IKELEY LIBRAR RAILROAD COMMISSION OF THE STATE OF CALIFORNIA REPORT T BY THE . Joint Committee on Inductive Interference TO THE Railroad Commission of the State of California Approved by the Railroad Commission on July 30, 1914 CALIFOBNIA STATE PRINTING OFFICE 1914 REPORT BY THE Joint Committee on Inductive Interference TO THE Railroad Commission of the State of California Presenting the results accomplished to date in the study of inductive interference and including recommendations for rules designed to prevent or mitigate inductive interference to communication circuits by power circuits San Francisco, California, July 7, 1914 CALIFORNIA STATE PRINTING OFFICE 1914 12191 CS/ LETTER OF APPROVAL. SAN FRANCISCO, July 30, 1914. Joint Committee on Inductive Interference: GENTLEMEN: We desire to acknowledge receipt of yours of the 7th instant, transmitting the report of the Joint Committee on Inductive Interference, and also of yours of the 23d instant, referring in greater detail to future work of your committee, and to thank you for the same. . The Commission realizes the arduous labor which your committee and the individual members thereof have performed in seeking to ascertain the causes of inductive interference between power and communication circuits, and to prescribe rules and regulations for preventing or minimizing such interference, and extends to the committee, and each member thereof, its congratulations on the results accomplished and its thanks for the scholarly, scientific, painstaking manner in which the work has been performed. The Commission has adopted the rules as proposed by your com- mittee and has added two new rules, one dealing with the applicability of the rules to existing and future construction and the other declar- ing the principle that these rules shall be subject to the laws of this State and the orders of this Commission, now or hereafter in effect. The Commission's order will be published as a general order. Your report will be printed by this Commission for free distribution. While the general conclusions will be given to the press in the usual course, we shall be glad to have the report printed in full in the Pro- ceedings of the American Institute of Electrical Engineers. The Commission hereby requests your committee to continue its work along the lines indicated in your report and your letter of the 23d instant, and authorizes the raising of the necessary funds by assessment as heretofore, with the understanding that the Commission will assign one of its stenographers to the work of the committee, and that our engineering department will at all times co-operate with your committee. With sincere appreciation for the work hitherto performed and assurances of continued interest in the work to which you will now devote yourselves, We remain, respectfully, RAILROAD COMMISSION, JOHN M. ESHLEMAN, H. D. LOVELAND, ALEX GORDON, MAX THELEN, E. 0. EDGERTON, Commissioners. LETTER OF TRANSMITTAL. SAN FRANCISCO, July 7, 1914. To the Railroad Commission of the State of California, San Francisco, California: GENTLEMEN : The Joint Committee on Inductive Interference sub- mits herewith a report based on its work to date, containing provisional rules which tend to improve conditions in respect to inductive inter- ference. The investigation undertaken by the committee has not been completed, but the results already obtained serve to point out a number of requirements and precautionary measures which should be complied with in future work. These have been embodied in the rules presented herewith, and it is the recommendation of the committee that these rules he made effective immediately without waiting for the completion of the investigation. The committee desires to explain, in respect to certain of the rules, that while the general character of their essential provisions is well understood, the information available at present is not sufficiently com- plete to make it possible to set definite quantitative limits and to make all the rules explicit, such as they should be in order to afford the maxi- mum reduction of inductive interference consistent with the burdens imposed by the rules. In a few instances, rules have been drawn with definite limits which have been set somewhat arbitrarily, in accordance with the committee's best judgment. Therefore, the rules are not put forth as being complete or final, but must be regarded as provisional and subject to such change as the results of further investigation and experience may determine. They are, however, recommended unani- mously by the committee as the best which can be formulated at this time, and thus having the support of all the interests represented on the committee, it is hoped that the rules will appeal to the Commission as being reasonable and proper. The report also outlines other experimental work, some of which is now in progress, which the committee considers essential in order that additional information may be acquired for amplifying and revising these rules to make them more definite and complete. Respectfully submitted, (Signed) RICHARD SACHSE, A. H. BABCOCK, R. W. GRAY, F. EMERSON HOAR, J. E. WOODBRIDGE, J. P. JOLLYMAN, P. M. DOWNING, H. A. BARRE, C. H. TEMPLE, A. H. GRISWOLD, R. W. MASTICK, V. V. STEVENSON, J. A. KOONTZ, A. L. WILSON, Joint Committee on Inductive Interference. Honorary Members : HOWARD S. WARREN. JAMES T. SHAW. REPORT BY THE Joint Committee on Inductive Interference to the Railroad Commission of the State of California. SCOPE. This report presents briefly an account of the formation of this committee, its activities and results accomplished to date, and recom- mendations for such rulings by the Railroad Commission of the State of California as the committee believes are justified at this time; together with a technical discussion in explanation of the results and recommendations. HISTORICAL. The formation of the Joint Committee on Inductive Interference was the outgrowth of certain differences