to avoid the likelihood of masts falling on the roadways as far as possible. III. Special Regulations to be complied with when Safety Nets ARE to be omitted. When the use of safety nets is to be avoided by employing specially con- servative structures as indicated in § 22 {h) and (k), the lines must only be sub- jected to haK the stress permitted under I. {a). At the same time the masts lying between spans of unequal tensions must be designed to withstand the maximum one-sided pull. By specially shaping the insulators or specially attaching the line wires or by other suitable devices provision must be made so that, in case an insulator breaks, the wire will not fall or will, at least, be earthed before falling. INDEX A POLES, 59 fixing, 98 foundations for, 98 method of construction, 109 Active earth pressure, 80 Additional ice and snow load, 17, 123 Aesthetic requirements, 239 Agreement, draft, with local authority, 224 with contractor, 272 Allowable stress in masts, 39, 290 Aluminium, allowable stress in, 4 and copper compared, 4, 5, 6, 8 bi-metallic clamp for, 159 binding in of, 150 effect of atmosphere on, 7, 159 erecting, 204 mechanical joints in, 158, 159 modulus of elasticity of, 4 physical constants of, 4 soldering, 158 specific conductivity, 4 thermal expansion coefficient, 4 wii-e table, 8 Anchor plates, construction, 96 Anchoring masts, 96 Angle of repose of soil, 80 Arbitration for damage in erection, 223 Arcing at bad earth connection, 168 Arcs on insulators, guard ring protection, 147 Arms, cross, 73, 76, 111, 112 Arrangement of wires on poles, 160 Artificial straining of lines by weights, 123 Atmosphere, effect on aluminium, 7, 159 B. Base plates for struts, 63 Beams for constant bending stress, 46, 47 Bending and compression combined, 44 moments, 45, 46, 47, 182 stress on masts, 38 Bi-metallic clamp for aluminium, 159 Bi-metallic wire, 9 Binding-in, danger to cattle from binding wire scrap, 152 detailed instructions for, 150 materials for, 149, 150 to insulator, 149 Birds, perches provided for, 133, 160, 170, 196 settling on cross-arms, 115 Board of Trade regulations for, erection and operation of overhead lines, 285 height of wires from ground, 160 wind pressure allowance, 34 wooden poles, 39 Brackets, wall, 242 Branch connections, 153 Branch line junctions, 259 Breakage of wire, one-sided, 39, 79 Bridge structure for safety crossings, 133, 174 Bronze, allowable stress in, 6 long spans of, 7 physical constants of, 6 Brush discharge between lines, 160 Buildings, crossing of, by H.T. lines, 210 C. Cable, easier to erect, 208 preferable to solid wire, 3 span used with insulated, 243 telephone, sag due to, 27 Caisson, use of, in excavating, 197 Candle power of street lamps, 267 Capacity, electrostatic, of line, 160 Cardan's method of solving cubic equations, 19 Carriage of masts, 198 Catenary curve, 10 Cement mortar, 103, 104 Centre suspension of street lamps, 267 Channel iron cross-arms, 76 Charge for leave to erect mast, 223 Chemical effects on aluminium, 6 Clearance above roadways, 160, 285, 287 Clinometer, 215 Coating of tar for iron masts, 99 Coke used for earthing, 168 Collapse of struts, 44, 45, 51 Comparison of copper and aluminium, 4, 5, 6 Compass, telescopic, 216 Compression combined with bending, 44 Compression, law of, 44 Compressive stress on masts, 38 Concrete, allowable stress in, 94, 105 composition, 103, 104, 105 design of foundations of, 84 292 INDEX Concrete — continued. effect of various shapes of foundations, 84 foundation work, 103, 104 fraud in connection with, 106 in damp situations, 197 in frosty weather, 106 interference with agricultural work, 99 not suitable for wooden poles, 98 precautions necessary with, 103 preparation of, 104 properties and method of hardening, 103 ramming, 104 reinforced for mast feet, 99 substitutes