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DOCUMENTS
DIPT-
Joint Committee on Inductive
Interference
TO THE
Railroad Commission of the State
of California
Presenting the results of an investigation of inductive interference by power circuits
with communication circuits and including rules recommended for
the prevention or reduction of such interference
SAN FRANCISCO, CALIFORNIA
SEPTEMBER 28, 1917
CALIFORNIA STATE PRINTING OFFICE
SACRAMENTO
1918
GIFT OF
FINAL REPORT
OF THE
Joint Committee on Inductive
Interference
TO TNI:
Railroad Commission of the State
of California
Presenting the results of an investigation of inductive interference by power circuits
with communication circuits and including rules recommended for
the prevention or reduction of such interference
SAN FRANCISCO, CALIFORNIA
SEPTEMBER 28, 1917
t'ALll OUXTA STATE PRINTING OFFICE
SACIIAMKNTO
1918
=;0t> *
TABLE OF CONTENTS.
PAGE
INTRODUCTION 7
LETTER OF TRANSMITTAL 9
REPORT 11
PART ONE.
History of Committee's Organization and Work.
\'\n niation of Committee^. 12
I Vrsonnel __ 12
< >rganizution 13
Investigations , 14
.I'M nances _: 18
PART Two.
Explanation of Problem and Summary of Results.
Nature of Subject : . 19
Summary of Facts Established 21
I. Primary Cause 21
1^, Interference to Telephone Circuits Harmonies 21
::. Interference to Telegraph Circuits 22
I. balanced and Residual Components 22
5, C;MIS-S and Remedies for Residuals 23
(i. factors Affecting Intensity and Magnitude of Induction 25
(a) Dimensional Factors 25
(&) Electrical Factors ^ 25
7. Transpositions 26
8. Unbalance of Communication Circuits 27
9. Transients and Abnormal Conditions 27
10. Nonessential Features Cause Greatest Interference 28
Guiding Principles for Preventing Interference 28
PART THREE.
Revised Rules Recommended by Committee.
Reasons for Revising Rules _ 39
Text of Revised Rules _ 30
I. GENERAL PROVISIONS _ 39
(a) Applicability of Rules 30
(&) Co-operation 30
(c) Principle of Least Cost 31
(ri) Existing Parallels 31
(e) Saving Clause _ 31
387732
TABLE OF CONTENTS.
Text of Revised Rules Continued. PAGE
II. DEFINITIONS 31
(a) Class H Power Circuit 31
(b) Electrically Connected 31
(c) Signal Circuit 31
(<7) Communication Circuit 32
(e) Line 32
(f) Parallel 32
(g) Configuration 32
(h) Transposition 32
() Barrel 32
O') Discontinuity 32
(fe) Co-ordination 33
(Z) Balanced and Residual Voltages 33
(m) Balanced and Residual Currents 33
III. LOCATION OP LINES 33
(a) Avoidance of Parallels '. 33
(b) Notice of Intention 33
(c) Distance Between Lines 34
(rf) Length of Parallels 34
(e) Discontinuities 34
IV. DESIGN AND CONSTRUCTION OF LINES 34
(a) General Requirements 34
(b) Arrangement and Spacing of Power Conductors 34
(c) Transpositions General _ 35
() Constant-Current Lighting Circuits 58
(c) Power Circuits of Lower Voltages 59
(( UK SI alt of California:
GENTLEMEN: Nearly five years ago the Joint Committee on Inductive
Interference was organized under the auspices of the Railroad Com-
mission of the State of California and began its investigations with the
object of developing a better understanding of the subject of inductive
interference and particularly of acquiring information necessary for
establishing regulations which could be accepted by all interests con-
cerned as being effective, comprehensive and reasonable.
In July, 1914, the Joint Committee rendered to the Railroad Com-
mission a preliminary report containing an account of its investigations
up to that time and including provisional rules which were recom-
mended for immediate adoption. These rules were approved and are
embodied in your General Order No. 39, which is now in effect in
California.
In its preliminary report the Joint Committee outlined further
investigations which it seemed important to make preparatory to putting
the rules into more permanent shape and asked your authorization to
carry on these investigations. This request being approved, the work
was resumed and much additional information has since been obtained.
The past few months have been devoted to the preparation of a report
outlining briefly the main features of all work done by the Joint Com-
mittee. This report, which is submitted herewith, contains a draft of
revised rules which are believed to represent a substantial improvement
over the rules contained in the 1914 report. The Joint Committee
recommends that these revised rules be approved by the Railroad Com-
mission and issued as a new order to supersede General Order No. 39.
An attempt was made to meet the urgent demand for information
defining the limiting relationships between power and communication
lines which constitute a parallel. It was found, however, that sufficient
information was not at hand to satisfy all parties as to a basis upon
which these relationships cguld be set forth definitely.
While the subject of inductive interference offers an almost inex-
haustible field for further investigation, the Joint Committee feels that
the object for which it was formed has now been substantially
accomplished and it seems unnecessary that its work should be further
prolonged. The rules recommended will constitute, it is hoped, a
.satisfactory basis of procedure for several years at least, in dealing
with cases of inductive interference, assuming a proper spirit of
co-operation among the companies concerned. The rules themselves
definitely call for and emphasize the necessity for this co-operation.
In course of time, no doubt, the experience gained in the application of
these rules, together with advances in the art, will make it desirable
that the rules be changed in some respects, but such changes are thought
to be a matter of the more or less distant future and not to require the
continuance of the Joint Committee.
10 FINAL REPORT ON INDUCTIVE INTERFERENCE
From time to time during the course of this investigation Technical
Reports have been prepared setting forth in detail the matters which
have been studied and the results and conclusions derived. There are
in all 71 of these Technical Reports which are listed in Appendix II ot f
the report herewith transmitted. As they contain a large amount of
valuable information and are in fact the only records of most of the
data derived by the Joint Committee, it is particularly important that
they be rendered available to persons interested. To this end, the
Joint Committee has selected 30 of the most valuable of these Technical
Reports and earnestly recommends to the Railroad Commission that
these be printed in a suitable volume by the State of California and
offered for sale to libraries, colleges, companies, societies and individuals
interested. The reports recommended for publication are so desig-
nated in the list given in Appendix II.