involving power, communication, and railroad interests which were brought to the attention of the Rail- road Commission of California. As an alternative to contesting the issue at that time it w r as agreed by the power and communication com- panies, with the approval of the Commission, that a joint investigation should be made to obtain certain information essential to a proper solution of the difficulties. The Commission desired that the matter be thoroughly investigated before passing upon the general principles involved in these difficulties. To this end a general conference was called to select representatives to form a "Joint Committee" empowered to conduct tests, experiments, and investigations, the results of which would serve as a basis of recommendations for rules and regulations to be issued by the Commission, tending to minimize inductive interference and physical hazard arising from parallelism of different classes of circuits. This conference was held December 16, 1912. As a result the Joint Committee on Inductive Interference, representing the Rail- road Commission and railroad, power, and .communication interests of the State, was organized and authorized by the Railroad Commission of California to conduct the desired investigation. The personnel of the committee selected is given below. Representing Railroad Commission: Mr. R. A. Thompson, Chief Engineer. Mr. A. R. Kelley, Assistant Engineer. Mr. James T. Shaw, Assistant Rate Expert. Mr. F. Emerson Hoar, Assistant Rate Expert. Representing Railroad Interests: Mr. A. H. Babcock, Consulting Electrical Engineer, Southern Pacific Company. Representing Telephone and Telegraph Interests: Mr. A. H. Griswold, Plant Engineer, The Pacific Telephone and Telegraph Company. Mr. R. W. Gray, Division Superintendent, Western Union Tele- graph Company. 6 REPORT ON INDUCTIVE INTERFERENCE. Mr. C. H. Temple, General Manager, United States Long Distance Telephone Company. Mr. L. M. Ellis, General Manager, Union Home Telephone Com- pany. Representing Power Interests: Mr. H. A. Barre, Electrical Engineer, Pacific Light and Power Corporation. Mr. Louis Elliott, Engineer, Great Western Power Company. Mr. P. M. Downing, Engineer, Pacific Gas and Electric Company. Mr. J. E. Woodbridge, Chief Engineer, Sierra and San Francisco Power Company. The organization and personnel of the Joint Committee on Inductive Interference were approved by the Railroad Commission on January 6, 1913, and the committee thereupon proceeded with the necessary tests and investigations. For the more efficient conduct of its work the Joint Committee was divided into several smaller subcommittees, each assigned to and respon- sible for certain branches of the investigation. The present organiza- tion of the Joint Committee is given on a chart presented as Ap- pendix VI. Since the formation of the committee, through additions, resignation or death, the personnel of the committee has changed as follows : Mr. Louis Elliott resigned and Mr. J. A. Koontz, Engineer of the Great Western Power Company, was appointed in his place. Mr. V. V. Stevenson, Electrical Engineer of the Postal Telegraph Cable Company, and Mr. L. N. Peart, General Superintendent of the San Joaquin Light and Power Company, were added to the original membership by action of the committee. Mr. R. A. Thompson, Chairman of the Joint Committee, resigned. Mr. W. C. Earle, his successor as Chief Engineer of the Commission, was elected to membership and chairmanship. Subsequently Mr. Earle resigned and Mr. Richard Sachse, Acting Chief Engineer of the Rail- road Commission, was elected to membership and chairmanship. Mr. L. M. Ellis resigned and Mr. R. W. Mastick, Transmission and Protection Engineer of The Pacific Telephone and Telegraph Company, was elected to membership. Mr. H. S. Warren, Electrical Engineer of the American Telephone and Telegraph Company, was elected to honorary membership. Mr. James T. Shaw, Secretary of the Joint Committee, resigned. Mr. A. R. Kelley was elected to the office of secretary. The vacancy in membership created by the resignation of Mr. Shaw was later filled by the election of Mr. A. L. Wilson, Assistant Rate Expert of the Railroad Commission. Mr. James T. Shaw was elected to honorary membership. The death of Mr. L. N. Peart created a vacancy in membership which was filled by the election of Mr. J. P. Jollyman, Engineer of Electrical Construction of the Pacific Gas and Electric Company. Mr. A. R. Kelley resigned. To date the vacancy created by Mr. Kelley 's resignation has not been filled nor has a permanent secretary been elected. Early in its work the Joint Committee established a field engineering staff, reporting to the Subcommittee on Tests, to conduct the necessary tests and investigations. This field staff was composed of engineers REPORT ON INDUCTIVE INTERFERENCE. 7 in the employ of The Pacific Telephone and Telegraph Company and the American Telephone and Telegraph Company, and was later aug- mented by the addition of two engineers and a stenographer engaged by the Joint Committee. Previous to the formation of this committee in December, 1912, The Pacific Telephone and Telegraph Company had started an investigation (-f inductive interference between the lines of the Coast Counties Gas and Electric Company and the lines of the telephone company in the neighborhood of Morgan Hill in Santa Clara County. This investiga- tion was completed by the Joint Committee and its results have been considered in connection with other work carried out by the Joint Committee. In January, 1913, the Joint Committee established its field staff at Salinas, to investigate parallels on the lines of the Sierra and San Francisco Power Company north of Salinas and on the line of the Coast Valleys Gas and Electric Company south of Salinas, both of these power lines being parallel with the lines of The Pacific Telephone and Telegraph Company, the Western Union Telegraph Company and the Southern Pacific Company's