0^ ■ssipfes fXlXi )WLE \S[ \ ^c 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/cu31924031489978 Cornell University Library arV18779 The protection of railroads from overliea i 3 1924 031 489 978 olin.anx BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henrg W. Sage 1891 .^.,.'2-.H<\.»4.>S.5- .»,S\v;.u^. 97S5-a The D. Van Nostrand Company intend this book to be sold to the Public at the advertised price, and supply it to the Trade on terms which Mrill not allow of reduction. THE PROTECTION OF RAILROADS FROM OVERHEAD TRANSMISSION LINE CROSSINGS THE PROTECTION OF RAILROADS FROM OVERHEAD TRANSMISSION LINE CROSSINGS By FRANK F. FOWLE, S.B. Consulting Electrical Engineer NEW YORK D. VAN NOSTRAND COMPANY 23 Murray and 1 9^9 27 Wabren Sts. T PE -4i / / ht'Y 5 5 Copyright, 1909, fiy D. Van Nostrand Co. TAe PUmpton Press Norwood Mass. V.S.A. PREFACE The rapid progress made in the development of high- tension power transmission has given rise to a number of new problems which, though of secondary importance to that of transmission, are quite important to the electrical field as a whole. One of the most important of these is the problem of safeguarding persons and property from exposed transmission lines. It is coming to be regarded more and more that high-tension transmission lines are agencies of the most dangerous character and an increas- ing amount of attention is being paid to the subject of protection at all places of hazardous exposure. The general problem of protection is a broad one and naturally resolves itself into a number of typical cases. The protection of railroad property and the lives of rail- road employees at overhead crossings of high-tension lines forms one of these cases and is the only typical case consid- ered in this volume. The subject-matter was not prepared with the original intention of publication in book form, but was arranged for presentation before the Association of Railway Telegraph Superintendents at their annual meet- ing of 1908. The author does not believe it feasible to prepare a general specification which will cover all crossings of high- tension lines with steam railroads, but considers it prefer- able to indicate or outline a form of specification which shall cover the general principles involved, the materials to be used, and the types of construction, without going into VI PREFACE such details as naturally pertain to individual crossings. The latter must necessarily be worked out for each individ^ ual case by those familiar with the local conditions. The subject has been treated along the lines of this policy. Much of the treatment appUes to other typical cases in the general problem of protection, but the other cases were not considered here, owing to the fact that the material was arranged for presentation to steam railroad ofScers prima- rily. Should the reception accorded this volume indicate sufficient general interest in the subject, the author will gladly attempt to enlarge the treatment at some future time. Owing principally to the fact that the subject-matter was not completed in time for presentation at, the 1908 annual meeting, arrangements were later completed for publica- tion in this form. The author's thanks are due especially to Mr. John L. Davis, Superintendent of Telegraph of the Chicago and Eastern Illinois Railroad, to Mr. Chas. Selden, Superintendent of Telegraph of the Baltimore and Ohio Railroad, and to Mr. E. A. Chenery, Superintendent of Telegraph of the Missouri Pacific Railway, for their active interest and cooperation in arranging for publication. Frank F. Fowle. Chicago, III., April 1$, 1909. CONTENTS PAGE Protection or Life and Property on Railroads i What the Bangers are ... .... .2 Effects of the Failure of a High-tension Transmission Line at A Railroad Crossing . . 7 Examples of High-tension Practice in Line Construction Failures of High-tension Lines Screen Protection . Bridge and Catenary Types of Reinforced Crossing . Underground Crossings ... Discussion of Proposed Type of Crossing . ... Typical Crossing .... ... .... 9 IS 30 37 43 45 68 THE PROTECTION OF LIFE AND PROPERTY ON RAILROADS FROM OVERHEAD CROSSINGS OF HIGH-TENSION POWER TRANSMISSION LINES A Paper Prepared for the 2'jih Annual Convention of the Association of Railway Telegraph Superintendents at Mon- treal, June 24 to 26, 1908. The subject of protection to life and property on rail- roads from overhead crossings of high-tension power trans- mission lines is one of growing importance to the members of the Association, and with the permission of the Chairman of your Topic Committee the writer has therefore deviated from the subject of "The Telephone," which was originally assigned to him by the Committee, in order to present for your consideration a new type of protected high-tension crossing. This type secures the desired elements of pro- tection through mechanical and electrical reinforcement of the power line and the use of incombustible materials throughout its construction; it is a complete departure from the screen or basket network form of protection now in considerable use. It is the purpose of this paper to show that the type of crossing which the writer proposes not only affords all the protection which can be reasonably demanded, but that its construction is in general conformance with the latest and most approved practice in the construction of high-tension power transmission lines; and therefore that it promises to be a type upon which both power and telegraph 2 HIGH-TENSION POWER TRANSMISSION LINES engineers can agree with mutual satisfaction and con- fidence. At the present time there is a wide diversity of practice among the raibroad companies throughout the coimtry in protecting such crossings, which ranges between the extremes of no protection whatever and ths provision of an overhead steel bridge structure which supports the high-tension wires over and across the right-of-way. High-tension trans- mission lines are now coming to be universally regarded as highly dangerous and extra hazardous in character, and the best engineering practice recognizes this danger and pro- vides every reasonable safeguard against accidental contact between such lines and all other objects, persons and prop- erty. Telephone engineers were early in recognizing the danger, when the art of long-distance power transmission was in its early stages and operating voltages did not exceed 10,000 to 12,000 volts; they devised at that time the screen form of protection, before high-tension phenomena were known to any extent or the present construction methods had been developed. The transmission art has developed very rapidly during the past ten years, until at the present time potentials of 66,000 volts are in common use and thou- sands of kilowatts are transmitted over a single circuit. In consequence the dangers have greatly increased and it has become imperative that all the known safeguards and pro- tective devices be employed. WHAT THE DANGERS ARE The probability of fatal injury to human life from physical contact with a circuit whose potential exceeds 1,000 volts is well known to be very great; beyond 2,000 volts this proba- HIGH-TENSION POWER TRANSMISSION LINES 3 bility becomes very nearly a certainty, and it may be noted that the potentials employed in the electrocution of criminals do not exceed the latter figure. The probabiUty of serious personal injury from physical contact with a circuit whose potential exceeds 250 volts, and ranges anywhere between this value and the lower limit stated above, is also well understood. As regards death from electrical shock, there are many instances on record which were caused by 2,300- volt, alternating-current circuits; this voltage is now in wide use for primary circuits in city distribution systems for commercial light and power. The 3, 300- volt, alternating- current trolley system has been fatal in a number of in- stances. There is no sensible difference, so far as fatal consequences are concerned, between voltages of 2,300 and 23,000; both are so dangerous that they cannot be differently classed in this respect. There are on record a few cases of shock and burn from circuits of 20,000 to 30,000 volts potential, in which the victim recovered, but such instances are rare; in explanation of these it should be stated that the degree of injury is not measured by the voltage of the cir- cuit with which the victim comes in contact, but by the amount of current which his body receives. Among the records of serious injuries there are many cases of burns received from circuits of 500 to 600 volts direct current; this voltage is in wide general use for street, elevated and interurban electric railways and in many instances it has been fatal to animals. Mr. Wm. Carroll, City Electrician of Chicago, says in a recent report to the Council Committee on Finance,* in discussing the dangers from overhead elec- trical circuits, "There have been over seventy persons killed 1 Report of Mr. William Carroll, City Electrician, to Hon. Frank I. Bennett, Chairman Committee on Finance ; Chicago, 111., May 27, 1908. 4 HIGH-TENSION POWER TRANSMISSION LINES in this city during the past seven years from high-potential wires and apparatus." The writer, during a recent investi- gation in the State of Illinois, learned of many cases of death and injury, the former on circuits ranging from 2,300 to 33,000 volts and the latter from 500 to 33,000 volts. Whenever any electrical circuit receives, accidentally or intentionally, a voltage much greater than that for which it was designed, and in consequence currents greater than it can safely transmit, overheating and possible destruction of the circuit may result, with the possibility of setting fire to buildings and causing injuries to persons at various places along the circuit, even at places remote from the point of application of the dangerous and excessive voltage. Pro- tective devices of one or another sort are in common use to prevent such occurrences, but their operation is uncertain and sometimes ineffective. Wherever circuits designed and constructed to operate at low potentials are exposed to cir- cuits of much higher potentials in such a manner that physi- cal contact with the latter is not impossible, there is an element of danger. Telephone and telegraph systems are classed in the National Electrical Code as Signalling Sys- tems, and it is specified that in installations where the cur- rent carrying parts of the apparatus installed are capable of carrying indefinitely a current of ten amperes, the interior wiring shall conform in general to that for an electric light or power installation of 0-600 volts potential and be pro- tected by fuses rated at not over ten amperes; in case the apparatus cannot carry ten amperes indefinitely, an approved protective device must be placed as near as possible to the entrance to the building, with the wiring on porcelain knobs as far as the protector and a copper ground wire not smaller than No. 18 