The official copies of the Technical Reports, other original records,
and the correspondence files of the Committee are ready to be placed in
the custody of the Commission.
In transmitting this report, which marks the conclusion of its work,
the Joint Committee desires to express its satisfaction at the degree
of success which has been reached in composing the differences formerly
existing in California between the power and communication interests,
differences due principally to lack of familiarity with the physical
aspects of this subject. While it would be too much to say that there
is now complete unanimity of opinion on every feature, still the Joint
Committee has been able, after careful consideration of all information
available, to agree unanimously upon the present report. In the
opinion of the Committee the results accomplished emphasize strongly
the superiority of a co-operative investigation of this kind, whereby the
fundamental facts are ascertained and acted upon, as compared with
litigation or other methods of arbitrary settlement without the benefits
derived from such investigation.
In conclusion, the Joint Committee takes pleasure in acknowledging
that the credit for what has been accomplished is due primarily to the
Railroad Commission of the State of California which has consistently
held to the policy of a co-operative investigation and has cordially sup-
ported this Committee in its work.
Respectfully submitted.
(Signed)
A. II. GRISWOLD, JAMES T. SHAW,
II. A. BARRE, RICHARD SACHSE,
J. E. WOODBRIDGE, F. EMERSON HOAR,
J. L. ORD, ARTHUR F. BRIDGE,
V. V. STEVENSON, A. L. WILSON,
R. W. MASTICK, JOHN A. KOONTZ,
HOWARD. S. WARREN, J. P. JOLLYMAN,
C. H. TEMPLE, P. M. DOWNING,
A. H. BABCOCK.
FINAL REPORT
OF THE
Joint Committee on Inductive Interference to
the Railroad Commission of the
State of California
INTRODUCTION.
This report embodies the results of an investigation by the Joint
Committee on Inductive Interference, extending over a period of
approximately four years. The task undertaken by this Committee
was a study of the problem of interference with communication (signal)
circuits caused by the inductive effects of neighboring power (electrical
supply) circuits, including field experiments and tests necessary to
determine the underlying physical facts, and the preparation of recom-
mendations to the Railroad Commission of the State of California for
its guidance in making rulings designed to prevent or mitigate such
interference.
A previous report by this Committee to 'the Railroad Commission was
rendered on July 7, 1914, embodying the results of the first two years
of the investigation and including provisional rules for the prevention
or mitigation of inductive interference, based on the information
available at that time. The rules therein recommended were adopted
by the Railroad Commission in its General Order No. 39, effective
August 20, 1914. The previous report also contains an outline of
further investigations which this Committee considered essential in
order that additional information might be acquired for amplifying
and revising the rules to make them more definite and complete. These
further investigations have now been carried out, as far as practicable,
Miid certain additional work directed toward the same end has also
been done.
Having completed Hie field investigations and having carefully
analyzed and studied all information accumulated, the Committee now
presents its final report, including revised rules for preventing or
reducing inductive interference. The new rules are not radically dif-
ferent in substance from, the rules formerly recommended and now in
effect in California, but are considerably changed in form and arrange-
ment. They are more specific as to certain of the requirements, more
complete in several respects, less arbitrary in setting physical limits
to be observed, more clearly expressed and, it is believed, better adapted
to the conditions of practical use.
This report, which contains an account of the Committee's work
from the beginning, including that covered by the preliminary report,
is divided, for convenience, into three parts as follows:
Part One gives a historical account of the Committee's formation
and activities.
Part Two presents, in nontechnical language, so far as possible, the
nature of the subject, a brief resume of the principal facts established
or agreed upon by the Committee, and a concise statement of the
physical principles underlying preventive or remedial measures.
Part Three contains the Committee's final recommendations for rules,
together with explanations of the same in detail,
12 FINAL REPORT ON INDUCTIVE INTERFERENCE
PART ONE.
HISTORY OF COMMITTEE'S ORGANIZATION AND WORK.
Formation of Committee.
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
Railroad Commission of the State of California. As an alternative to
contesting the issue at that time it was agreed by the power and com-
munication companies, 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 due to inductive interference.
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, experi-
ments, and investigations, the results of which would serve as a basis
of recommendations for rules and regulations to be issued by the Com-
mission, 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 Com-
mittee on Inductive Interference, representing the Railroad Commis-
sion and railroad, power and communication interests of the state,
was organized and authorized by the Commission to conduct the desired
investigation.
Personnel.
The personnel of the Committee selected is given below.
Representing the 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.
Mr. C. H. Temple, General Manager, United States Long Distance
Telephone Company.
Mr. L. M. Ellis, General Manager, Union Home Telephone
Company.
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 Compan5\
Mr. J. E. Woodbridge, Chief Engineer, Sierra and San Francisco
Power Company.
FINAL REPORT ON INDUCTIVE INTERFERENCE 13
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, who succeeded Mr. Earle as Chief
Engineer of the Railroad Commission, was elected a member and
Chairman of the Committee.
.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 w r as 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 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, and Mr. A. F. Bridge, Assistant Electrical
Engineer of the Railroad Commission, was elected to membership and
to the office of Secretary.
Mr. R. W. Gray resigned and Mr. J. L. Ord, Division Plant Superin-
tendent of the Western Union Telegraph Company, was elected to
membership.
Organization.
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 its tests and
investigations.
For the more efficient conduct of its work the Joint Committee was
divided into several subcommittees, each assigned to and responsible
for certain branches of the investigation. The present organization of
the Committee is given on a chart presented as Appendix V.
Early in its work the Committee established a field engineering staff,
reporting to the Subcommittee on Tests, to conduct the necessary tests
and investigations. This field staff was at first composed of engineers
in the employ of The Pacific Telephone and Telegraph Company and the
American Telephone and Telegraph Company, and was later augmented
by the addition of two engineers and a stenographer, engaged by the
Committee. Since August, 1914, the stenographer has been provided
by the Railroad Com mission. In November, 1914, a third engineer was
839628
14 FINAL REPORT ON INDUCTIVE INTERFERENCE
engaged by the Committee and three were retained in its employ for
nearly a year.
The Committee wishes to express its appreciation of the able manner
in which Mr. L. P. Ferris has supervised the analytical and theoretical
work.