signalling system. The investigation at Salinas continued from January, 1913, until July, 1913. The specific work undertaken at Salinas was : a determination of the magnitude and characteristics of the induction produced in the com- munication circuits, the factors in the power circuits causing this induc- tion, the quantitative relationship of cause and effect, and a comparison of the effects on the parallels north of Salinas with the neutral of the power circuit alternatively grounded beyond one end of the parallel and beyond both ends of the parallel. In July, 1913, the field headquarters were moved to Santa Cruz. At this point the committee desired to test the relative merits of various schemes of transpositions for both power and telephone circuits, and to complete the investigation begun at Morgan Hill on the system of the Coast Counties Gas and Electric Company, which system is of a different character from that studied at Salinas. A mathematical study of transpositions in general, and particularly of those for the parallel between Santa Cruz and Watsonville, has been completed. The experi- mental study of these schemes of transpositions has not yet been completed. Owing to the peculiar nature of the experimental work and the refinements required, suitable apparatus was not easily obtainable and in many instances it was necessary to design and develop special apparatus for certain of the tests. A considerable amount of time has necessarily been spent at all points of the tests in choosing from the almost innum- erable things which could be investigated with profit, those of greatest value which could be carried out with the means at hand. In the course of its investigations the committee has prepared a series of fifty technical reports which present and discuss in detail the various features of the work, the methods and apparatus employed and the results accomplished. These reports, which are on file at the committee headquarters in the offices of the Railroad Commission of California, are listed in Appendix V. 212191 8 REPORT ON INDUCTIVE INTERFERENCE. RESULTS ACCOMPLISHED. The following paragraphs summarize very briefly the principal results accomplished to date. These statements of results are accom- panied by brief explanatory comment upon the conclusions reached. The reasons for and explanations of these conclusions are given in more detail in the appendices, to which reference is made. 1. Interference to telephone circuits under normal operating condi- tions of power circuits arises almost wholly from the harmonic voltages and currents of the power system. (See Appendix I.) This is due chiefly to the fact that the frequencies of the harmonics generally present in the voltages and currents of power systems cover a considerable portion of the range of the voice frequencies, particularly those frequencies at which telephone instruments and the human ear are of maximum sensibility. Extraneous currents of frequencies approach- ing the average voice frequency have a more injurious effect upon tele- phone conversation than currents of lower frequencies. 2. The effect of induction of the fundamental frequency on telephone circuits is comparatively unimportant unless it is of magnitude sufficient to constitute a physical hazard. (See Appendix I.) This is due to the fact that the fundamental approaches the lower limit of audible frequencies, at which the telephone and the human ear are not efficiently responsive. 3. Interference to telegraph and other signalling circuits is due prin- cipally to the fundamental and lower harmonics. (See Appendix I.) Telegraph receiving instruments are relatively insensitive, as com- pared with the telephone, to the higher harmonics, but are sensitive to disturbances of lower frequencies, such as the fundamental and lower harmonics which more nearly approach the normal operating frequency of such circuits. 4. The power circuit currents and voltages may be divided into two factors: balanced and residual, of which, for equal magnitude, the latter in general produce the greater inductive interference. (See Appendix II.) Residual currents and voltages act inductively in a similar manner to single-phase currents and voltages acting in a circuit composed of the line conductors in parallel with earth return, which is a condition favorable to very large induction. Moreover, such a circuit which in- cludes the earth as one side can not be transposed. Transpositions in the power circuit can not reduce the inductive effect of residuals except as they reduce the magnitudes of the residuals themselves, which they do in some cases. The inductive interference arising from such currents and voltages can be reduced only in the case of metallic cir- cuits, such as telephone circuits, by transposing these circuits. It is, therefore, important that the telephone circuits be transposed at fre- quent intervals throughout parallels and carefully balanced throughout their entire length, and that the residual currents and voltages be kept sufficiently small to give negligible induction in telephone circuits so arranged. 5. Inductive interference to communication circuits, arising from the balanced voltages and currents can in a large measure be prevented by means of an adequate system of transpositions applied to both power and communication circuits (assuming the latter are metallic) and located with due regard to each other. REPORT ON INDUCTIVE INTERFERENCE. 9 This is accomplished partly by creating mutually neutralizing induc- tive effects in neighboring lengths of each side of the communication circuit or circuits by transposing the power circuit, and partly by equalizing the inductive effects on the two sides of the communication circuit or circuits by exposing each side equally to the influence of the power circuit by transposing the communication circuit. 