for, 100, 102 use under water, 105 water collection on, 99 Conductivity, aluminium, 4 bronze, 6 copper-clad steel, 7 specific, of copper, 3 steel, 7 Conductor, aluminium, 4 . bi-metallic, 9 bronze, 6 copper, 3 copper-clad steel, 7 insulated copper, 8 materials for, 3 steel, 7 Constant stress in overhead lines, 123 Contraction, elastic, effect on sag, 17 Contractors, agreement with, for erection, 276 Copper, allowable stress in, 3 compared with aluminium, 4 insulated cables, 8 medium and hard drawn, 4 modulus of elasticity, 3 physical constants of, 3 soft, 4 specific resistance, 3 thermal expansion coefficient, 3 wire table, 8 Copper-clad steel (Monnot metal), 3 Copper sulphate treatment for poles, 107, 108 Corner poles or masts, attachment of wires to, 149 design of, 38 load on, with aluminium, 5 load on, with copper wire, 5 on distribution systems, 266 resultant pull on, 191 stress in, 41 Cost, effect of length of span on, 22^ effect of type of mast on, 236 of concrete foundations, 86 of erection work, schedule of, 277, 282 of impregnatuig wooden poles, 110 Cost — continued. of local distribution schemes, 269 of Mannesmann tubes, 118 relative of copper and aluminium, 6 relative of copper and aluminium, curve, 6 Couplings, automatic Ime disconnecting, 123, 171 Crane, derrick, use of, in erection, 201 Creosoting wooden poles, 107 Crippling stress, on masts, 38 on struts, 44, 45, 61 Critical length of span, 18, 26 of struts, 45, 61 Cross-arms, attachment of, 115 bar iron, 115 bent, 116 channel iron, 112 design of, 73, 76, 111 for strain insulators, 112 on roof standards, 251 stiffening, 116 stress on, 185 Crossing or transposition of wires, 164 Crossings, arrangement of wires at, 160, 169 at right angles and on slant, 210 bridge structure for, 133, 174 complete calculation for, 177 of hollows, sag calculation, 19 over land or houses, 222 reducing number of road, 260 road, height of pole for, 23 river, stress on masts at, 42 rivers, 127, 130, 133 safety, 133 Cross -shaft, 213 D. Dangerous section of poles, 59, 61, 109 " Dead end " masts, 38 design of, 69 " Dead ending " a line, 158 Deflection, amount of at top, 68 calculation of, 72, 185 of masts through line breakage, 38 of wire through wind, 160 Depth of, poles in various soils, 96 wooden pole in ground, 90 Derrick crane, use of, in erection, 201 Deviation of wire through wind, 160 Diagonals of iron masts, 66, 68, 69, 71 Dial, miners', 216 Diameter of iron roof standards, 245 of poles, 50, 52 INDEX 293 Dip, or sag of line, 10. See Sag. Distance between street lamps, 267 between wires, 160 Distributing point, fuses a^, 256 masts, 43 ring cross-arms at, 256 switches at, 256 Distributing systems, local overhead, 239 Double insulators, for line transposition, 164 Double poles of A form, 59 of H form, 60 Draw-tongs, use of in erection, 204 Drilling holes for poles, 197 Duplex suspension insulator chains, 196 Dynamometer for tension measurements, 204 E. Earth plates, 167 Earth pressure, active and passive, 80 lateral, 80 specific, 81 Earth, specific weight of, 82 Earthing, bad, cause of arcing, 168 by band iron electrodes, 168 by iron pipe, 167 by overhead wire rope, advantages, 211 by through -running cable, 168, 211 by use of coke, 167 by use of salt, 168 by wire netting, 167 eyes, 170 hoop guards, 196 loops, 196 nets, 171 object of, 166 of neutral on three-wire system, 166 of roof standards, 244 of stay wires, 98 plate and wire connection, 166, 167 plate material, 167 wire, laying, 166 wire, protection of, 167 Economical span, 226 Elastic contraction, effect on sag, 17 English law on overhead lines, 286 on postal lines, 225 on wayleaves, 222 Erection of line wire, 202, 209 of poles and masts, 197, 198 on private property, 222 on public property, 224 Euler's formulse for struts, 45 Examples. See " Numerical