Investigations.
Previous to the formation of the Joint Committee in December, 1912,
The Pacific Telephone and Telegraph Company had started an investi-
gation of 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 inves-
tigation was completed by the Committee and its results have been
considered in connection with other work carried out by the Committee.
In January, 1913, the Committee established its field staff at Salinas,
to investigate parallels on the line 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 con-
tinued from January, 1913, until July, 1913.
The work undertaken at Salinas was for the purpose of determining
(1) the magnitude and characteristics of the induction produced in the
communication circuits, the factors in the power circuit causing this
induction and the quantitative relationships involved; and (2) the effect
of the condition of the neutral (grounded or nongrounded) of the
autotransformers at Salinas, on the induction in the communication
circuits.
Tests were made of the induction in the communication circuits, both
north and south of Salinas, under operating conditions of the power
circuits and with the neutral at Salinas alternatively grounded and
nongrounded. Tests were also made with special methods of energizing
the power circuit, in order to determine the relative importance of
various factors in causing the induction. This determination was also
made by theoretical methods, by computation of induction based upon
the dimensions of the parallel involved, and the results compared with
those of the tests. Instrument transformer equipment was investi-
gated in order to determine the errors thereby introduced in measure-
ments of the power-circuit voltages and currents.
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 dif-
ferent 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 was completed.
During the time the field headquarters of the Committee were at
Santa Cruz, the report of the Committee to the Railroad Commission,
*For definition of "transposition" see page 32.
FINAL REPORT ON INDUCTIVE INTERFERENCE 15
dated 'July 7, 1914. was presented. This report contained ;m account
of the formation of the Committee, its activities and the results accom-
plished up to that date, and also included such recommend jit ions for
rulings by the Railroad Commission, as seemed justified to the
( omiiiittee at that time. In addition, there was given a program of
future work designed to put the Committee in possession of informa-
tion which would permit making the recommended ridings more definite
and complete.
In a letter of acknowledgment to the Committee the Railroad Com-
mission approved the program of future work which was laid down
in the report and authorized the continuance of the Committee's
investigations. This program comprised experimental studies both of
transpositions and of residual* voltages and currents of power circuits.
The study of transpositions included: (1) the determination of the
practical effectiveness, in reducing induction, of systems of power and
communication circuit transpositions properly co-ordinated with each
other, with consideration of different lengths of balanced sections; (2)
the influence of imperfect electrical balance of communication circuits
in impairing the effectiveness of transposition systems, and (3) practical
effectiveness of transpositions in a power circuit isolated from ground in
balancing the voltages between the several conductors and ground, with
consideration of the relative efficiency of barrels** of different lengths.
The study of residual voltages and currents included an experimental
investigation of different types of power system connections and
apparatus with respect to the production of residual voltages and cur-
rents, of means to be employed to limit their magnitudes and the
determination of the minimum values which will produce harmful
inductive interference.
The work outlined in this program was continued at Santa Cruz until
November 24, 1914. The experimental study of the effectiveness of
transpositions in power and communication circuits undertaken at this
point could not be carried out, due to lack of suitable equipment. An
investigation of the effect of various transformer connections and of
the magnetic density employed in transformer iron on the residual
voltages and currents introduced in grounded-neutral networks by such
transformers was begun. In addition, from Santa Cruz as head-
quarters, measurements were made with portable apparatus at various
points on the systems of the Coast Counties Gas and Electric Company,
the Sierra and San Francisco Power Company, and the Pacific Gas and
Electric Company in order to study their characteristics with respect
to residual voltages and currents.
On November 24, 1914, the field headquarters and laboratory of the
Committee were moved to San Fernando. This location offered a
number of advantages for experimental work, the chief one being the
presence of an unused thirty-seven mile 15,000-volt line of the Pacific
Light and Power Corporation which was available for testing at all
times. A telephone circuit carried on the same poles with the 15,000-
volt circuit was also available, constituting a parallel for experimental
purposes. In addition, transformers were loaned by the Pacific Light
and Power Corporation which, with other transformers already pro-
*For definition of "residual" see page 33.
**For definition of "barrel" see page 32.
16 FINAL REPORT ON INDUCTIVE INTERFERENCE
vided by the Sierra and San Francisco Power Company, gave oppor-
tunity for carrying out transformer studies.
The principal experimental work undertaken at San Fernando
comprised studies of the factors affecting the residual voltages and
currents of power circuits, the effectiveness of transpositions in
balancing power circuits and in neutralizing inductive effects, and the
magnitude of inductive effects in short, uniform, nontransposed sections
of parallel. Concerning the residual voltages and currents of power
circuits isolated from ground, the investigations included the effects of
transpositions, leakage, accidental grounds and frequency of alter-
nations. Concerning the residuals of grounded-neiitral circuits the
investigation included the effects of magnetic density of the trans-
formers and of various connections of the transformer banks. In
preparing for the latter study a difficulty was encountered, due to large
double-frequency residual voltages and currents, apparently peculiar
phenomena previously unrecorded and probably of rare occurrence in
practice. These were investigated to a very limited extent, for the
purpose of devising means to overcome them so that the programmed
tests might be carried out.
The availability of both an idle power circuit and an idle telephone
circuit, the conditions of which could be varied for experimental pur-
poses, gave an excellent opportunity for studying induction and the
effect thereon of both power and telephone circuit transpositions and
of telephone circuit unbalances. An extensive series of tests was made
to determine the ratios of induced voltage in the telephone circuit to
inducing voltage or current in the power circuit under different con-
ditions of operation of the power circuit, for a short, uniform, non-
transposed section of parallel. These ratios, termed coefficients of
induction, were also obtained independently by calculations based upon
physical dimensions of the line. The results of the two independent
determinations were compared to ascertain the practicability of obtain-
ing coefficients of induction for other cases by computations thereby
eliminating the necessity for tests. Advantage was taken of the
opportunity at San Fernando to measure the residual voltages and
currents of the Pacific Light and Power Corporation's 15,000- volt
system to supplement the similar measurements previously made on
other systems.