6. Abnormal conditions and at times switching operations produce transient disturbances of a very severe character. This is due to the fact that abnormal conditions almost invariably give rise to residuals of large magnitude, often including high har- monics. Abnormal occurrences incident to electrical power transmis- sion do not give warning of their occurrence, and since they can not be produced artificially on transmission systems without subjecting the apparatus to great risk or danger, it has been deemed unwise to attempt any experimental tests of these effects. This conclusion is therefore drawn from general experience and data of actual occurrences collected by the committee. RULES RECOMMENDED BY THE COMMITTEE. The following are the rules which the committee, as the result of its study to date, recommends be issued at this time to govern the future construction and operation of power and communication circuits which are or are proposed to be so located as to create a parallel, as herein- after defined : OUTLINE OF RULES. DEFINITIONS. a. Power Circuit. b. Communication Circuit. c. Telephone Circuit. d. Line. e. Parallel or Parallelism. f. Residual Current. g. Residual Voltage. h. Transposition. I. AVOIDANCE OF PARALLELISM. II. CONDITIONS UNDER WHICH PARALLELISM WILL BE PERMITTED. a. Minimum Horizontal Separation. b. Balance of Power System. c. Limitation of Residual Currents and Voltages. d. Transpositions Inside Limits of Parallel. e. Transpositions Outside Limits of Parallel. f. Uniformity of Parallel. g. Transformer Connections. h. Switch Equipment. i. Switching. j. Use of Air Switches. k. Abnormal Conditions. 1. Devices for Indicating Abnormal Conditions on Systems Isolated from Ground. m. Procedure Under Abnormal Conditions. n. Ammeters in Neutral Ground Connections. o. Charging Electrolytic Lightning Arresters. p. Wave Form of Rotating Machines. q. Exciting Current of Transformers. III. PROVISIONS APPLYING TO EXISTING PARALLELS. IV. WAIVER OF CONDITIONS BY COMMUNICATION COMPANY. V. PARALLELISM WITH ALTERNATING CURRENT RAILWAYS. 10 REPORT ON INDUCTIVE INTERFERENCE. DEFINITIONS. The following definitions are given of certain technical terms em- ployed herein : a. Power Circuit. The term "power circuit" includes any over- head constant potential alternating current power transmis- sion or distribution circuit or electrically connected network which has a voltage of five thousand volts or more between any two conductors, or of three thousand volts or more be- tween any conductor and ground. b. Communication Circuit. The term "communication circuit" includes any overhead, open wire telephone, telegraph, or signalling circuit which is used in the service of the public. c. Telephone Circuit. The term "telephone circuit" includes any inter-exchange metallic telephone circuit, and therefore ex- cludes subscribers' circuits. This term also includes any metallic telephone circuit operated by any railroad or other company for dispatching purposes or for public use between separate communities. d. Line. The term "line" means any circuit or aggregation of circuits carried on poles or towers. e. Parallel or Parallelism. The terms ' ' parallel " or " parallelism ' ' refer to cases where a power line and a communication line follow substantially the same course, or are otherwise in prox- imity for a sufficient distance, so that the power circuit is liable to create inductive interference in the communication circuits. f. Residual Current. 'The term "residual current" denotes the vector sum of the currents in the several conductors of a power circuit. g. Residual Voltage. The term "residual voltage" denotes the vector sum of the voltages to ground of the several conductors of a power circuit. h. Transposition. The term "transposition" denotes the inter- change of position of the several conductors of a circuit. I. AVOIDANCE OF PARALLELISM. Every reasonable effort shall be made to avoid new parallelism. The party proposing to build a new communication' or power line, which will create a parallel, or generally to reconstruct an existing line in- volved in a parallel shall give due notice (at least thirty days wherever possible) of its intention to the other party, including detailed informa- tion as to the location and character of the proposed line. If a plan can be devised and agreed upon by the two parties for maintaining an adequate separation between the two classes of lines so as to avoid inter- ference, this shall be done. In case it is impracticable to secure adequate separation between a power line and a communication line, parallelism will be permitted, subject to the conditions set forth in II. II. CONDITIONS UNDER WHICH PARALLELISM WILL BE PERMITTED. a. Minimum Horizontal Separation. The minimum horizontal sep- aration between the power line and communication line shall be equal to REPORT ON INDUCTIVE INTERFERENCE. 11 the height of the taller line. The only exceptions to this provision are angle crossings and other unavoidable cases of close proximity, and in all such cases the power line shall be kept above the communication line and constructed in conformity with the National Electric Light Association's specifications for overhead crossings or other approved equivalent which may be agreed to by both companies. b. Balance of Power System. The power company shall exercise due diligence to keep the currents in, and the voltages to ground of, the conductors of any power circuit involved in a parallel as closely balanced as practicable. In all cases where telephone circuits are involved, special consideration shall be given to the prevention or elimination of harmonics in the residual current and in the residual voltage. c. Limitation of Residual Currents and Voltages. Pending addi- tional rules on specific means other than those given herein, the parties concerned shall endeavor to agree upon the means to be employed for the prevention or limitation of residual currents and voltages, and in the event of disagreement the matter shall be referred to the Railroad Commission of the State of California. d. Transpositions