examples. ". Excavation, shape of, for masts and poles, 197 Excavation work, for poles and masts, 197 use of caisson for, 197 when water is present, 197 Exceptionally heavy ice and snow load, 123 Expansion coefficient, thermal, of aluminium, 4 of bronze, 6 of copper, 3 of copper-clad steel, 7 of steel, 7 F. Factor of safety, definition, 44 in concrete, 95 in iron masts, 121, 184, 286 in joints, 153 in line, 3, 4 in special safety suspensions, 195 in wooden poles, 53, 54, 286 Falling wires, danger from, 169 Faults to be looked for in new wire, 202 Feet, deterioration of wooden, 115 for masts, 115 iron, 116 replacing rotted, 116 reinforced concrete, 115 Fixing double poles of A and H form, 98 iron masts by foundation bolts, 99 poles in the ground, 96 stay wires, 96, 98 struts in the ground, 96 Flexible supporting masts, 38, 78 Foundation blocks, composition, 103 concrete, 84 forces in, 93 Foundations, bolts for fixing iron masts to, 99 depth of, for wooden poles, 90 erecting masts on finished, 201 for masts, 78, 82, 84, 90 strengthened with broken stone, 96 Frost, effect on foundations, 99 effect on stress in wires, 16, 123, 180 Frequency of line vibrations, 206 Fuses at distributing points, 256 for portable motor plugs, 261 for street lamps, 266 G. Galvanised iron and steel wire rope, 9 Galvanising iron structures, 123 Gear, lowering, for street lamps, 267 294 INDEX German law on postal lines, 177, 225 on wayleaves, 222 German rules for line erection. See Verband Deutscher Electrotechniker. Gould's safety coupling, 170 G.P.O. rules for crossings, 160, 169, 177, 225 for wind pressure, 34 for wooden pole erection, 39 Ground, depth of pole in, 90, 96 Ground pressure, allowable, 78 lateral, 80 specific, 81 symmetrical and unsymmetrical, 79 under base plate, 63 Grouping of wires on masts, 162 Guard ring protection for insulators, 145 H. H POiE, 60, 109 fixing, 96 foundations for, 98 relative strength, 109 Half-time street lighting, 262 Hard drawn copper, 4 weakened by soldering, 153 Height of line above roadway, 160, 260, 285, 286, 287 of mast requu-ed at crossing, 181 of street lamps, 267 Hesse's safety coupling, 170 Hexagon arrangement of wires on mast, 163 Hinged feet for iron masts, 133 High-tension lines crossing buildings, 210 Hoop guard for falling wires, 196 Horizontal distance between wires, 160 table of, 161 House service wires, 257, 259 Humming, of wires, 244 packing to prevent, 247 Ice, additional load due to, 17, 177 effect on sag, 16, 180 load, exceptional, 123 Impregnating wooden poles, 107 Independent poles, 50, 52, 56 Inductance of lines, 160 Induction, self and mutual, 162 Inertia, moment of, 46 tables of, 48, 49 Inhabitants, number of, related to cost of distribution scheme, 271 Insulated copper conductors, effect on steel supporting rope, 27 sag and stress in, 25 table of, 8 Insulator pins, 73, 111 attachment of insulators to, 147 conical, 113 cylindrical, 113 for top of pole, 113 load tests on, 114 swan-neck type, 112 Insulators, attachment, instructions for, 147 to pins, 147 with cement, 147 with hemp, 147 Delta type, 140 development from telegraph insulators, 140 disadvantages of glass, 134 double, for line transposition, 164 double petticoat type of, 140 for insertion ia stay wires, 98 for leading-in wires, 257 for oveihead lines, 134 grouped at distributing points, 258 increase of weight with volts, 141 inverted type, 140, 252 kind of porcelain used, 134 leakage losses from, 136 mechanical properties of, 136, 139 mounting suspension type of, 148 other materials for, 139 protection of, with guard rings, 145 replacement of, 148 self-cleaning, 139 suspension, , arrangement on mast, 133 as strain insulator, 143 chain type, 143 examples, 144 metal cap type, 143 on corner masts, 144 type of, 139, 142 type, shapes of, 143 triple safety suspension type of, 173 