On June 17, 1915, the headquarters of the field staff were moved to
San Francisco, where the work of analyzing the San Fernando data
was completed. At San Fernando the Committee's energies were
largely centered on completing the experimental work, before the power
line was required for service.
It was endeavored to make the analyses and reports as thorough and
complete as possible. For each of the subjects experimentally investi-
gated at San Fernando, theoretical studies were undertaken at San
F'rancisco, which in some cases were much extended in scope over that of
the corresponding experimental work. Where possible, the experi-
mental and theoretical results were compared. The effects of circuit
configuration, or arrangement and relative location of conductors,
transpositions and frequency on the residual voltages and currents due
FINAL RKI'OUT ON INDUCTIVE INTERFERENCE 17
to the line unbalance of power circuits isolated from ground, and 1lic
effects of accidental grounds, were investigated from a theoretical
standpoint. A study was made of the relation of magnetic density of
the transformer iron and of transformer connections to the residual
voltages and currents of triple frequencies thereby introduced into
connected circuits. A report was prepared giving formulas for th<-
computation of coefficients of induction in communication circuits
paralleled by power circuits, including an explanation of the derivation
of the formulas and convenient forms which had been developed for
systematically carrying out such computations.
To determine the effect of configuration and relative position of
power and communication circuits on the induction in the latter, an
extensive series of computations based upon the dimensions of assumed
cases of parallelism, was carried out. The results of this study comprise
214 curve sheets, containing over 3,000 curves, by the aid of which the
values of the coefficients of induction for those cases of parallelism
Avhich occur most commonly may be determined.
Among the important reports prepared at San Francisco is one
reviewing previous work and presenting new data on the subject of
co-ordinating power-circuit and telephone-circuit transpositions as a
means of reducing interference. A new telephone transposition sys-
tem developed by the American Telephone and Telegraph Company,
largely in response to the need for a system of telephone transpositions
having increased flexibility in respect to co-ordination with power-
circuit transpositions, is described and its use illustrated. As examples
of co-ordinated transposition systems, plans are presented for all the
parallels which have been experimentally investigated by the
Committee.
Apparatus suitable for use in the experimental work of the Com-
mittee was not easily obtainable and in many instances it was necessary
to design and develop special apparatus for certain of the tests. In
cases where apparatus was not available for measuring desired
quantities directly, it was necessary to develop methods of measurement
whereby they might be obtained indirectly.
In deciding from time to time upon its program for future work, the
Committee has found it necessary to formulate and consider in detail
many plans of experimentation which have never been carried out. It
has not always been easy to decide upon the best location for carrying
on a particular investigation when each of the several different possible
locations possessed certain advantages. To decide between them or to
choose between different programs of work has meant that the several
plans under consideration had to be worked up in considerable detail
before the preponderance of advantage in favor of some one procedure
could be established. In several cases plans for work regarded as
particularly desirable had to be given up because they were found to
be too laborious, or for other reasons were not feasible.
In the course of the investigation seventy-one Technical Reports have
been prepared, which describe in detail the various features of the work,
the method and apparatus employed and the results accomplished.
These reports, some of which are recommended for publication, are
listed in Appendix II.
18 FINAL REPORT ON INDUCTIVE INTERFERENCE
At the request of the Committee, laboratory investigations were made
in New York by the American Telephone and Telegraph Company, the
Postal Telegraph-Cable Company and the Western Union Telegraph
Company to determine the detrimental effects of extraneously induced
currents on the operation of telephone and telegraph circuits. Reports
of the results of these investigations were submitted to the Committee.
These are also listed in Appendix II.
At various times during the course of its work, the Committee has
contributed discussions before the American Institute of Electrical
Engineers. The Committee's report of July 7, 1914, was presented at
the Spokane Convention of the Institute in September, 1914, and later
at meetings of the San Francisco and of the Los Angeles sections of the
Institute. On each of these occasions considerable discussion was
brought forth. In June, 1915, at the Deer Park Convention in connec-
tion with papers presented on the subject of irregular power -circuit
wave-forms, the Committee submitted a discussion from the standpoint
of inductive interference. In September, 1915, at the Panama-Pacific
Convention the Committee submitted a discussion in which the progress
of the work from July, 1914, to September, 1915, was described. In
September, 1916, at the convention of the Institute held in Seattle,
the Committee submitted a discussion of a paper presented on the
subject of irregular wave-forms.
Finances.
The funds required for carrying on the work of the Committee were
contributed by various telephone, power and telegraph companies.
Such contributions were made at the start of the investigation and
immediately after the report rendered to the Commission on July 7,
1914. Further support was given in the furnishing, by the railroad
companies, of free transportation to the Committee members and
employees while on Committee business, and of the Committee's equip-
ment ; by the telephone companies of the services of their engineers on
the work of the field staff; and by the Railroad Commission of the
stenographer and stationery supplies. The time which the Committee
members devoted to the work was without cost to the Committee.
It is estimated that the total cost of the investigation is more than
$100,000.
FINAL KEPOKT ON INDTCTIVI-; I NTKKFKUKNCE 19
PART TWO.
EXPLANATION OF PROBLEM AND SUMMARY OF RESULTS.
Nature of subject.
The object sought herein is to describe \vluM inductive interference is.
using as far as practicable nontechnical terms, for the benefit of those
not familiar with electrical theory.
The transmission of power electrically by wire circuits in either
large or small quantities requires a current of electricity. Also, to
make electricity flow, there must be in the circuit a voltage or, in other
words, electric pressure, as all circuits offer more or lass resistance or
impedance to an electric current. If the voltage is produced directly
by a battery it forces the electric current around the circuit in one
direction only. Such current is called direct or continuous. Con-
tinuous voltage and current may also be produced by an electric
generator, and this is the common practice for street railways,
but on most other power lines the generators produce an alternating
voltage, that is a voltage which during each short interval of time
known as a period (usually not longer than one twenty-fifth of a
second) varies in value from zero up to a maximum, then diminishes to
zero, increases to a maximum in the opposite direction, and then
diminishes again to zero, repeating this cycle of variations through suc-
ceeding equal periods. Thus, the voltage and the corresponding cur-
rent change in direction or alternate twice each period. The number
of periods or cycles per second is called the frequency.