Inside Limits of Parallel. An adequate system of transpositions shall be installed in the power circuit (or circuits), and in the communication circuit (or circuits) provided the latter is metallic. When both circuits are transposed the transpositions in both the communication and power circuits shall be located with due regard to each other. Every reasonable effort shall be made by both parties concerned to fix the limits of the parallel and the location of crossings, branch lines, and connected apparatus so as to facilitate the application of an effective transposition scheme. In the case of a parallel between a power line and a telephone line the company owning or operating the telephone line involved shall have the right to specify the number, type (in respect to electrical char- acteristics) and location of the transpositions in the power circuit, subject to the following limitations : 1. For power circuits of 50,000 volts or over the average distance between successive transpositions shall not be required to be less than one mile and the minimum distance between any two succes- sive transpositions shall not be required to be less than two thirds of a mile. 2. For power circuits of less than 50,000 volts the distance be- tween successive transpositions shall not be required to be less than one sixth of a mile. The transposition system of the telephone circuits shall be modified where necessary in order that the power and telephone circuits shall be, as nearly as practicable, mutually non-inductive. For short parallels less than six miles in length (or short sections of longer parallels which have to be treated independently because of abrupt change in conditions) with power circuits of 50,000 volts or over, where it is impracticable to obtain an adequate balance by the location of transpositions in accordance with the limit specified above, the com- pany owning or operating the telephone line involved shall have the 12 REPORT ON INDUCTIVE INTERFERENCE. right to specify the number, type and location of transpositions, pro- vided the distance specified between successive transpositions is not less than one half mile. When necessary (due to variations in lengths of telephone transposi- tion sections) in order to secure an adequate balance, a reduction of 10 per cent in the limiting distances between successive power circuit transpositions as given above shall be allowed. In the case of a parallel between a power line and a telegraph line or other grounded communication circuit, the location of the transposi- tions in the power line shall be with due regard to the limits of the parallel in order to form as nearly as practicable a balanced system. The location and type of such transpositions shall be as specified by the communication company, subject to the condition that the trans- po-itions in the power circuit may not be required to be less than one mile apart. In no case shall the power company be required to relocate poles or towers far the transpositions. The parties concerned in any proposed parallel shall endeavor to agree upon a transposition scheme for such parallel in accordance with the above. In the event of a disagreement, the matter shall be referred to the Railroad Commis-ion of the State of California. e. Transpositions Outside Limits of Parallel. In addition to trans- positions within the limits of a parallel, as provided in "d" hereof, each new power circuit isolated from ground (or extension of such existing circuit) which is constructed subsequent to the date when these rules become effective, shall be transposed throughout its entire length in such manner as to balance the electrostatic capacities to earth of its several conductors, so as to avoid inequalities among the voltages to earth of the several conductors, which would create inductive inter- ference. Such transpositions shall not be more than eight miles apart, provided, however, that circuits less than three miles in length are not required to be transposed until they are extended to a greater length; except that extensions or spurs from existing lines, the electrostatic capacities to earth of whose conductors are balanced, shall be so con- structed as not to change materially the balance, of the existing lines to which they are connected. f. Uniformity of Parallel. To facilitate the application of effective transpositions, both parties shall endeavor to maintain uniform separa- tion, uniform arrangement of conductors and uniform relative location of the two classes of circuits within the limits of a parallel. However, when it is feasible to secure a substantial increase of separation between the two lines for a considerable portion of a parallel this shall be done, as such an increase of separation is of more benefit than uniformity. g. Transformer Connections. (1) On any power circuit involved in a parallel, no grounded single-phase, or grounded open-star transformer connections shall be employed. NOTE. This does not apply to railroads operating alternating current trolleys with ground return which are covered by V. (2) On a power circuit involved in a parallel no star-connected trans- formers or auto-transformers with grounded neutral shall be employed, unless delta-connected secondary or tertiary windings or other equiva- REPORT ON INDUCTIVE INTERFERENCE. 