trouble with large sizes of, 140 Interference of H.T. line with telephone line, 164 Intermediate or supporting masts, 38, 209, 228 Inverted insulators, 140, 252 Iron ropes, galvanised, 9 Iron masts. See Masts. Iron-work towers. See Lattice-work towers. Jaws for straining up wires, 204 INDEX 295 Joints, at '' dead ends," 158 careless work at, 157 conical grip type of, 155 factor of safety at, 153 in aluminium, 6, 158 ia new wire, 202 made by terminals, 259 mechanical in aluminium, 159 oxidation of, 158 resistance of, 153 rivetted, 155 soldering, 153, 158 spiral metal sheath, 153, 155 strength of, 154 — 157 tension at, 153 with branch lines, 153, 259 K. Kinks, removal of from wire, 208 Kyanising wooden poles, 107 Lighting, arrangement of leads for, 263 candle power of lamps for, 267 distance between lamps, 267 fuses for, 266 height of lamps, 267 street, 242, 262 time switches for, 263 Lightning protection by earthed roof standards, 244 by overhead earthed wire, 211 with suspension insulators,. 144 Line. See Wire. attachment to insulators, 149 Line breakage, deflection of mast by, 38 disconnecting couplings for, 123, 171 Local authority v. private company, 270 Local overhead distributing systems, 239 cost of, 269 Long spans in bronze wire, 7 in steel wire, 7 Loop, earthing, 196 Lowering gear, 268 Labels for masts, 107, 121, 198 Lamps. See Lighting. Lamps, street, 243, 262 Lateral bending stress, 38, 44, 51 Lateral earth pressure, 80 Lattice-work iron masts, 118 calculation of, 41, 66, 181 construction of, 121 tapering, 121 Lattice-work structures or towers, 118, 121 allowable stress in, 121 cost of, 122 examples of, 127, 130, 133 factor of safety in, 121, 184 flat, 122 hinged feet for, 133 quadrangular, 122 rivet hole preparation, 121 Law on overhead lines, 285 on Post Ofiice lines, 225 on way leaves, 222 Laying out the route of a line, 212, 219 Leading-in wires, 257 Length of span, critical, 18, 25 in local distributions, 243 most economical, 226 with cables, 243 Length of wire, for given span, 11 manufactured, 202 Level, telescopic, 213 Levelling staff, 212 Life of wooden poles, 107, 236 M. Main stays of iron masts, 66, 70 Mannesmann tube masts, 118 Maps, use of, 219 Masts, allowable stress, 121 anchoring, 96 angle iron, 95 annual charges for, 223, 233, 236 carriage of, 198 construction, 121 corner, 38 cost of, 122, 223 " dead end " or strain, 38 deflection of through line break, 38, 68 design of, 66, 69, 181 erection of, 197 factor of safety, 121 feet for, 115 fixing by foundation bolts, 99 flexible, 38, 78 foot protection, 99 for local distribution systems, 241 for suspension insulators, 133 foundations, 78 galvanising, 123 lattice work, 118, 122 Mannesmann tube, 118 multiple tubular, 118, 121 on private property, 222 on public property, 224 painting, 123 rivet holes for, 121 simple tubular, 118 spiked collars for, 121 298 INDEX Schedule of prices for erection work, 277 Screens, safety, 133 Section switches, use of, 211 Service, house, wires, 257 Shearing stress, 46 Single phase line, arrangement, 163 Slope, natural, of soil, 80 Snow, additional load due to, 17, 123 Soldering, aluminium, 158 composition used for, 158 flux for, 158 on roofs, 253 relative strength of joints, 156 weakening of H.D. copper by, 259 Soft copper, 4 Softness of aluminium, 5 Soils, allowable pressure on, 78 angle of repose of, 80, 82 speoiflc gravity of various, 82 Solid wires and cables compared, 3, 4 Spans, critical length of, 18, 25 length of wire for given, 1 1 long, in bronze and steel, 7 most economical, 226 usual in local distribution systems, 243 Specification for erection of overhead line, 272 Specific conductivity of aluminium, 4 of bronze, 6 of copper, 3 of copper-clad steel, 7 of steel, 7 Specific gravity of aluminium, 4 of bronze, 6 of concrete, 84, 185 of copper, 3 of