The voltage associated with any electric circuit is accompanied by an
electric field of force, or condition of stress, in the surrounding space,
whose intensity is proportional to the voltage. At the same time the
corresponding electric current is accompanied by a magnetic field of
force which occupies the same surrounding space and whose intensity
is proportional to the current. Thus any changes in the magnitude or
direction of the voltage and current, such as the alternations described
above, are accompanied by corresponding changes in their fields. The
intensity of these fields of force, in general, diminishes very rapidly
with increasing distance from the circuit.
Conversely, any other circuit within these fields of force will have
voltages and currents set up or ''induced" in it, when changes occur
in the fields, that is, when the voltage or current of the first circuit
changes. Power circuits of the alternating-current type, most com-
monly employed in power transmission, having their voltages and
currents continually varying, will continually induce voltages and cur-
rents in a neighboring communication circuit. These induced voltages
and currents are evidence of the absorption of energy from the fields.
Thus one circuit influences another by the transfer of energy from the
one to the other, without any contact between the wires of the two cir-
cuits. This phenomenon, termed "induction," has long been known,
and has many useful applications in electrical engineering.
To transmit signals over a communication circuit it is necessary thai
the power used, and thus the voltage and current, vary from instant to
instant. In telephone circuits this variation is extremely complex, the
20 FINAL REPORT ON INDUCTIVE INTERFERENCE
current which reproduces the human voice in a distant telephone con-
sisting of a number of component simple currents varying in frequency
from about 100 to 4,000 cycles per second. For telegraph circuits the
variation is much less complex and the frequencies of the important
components of the voltage and current are less than 300 cycles per
second. In both cases the signalling impulses are sent and received by
delicate mechanisms, and the amounts of power required are exceed-
ingly small, particularly for telephone circuits. When communication
circuits are in the field of influence of a power circuit the rate at which
energy is transferred to them by induction may be comparable with,
or even larger than, the power required for their operation, although
entirely inappreciable compared to that of the power circuit. For
example, the power required to operate a small incandescent lamp is
sufficient, if directly applied, to cause a loud noise in several million
telephone receivers.
For power circuits of the type most commonly used in California,
the frequency is either 50 or 60 cycles per second. This is the funda-
mental frequency of the voltage and current, representing useful power,
but there are also present in power circuits other voltages and currents,
usually of relatively small magnitude, of various higher frequencies up
to several hundred cycles per second. These higher frequencies or
harmonics of the fundamental frequency, are the chief cruise of inter-
ference to telephone circuits, since they are of the frequencies of the
sound-waves of the human voice, at which the telephone is most
sensitive. On the other hand, the chief interference with telegraph
circuits is caused by the fundamental or useful frequency of the power
circuits, which most nearly corresponds to the frequency of the tele-
graphic impulses.
The disturbances thus caused in telephone circuits manifest them-
selves as humming noises which impair the intelligibility of conversa-
tion and cause annoyance. In telegraph circuits, chattering of the
relays is caused, the intelligibility of signals is impaired, and the speed
and ease of transmission are reduced.
Under abnormal conditions the inductive disturbance due to a power
circuit may be very greatly increased. When sudden changes take
place in the conditions of the power circuit such as those caused by
energizing or de-energizing the circuit, or when a wire breaks and falls to
ground, relatively large amounts of energy may be suddenly introduced
into the communication circuits. These momentary impulses may be
sufficient to constitute a physical hazard, to operate protective devices or
to cause severe acoustic shocks to telephone operators or users.
Briefly, then, inductive interference may be defined as the impair-
ment of the serviceability of communication circuits resulting from the
transference of energy into them, through intervening space, from
near-by power circuits. The study of inductive interference deals
with the factors affecting the magnitude and character of the induction
and their relationships, the attendant detrimental effects on communi-
cation circuits and the means to be employed in overcoming or
mitigating such interference.
FINAT, RKI'OKT <>N I MMTTIVK 1 NTlvKFKRENCE 2l
Summary of facts established.
It serins desirable to summarize briefly Hie principal technical facts
regarding inductive interference, which may now be considered as
established. Only the most important points are mentioned here, a
detailed technical discussion being given in the Technical Reports.*
1. Primary Cause.
As previously shown in discussing the "Nature of Subject," the
primary cause of inductive interference is the presence, about the
power circuits, of fields of influence which vary in intensity from
instant to "instant, usually in periodic or cyclic fashion. Communica-
tion circuits in regions where these fields are of appreciable strength
absorb energy therefrom, by "induction." When the rate at which
the energy is thus absorbed is of the same order of magnitude as the
power required for the transmission of signals, the impulses received
at the terminals of the communication circuit are distorted and the
serviceability of the circuit is impaired.
2. Interference to Telephone Circuits Harmonics.
Under normal operating conditions of the disturbing power circuits,
interference to telephone circuits, manifested by a humming noise from
the telephone receivers, is due almost entirely to the higher harmonics
of the power-circuit voltages and currents; for the reason that such
harmonics cover a considerable portion of the range of frequencies of
human speech, at which telephone apparatus is most sensitive.
Except when the interference is very slight or very severe, the detri-
mental effect of extraneous current in a telephone receiver increases
approximately in direct proportion to the magnitude of the current.
Increasing the frequency causes a very rapid increase in the detri-
mental effect (roughly as the square of the frequency) up to about 800
periods per second, beyond which there is a gradual decrease. When
several frequencies are present in the extraneous current, the resultant
detrimental effect is considered roughly proportional to the square root
of the sum of the squares of the separate effects of the several single-
frequency components, though this relation has not been definitely
established.
The higher harmonics, which are irregularities of the voltage and
current waves of power circuits, usually result from:
(a) design and construction of generators and motors, whereby
pure sine wave shapes are only approximated;
(b) the use of iron in transformers under conditions approach-
ing magnetic saturation, thereby causing distortion of the current
and voltage waves j
((') the presence of electric arcs in the circuit, as in some street-
lighting systems.
The higher harmonics which commonly occur in alternating-current
systems are odd integral multiples of the fundamental frequency.
They are of sufficient magnitude to be of importance, often as high as
nineteen times the fundamental frequency, and have been observed as
*Listed in Appendix II.