B. & S. gauge from the ground plate of the HIGH-TENSION POWER TRANSMISSION LINES 5 protector to earth or to water or gas pipes (see National Electrical Code, Class E, rules 64 to 67). The National Code gives the following classification of systems by volt- age:— "Low-Potential Systems, 550 volts or less." "High-Potential Systems, 550 to 3,500 volts." " Extra-high-Potential Systems, over 3,500 volts." Telephone and telegraph systems may therefore be properly classed as Low-Potential Systems. The protective device in common use consists of a fuse in each line wire and an arrester from line to ground, with the gap so pro- portioned that the arrester will operate at a few himdred volts, — 350 volts is the value used widely in telephone work, with fuses rated to blow at seven amperes or there- abouts. The writer does not beUeve as a general poUcy that such protective devices should be depended upon to protect human life, in the event of physical contact with systems whose potential exceeds 1,000 volts; the most that is usually claimed for them is 2,500 volts and at the latter voltage the device is sometimes destroyed in performing the function expected of it. The theory of their action is that a potential to groimd of 350 volts or more wiU break down the air gap to ground and estabhsh an arc, through which sufficient current will flow to operate the fuses and thus open the line circuit. Within the writer's experience, a cross occurred between a telephone line and one side of a 2,300- volt, alternating-current circuit, which established an arc to ground at the arrester; sufficient current did not flow to ground to blow the fuse and in consequence an office cable was burned out and several pieces of apparatus destroyed before any one could open the line at the arrester frame. The successful operation of these devices depends upon the 6 HIGH-TENSION POWER TRANSMISSION LINES ability of the ground wire to carry safely any current which the fuse can carry even momentarily, and it depends also upon the good condition of the ground wire at all times. If the foreign potential is sufficient to establish an arc through or around the fuse, the device fails entirely and destructive action will ensue. On account of the uncertainty of the operation of these devices and the fact that their operation further depends , upon their proper care and maintenance, which involves a human agency, the writer does not believe it to be policy to depend upon them for potentials exceeding I, coo volts. Few persons would care to submit themselves to the test of using a telephone protected by one of these devices, for the purpose of illustrating the effect upon both the apparatus and the user, of a cross between the telephone line and a 2, coo- volt power circuit, or one of only 1,500 volts. It is true that a cross may not in every case impress the full potential of the power circuit upon the telephone line, but it is not possible to predict in any case just what potential will be impressed and therefore the safe course is to assume the fuU potential. The writer therefore believes that 1,000 volts can be justified as the safe limit. Railroad rights of way are commonly occupied by tele- graph, telephone, and signal wires and cables upon one or more pole lines; the ownership of the telegraph and tele- phone wires is quite commonly divided between the railroad company and telegraph or telephone companies. There are in addition, at scattered points along the right of way, signals and signal structures, pipe lines and wires at signal interlocking plants, wooden trunking for signal wires, and electric light circuits, which may or may not be exposed to the danger of physical contact with high-potential wires. The failure of a transmission line at or near the point of HIGH-TENSION POWER TRANSMISSION LINES 7 crossing may permit one or more of the high-potential wires to fall, or to slack down so as to constitute an obstruction to railroad traffic; the several ways in which the Une may fail will be considered in detail later, but the results of such failures may be enumerated as follows: — THE EFFECTS OF A FAILURE OF A HIGH- TENSION TRANSMISSION LINE AT A RAILROAD CROSSING I. A high-potential wire may come into contact with the telegraph, telephone or signal wires and thereby spread a very dangerous and destructive potential over a large zone extending many miles from the scene of failure and inclu- ding many railroad and telegraph offices and many signals and their auxiliary apparatus. This potential may cause buildings and apparatus to take fire and it may kill employees at places so remote from the scene of failiure as to exclude any possibility of forewarning whatever; the lives of the public even may be endangered where public telephone lines are on the right-of-way. The signal system may be wholly or partially destroyed and the signal service interrupted for a period of several hours or days. II. A high-potential wire may fall across the tracks and thus interfere with railroad trafiic in both directions imtil a competent person can arrive to repair the line; in the meanwhile the lives of train crews and the traveling public may be endangered. III. A high-potential wire may slack down somewhat, and though not enough to blockade traffic it may not be high enough to clear the heads of trainmen on the tops of freight cars or trains and may thus endanger their lives. 8 HIGH-TENSION POWER TRANSMISSION LINES The occurrences enumerated above may result from elec- trical or mechanical failures of the high-tension wires or their supporting structures, in such a manner as to let the high-tension wires slack down or fall. In case a high- tension wire parts and both ends fall upon the railroad company's property, it may be noted that when the trans- mission line is supplying power to synchronous motors or rotary converters, both ends of the broken wire will be alive. The overload circuit-breakers which protect the generating stations from everloads and short-circuits on the power lines may, of course, prevent any serious results from a Une failure at a railroad crossing which occurs in such a manner as to produce a complete short-circuit; but the circuit-breakers will not act until the overload exists, and the property of the railroad company or the persons of their employees may be in the path of the short-circuit and subject to its destructive action. Moreover, a complete short-circuit wiU not be the result of every line failure and therefore the power circuit-breakers cannot be relied upon for protection to the railroad company's property and the lives of their employees. The failures enumerated above are neither remote nor conjectural as regards the probabihty of their occurrence with types of transmission lines which were standard in this country only five or six years ago and are stiU employed to a considerable extent. The art of long-distance power trans- mission has developed very rapidly in the last few years, until the standard practices of to-day contrast strongly with those of 1903 ; many Hues constructed at or prior to the latter date are still in operation and furnish excellent proof of the weaknesses which led to the abandonment of wooden pole construction and the adoption of metal construction through- out, as the most approved practice. HIGH-TENSION POWER TRANSMISSION LINES EXAMPLES OF HIGH-TENSION PRACTICE IN LINE CONSTRUCTION The transmission lines built in this country prior to 1903 employed wooden supporting structures entirely, and Fig. I. — 3 6, 000- volt, 3-phase transmission line; wooden poles, cross-arms and pins and glass insulators; single-strand copper wire No. 2 B. & S.; built in 1902. not only was wood regarded as an economical material but its insulating qualities were thought to be an advantage in operating high-tension lines. "Wood construction had been lO HIGH-TENSION POWER TRANSMISSION LINES found very satisfactory for the moderate voltages employed in the earliest transmission lines, but at that time the phenom- ena which occur with the high voltages and large energy Fig. 2. — i2,ooo-volt, 3-phase transmission line; wooden poles, cross-arms and pins and porcelain insulators; built in 1907. capacities now employed were unknown. Figures i and 2 illustrate the general type of line with wooden poles, cross- arms and pins, glass or porcelain insulators, and the wires secured to the insulators with tie-wires. The wires were HIGH-TENSION POWER TRANSMISSION LINES II usually single strand, sometimes as small as No. 4 B. & S. gauge. As transmission lines were constructed for higher and higher voltages, it was found in experience that severe arcing phenomena were encountered, which acted destructively on wooden structures. The burning ofE of pins, cross-arms and poles became of not uncommon occurrence. Much of this trouble arose from the use of line insulators which were not properly designed for the purpose to which they were put, lacking adequate mechanical and electrical strength. The development of the modern high-tension porcelain insulator produced an expensive type, very expensive indeed as compared with the type of insulator used for low-voltage work and for telephone and telegraph lines. It appeared desirable to reduce the number of insulators to a minimum and therefore to employ as long spans as possible. A study of the economy of such a procedure developed the modern type of high-tension line, with long spans supported on steel towers and metal construction throughout. A line of this character was constructed in Mexico in 1903 for the Guana- juato Power and Electric Company, and since that time numerous lines of the same general type have been con- structed in this country. Figs. 3, 4 and 5 show a recent type of transmission Une, which consists of stranded alumi- num cables supported on porcelain insulators; the poles, cross-arms and pins are steel and the foxmdations are con- crete. This line was erected by the Sanitary District of Chicago and transmits power from Lockport to Chicago, 111. A type of structure which has been used considerably is patterned after the galvanized steel windmill towers which are widely used in this country. These towers are built up from 12 HIGH-TENSION POWER TRANSMISSION LINES Fig. 3. — 44,000-volt transmission line, with steel towers, cross-arms and pins; 350-foot spans; erected in 1907. HIGH-TENSION POWER TRANSMISSION LINES 13 Fig. 4. — ■ Upper half of steel pole used in 44,000-volt trans- mission line. 14 HIGH-TENSION POWER TRANSMISSION LINES Standard steel sections, such as angles, flats, rounds, channels and pipe; the most economical design of tower is one in which the width at the base bears an approximate ratio to the height Fig. 5. — Another view of pole in Fig. 4. Note the ground wire at the apex of the pole. of one to four or five. A general discussion of tower design will be found in two papers by D. R. Scholes in the Trans- actions of the American Institute of Electrical Engineers, mentioned among other references at the end