13 lent means are used of suppressing the third harmonic components of the residual voltages and currents introduced by the transformers. (3) Where single-phase loads are connected to a polyphase power circuit involved in a parallel the power company shall endeavor to arrange successive connections of this type so as to equalize the loads upon the several phases. (4) On a three-phase circuit involved in a parallel, the power company shall use, wherever practicable, a closed-delta connection in preference to an open-delta connection, and where the latter is employed an effort shall be made to distribute such connections equally upon the several phases. h. Switch Equipment. A power circuit involved in a parallel shall be equipped, between the source of supply and -the parallel, with oil switches, all poles of which shall be mechanically interconnected for simultaneous action. With the exception of stations where an operator is constantly on duty, these switches shall be rendered automatic for short-circuits, grounds, and abnormal neutral currents. i. Switching. All switching on all parts of a system connected to a circuit involved in a parallel, which causes harmful transient disturb- ances in communication circuits, shall be done by means of oil switches, all poles of which are mechanically interconnected for simultaneous operation. j. Use of Air Switches. The use of air switches, on a power circuit involved in a parallel, is prohibited except for purposes of isolating sections of dead line, or for disconnecting transformers under no load. This applies to the entire power system, any circuit of which is involved in a parallel, unless such switching is so remote as not to cause harmful transient disturbances in the communication circuits. k. Abnormal Conditions. A power circuit involved in a parallel shall not be operated at any time with an open, grounded or short- circuited line wire or wires or transformer winding. 1. Devices for Indicating Abnormal Conditions on Systems Isolated from Ground. If a power circuit involved in a parallel is electrically isolated from ground, reliable indicating devices "shall be installed at its source of supply to inform the operator immediately of abnormal conditions, such as grounds and wherever possible, open-circuits, which have not operated automatic switches. Upon indication of trouble by such devices, the operator shall immediately open the oil switches and proceed in the manner outlined in "m. " m. Procedure Under Abnormal Conditions. In case of the opening of an oil switch due to an abnormal condition in a power circuit involved in a parallel, or any circuit supplying or supplied by the same, such switch may be closed once; if opened a second time due to the continuance of the fault or abnormal condition, said switch shall not be closed again until the line has been sectionalized. The fault may then be located by energizing sections of line, provided that further sectionalization of the line be done in such sequence as to cause the minimum disturbance to parallel communication circuits, and provided further that where practicable the faulty section of line shall be ener- gized but once in this process of sectionalization, where the fault exists within or beyond the parallel, until such fault is remedied. 14 REPORT ON INDUCTIVE INTERFERENCE. n. Ammeters in Neutral Ground Connections. Wherever a neutral ground connection is employed on a circuit involved in a parallel an ammeter, suitable for measuring as accurately as practicable the cur- rent in the neutral under normal operating conditions, shall be installed in all neutral connections at the main generating and substations on the power system electrically connected to the circuit involved in the parallel. The power company shall maintain a record of hourly meas- urements of the neutral current at all such points. o. Charging Electrolytic Lightning Arresters. Where a power sys- tem is equipped with electrolytic lightning arresters so charged as to cause inductive interference in communication circuits, the method of charging the arresters shall be modified to eliminate the disturbances as far as possible. The charging of such lightning arresters shall be done at such time as to give the minimum liability of interference with communication circuit operation, preferably between the hours of 2 a. m. and 4 a. m. p. Ware Form of Rotating Machines. The power company shall make every effort to obtain generators and synchronous motors for use on all parts of the system, giving, as nearly as rea-:onably possible, pure sine waves of voltage at fundamental frequency. In no case shall the deviation from a pure sine wave exceed the limit set forth in the Standardization Rules of the American Institute of Electrical Engineers. q. Exciting Current of Transformers. In order that the wave shapes of voltage and current may be distorted as little as practicable by transformers, the main line transformers employed on circuits in- volved in a parallel and on future extensions of such circuits shall have an exciting current as low as is consistent with good practice, and in no case shall the exciting current at rated voltage exceed ten per cent of the 'full load current. Such transformers shall not be operated at more than ten per cent above their rated voltage. III. PROVISIONS APPLYING TO EXISTING PARALLELS. The following sections of II shall apply also to power circuits involved in existing parallels: b, i, j, k, 1, m, o, p, and q. Also, g-3 and g-4 shall apply to existing parallels to the extent that transformers added hereafter shall be connected as provided in said rules. IV. WAIVER OF CONDITIONS BY COMMUNICATION COMPANY. At the option of the company operating the communication circuit or circuits any of the provisions of II and III may be waived, provided that such waiver does not increase the hazard. V. PARALLELISM WITH ALTERNATING CURRENT RAILWAYS. It is recognized that railroads operating alternating current trolleys with ground return create serious inductive interference with parallel communication circuits. In the present state of the art, no means for completely overcoming inductive interference from such parallels is known, hence, they are to be avoided if possible and where unavoidable, the responsibilities arising therefrom must be settled by mutual agree- ment or in case of inability to agree the matter shall be referred to the Railroad Commission of the State of California. REPORT ON INDUCTIVE INTERFERENCE. 