copper-clad steel, 7 of soils, 82, 185 of steel, 7 Specific resistance of copper, etc., 3, 4, 6, 7 Stability of poles and masts, 78 Standards, iron roof, 242 wall, 245, 250 Standing boards on roofs, 249 Stayed poles, 56 Stay tighteners, 65 Stay wires, earthing, 98 coUar for, 64 flxiag, 96 forces on, 57, 64 for roof standards, 250 insulators for, 98 Steel, allowable stress in, 7 long spans in, 7 Steel — continued. modulus of elasticity of, 7 physical constants of, 7 specific conductivity of, 7 thermal expansion coefficient of, 7 wire rope, 9 wire rope supporting cable, 27 Steel measuring tape, 212 Straight line arrangement of three-phase con- ductors, 163 Strain insulators, 98, 143, 158 Strain masts, 38 design of, 69, 78 occasional, in wooden pole line, 209 Straining lines artificially by weights, 123 Straining up the line wire, 204 Stress, allowable in, aluminium, 4, 285, 289 bronze, 6 cast-iron structures, 290 concrete, 94 copper, 3, 285, 288 copper-clad steel, 7 insulated cables, 25 soil, 78 steel, 7, 9 wood, 56, 289 wrought-iron masts, 290 and sag curve for H.D. copper, 28, 29, 179 and sag tables for H.D. copper, 30, 31, 32, 33 bending, 45 compressive, 44 compressive combined with bending, 44 crippling, crumpling, or lateral bending, 44 for equal sag at - 5° C. and + 40° C, 22 on independent pole, 50 on maste, 38 on wall and roof standards, 245 sheariag, 46 tensile, 44 torsional, 46 Stretching screws, 65 Street lighting, 243, 262 Struts, angle to pole, 96 collapse of, 45 critical length of, 45, 51 Euler's formulae for, 45 fixing in ground, 96 for roof standards, 250 Strutted poles, 58 Supporting structures, design of, 38 flexible, 38 forces on, 38, 78 galvanising, 123 protecting wilii paint, 123 Surface water rising, effect on foundations, 81 INDEX 299 Suiveying instruments, 212 Surveying the route of a line, 219 Suspension, central for street lamps, 267, 268 triple insulator type of, 173, 177 Suspension type insulator, 142 advantages of, 142 at corner points, 144 chain type, 143 erection of line with, 204 lightning protection through, 144 masts for, 133 metal cap type, 143 shapes of, 144 used for high voltages, 139, 140 Swan-neck insulator pins, 112 collars for, 251 Swing, frequency of line, 206 of line due to strong wind, 161 synchronous, of various lines, 160 Switches, at distributing points, 256 for part-time lamps, 262 time, 263, 264, 265 Switchiag stations, position of, 209 Symmetrical ground pressure, 79 Synchronous swinging of liues, 160 Tables, No. 1, p. 3 ; No. 2, p. 4 ; No. 3, p. 6 ; No. 4, p. 7 ; No. 5, p. 7 ; No. 6, p. 8 ; No. 7, p. 8 ; No. 8, p. 9 ; No. 9, p. 30 ; No. 10, p. 31 ; No. 11, p. 32 ; No. 12, p. 33 ; No. 13, p. 37 ; No. 14, p. 44 ; No. 15, p. 47 ; No. 16, p. 48 ; No. 17, p. 82 ; No. 18, p. 104 ; No. 19, p. 105 ; No. 20, p. 105 ; No. 21, p. 109 ; No. 22, p. 110 ; No. 23, p. 119 ; No. 24, p. 122 ; No. 25, p. 122, No. 26, p. 141 ; No. 27, p. 141 ; No. 28, p. 153 ; No. 29, p. 154 ; No. 30, p. 156 ; No. 31, p. 157 ; No. 32, p. 157 ; No. 33, p. 161 ; No. 34, p. 188 ; No. 36, p. 189 ; No. 36, p. 190 ; No. 37a, p. 192 ; No. 37b, p. 194 ; No. 38, p. 207 ; No. 39, p. 221 ; No. 40, p. 228 ; No. 41, p. 235 ; No. 42, p. 238 ; No. 43, p. 246 ; No. 44, p. 246. Tacheometer theodolite, 215, 218 Taper in masts, 121 in poles, 50, 56, 107 Tar coating for iron, 99 Telescopic compass, 216 Telescopic level, 213 use of, in sag measurement, 215 Telephone cables, avoidance of interference with, 164, 211 erection on steel rope, 205 sag in steeLxope supporting, 27 Temperature, effect on sag and tension, 16 minimum to be allowed for, 17 Tensile stress. See Stress. Tension and sag, adjustment of, 205 curves for, 28, 29, 179 effect of temperature on, 16 in conductors, 10 Terminal clamps, for joints, 163, 259 for wires, 160 Terminal masts, 38 design of, 69 Theodolite, 