of this paper. General type of transmission line tower, for one circuit. Scale i Inch=i Foot, Fig. 7. General type of transmission line tower, for two circuits. HIGH-TENSION POWER TRANSMISSION LINES 1 5 An interesting paper by Guido Semenza in the Institute Transactions (see list of references) on European high- tension practice shows that the general design consists of iron or steel poles, cross-arms and pins, concrete founda- tions and porcelain insulators cemented to the pins; this style of construction obtained in Europe before it was adopted in this country. The Guanajuato line is a loi-mile transmission at 60,000 volts, on steel towers, 12 per mile, with stranded copper conductors equivalent to No. i B. Sz: S. gauge.. Mr. Norman Rowe, in a paper describing some experiences on this line with lightning, describes a change in its construction by which a ground cable was erected at the top of the line for protection against lightning, with generally satisfactory results (see lists of references). Fig. 6 shows the general style of tower used on the . Guanajuato line, and Fig. 7 a type of tower for two transmission circuits. Fig. 8 shows a steel terminal-pole for a high-tension line at the junction of the overhead line with underground cable; the pole is set in a concrete foundation. FAILURES OF HIGH-TENSION LINES The arcing phenomena which occur with very high volt- ages and the destructive character of such arcs when they occur upon circuits connected to sources of very great energy are such as to create dangerous exposures where high-ten- sion lines are carried upon wooden structures in proximity to low-voltage wires and on public streets and highways. These arcs are severe enough to burn off pins, cross-arms and poles where wooden construction is employed; not infrequently they melt the line or phase-wires of the trans- mission circuit and permit them to fall. Such destructive i6 HIGH-TENSION POWER TRANSMISSION LINES Fig. 8 — Riveted box-column type of steel pole. Note use of strain insulators supported upon through-pins attached to cross-arms above and below. HIGH-TENSION POWER TRANSMISSION LINES 1 7 arcs are frequently caused by lightning strokes, which puncture or destroy the insulators and permit the phase- wires to fall on the cross-arms, or which discharge over the surface of the insulators to the cross-arms and thence down the poles to ground, permitting the power current to follow behind the discharges and thus estabhsh arcs. Every volt- age has a definite striking distance through air, between clean needle points; but after the air has broken down and a discharge taken place, the initial resistance of the air to puncture has very greatly diminished and the discharge path has become conducting, so that the power current flows across between the needle points and forms an arc. Such an arc will not break until the needle points have been separated by a distance equal to many times their original separation; if the arc is maintained by a circuit fed from a very large source of energy, the amount of energy dissipated at the arc is correspondingly great and the arc becomes viciously destructive, easily destroying all inflammable materials and even fusing metals. The following table of striking distances in air for different voltages is the standard of the American Institute of Electrical Engineers. (See page 18 for table.) The above table shows the voltages necessary to break down the air dielectric and establish arcs, for distances up to 30 inches; once an arc is established, however, the dis- tance must be increased many times before the arc will extinguish itself. On this account, and also to avoid as much as possible the starting of arcs by external agencies, such as branches of trees, kites, large birds, etc., which may form a cross between the wires, the separation is made much greater than would be called for on the basis of the sparking distance in air at the voltage of operation. No hard and i8 HIGH-TENSION POWER TRANSMISSION LINES TABLE OF SPARKING DISTANCES IN AIR BETWEEN OPPOSED SHARP NEEDLE-POINTS, FOR VARIOUS EFFECTIVE SINU- SOIDAL VOLTAGES Voltage Inches Voltage Inches 5,000 0.225 140,000 13 -95 10,000 0.47 150,000 15.0 15,000 0.725 160,000 16.05 20,000 I.O 170,000 17.10 25,000 1-3 180,000 18.15 30,000 1.625 190,000 19.20 35.°°° 2.0 200,000 20.25 40,000 2-45 210,000 21.30 4S.OOO 2-9S 220,000 22.3s 50,000 3-55 230,000 23.40 60,000 4-65 240,000 24.25 70,000 S-8S 250,000 25-50 80,000 7-1 260,000 26.50 90,000 8-35 270,000 27.50 100,000 . 9.6 280,000 28.50 110,000 IO-7S 290,000 29.50 120,000 11.85 300,000 30-5° 130,000 12.90 Fig. 8 -A. fast rule can be given for minimum separation, as the ten- dency at present is to increase the separations which were used a few years ago. A table of recommended separations is given later in a proposed general specification for crossings. A high-tension line which crosses a railroad right-of-way on wooden structures may fail at the crossing in any one of a variety of ways, as enumerated below. I. Failure of Poles, Cross-Arms and Pins: — (a) High winds or the combination of high winds with sleet and wet snow may break the poles, arms or pins, or it may uproot the butts of the poles. HIGH-TENSION POWER TRANSMISSION LINES 19 (b) Such a failure as {a) may first occur in the near vicinity of the crossing, with the result that the line which remains standing is placed under a heavy longitudinal stress which in turn causes a failure at the crossing. (c) Forest fires or ground fires set by locomotive sparks or hot cinders may consume the poles, arms and pins. {d) Trees or large branches may fall on the line during storms and break the poles, arms and pins or cause the butts of the poles to uproot. (e) Arcs from a phase-wire to a pole, arm or pin will set fire to the same and either destroy or greatly weaken the supporting structure. (/) Lightning may shatter the poles, arms and pins and thus weaken or destroy the supporting structure. (g) Derailments of trains may cause locomotives or cars to collide with the poles and demolish the same. (h) The failure of a phase- wire may permit the same to come into contact with poles or arms and thus start arcs which will weaken or destroy the supporting structure. (j) Washouts may weaken the earth around the butts of the poles or expose them altogether and remove their sup- port. II. Failure of Phase-Wires: — {a) High winds or the combined effect of high winds with sleet and wet snow may stress the phase-wires beyond the elastic limit and permanently stretch them or may stress them to the breaking point and permit the spans to fall. (&) Such a failure as II. (a) may first occur in the near vicinity of the crossing, with the result that the tie-wires securing the phase-wires to their insulators may loosen or break and permit the phase-wires to slack down over the 20 HIGH-TENSION POWER TRANSMISSION LINES railroad crossing, or possibly set fire to the supporting struc- tures at the crossing. (c) Branches of trees, sticks, twigs, kites, small pieces of wire and miscellaneous missiles may lodge on the phase wires and start arcs between them or between the wires and the poles, arms and pins, which may melt off the wires and permit the spans to fall. {d) Lightning may melt off the phase-wires and permit the spans to fall. (e) Imperfect joints in the phase-wires may give way when the spans shorten under low temperatures or fail under the repeated bending stresses caused by the wind swinging the wires. (/) A broken or punctured insulator will permit the for- mation of an arc between the phase-wire and the pin or cross-arm, which may melt off the wire and permit the span to fall. {g) The wires of any other line which may fall on the transmission line or be blown upon it by high winds may cause short-circuits and start arcs which will melt off the phase-wires and permit the spans to fall. {K) Excessive currents on copper wires, sufficient to over- heat them, may anneal them and greatly impair their tensile strength, causing them to stretch or break in service. {i) Coal gases from locomotives may deteriorate the phase-wires and in time so reduce their tensile strength as to cause fractures. (7) The deposits of soot and dirt, which accumulate on insulators in the neighborhood of frequent locomotive traffic, may cause the formation of arcs which will melt off a phase-wire and permit the span to fall. HIGH-TENSION POWER TRANSMISSION LINES 21 III. Failure of Insulators: — (a) Lightning may puncture, crack or shatter insulators. (6) Insulators are sometimes cracked or shattered by mischievous or vicious persons with stones or firearms. (c) Permanent internal stresses are caused in manufac- ture by unequal shrinkage which results from uneven and imperfect cooling; rupture may occur at any subsequent time without warning. {d) Puncture may be caused by excessive line potentials which result from surges, due to opening the transmission circuit under heavy overload or short-circuit. In the twenty-three different failures enumerated above, sixteen are due to external causes and seven to internal; mechanical weaknesses resulting from the use of defective materials and careless installation are also among the pos- sible causes of failure, but are omitted above because it is desired to emphasize the failures which can be overcome only by proper design. A few examples of high-tension failures are given below, in order to show that such failures have actually occurred and also to illustrate their destruc- tiveness. The voltages used on arc-light circuits are sometimes as high as those employed in power transmission and are dangerous. The following case of failure occurred within the writer's personal experience. A direct-current series arc-light cir- cuit, 9.6 amperes, 102 lamps in operation, became crossed with a telephone toll-line in Milwaukee, Wis., on October 3, 1907. Between the point of cross and the nearest generator terminal there were 18 arc-lamps; the arc circuit was not groimded and had been operating normally so far as known. This distance from the generator to the extreme end of the arc circuit was about 5 miles. A test office at Truesdell, 22 HIGH-TENSION POWER TRANSMISSION LINES Wis., about 35 miles distant, was set on fire and considerable damage done; one of the telephone wires was melted off at two places, where it came into contact with the foliage of trees. The operation of the arc circuit was not sufficiently hampered to put it out of commission and the office building at Truesdell was saved by cutting the office bridle-cables outside of the building. Accidents have occurred which did not result directly from the failure of a circuit which transmitted power at a dangerous voltage, but from the failure of another circuit which was normally harmless of itself, but served to transmit or distribute the dangerous potential to places where it en- dangered life and property. Such a case as this occurred in Bloomington, 111., on October 26, 1907, when a lamp- trimmer was