15 DISCUSSION OF RULES. It will be noted from the definitions that the terms "power circuit" and ''telephone circuit" are used in these rules in a spiral, restricted sense. (I) The first and most obvious means of preventing inductive inter- ference is to avoid the close association of power and communication circuits. Further, it is recognized that in no other way can complete freedom from interference be secured. While, with the ever increasing iu1 work of electrical circuits of all kinds, adequate separation to avoid interference is becoming increasingly difficult to maintain, the com- mittee feels that the importance of such separation justifies its being made the first premise in rules designed to prevent inductive interference. Notice, sufficiently in advance, should be given the other party or parties concerned in any proposed parallel in order that thorough consideration may be given by both parties to possible means of avoiding the parallel or, in case the parallel can not be avoided, to the necessary remedial measures to be employed. (Il-a) The best insurance against physical hazard in case of close proximity is to maintain a separation equal to the height of the taller line, thus avoiding the possibility of physical contact in case of failure. In the case of crossings and unavoidable cases of close proximity for short distances extra strength construction is necessary as a precaution against failure. (II-b-c) As has been pointed out under the heading "Results Ac- complished," and more fully explained in Appendix II, residual voltages and currents are particularly troublesome factors in causing interference. Means to eliminate or reduce such residuals in power systems are highly important and while information at this time does not enable the committee to formulate as explicit a rule as is desirable, yet the importance of the subject justifies its inclusion in the rules. The acquisition of further information on which to base a more explicit rule upon this subject is a most important problem, the experimental study~of which is discussed in the following section of this report. (Il-d) Transpositions properly located in both power and communica- tion circuits offer the most reliable and effective means for preventing interference from balanced voltages and currents of power circuits. While the inductive effects increase in severity for the higher voltage circuits, due in part to the increased separation of the line conductors, which renders more frequent transpositions desirable, the mechanical difficulties involved are so great as to overbalance the other reasons and the rules, therefore, provide for less frequent transpositions in the higher voltage circuits than in the lower voltage 'circuits. A further reason for frequent transpositions in the lower voltage circuits is the necessity of a flexible system of transpositions applicable to short parallels which generally occur with such circuits. (Il-e) The provision requiring transpositions outside the limits of a parallel on systems electrically isolated from ground is an explicit measure for carrying out the purpose of the more general provision given under II-b-c, "Balance of Power System" and "Limitation of Residual Voltages and Currents." 312191 16 REPORT ON INDUCTIVE INTERFERENCE. (Il-f ) Non-uniformity of separation and type of construction within the limits of a parallel are inequalities which can not in many cases be taken into account in the design and layout of transposition schemes. Such inequalities tend to nullify the effectiveness of the transpositions, hence it is desirable that they be avoided. A precautionary statement is included in the rule in order that the possibility of securing a wide separation for a considerable portion of a parallel may not be sacrificed for the sake of absolute uniformity throughout the entire length. (Il-g) Some types of transformer connections and methods of opera- tion give rise to large residual voltages and currents and certain pro- visions of the rules are designed to prohibit or restrict the use of such connections and methods of operation. Thee rules may be considered as explicit provisions complying with the general provision in II-b-c, "Balance of Power System" and "Limitation of Residual Voltage and Currents." The sufficiency of these specific provisions as an insur- ance against harmful residual voltages and currents is subject to future determination. The present information of the committee does not warrant the definite recommendation of any one type of connection or method of operation as best from the standpoint of inductive interference. This is true as to the relative merits of the two general types of systems, the grounded neutral and the isolated system. The advantages and di-advantages of these general types and any modifications of these types are dependent upon their inherent characteristics in respect to residuals and the limitations and control of residuals under both normal and abnormal conditions. Both types are on an equality with respect to the interference caused by balanced voltages and currents. (Il-k) Continued operation under certain abnormal conditions is possible in some power systems. In particular, it is possible to operate a grounded star-connected system with one phase open, and it is possible to continue the operation of an isolated system when one phase becomes grounded accidentally. The former gives rise to a large resid- ual current and the latter to a large residual voltage, both of which are liable to render parallel communication circuits inoperative. For these reasons the rule prohibits such operation which, aside from the con- sideration of inductive interference, does not constitute good practice in power system operation. (II-h-1-n) To provide that operation under the abnormal conditions mentioned above may not continue without the knowledge of the power company, the rules specify that devices for indicating grounds shall be installed on isolated systems. With respect to grounded star-connected systems, the rules specify with certain exceptions the automatic opening of switches by abnormal neutral currents. In such systems ammeters are required in all main neutral ground connections. Such ammeters, read regularly, afford means of detecting abnormal neutral currents and are of value in showing the degree of balance of the system, as the neutral current is easily affected by unbalanced conditions. (Il-m) Accidental causes give rise to occasional abnormal conditions. These can only be guarded against by good construction and mainten- ance, and careful operation which, however, can not prevent entirely such occurrences. When trouble develops on a power circuit involved in a parallel, it is always liable to cause serious interference to the REPORT ON INDUCTIVE INTERFERENCE. 