439828
22 FINAL REPORT ON INDUCTIVE INTERFERENCE
high as the 35th order. High frequency voltages and currents also occur
in direct-current systems. Harmonics (other than the fundamental
or first harmonic) are not essential to the functioning of power systems
and may be sources of trouble therein.
Induction of the fundamental frequency of power circuits (below 100
cycles per second) is the cause of very little interference to telephone
circuits, except when its magnitude is sufficient to constitute a physical
hazard, or to operate grounded signalling devices, as both the human
ear and telephone apparatus are much less sensitive to these relatively
low frequencies.
3. Interference to Telegraph Circuits.
Under normal operating conditions of the disturbing power circuits,
interference to telegraph circuits, manifested by the reduction in clear-
ness and maximum speed of signalling, is due to induced currents of
fundamental frequency and, to a limited extent, frequencies of the
lower harmonics (chiefly the third).
Telegraph receiving instruments are readily responsive to these
frequencies, because they approach the normal operating frequencies
of telegraph transmission. Telegraph instruments are not sensitive to
the higher harmonics. Other signal circuits (telephone circuits
excluded), in general, resemble telegraph circuits in being most affected
by induced currents of fundamental frequency.
4. Balanced and Residual Components.
In analyzing inductive effects, it is convenient to divide the power-
circuit voltages and currents into two general classes: (1) "balanced,"
with respect to the earth as a neutral conductor or point of reference,
and, (2) "residual," completely unbalanced with respect to the earth,
i.e., employing the metallic power-circuit conductors, as a group, for one
"side" and the earth as the other side of their circuit.
' * Balanced ' ' current components in the several conductors of a power
circuit are such that at every instant their algebraic sum is zero. The
algebraic sum of the total currents in the several conductors of a power
circuit at any instant is the "residual" current. Similarly, the
"balanced" voltages of the several conductors are such that their alge-
braic sum is zero at every instant, while the algebraic sum of the
total voltages to ground at any instant is the "residual" voltage.
As an example, a trolle}^ circuit, consisting of an overhead trolley
wire and "return" through rails and earth, is completely unbalanced
with respect to the earth, its total voltage and current being residual.
On the other hand, a two-wire circuit having no metallic connection to
earth and its two sides symmetrical with respect to the earth's surface
and not in close proximity to other circuits or objects, would have no
residuals, the voltages to earth of the sides of the circuit being equal and
opposite and the currents wholly confined to the metallic conductors and
therefore equal and opposite, i.e., in both cases balanced.
This classification of the voltages and currents is of basic importance,
since there is no generally applicable relation between balanced and
residual components or their inductive effects, and furthermore since
the remedies for induction from balanced and residual voltages or cur-
rents are often fundamentally different.
FINAL REPORT ON INDUCTIVE INTERFERENCE 23
The circuHs commonly employed in power transmission jind
distribution ordinarily have both classes of voltages and currents in
sufficient magnitude to require attention. With exceptions, such as
trolley circuits above mentioned, the balanced components of funda-
mental frequency are the useful energy-transferring agents, while the
residuals are the result of incidental differences between ideal design
and construction of line and apparatus, giving perfect balance, and
design and construction which approach this condition sufficiently
for commercial operation, disregarding inductive effects.
Both balanced and residual voltages and currents contain harmonics,
but the general tendency is that the residuals contain greater percent-
ages of harmonics than do the balanced components. Besides, under
some conditions (discussed in 5 below), a series of harmonics, odd
multiples of three times the fundamental frequency, appear as residuals,
but not in the balanced components.
Inductive effects from residuals are usually of greater intensity than
those from balanced voltages or currents of equal magnitude. The
ratio of effects from these two sources is exceedingly variable, ranging
from about two to several thousand. The relatively greater induction-
producing power of residuals is due to the fact that the residual com-
ponents associated* with the several conductors are all "in phase" and
their inductive effects therefore cumulative, whereas the several
balanced components are "out of phase" (by 120 degrees in a three-
phase system) and hence their resultant induction is a differential
effect, i.e., the inductive effects due to the balanced components partially
neutralize one another.
5. Causes and Remedies for Residuals.
Unbalances or inequalities among the admittances to ground of the
several conductors of a power circuit cause residuals of the frequencies
present in the voltages between conductors.* In a system without
metallic connection to earth a residual voltage is produced. With a
grounded-neutral system a residual current is produced and the residual
voltage due to unbalanced line admittances is greatly reduced. Unbal-
anced admittances are caused by : ( 1 ) differences of position of the
conductors with respect to ground and to one another, being a function
of the configuration, height above ground, location of ground-wires and
other neighboring objects, and to a small extent, of size of conductors ;
and (2) differences in insulation resistance, as may be due to defective
insulators. Transposing the conductors, which tends to equalize their
relations to ground and to one another, is an effective remedy for
unbalanced capacitance. Such transpositions must be located with
proper regard to changes in configuration, and at short enough
distances from each other so that there is no material difference in the
electrical conditions at two such points at any given instant. Of
commonly occurring configurations the equilateral triangular is most
nearly balanced, hence causes the least residuals due to unbalanced
capacitances, while the plane configurations, especially the unsym-
metrical horizontal, are the worst in this respect. The remedy for
*It is to be noted that the unbalances here referred to are not unbalances such as
those due to single-phase loads between line conductors.
24 FINAL REPORT ON INDUCTIVE INTERFERENCE
unbalanced insulation resistance lies in careful maintenance ; for a well
constructed and maintained system this is usually not an important
source of residuals.
In a power system having loads connected between the several con-
ductors and ground (as in a star-connected system with grounded
neutrals), differences among the loads of the several phases may cause
residual voltages and currents, due to part of the load being supplied
through a circuit consisting of conductors with ground "return."
Inequalities of ratios or impedances among the transformers of a bank
also cause residuals in such a circuit. The evident remedy is careful
equalization of loads, and the use of like transformers. Removal of
the ground path for unbalanced load currents, allowing only one
neutral ground, is the most effective and reliable remedy for this source
of residual current.