killed while trimming an arc-lamp, by current from a commercial lighting and power circuit which was trans- mitted to the arc circuit by a telephone wire which failed in some manner and crossed the arc circuit with the commercial circuit; the voltage in use for primary distribution was approximately 2,300 volts, 60 cycles, alternating current. A case was recently related to the writer, in which light- ning struck a 33,000-volt, three-phase line, of No. 2 B. & S. gauge copper on porcelain insulators, with wooden poles and cross-arms and iron pins, which set fire to a pole and melted off one of the phase-wires, permitting it to fall to the ground. Power was being supplied at the time from a 4,000 K.W. generating station about 65 miles distant, through step-up transformers which were delta connected. A port- able sub-station was located about 12 miles north of the broken phase-wire ; the Ughtning arresters discharged severely and the power current followed the discharge, setting up such severe arcs that the sub-station took fire and was destroyed. HIGH-TENSION POWER TRANSMISSION LINES 23 A high-tension pole-line failure at a railroad crossing occurred in May, 1907, during a heavy wind and rain storm. The pole-line carried a 33, 000- volt, single-phase transmission circuit and a 3, 300- volt, alternating-current trolley, on a line of interurban railway. At the place of failure the poles were set in an earth embankment on the approaches to a bridge crossing over the steam railroad. The butts of the poles were uprooted by the wind pressure on the poles and wires, which let the high-tension wires down across the steam tracks so as to impede traffic; trains were delayed in each direction for several hours. The writer was informed that the high-tension circuit was not alive at the time of the failure. One of the most severe cases of failure and burn-out occurred in December, 1903, near Joliet, 111., and is of great interest in connection with this subject. The following quotation is taken from the Electrical World and Engineer of December 26, 1903, Vol. XLIL, page 1060, under " Current News and Notes." "RUIN CAUSED BY BREAK OF HIGH-TENSION WIRE "Through the breaking of a wire of the Chicago and Joliet Electric Railway, three raihoad stations situated several miles apart were set on fire, while those atLemont and Spencer were destroyed, and telegraphic communication with Joliet was almost entirely cut off. The circuit which feeds the power houses at Lemont and Summit runs from the plant of the Economy Light and Power Company at the Jackson Street dam of the drainage channel in Joliet. The pressure carried is 15,000 volts. The break occurred 24 HIGH-TENSION POWER TRANSMISSION LINES where the circuit passes six feet above the wires of the Santa Fe Railroad. When the contact was made, the current burned out every fuse in the Joliet office of the Western Union Telegraph Company and ended communication between Chicago and St. Louis on the Chicago and Alton Railroad; between Chicago and Kansas City on the Santa Fe and to all points west of Joliet on the Rock Island. The current entered the telegraph station on the Santa Fe at Lemont and set it on fire. The station was deserted at the time and in it were stored 500 pounds of dynamite await- ing shipment. The heroic efforts of Mayor Keig, of Romeo, and several others, saved the village from destruction. In several other stations on the line of the Michigan Central, Santa Fe, Rock Island and Alton Railroads damage was done to the telegraph system and wires and instruments were rendered useless." Instances of high-tension burn-outs are numerous and many examples might be presented, but space will not per- mit. The effects of high-tension burn-outs on wooden pole lines are well illustrated in Figs. 9 to 19, inclusive; this line is operated at about 36,000 volts, 60 cycles, 3-phase. The construction is No. 2 B. & S. gauge copper wire, wooden poles, arms and pins and glass insulators. It is proper to note that the insulators used on this line would not be accepted to-day for an operating voltage in excess of 10,000 to 12,000 volts. Porcelain insulators would be chosen for this line if it were being built at the present time, of the built-up type made in two to four sections cemented together and to a metal pin, of iron or steel. The use of a better insulator would have decreased the number of failures and burn-outs, but not eliminated them. The use of metal instead of wood for pins prevents the destruction of the pins by arcing, but HIGH-TENSION POWER TRANSMISSION LINES 25 Fig. g. — Showing result of arc from a phase-wire to the cross- arm, burning off the pin and permitting the phase-wire to sag about lo feet below its normal support. 26 HIGH-TENSION POWER TRANSMISSION LINES it transfers the seat of combustion from the pin to the cross- arm. The burn-outs illustrated in Figs. 9 to 19 do not pre- vent the operation of this line at the present time. Such Fig. 10. — Showing charred pole-top. The original top has been burned off and the cross-arm lowered. accidents, .when they occur upon private right-of-way, do no harm except to the power system itself and are not danger- ous to the lives and property of the public or to the property HIGH-TENSION POWER TRANSMISSION LINES 27 of other companies. At points of exposure, however, the writer believes that such lines constitute a danger which requires every reasonable safeguard to be adopted. Force- s, 4 Fig. II. — Showing the effect of a broken insulator; top of pin burned off and phase-wire sagging 5 or 6 feet below the cross-arm. lain insulators and metal pins of the most approved type are not adequate safeguards; on a 3 3, 000- volt line so equipped the writer has seen the results of burn-outs which are well 28 HIGH-TENSION POWER TRANSMISSION LINES Fig. 12. — Showing pole burned off about 6 feet up from the ground, and upper portion of pole lashed to the stump with iron wire to serve as a support. HIGH-TENSION POWER TRANSMISSION LINES 29 Fig. 13. — Showing stump of burned pole; burned cross-arm with remnant of pole-top attached shown leaning against stump, and also a broken insulator and a pin. 30 HIGH-TENSION POWER TRANSMISSION LINES typified by the illustrations given in this paper, and their occurrence, although less frequent, is nevertheless a fact. / ^ y / ■'"■^ ^fc^ A-^ ^ / / / / ^ / / 5^ "l Fig. 14. — Showing charred pole-top and cross-arm lowered about 3 or 4 feet to secure a safe support. SCREEN PROTECTION The fundamental idea in protection is that of safety, and in all mechanical structures safety is obtained by so design- ing them that the maximum working stresses do not exceed HIGH-TENSION POWER TRANSMISSION LINES 31 Fig. 15. ^Showing pole burned off and upper half supported on the stump, pulling the phase-wires down about 15 feet below their normal position. 32 HIGH-TENSION POWER TRANSMISSION LINES Fig. i6. — Showing pole burned off; the pole-top and cross- arm were lying on the ground. Note the insulator still attached to one of the phase-wires and the top of an insulator attached to the middle phase-wire. HIGH-TENSION POWER TRANSMISSION LINES 33 a small fractional part of the breaking stresses; this is equally true as regards electrical stresses. The theory of screen protection is restriction rather than prevention; it presumes 1 1 i 4 iflN i 1 / j ' ; ! 1 1 .'■ \ \ T^ \ \ \ \ \ \ \ \ \ \ \ ; j / ut Bf^BS^*^ Fig. 17. — Showing charred pole-top; 3 or 4 feet were burned off the top of the pole. that the high-tension line is likely to fail in some manner which will result in physical contact with an exposed low- potential system and seeks to prevent the contact but not 34 HIGH-TENSION POWER TRANSMISSION LINES 1 ; — :^ :--_ — ^^ -—L^ .,i_-' .'. Fig. i8. — Showing charred stump of pole which was burned off. HIGH-TENSION POWER TRANSMISSION LINES 35 Fig. 19. — Showing burned pole-top; cross-arms were lowered to avoid the weakened portion of the pole. 36 HIGH-TENSION POWER TRANSMISSION LINES the failure. The screen, therefore, must be capable of sus- taining indefinitely the potential of the high-tension line and transmitting its current safely, or it must act in some way which will shut down and prevent further operation of the high-tension line until the failure has been repaired, at the same time completely screening and isolating the low- potential system. The writer believes that it is better engineering to seek the means of preventing failures than the means of restrict- ing them after they occur; only when no means of prevention can be obtained should restrictive means be adopted as the primary source of protection. Aside from this fundamental objection to screens there are other objections to them; regardless of whether the screens are erected on the high- potential or the low-potential line, they increase the load on the poles from sleet and wind pressure, and in order not to reduce the factor of safety the poles must be reinforced by guys or braces; in some cases larger poles must be set. A screen on the high-potential line may, under the condition of sleet load and wind pressure, result in making the cross- ing weaker than the rest of the line and therefore the point most likely to fail. The addition of a screen to a pole-line, instead of increasing the strength of the line does the opposite, by increasing the burden the line must sustain; it is true that the line may then be reinforced so as to have finally a greater factor of safety than initially, but only at considerable expense. The writer does not beUeve in this form of protection except as a last resort when no better method can be employed. It is not the purpose here to enter into a detail description of the various types of screens which have been made use of; the ideas in screen design have been in a pretty constant state of evolution and there are few crossings protected in HIGH-TENSION POWER TRANSMISSION LINES 37 precisely the same way. Figs. 20 to 23 show several types of screen protection. The cost of a screen varies greatly with the local condi- tions to be met; the limits of cost may be placed as low as $50 and as high as $500, dependent upon the length of span, the amount of reinforcement necessary and the design of the screen. The type of screen illustrated in Fig. 23 is relatively cheap, but its successful operation assumes that a broken phase-wire will fall vertically downward and rest on the screen; this will not necessarily happen because a high wind may blow the phase-wire clear of the screen or the wire may "whip" to one side as the result of residual spring tension. Such a screen, to be effective under all conditions, must be provided with vertical sides so as to form a box or basket network enveloping the high-tension wires except at the top; the cost is thereby increased and the poles must sustain much greater stresses resulting from sleet load and wind pressure. BRIDGE AND CATENARY TYPES OF REINFORCED CROSSING The extreme of mechanical reinforcement is an overhead bridge which supports the high-tension conductors across the right-of-way. The type of structure which has been used for this purpose is in general similar to the structures used for supporting automatic block and interlocking signals. When properly designed there is no question as to the safety of such crossings, but they are very expensive and they may obstruct a clear view of signals. Their cost will vary greatly with the length of the cross-over span and in this connection it should be remembered that practically the full width of 38 HIGH-TENSION POWER TRANSMISSION LINES Fig. 20. — ^ Showing an early type of screen attached to the poles of the high-tension line; also showing screens of iron wire on the telephone poles with grounded steel cables about 24 inches above each screen. HIGH-TENSION POWER TRANSMISSION LINES 39 Fig. 21. — Showing same types of screen as Fig. 20. 