17 coirimunication circuits, if the exposure is severe. In the present state of the art, the method of fault location on power circuits is a process of repeated sectionalization and energization of the faulty line until the fault is located within certain limits. This process causes repeated interruptions with loss of time in the operation of the communication circuits, and in the case of telephone circuits is accompanied sometimes by injury to the operators. It should be explained that the loss of time is much greater than the duration of the disturbance, owing to the time required to restore the protective devices on the communication circuits to their normal condition. No method of locating faults on power cir- cuits is known which meets the requirements of practice and yet avoids the disadvantages of the present method. The inductive disturbances due to fault location can be to a considerable degree ameliorated by disconnecting the faulty line from the rest of the system and energizing this line by a single generator at such excitation as may be necessary to overcome the insulation of the fault. Whenever practicable this method is employed by power companies ; hence, it has not been thought necessary to cover it by a specific rule. In view of these facts, the committee is recommending the limitation of the present practice in this regard so as to avoid, as far as seems practicable, the repeated interruptions to communication circuit opera- tion. It is highly desirable that some better method of fault location be developed, not only because of the attendant consequences of the present method on communication circuits, but also because of the abnormal strains to which the power apparatus is necessarily subjected. (II-h-i-j) Normal switching operations on power circuits produce at times severe transient disturbances in parallel communication circuits. The commonly recognized fact that oil switches produce less severe transient disturbances in power circuits, affords the basis for the pro- visions in the rules dealing with switches and switching. The auto- matic features required are designed to prevent continued operation under abnormal conditions. (II-o) Transient disturbances of severe nature to telephone circuits are sometimes caused by the charging of electrolytic lightning arresters. There are available methods of diminishing the transients due to this cause, and a general provision to the effect that such methods shall be employed when necessary is included in this rule. It is further pro- vided that the charging of arresters should be done at times when the telephone circuits are least used. (II-p-q) Fundamentally, interference to telephone circuits by power circuits in normal operation is largely due to the existence of harmonics in the currents and voltages. While the complete elimination of these harmonics seems impracticable, still beneficial results may be obtained by practical efforts in this direction, and the committee feels that the two general provisions as to the wave form of rotating machines and the exciting current of transformers are of great importance both from a practical standpoint and also as enunciating a general principle. The matter of generator wave form particularly is of importance for all types of systems. The provision with reference to the exciting current of transformers, while desirable in all cases, is particularly so on grounded star-connected systems. 18 REPORT ON INDUCTIVE INTERFERENCE. (III) Certain of the measures in II, particularly those referring to power system operation, which are helpful in mitigating inductive inter- ference have been recommended to apply to existing parallels. (IV) Since these rules are designed for the protection of communica- tion circuits, it is proper that the companies operating such circuits be given the right to waive any measures of protection which they may in any particular case consider unnecessary. (V) The committee has undertaken no investigation of cases of par- allelism with alternating current railways, but as the seriousness of this class of exposure is recognized, it was thought desirable that it be referred to specifically. FUTURE WORK. The further work necessary in order to secure the information essen- tial as a basis of determining more explicit and effective rules than those herein recommended, is particularly concerned with the subjects of transpositions and residual voltages and currents. In order to cover these subjects in as effective and economical a manner as possible it is thought that the procedure should be along the following lines: 1. Experimental study of transpositions which includes the deter- mination of : (a) The practical effectiveness of transpositions in both power and communication circuits as a means of reducing induction aris- ing from balanced voltages and currents ; involving considerations of different co-ordinated transposition schemes, particularly with different lengths of power circuit barrels. (6) The practical effectiveness of transpositions in communica- tion circuits as a means of reducing inductive interference arising from residual voltages and currents ; involving considerations of different systems, particularly different lengths of balanced com- munication circuit transposition sections. (