When a transformer bank in a three-phase system is connected in
star with neutral grounded, harmonics of three times the fundamental
frequency, and odd multiples thereof, appear as residuals, on the
grounded -neutral side. This is because of the variation of the per-
meability of the transformer iron with varying magnetic density,
causing harmonics in transformer exciting currents or in their induced
voltages. As the triple-harmonic components are "in phase" in the
three transformers, triple-harmonic residual voltages and currents are
produced if the neutral is grounded. Delta-connected windings on such
a transformer bank provide a shunt path for these triple-harmonic com-
ponents of the exciting current, and greatly lessen the residuals which
might otherwise be caused on the grounded neutral side. Since the
magnitude of these residuals decreases very rapidly as the maximum
magnetic density is reduced, lowering the voltage impressed per turn
of the transformer winding, or substituting transformers of lower mag-
netic density, is a very effective remedy. Isolating the neutral of a
transformer bank eliminates it as a source of triple-harmonic residuals.
Generators with star-connected armature windings may cause
residuals due to: (1) inequalities among the voltages induced in the
several windings; (2) departure from ideal phase differences, 120
degrees for a three-phase generator; (3) triple-harmonic voltages of the
three windings being in phase, between neutral and line terminals.
When a generator is connected to the line either directly or through
auto-transformers, residuals are thus caused only if the generator
neutral is grounded. When connected to the line through transformers
residuals will result from these causes if the transformer bank is star-
star connected with line-side neutral grounded and station-side neutral
connected to generator neutral. The remedies are: (1) careful design
and construction, (2) avoidance of grounded neutral or transformer
connections permitting transformation of generator residuals to line
(as by the use of a delta connection on the generator side of the
transformers).
The grounding of transformers, transformer banks or generators at
unsymmetrical points of their windings unbalances the electrically con-
nected circuit and thereby causes a residual voltage and current. The
remedy is obvious.
I'M N Ah RKI'OKT o.\ I.XWCTIVE INTUKKKKKNCE 25
6. Factors Affecting Intensity ami Mat/nil mlc of Induction .
(a) Dimensional Factors. In general, as the horizontal separation <>!'
power and communication lines is increased, the induction decreases at
a rate varying, roughly, from direct proportionality to about the third
power of the separation. That is, doubling the separation reduces the
induction from unity to some value between one-eighth or less and
one-half. The rate of decrease is less for magnetic induction than for
electric induction and less for induction in grounded circuits than for
induction in metallic circuits. When the disturbed and disturbing cir-
cuits are very close together, the rate of decrease may be less than
stated above and in some instances, notably with the vertical-
configuration power circuit, there may be an increase of induction at
first, as the separation increases.
Other things being equal, the magnitude of the induction increases
nearly in direct proportion to the length of parallel.
The configuration of a power circuit has a large influence on the
intensity of the induction from balanced voltages and currents, but a
very small influence on induction from residuals. No one configuration
commonly employed can be selected as universally superior, since the
one giving the least inductive effect depends on the spacing of the con-
ductors and the relative position of the two classes of circuits, also
upon the type of induction, electric or magnetic, which preponderates.
Induction from balanced components increases nearly in direct pro-
portion to the spacing of power conductors, but induction from
residuals, particularly residual current, is only slightly affected by the
conductor spacing.
The intensity of the direct induction in metallic communication cir-
cuits depends largely upon the arrangement of the conductors and
increases in direct proportion to their spacing (for two-conductor
circuits in a given plane) ; but the inductive effects on the conductors
as a group, with reference to the earth as a neutral conductor, are only
slightly affected by the spacing or arrangement.
(b) Electrical Factors. The induced current in a communication
circuit increases in direct proportion to the magnitude of the voltage or
current in the power circuit which causes it, and approximately in pro-
portion to the frequency of the inducing voltage or current.
As stated above, residual voltages or currents produce much more
intense inductive effects than balanced voltages or currents of the same
magnitude.
The magnitude of the induced current is considerably affected by
the amount and character of line and of terminal apparatus between
the parallel or source of disturbance and the receiving instrument.
The primary effect of such sections of unexposed line and apparatus is
to diminish the received current.
Several communication conductors on one line tend to shield one
another. It is generally assumed that ground-wires, as commonly
employed for lightning protection on power lines, are shielding agents.
This is true with respect to inductive effects from residual voltages and
currents, but such ground-wires may increase the intensity of the
induction from balanced voltages and currents, by distortion of the
electric and magnetic fields about the power circuit.
26 FINAL REPORT ON INDUCTIVE INTERFERENCE
Using the well-known laws of electricity and magnetism, it is pos-
sible to determine by computations the effect of these various factors,
both dimensional and electrical, in simple practical cases. Even with
the simplifying assumptions allowable, the work is usually tedious. In
complex cases as when the simultaneous action of all factors is to be
considered, quantitative results are best obtained by experimental
means.
7. Transpositions.
One of the most valuable means of overcoming inductive interference
under normal operating conditions of power circuits is to transpose the
conductors of each circuit, so as to equalize their relations to all other
circuits and to earth.
Transpositions in a power circuit tend: (1) to equalize the
capacitances of its conductors to ground, thereby removing a source of
residuals, and (2) to cause the inductive effects from the balanced
voltages and currents to neutralize one another in neighboring lengths
of a parallel communication line. Transposition of a power circuit does
not reduce induction from residuals, except as it may do so indirectly by
a reduction in the magnitude of the residuals as just noted.
Transpositions in a communication circuit tend: (1) to equalize the
capacitances of its conductors to ground; (2) to lessen the induction
among the several communication circuits of a line (known as "cross-
talk" on telephone circuits) ; and (3) to equalize the inductive effects on
the two sides of the circuits, due to near-by power circuits. Such trans-
positions do not protect the circuit against voltages induced between
the circuit as a whole and ground or along the conductors as a group.
In order that transpositions shall be most effective they must be
carefully located, within sections where the intensity of the inductive
effects is uniform, with respect to points where the induction changes,
called points of discontinuity. The transpositions in each class of line
must also be located with regard to the transpositions of the other class
of line, i.e., the transpositions in the power and communication lines
must be co-ordinated.