40 HIGH-TENSION POWER TRANSMISSION LINES ; z 3t m 1 1 Fig. 22. ■ — Showing grounded screen on telephone poles; angle- iron uprights on fixture in background are to prevent high-tension wires from "whipping" over onto unpro- tected span of telephone line. HIGH-TENSION POWER TRANSMISSION LINES 41 Fig. 23. — Showing a type of screen frequently used at rail- road crossings; note that there are two high-tension lines, one of which is provided with no safeguards whatever. 42 HIGH-TENSION POWER TRANSMISSION LINES the right-of-way must be protected, or at least that portion of it occupied by tracks and pole-lines. The cost of such structures might not be prohibitive in the case of important lines transmitting very large amounts of power, but would be burdensome upon less important lines of lighter con- struction. The minimum cost of a bridge crossing erected and in service is estimated at approximately $400 to $500, and for long spans of 75 to 100 feet it would be very much more. This type of crossing has not been used to any great extent. Another type of reinforced crossing has been adapted from the catenary trolley, which is in considerable use on electric traction systems operatuig at a trolley voltage of 3,300 volts or higher. The general form of the catenary construction is illustrated in Fig. 24. The messenger M is a stranded steel cable; the phase-wire P is supported from the messenger by the hangers H. The messenger is pro- portioned to sustain the load of the entire cross-over span with the desired factor of safety, under the specified con- ditions as to sleet load and wind pressure. The entire ele- ment of safety lies in the. ability of the messenger to sustain its load under all conditions; the factor of safety can be increased by substituting the messenger for the phase-wire and dispensing with the whole structure which is suspended from the messenger. The stresses in the messenger due to sleet load and wind pressure will thereby be made less than half their former value. Therefore it is not apparent that there is any economy in the catenary design so far as simple mechanical strength is concerned. The catenary trolley came into use with high speeds and high trolley voltages in electric traction, because it was necessary to secure a hori- zontal troUey-wire supported frequently enough to avoid 2 2 "5 *. II H a HIGH-TENSION POWER TRANSMISSION LINES 43 any appreciable flexure at the points of support. The writer considers the catenary type of crossing superior to screen protection, as being stronger and more simple; but capable of further simplification resulting in even greater strength and safety. UNDERGROUND CROSSINGS In view of the extended use of underground cables for potentials as high as 25,000 volts, the possibilities of under- ground crossings should not be passed by. Such crossings have been regarded with little, if any, favor in this country for over-head transmission lines. There are two important reasons contributing to this state of opinion; first, the interruptions to power service which might be caused by light- ning punctures in the cable and, second, the cost and annoy- ance of maintaining such cables at various isolated places on a power transmission system; in other words, it is feared that such crossings would constitute the weakest points in the system. This has not been the case, apparently, in Swiss practice, and the following is quoted from a paper on "Electrical Transmission Plants in Switzerland," by Enrico Bignami, before the International Electrical Congress at St. Louis in 1904; it will be found in Vol. II of the Proceedings, Section D. "As regards the crossings of railroads by wires, we find, in the case of transmission lines, that in 108 out of 120 stations considered, there are 240 crossings, or 67 per cent., above railroads; 118 crossings, or 33 per cent., under railroads. These last crossings are those of highest tension. Almost all these transmission lines have lightning arresters of the horn type. Of distributing wires we find 290 crossings of 44 HIGH-TENSION POWER TRANSMISSION LINES low-tension wires, or 73 per cent., above raikoads, and no, or 27 per cent., under railroads. We have, then, in all, 530 crossings, or 70 per cent., above railroads, and 228, or 30 per cent., below." The highest operating voltage on any of the Swiss lines was given as 25,000 volts. The high-tension underground practice in this country is ^'ery well summarized in a paper on "Underground Electrical Construction," by Louis A. Ferguson, before the 1904 International Congress; it will be found in Vol. II of the Proceedings, Section E. A more recent treatment is contained in the treatise by Henry Floy, entitled, "High-Tension Underground Electric Cables." The table in Fig. 25 is condensed from one of the tables given by Mr. Floy. SOME COiMPANIES OPERATING HIGH-TENSION, THREE-CONDUCTOR CABLES S3 id < "3 > 1 1 1^ 1° *ow 1^1 131 11 13 1 *o Thickness of Insulation in Sixty-Fourths of an Inch Name of Company 8 III ri New York Edison Co... Commonwealth Edison Co., Chicago Interborough Rapid Transit Co., New York New York Central and Hudson River R. R. Co., New York United Railways and Electric Co., Baltimore Commonwealth Edison Co. Chicago 200. 340. 320. 1 ''^ loo- ■ 125. SS- IS- 3- 3- 3-4 6,600 9,000 11,000 n.ooo 13.200 20,000 23,000 25,000 No Yes No Yes 250,000 0000 000 0000 000 0000 00 2 2 2 0000 8 { 8 Armored 8 8 10 8 6 8 8 6 Rubber Paper Paper Rubber Pa^r Cambric Pajier Cambric Rubber Paper 12 10 12 14 13 14 14 12 14 18 12 14 iS 20 12 10 8 14 13 12 14 12 10 14 Edison Illuminating Co., Detroit 6 St. Paul Gas Light 1 Co., St. Paul / Montreal Light, Heat and Power Co.. Montreal . . 10 8 12 Fig. 25 HIGH-TENSION POWER TRANSMISSION LINES 45 Under the most favorable circumstances, underground crossings are limited to voltages of 25,000 or less, until cables can be insulated for reliable operation at higher volt- ages and satisfactory lightning protection can be assured. The conditions of unusually long crossings, duplicate trans- mission lines and accessibility or convenience of location of lightning arresters for frequent inspection and cleaning, make it seem feasible to employ underground crossings for lines of 25,000 volts or less. These conditions would be encountered, for the most part, in the vicinity of population centers. An instance of a cable crossing for arc-light cir- cuits is illustrated in Fig. 26, which shows one of the cable- poles at the crossing. The arc-Hght wires enter the cable just below the cross- arm, and the cable is encased in iron pipe down the pole in the picture, along beneath the railroad bridge and up the cable-pole on the other side of the tracks. The pole-tops are tied together by a single strand over the tracks to take up the stresses of the dead-ended wires. The voltages on arc-light circuits reach 9,000 to 10,000 volts. DISCUSSION OF PROPOSED TYPE OF CROSSING The proposed type of crossing is so reinforced electrically and mechanically as to make a failure an extremely remote possibiUty, or practically impossible. A high-tension Hne should be least likely to fail at the points of dangerous exposure, railroad crossings being a specific case. So far as possible the railroad company's specification for high- tension crossings should be general in its nature, in order to allow the power company the greatest possible latitude in the details of design and construction and to permit them 46 HIGH-TENSION POWER TRANSMISSION LINES to follow their own standards wherever admissible. The most general form of specification would cover the follow- ing points: i pi i i.JJJi iiii . iii J P Fig. 26. — Railroad crossing of arc -light circuits, in cable under tracks. 1. Extreme conditions of sleet load, wind velocity and temperature to be sustained by the wires and their support- ing structure. 2. Precautions to be observed in relation to fire risks and electric arcs or the effects of arcs. HIGH-TENSION POWER TRANSMISSION LINES 47 3. Factor of safety in mechanical stresses in towers, poles, cross-arms, pins, insulators, cables and wires. 4. Factor of safety in electrical stresses in aU insulators and dielectrics. These fundamental requirements are discussed in what follows, under the sub-headings of a provisional form of specification. It is not possible to draw a specification which will meet local conditions over the entire country and this provisional specification is intended only as a guide. Classification of Crossings by Voltage. The voltages commonly employed in the transmission and distribution of electric energy for lighting, power and railway uses may be classified as follows, in Fig. 27. Voltages D. C. A. C. High-Tension Transmission at Constant Potential 5000 and upwards Primary Distribution for Com- mercial Service at Constant Potential no to 125 220 to 250 550 to 600 1050 to IIOO 2100 to 2300 Primary Distribution for Street Lighting at Constant Cur- variable up to 9000 variable up to gooo rent or 10,000 or 10,000 Secondary Distribution for Commercial Service at Con- no 220 stant Potential 440 Third Rail, at Constant Po- tential 550 to 650 Overhead Trolley at Constant Potential 550 to 650 3300 6600 11,000 Fig. 27 48 HIGH-TENSION POWER TRANSMISSION LINES The Standardization Rules of the American Institute of Electrical Engineers recommend the following voltages for transmission lines: — 6,600 22,000 44,000 88,000 11,000 33,000 66,000 and allow a range of 10 per cent, variation in terminal volt- age to cover line-drop in transmission. The difhculties of insulation and of protection against arcing increase with the voltage. The factor of safety in electrical stresses can be increased at the lower transmission voltages over what is obtainable with the higher voltages. Wooden poles can be made fairly safe at the lower voltages, although steel construction is superior in several ways and therefore safer. Wooden construction cannot be made totally impregnable to fire risks, but the known safeguards, when applied to hnes shown in Figs, i and 2, will greatly reduce the hazards from such lines at railroad crossings. The fire risk on wooden construction from external