216 construction of, 216 measurement of angles by, 216 measurement of distances by, 218 measurement of heights by, 217 tacheometer type of, 215 Thermal expansion coefficient, of aluminium, 4 of bronze, 6 of copper, 3 of copper-clad steel, 7 of steel, 7 Thermometer, use of, in erection, 205 Three-phase lines, arrangement, 163 Through -runniag earthing wire, 168 Time switches, 263 Tipping slope for mast erection, 200 Tools, list of, for overhead line work, 282 Torsional stress, 46 Towers. See Lattice-work towers. Transformer stations, position, 209 Transposition of wires, 164, 211 Trap doors in roofs, 247 Trees, avoiding, in line erection, 210 Triangular arrangement of three-phase lines, 163 Triple insulator suspension, 173 Truss rod of H. pole, 60 Tubular iron masts, 118, 121 allowable pull on, 245 resisting moments of, 246 Turntable, use of, in line erection, 202 Twisting stress, 38, 46 Tying-in wire for insulators, 149 U. Ulbeichts' safety network conductor, 133 Unsymmetrical ground pressure, 79 V. Velocity of wind, relation to pressure, 35 Verband Deutscher Elektrotechniker, collected regulations on overhead line work, 287 , rules on — clearance of line above ground, 1 60 diameter of wood poles, 51 800 INDEX Verband Deutscher Elektrotechniker — eon- tinued. rules on — continued. earthing of stay wires, 98 joints in overhead liaes, 158 maximum span with wooden poles, 209 minimum clearance of broken wire, 174 smallest allowable wire, 261 specially secure construction, 173 stress in aluminium wire, 4 stress in copper wire, 3 stress in iron structirres, 121 stress on poles, 39 temperature conditions for stress calculation, 17 wind pressure, 34 Yertical distance between lines, 160 Yibration frequency of line, 206 W. "Wall brackets, iron, 242, 245 "Wall standards, 245, 250 "Water collection on concrete, 99 "Water-tight joints in roof, 246 "Wayleaves, obtaining, 210 English and German law on, 222 "Weight of insulators, 142 "Weightiag lines at ends for constant sag, 123 "Whirlwinds, effect of, 161 "Wind, Board of Trade rules for, 34 effect on sag, 16 example of effect of, 36, 161 G.P.O. rules for, 34 Newton's law of pressure of, 35 pressure of, on line and on masts, 34, 182 pressure on copper and aluminium lines compared, 5 relation between pressure and speed, 35 V.D.E. rules for, 34 "Windlass, use of, in erection, 200 "Wire, attachment to insulator, 149 bi-metallic, 9 binding-in, 149, 152 binding-in aluminium, 150 breakage, effect of, 38, 79 clamps for, on suspension insulators, 152 insulated, 8 material for binding, 150 properties of — aluminium, 4, 8 bronze, 6 copper, 3, 8 Wire — eontinu ed . properties of — continued. copper-clad steel, 7 steel, 7 rope, 9 terminal clamps for, 152 Wires, arrangement of, above postal lines, 160 at crossings, 160 on poles, 160, 245 birds, effect of, on, 161 brush discharge from, 160 checking tension of, 205 contact between, 160 distance between, 160 electrostatic capacity of, 160 erecting aluminium, 204 erecting copper, 202 erecting on roofs, 254 erecting on suspension insulators, 204 experiments on safe distance, 161 failing, danger from, 169 for part time and whole time lighting, 262 frequency of vibration of, 206 grouping on masts, 162 horizontal distance between, 161 house service, 257 humming of, 244 inductance of, 160, 162, 163 jointing, 163, 259 manufactured length of, 202 removal of kinks in, 208 self and mutual induction of, 163 single-phase and three-phase, 163 straining up, 204 synchronous swinging of, 160 transposition of, 164 vertical distance between, 160 wind, effect of, on, 160, 161 Wooden poles. See Poles. Young's modulus of elasticity, for aluminium, 4 for bronze, 6 for copper, 3 for copper-clad steel, 7 for steel, 7 Zinc chloride impregnation of poles, 107 BRAHBIIRY, AGNtW, & CO. LD., PKINTEES, LONDON AND lONERIDOE.