causes is independent of the voltage, but the risk from internal causes increases with the voltage. Based on this considera- tion, ah transmission lines are divided hereafter into two classes, those of 11,000 volts or below and those above 11,000 volts. Sleet Load. Sleet storms occur only in certain parts of the country and the most accurate knowledge of their severity in any given locality can be obtained from the records of the Weather Bureau and the observations of persons who have long resided in the territory. Some engineers allow for a sleet accumulation of one-half inch in diameter. The writer has seen it one inch on small wires and known it to remain for a period of several days. Low temperatures HIGH-TENSION POWER TRANSMISSION LINES 49 cannot accompany the formation of sleet, which takes place at 0° C. or 32° F.; an appreciable drop in temperature wiU terminate the formation of sleet, but the accumulated sleet will remain on the wires. A considerable drop in tempera- ture may follow a sleet storm, but is not of usual or frequent occurrence. A sleet load of one inch diameter is recom- mended as a minimum safe requirement, the weight of which should be computed at 57.5 pounds per cubic foot. Sleet storms of such severity may be expected every few years in the sleet storm zones. Accumulations of wet snow wiU also occur, especially when driven by a moderately high wind; it is almost impossible to calculate the probable weight and distribution of such a load, because the manner of its formation is very indefinite and because the weight of snow will vary from 5 pounds per cubic foot for dry snow freshly fallen to 50 pounds per cubic foot for wet snow packed down by rain.^ Sleet will accumulate on all exposed surfaces, whereas snow will accumulate principally on the surfaces exposed to wind pressure. The assumption of a one-inch sleet load, however, may be regarded as pro^'iding against failures from snow loads, to the fullest extent jus- tifiable by experience. It has sometimes been stated that high-tension lines are immime from sleet because the phase-wires are usually at a higher temperature than the atmosphere, due to the dissipa- tion of energy. A calculation of the radiated energy in watts per square inch of conductor surface, resulting from- the resistance loss in the phase-wires on a typical line having not in excess of 10 per cent, drop in voltage at fuU load, does not indicate a rise of temperature exceeding 2 or 3 degrees 'See "Applied Mechanics," by G. Lanza, 7th revised edition, Sec. 130, page 151, Stresses in Roof-Trusses. 50 HIGH-TENSION POWER TRANSMISSION LINES Fahrenheit. Direct discharge into the atmosphere, known as brush discharge or corona, does not occur upon properly designed lines and is rarely observed. Therefore it is not apparent why there should be no sleet formation upon high- tension wires; and as a matter of fact, operatives in charge of lines with voltages as high as 33,000, on copper wires, have informed the writer that they do experience sleet. It has been stated that aluminum wires do not accumulate as much sleet as copper, iron or steel wires, because of the greasy character of the oxide which forms and remains on the surface of aluminum wires; no statistical data is at hand to bear out this contention and therefore it does not appear to be proper to modify the recommended sleet load given above. This subject should be investigated because the proven immunity of aluminum from sleet would be a prominent factor in its favor. Wind Pressure. The subject of wind pressure has re- ceived the attention of numerous investigators. The rela- tion between the average velocity and the pressure of wind is known approximately. The table in Fig. 28 is taken from A. R. Wolff's treatise, "The Windmill as a Prime Mover." Smeaton's formula expressing the relation between ve- locity and pressure is P = .oo5F^ (i) where P = pressure in pounds per square foot, and F = velocity in miles per hour. The formula expressing the results of the tests by the Weather Bureau is P = .004 V (2) Langley's results gave P = .0036 V^ (3) HIGH-TENSION POWER TRANSMISSION LINES 51 ROUSE-SMEATON TABLE Temperature about 45° F. Velocity of Wind Miles per Hour Pressure Pounds per Square Foot Remarks I 2 3 4 S 10 IS 20 25 30 35- 40 45 5° 60 80 100 .005 .20 .044 .079 .123 .492 1. 107 1.968 3-075 4.429 6.027 7-873 9-9^3 12.30 17.72 31-49 49.20 Hardly perceptible Just perceptible Gentle, pleasant wind Pleasant, brisk gale Very brisk High wind Very high storm Great storm Hurricane Immense hurricane Fig. 28 Borda's experiments with wind pressure on cylindrical surfaces bear out the assumption that the pressure on a cylindrical surface is equal to one-half of the pressure on its projected plane surface when normal to the wind. The experiments of Mr. H. W. Buck with wind pressure' on a 950-foot span of stranded aluminum cable of .58-inch diam- eter, are in substantial agreement with the formula P = .0025P (4) for actual velocities up to 33 miles per hour; where P is the 'The Use of Aluminum as an Electrical Conductor, by H. W. Buck, International Electrical Congress, St. Louis, 1904, Vol. II., Section D, of Proceedings. 52 HIGH-TENSION POWER TRANSMISSION LINES pressure in pounds per square foot of projected cable area. It is worthy of notice that Mr. Buck's results bear out Smeaton's formula and Borda's assumption; his results were obtained, however, with a stranded cable, and for a smooth conductor the formula of the Weather Bureau with a correction factor of 0.5 is probably correct, which would be P = .002 V^ (5) The indicated wind velocities reported by the Weather Bureau are in error and are too high. The following table. Fig. 29, gives the correct actual velocities in miles per hour corresponding to reported or observed velocities: CORRECTED WIND VELOCITIES Miles per Hour Indicated Velocity, U. S. Weather Bureau Corrected Actual Velocity \j. 10 ■ 9.6 20 17.8 30 2S-7 40 33-3 50 40.8 60 48.0 70 55-2 80 62.2 90 69.2 100 76.2 Fig. 29 The following table of wind velocities observed at various cities in the country was given by Mr. Buck in his paper just referred to; these velocities were the highest on record from 1894 to 1904, at the places named: HIGH-TENSION POWER TRANSMISSION LINES OBSERVED WIND VELOCITIES 53 Place Wind Velocity Indicated Corrected "Ri^iTnarlc M D 72 78 90 78 84 76 60 56.6 60.8. Buffalo N Y 6g.2 New York Citv 60.8 Galveston Tex 65- 59.4 Salt Lake Citv 48. Fig. 30 Winds of extremely great velocity do not occur at very low temperatures and are not likely to occur with sleet storms. There is considerable discussion as to what velocity should be assumed in conjunction with sleet load; this, again, depends upon experience as found from the records of the Weather Bureau and the knowledge of old residents of the locaUty. High winds may occur, however, before the sleet melts and therefore it appears reasonable to assume actual velocities of 60 to 70 miles per hour, in conjunction with a temperature drop of 15° or 20° F. below freezing point. The stresses and deflections under load should be investigated also at velocities of 90 to 100 miles per hour, through the specified temperature range, without sleet. The pressures on inclined surfaces are computed from either of two well-known formulae which give substantially equal results for inclinations greater than 45°. Duchemin's formula is p = p, ''^^ (6) ^ ^' I + sin^ e ^ ' 54 HIGH-TENSION POWER TRANSMISSION LINES where p = intensity of normal pressure on inclined surface. />! = intensity of pressure on plane normal to the direction of the wind. 6 = angle made by the inclined surface with the direction of the wind. Duchemin's formula is recommended by Prof. W. C. Unwin. Button's formula is p = Pi (sin e)'-84™=»-' (7) The experiments with pressure gauges of various sizes have given results which indicate that the pressure per square foot is greater upon small surfaces than upon large ones, with the same wind velocity. Wind velocities are known to vary with the altitude above the earth, increasing with the higher altitudes. Seldom are wind velocities con- stant; instead they often vary greatly from moment to moment, the wind coming in puffs or gusts. The observed velocities are average values and not instantaneous maxima; no data is at hand bearing on the latter. The experience in this country in building light windmill towers, of a py- ramidal form, from structural steel shapes, is more or less of a guide in regard to wind pressures. Mr. D. R. Scholes, in a recent paper dealing with such structures for transmission- line towers,' states that the common type of windmill tower will fail theoretically under wind pressures of 40 to 50 pounds per square foot; from the fact that actual failures do occur, although very infrequently, he reasons that 40 pounds per square foot is a reasonably safe figure with a factor of safety of 2. According to Smeaton, this pressure corresponds to '"Fundamental Considerations Governing the Design of Transmission Line Structure,'' by D. R. Scholes; Proceedings of Amer. Inst, of Elec. Eng., June 29, 1908. HIGH-TENSION POWER TRANSMISSION LINES 55 an actual velocity of 90 miles per hour, and according to Langley, 105 miles per hour. The wind pressures allowed for in bridge practice vary from 30 to 50 pounds per square foot. Rankine calculated from chimneys that had stood for long periods, that the greatest average pressure which could be reaUzed on tall chimneys is 55 pounds per square foot. The pressure recom- mended for the supporting structures of high-tension Unes at raihoad crossings is 50 pounds per square foot, for stranded conductors 25 pounds, and for smooth cyhndrical surfaces 20 pounds per square foot of projected area; these pressures correspond to a wind velocity of 100 miles per hour. Pres- sures of half the foregoing values are recommended in con- junction with one-inch radius of sleet load at a temperature of 10° F.; the pressures in this case correspond to a velocity of 71 miles per hour. Temperatures. The temperature range can be judged from the Weather Bureau records, but the range of extreme temperatures so obtained wiU not be great enough. The temperatures in such localities as river-bottom lands will usually be several degrees colder on a frosty winter morning than the temperature at the Weather Observatory. The temperature directly in the sun's rays on a hot summer day will exceed considerably the Weather Bureau observation taken in the shade. Both of these extreme conditions must be met, and in such localities as the Middle States a range from 25° or 30° F. below zero to 150° F. above is pretty close to the actual conditions. Factors of Safety. The choice of a factor of safety depends upon a number of considerations, such as the use to which the structure or member is to be put, the reliabiUty of the structural material, the character of the load and the damage 56 HIGH-TENSION POWER TRANSMISSION LINES which would result from failure. Factors of safety com- monly vary from 3 for dead loads to 6 for live loads; 4 is often used in bridge work. The factor of safety is the factor by which the breaking or rupturing stress must be divided to obtain the greatest allowable working stress. A factor of 4 is recommended by Prof. Lanza for timber, provided that the breaking stresses are determined from tests upon full- size specimens; tests on small-size pieces have given values of tensile strength which cannot be attained in large members. A factor of safety of 2 is not sufi&cient, because the elastic limit is roughly 50 per cent, of the tensile strength and the maximum working stress would be likely to produce a per- manent " set" and alter the shape of the structure. A factor of 3 seems to be the minimum permissible, in which case the working stress is two-thirds of the elastic limit. A factor of 4 reduces the working stress to one-half of the elastic limit and amounts to a factor of safety of two as regards per- manent "sets" in the material and consequent alteration of shape; it also makes possible an actual test of the material or the structure to twice the working stresses, approximately, without injury if the material meets the requirements. In the case of timber structures it is not possible to test them to 50 per cent, of the tensile strength, usually because they cannot be erected before shipment under service con- ditions. Therefore a factor of safety of 6 to 8 seems to be warranted; this is advisable also from the standpoint of de- terioration of timber from exposure to weather and soil. Figures on tensile strength should be taken from tests of pieces as large as those to be used in the structure. The factor of safety for electrical stresses in insulating materials depends upon new considerations. Nothing is known about the existence of any elastic limit, which of HIGH-TENSION POWER TRANSMISSION LINES 57 itself would permit a low factor of safety. On the other hand the unreliabihty of glass and porcelain in respect to tensile strength, as compared with iron and steel, warrants a high factor of safety. The failure of an insulator on a wooden pole line would be likely to result in much greater destruction to the pole than .such a failure on a steel pole line; therefore a lower factor of safety is warranted with steel poles than with wooden ones. Insulators may fail by puncturing or by arcing over the surface; it is common prac- tice to subject them to potential tests of two kinds, a dry test and an artificial rain test. Each insulator should also be tested for mechanical strength, under conditions of stress identical in every way possible with those on the pole-line. Based on the maximum stresses sustained without injury or failure under actual test, factors of safety from 2 to 3 are recommended for porcelain insulators on steel poles and from 4 to 5 on wooden poles, for electrical and mechanical stresses. Structures. Steel structures are recommended on all lines of voltages above 11,000 and are preferable for those of lower voltages. The entire supporting structure — pole, cross-arms and pins — is to be of steel and thoroughly grounded. On lines of 11,000 volts or less, wood is believed to be safely permissible for the poles, but the cross-arms and pins should be steel and should be grounded. Suitable steel structures can be designed by the use of standard structural steel shapes; owing to the great varieties in design, space will not permit a discussion of the subject and standard text- books on Applied Mechanics should be consulted. When- ever possible the manufacturer should set up each pole or tower complete, for mechanical test under loads which wiU stress the material nearly to the elastic limit. It is common 58 HIGH-TENSION POWER TRANSMISSION LINES practice to employ galvanizing, but in the vicinity of frequent locomotive traffic, it deteriorates rapidly under the action of coal gases and therefore painting may be in many cases the superior protection against rust. For wooden structures the design recommended is an "A" fixture, or an "H" fixture braced against side swaying. It is the writer's belief, based on experience and theoretical con- siderations, that many failures on wooden pole lines occur as a result of the stresses which are set up by the swaying of sleet-laden wires; the swaying is communicated to the poles and after sufficient repetition the butts are loosened so as to permit the swaying to increase in amplitude and the poles finally snap off. Lines heavily loaded with sleet have been saved from failure by temporarily bracing and guying them so as to prevent swaying; in one instance a heavy line was saved from failure after a sleet storm, when the wind was rising, by sending linemen up the poles to dislodge the sleet, which was composed largely of snow-ice and was brittle, so that a sharp blow on a wire would dislodge it. Therefore an "A" fixture is much superior in this regard to a single- stick fixture or pole. As a preventive means against the possibility of setting fire to poles from forest fires or ground fires, it is recom- mended that the butts be set in concrete and that a concrete collar be formed around each pole for a distance substan- tially of one foot above the ground. Concrete will not add to the strength of the pole, but it will add weight at the butt and increase the bearing area, thereby increasing the resist- ance of the pole to overturning by uprooting of the butt. The structures should be designed to withstand the line stresses under the most severe conditions of assumed loading, and in the case of the steel structure to sustain the pull in the HIGH-TENSION POWER TRANSMISSION LINES 59 direction of the line occasioned by complete wire failures on either side of the crossing. The loads in the spans on either side of the crossing should be taken into full account, and in the case of lines with long spans it may prove economi- cal to shorten the spans immediately adjacent to the crossing on either side. The strength of structural materials is given below in Fig. 31, using average values. Material Tensile Strength Pounds per Square Inch Modulus of Elasticity for Tension and Compression Cast Iron 14,000 to 20,000 17,000,000 Wrought Iron .... 40,000 to 60,000 28,000,000 Mild Steel 5S,ooo to 70,000 28,000,000 to 30,000,000 Yellow Pine 4,000 to 7,000 1,600,000 (compression) White Cedar 4,000 to 7,000 Chestnut 6,000 to 9,000 Oak 3,000 to 6,000 1,300,000 (compression) Fig. 31 The figures given by different authorities for timber vary considerably, largely on account of the difference in the sizes of the pieces tested; it is most important to obtain, if possible, the results of tests on pieces of the same size as those to be employed in the structure whose design is in hand. A safe maximum working stress in timber for ordinary use is 800 to 1 ,000 pounds per square inch, but for the purpose under dis- cussion it is recommended that 600 to 800 pounds be the limit. Foundations. Concrete foundations are commonly em- ployed with steel structures in order to provide a secure base and to protect the steel-work below ground and give it long life. The resistance of a foundation to uplift is equal to its 6o HIGH-TENSION POWER TRANSMISSION LINES own weight plus the weight of earth resting upon it; it is customary to assume that the total volume disturbed by actual uplift is contained within an inverted truncated pyramid whose sides make an angle with the vertical vari- ously stated from 30° to 45°; the base of the foundation is taken as the small base of the pyramid. An angle of 30° is conservative and is believed to be safe for any type of founda- tion which has extended flat surfaces about the base to act as anchorages. In the case of a pole or foundation with per- pendicular sides, such an assumption does not seem war- ranted. The foundations for tower structures are made wider at the bottom than the top, more for the purpose of resisting uplift and consequent overturning of the tower than for sustaining excessive downward pressure. Insulators. The ordinary pin-type of insulator is not so satisfactory for sustaining heavy pulls as the strain-type of insulator which is mounted upon a pin which extends clear through the insulator and is supported at top and bottom. This type of insulator and pin does not place a torsional stress on the cross-arm tending to twist it; the cross-arm is subject only to bending stresses. Such insulators are not made for the highest voltages now in use, and for those voltages the ball or disc-type of strain insulator may be employed and enough of them connected in series to obtain the desired factor of safety. The latter type requires no pin; the axis of the insulator is in line with the phase-wire and when several such insulators must be linked in series, steel strand or guy rope may be used for the purpose; the strand from the insulator adjacent to the cross-arm may be passed through an eye-bolt attached to the arm or around a horizontal roller supported at each end. The pin-type of strain insulator is usually made in two 4 5 Fig. 31-A. — Types of High-Tension Insulators, i. Standard double petticoat. 2. Standard triple petticoat. 3._ Suspension type for high voltages. 4. Standard strain insulator. c. Disc tyoe of strain insulator; also used in place of No. 3. tSffl^S*?!©**' *^»- 62 HIGH-TENSION POWER TRANSMISSION LINES pieces cemented together, except in the smaller sizes. Porce- lain is the material which is recommended for all sizes. When the mechanical stresses are too great for one insulator to bear safely, two or more insulators may be arranged in multiple so as to divide the stresses. Every insulator employed should be subjected to the full mechanical and electrical tests. The mechanical tests should be made in conjunction with the pin which is to be used on the structures. The methods of making the electri- cal tests have not been standardized and are now the sub- ject of discussion among engineers. The dry test and the artificial rain test are in common use, but in different ways and under no common set of conditions. Some progress in this matter is to be reasonably expected in the near future. A very careful inspection of every insulator should be made to detect any flaws or defects. The maximum potential stress which the insulator must sustain is known from the line voltage and the chosen factor of safety. The maximum mechanical stress wiU be known from the calculations of sleet load and wind pressure upon the phase-wires, to determine the necessary conductor strength for a given length of span and amount of sag. The insulator should sustain without injury under actual test a stress equal to the elastic limit of the conductors. Conductors. Stranded conductors are recommended for two reasons: the tensile strength of hard-drawn wires per square inch increases with the amoimt of drawing and hence is greatest with the small wire sizes; in the case of a single solid conductor a flaw may result in a failure, whereas in a stranded conductor the possibility of a number of flaws in different strands within the length of a single span is very remote. For the same total weight of material per linear HIGH-TENSION POWER TRANSMISSION LINES 63 foot a stranded conductor is therefore stronger and safer than a solid conductor erected under equal conditions. The current-carrying capacity of the conductors over the crossing should be equal at least to the capacity of the conductors on the main part of the transmission line, and preferably should exceed the latter 25 per cent, to 50 per cent., so as to be less likely to melt off under lightning dis- charges. The relations between the tensile stresses in a conductor and the length of the span and the sag are too well known to consume space here in discussing them at length; any treatise on Mechanics or any standard Mechanical Engineers' Hand- book should be consulted. The tensile stress is decreased by shortening the span and increasing the sag; the effect of atmospheric temperature should be considered because all metals respectively expand and contract with high and low temperatures sufficiently to affect the sag and that in turn affects the stresses. Fig. 32 gives the strength of various materials used for wires and cables. Copper is one of the best materials for cables at railroad crossings and may be expected to have a long life. Tests on copper cables do not show a tensile strength per square inch quite as great as the strands, lack- ing about 10 per cent., but having greater elongation. The strength of copper wires may exceed slightly the value given, but the wire becomes harder and more susceptible to injury; the strength of small steel wires may exceed 200,000 pounds per square inch, but the wire is very hard and brittle and the conductivity very low. The use of high values of tensile strength is not recommended; it is believed to be the safest practice to specify moderate values and to subject full-sized samples of the material to actual test. 64 HIGH-TENSION POWER TRANSMISSION LINES Material Annealed Copper. Hard Drawn Cop- per ; . . . Hard Drawn Alu- minum Silicon Bronze . . . Phosphor Bronze. Galvanized Iron, BB Galvanized Steel . ioo% 62 15-8 26 15-6 13-2 E 3 32,000 S5.000 26,000 100,000 102,000 58,000 65,000 3S>°°° 14,000 3S.oo° 40,000 r 16,000,000 9,000,000 25,000,000 30,000,000 6^ .000,0095 000,0128 000,0068 .000,0064 1920° F. "57 2400 so; S.go 8.94 2.68 7-73 7-85 Fig. 32 Separation of Conductors. The proper separation of the phase-wires is of the utmost importance. The formation of arcs cannot, perhaps, ever be ehminated, but the effects of such arcs can be minimized by adopting the greatest separation feasible and the arcs themselves thereby made less stable. An examination of past practice in this regard will show a large variation of separation on different lines operating at the same voltage; the present tendency is toward greater separations. Under given conditions as to voltage and conductor sep- aration, there is a minimum diameter of conductor at which brush discharge into the atmosphere or corona commences. Prof. H. J. Ryan has investigated this phenomena and determined experimentally the law between conductor separation, diameter and operating voltage at which the phenomenon appears. The table in Fig. 33 is taken from Prof. Ryan's paper.' 'The Conductivity of the Atmosphere at High Voltages, by H. J. Ryan, Transactions of Am. Inst, of Elec. Eng., Feb. 29, 1904, Vol. XXIII. HIGH-TENSION POWER TRANSMISSION LINES 65 Diameter of Con- ductor in Inches Maximum Volts at which Atmospheric Conduction Loss Occurs Corresponding Effect- --Ive Volts (Sine Wave) Operating Pressure, 90 per cent, of Corresponding Effective Volts .058 .106 .192 .430 .710 .990 78.50° 118,000 157,000 235.5°° 314,000 392,000 55,°°° 83,3°° 111,100 116,600 222,200 277,700 50,000 75,000 100,000 150,000 200,000 250,000 Barometer 29.5 inches. Temperature 70° F. Conductor Separation = 48 inches. Fig- 33 The conductor separation at the crossing should be greater than the separations on the remainder of the line, unless this separation is already so ample that an increase would require very large towers at the crossing. Dead Ending and Tying. The ordinary tie is not highly reliable and it may loosen or lack sufhcient strength to pre- vent the phase-wire from sUpping through in the event of a fracture of the phase-wire. Clamps are more reliable than tie-wires, but in the present case it is believed to be safer to dead-end the conductors on the remainder of the trans- mission line at each end of the crossing and thus insure the safety of the cross-over span from a failure of a phase-wire in a near-by span. Sleeve joints are regarded as superior to other forms and are recommended. It is customary to reinforce the body of the conductor at and near the point of support on the insula- tor by extra servings of tie-wire or metal strips, to decrease the liability of melting the phase-wire by arcs, which are likely to weaken or destroy it. When strain insulators are employed this purpose may be accomplished by the use of 66 HIGH-TENSION POWER TRANSMISSION LINES larger cables at the ends of the spans or by wrapping the cables with a serving of soft metal, at and near the insulator. The factor of safety of i.ii employed in the table of Fig. 33 to obtain the operating voltage seems very low, considering that voltage changes of 10 per cent, are often allowed for in regulation. Copper cables smaller than about three-eighths of an inch diameter are not desirable from the standpoint of tensile strength, and with such a minimum limiting diameter there is ample margin below the critical pressure, with the highest voltages now in commercial use. The following table of conductor separations has been prepared from a study of various transmission lines and is believed to represent conservative practice and to be safe. Operating Voltage Minimum Separation 6,600 volts 24 inches 11,000 30 22,000 38 33,000 48 44,000 60 66,000 75 88,000 90 Fig. 34 Ground Cable. An overhead ground-wire on transmission lines is very generally accepted as a partial protection against lightning. Whether or not a transmission line is so equipped, such a ground-wire is recommended for railroad crossings and a steel cable not less than one-half inch diameter will serve the purpose. The cable should be securely connected to the structures so as to form a good electrical connection and the structures should be grounded. In the case of HIGH-TENSION POWER TRANSMISSION LINES 6'] wooden poles for lines of ii,ooo volts or less, the cable should extend down the pole at each end of the span and connect to a ground plate or coil; the steel cross-arms and pins should be grounded also. The point of attachment of the cable on wooden poles should be above the top cross- arm near the top of the pole. Low-Voltage Circuits on the High-Tension Line. Quite commonly there are telephone circuits and sometimes low- voltage power circuits on the same poles or structures which carry the high-tension conductors. Inasmuch as a failure of the high-tension wires at some other place might com- municate their potential to the low-voltage or telephone cir- cuits, which would cause them in turn to become a menace at the crossing, it is believed to be necessary to reinforce them so that in the event of such an occurrence they would be practically certain to fail elsewhere. Therefore it is recommended that the conductors of these circuits be stranded cables of at least twice the current-carrying capacity of the wires on the remainder of the line, and that they be dead- ended on strain insulators of the through-pin type. The factors of safety for mechanical stresses should be those already specified, but it is not regarded as necessary to provide insulators rated to operate at the full high-tension voltage. The insulators may be subjected accidentally to the full voltage, however, and therefore an insulator is recommended which will not fail under a test of the line voltage; based on the maximum stresses sustained under actual test without injury or failure, this is equivalent to a factor of safety of one. If the low-voltage circuit is em- ployed to transmit power, of course the insulators must meet the voltage requirements and possess the necessary tensile strength. 68 HIGH-TENSION POWER TRANSMISSION LINES TYPICAL CROSSING A typical crossing for a high-tension line carrying two 22,000-volt, 3-phase circuits is illustrated in Fig. 35. The entire crossing is no more unsightly in appearance than any transmission line of the same general type and there is a minimum obstruction of view along the right-of-way. The principal dependence for safety is upon the universally recognized principle in every form of structure, namely, a reasonable factor of safety. The advantages claimed for this type of crossing are strength, safety and simplicity. LIST OF REFERENCES TO THE TRANSACTIONS OF THE AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS Burning of Wooden Pins on High-Tension Transmission Lines, by C. C. Chesney, March 27, 1903, Vol. XXI. The Grounded Wire as a Protection Against Lightning, by Ralph D. Mershon, July i, 1903, Vol. XXII. Overhead High-Tension Distributing Systems in Suburban Districts, by Geo. H. Lukes, Dec. 18, 1903, Vol. XXII. Safeguards and Regulations in Operation of Overhead Distributing Systems, by W. C. L. Elgin, Dec. 18, 1903, Vol. XXII. European Practice in the Construction and Operation of High-Pressure Transmission Lines and Insulators, by Guido Semenza, Feb. 26, 1904, Vol. XXIII. The Conductivity of the Atmosphere at High Voltages, by Harris J. Ryan, Feb. 29, 1904, Vol. XXIII. Long Spans for Transmission Lines, by F. O. Blackwell, June 21, 1904, Vol. XXIII. Report of Committee on High-Tension Transmission, June 21, 1904, Vol. XXIII. Some Experiences with Lightning Protective Apparatus, by Julian C. Smith, Oct. 27, 1905, Vol. XXIV. Some Experiences with Lightning and Static Strains on a 33,ooo-FoZ< Transmission System, by Farley Osgood, May 28, 1906, Vol. XXV. I; •bI 5; > p ^ CL 8 o HIGH-TENSION POWER TRANSMISSION LINES 69 Protective Apparatus for Lightning and Static Strains, by H. C. Wirt, May 28, 1906, Vol. XXV. Transmission Line Towers and Economical Spans, by D. R. Scholes, June 26, 1907, Vol. XXVI. Lightning Rods and Grounded Cables as a Means of Protecting Trans- mission Lines Against Lightning, by Norman Rowe, June 26, 1907, Vol. XXVI. A New Type of Insulator for High-Tension Transmission Lines, by E. M. Hewlett, June 26, 1907, Vol. XXVI. Some New Methods in High-Tension Line Construction, by H. W. Buck, June 26, 1907, Vol. XXVI. The Transmission Plant of the Niagara, Lockport and Ontario Power Company, by Ralph D. Mershon, June 26, 1907, Vol. XXVI. Fundamental Considerations Governing the Design of Transmission Line Structures, by D. R. Scholes, June 29, 1908, Vol. XXVII. The Testing of High-Voltage Line Insulators, by C. E. Skinner, June 29, 1908, Vol. XXVII. 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