On account of the finite (though very short) time required for
electric waves to travel along the conductors, the electrical conditions
at a given instant will be different at different points along the lines,
being practically opposite at points one-half wave length apart; hence
transpositions laid out on a basis of uniform conditions do not produce
perfectly neutralizing and equalizing effects in adjacent sections of a
parallel. To be effective, therefore, the nominally balanced lengths of
a transposition scheme should be very short as compared to a wave
length at the frequencies of induction to be considered and guarded
against. The impairment of balance due to this effect varies approxi-
mately as the square of the length of nominally balanced section and
directly as the frequency of the induction. It is usually advantageous
to omit transpositions at the junction points of successive balanced sec-
tions, or barrels of the power circuit, as this lessens the impairment of
balance just mentioned.
The length of parallel within which a nominal balance should be
obtained in a scheme of transpositions designed to adequately reduce
FINAL KKI'ORT ON INDUCTIVE INTERFERENCE 27
interference, is usually determined by the points of discontinuity, the
lengths of sections thus required adequately meeting the requirement
above 'mentioned of securing balance within a small fraction of a wave
length. In long uniform parallels involving telephone circuits, bal-
anced sections with barrels in the power circuits three miles in length
are usually adequate. For such parallels involving telegraph circuits
longer barrels are permissible as only the wave length of fundamental
frequency need be considered.
Though transpositions afford a very practical and effective means of
mitigation for some inductive disturbances, they cannot be considered
as a complete remedy for interference even under normal operating
conditions.
8. Unbalance of Communication Circuits.
Differences in the admittances to ground or series impedances of the
Iwo conductors of a metallic communication circuit cause currents in
its terminal apparatus when a voltage is induced between its conductors
and ground or along its conductors in multiple. These unbalances
may be reduced to the smallest practicable values by transposing the
conductors and by proper design, construction and maintenance of
open-wire lines, cables and connected apparatus. A small amount of
unbalance is, of course, unavoidable. Since the induced currents here
considered are proportional to the product of the unbalance and the
induced voltage, it is necessary to restrict the amounts of either or
both these factors in order to sufficiently limit the induced currents in
the terminal apparatus of metallic telephone circuits.
9. Transients and Abnormal Conditions,
When a section of power circuit is energized or de-energized a sudden
change takes place in the electric and magnetic fields about the cir-
cuit. If the several conductors are not energized or de-energized at
exactly the same instant, large residual voltages and currents exist
momentarily. An extreme case of this sort occurs when single-pole
switches are operated successively, or when one pole of a switch fails
to operate.
When one conductor becomes grounded there is a sudden change
from a condition of approximate balance to one of large unbalance
which persists until the circuit is de-energized or the fault cleared.
At the time of such abnormal conditions of power circuits the induc-
tion in parallel communication circuits is greatly in excess of that
experienced under conditions of normal operation, sometimes causing
hazardous voltages, and acoustic shocks to telephone users. If the
protective devices of communication circuits are operated, service inter-
ruption continues for a considerable period after the initial cause has
subsided, until such devices are restored. Where telephone circuits are
affected the operating personnel may be temporarily demoralized by
severe acoustic shocks. An "arcing ground" on a power circuit not
normally connected to ground may continue for sonic lime with constant
repetition of the accompanying transients, and corresponding severe
disturbance.
28 FINAL REPORT ON INDUCTIVE INTERFERENCE
The means sometimes employed in handling faults in power circuits
of repeatedly re-energizing the faulty circuit, either to burn off a
"ground" or locate it by sectionalization, often greatly aggravates the
disturbance to communication circuits by repetition.
Abnormal conditions and severe switching transients are, aside from
their detrimental effects on communication circuits, very undesirable
from the standpoint of power-system operation. The frequency of
their occurrence can only be lessened by high-grade design and con-
struction and by careful operation and maintenance.
10. Nonessential Features Cause Greatest Interference.
It will be apparent from the foregoing that those features of power
and communication circuits, which have the greatest tendency to result
in interference, while not wholly avoidable, are nonessentials which
serve no useful end in the normal functioning of the circuits, and that
the necessary precautions for the prevention of inductive interference
are not incompatible with a high grade of service but to some extent
further that end. This circumstance is fortunate alike for the public,
the power companies and the communication companies.
GUIDING PRINCIPLES FOR PREVENTING INTERFERENCE.
The following are the basic physical principles which underlie the
rules recommended in Part Three and which should guide all efforts to
prevent inductive interference.
1. Avoidance of close proximity.
By no other means can complete freedom from interference be
secured.
2. Elimination or suppression of harmonics.
To the existence of harmonics is due practically all interference to
telephone circuits under the normal operating conditions of parallel
power circuits. Improvement in this respect may be effected by giving
due regard to its importance in the purchase of new equipment.
3. Limitation of residuals.
The intensity of the induction due to residual voltages and currents
is relatively more severe than that due to balanced voltages and cur-
rents and induction arising from residuals cannot be neutralized by
power transpositions. They can be lessened by balancing the line and
also the load, by the use of advantageous transformer connections and
by the avoidance of excessive magnetic density in the iron cores of
transformers.
4. Reduction of intensity of induction by favorable arrangements of conductors.
Within the latitude afforded by various practical configurations of
power and communication circuits the induction with some arrange-
ments is of much less intensity than with others. In cases of multi-
circuit power lines important advantage can be secured by care in fixing
the phase relations of the conductors of the several circuits.
FINAL REPORT ON INDI'CTIVK INTERFERENCE 29
5. Neutralization of induction by co-ordinated transposition systems.
By means of transpositions in both classes of circuits within a parallel
the phase or direction of the induction may be controlled so that
mutually neutralizing effects are created in neighboring lengths of cir-
cuit. To be effective the transposition systems of the two classes of
lines must be co-ordinated.
6. Balancing of metallic communication circuits.
Accurate balancing of metallic communication circuits, particularly
telephone circuits, tends to reduce the disturbing effect of induction
from parallel power circuits and other near-by communication circuits.
Unbalances are reduced by transposing the conductors and by careful
design, construction and maintenance of lines and apparatus.
7. High-grade-construction and care in the operation and maintenance of power
circuits.
No means are known, except increased separation of the two classes
of lines, whereby the severe momentary disturbances to communication
circuits, due to abnormal conditions on neighboring power circuits, can
be prevented; hence the importance of minimizing such occurrences by
high standards of construction, operation and maintenance.
30 FINAL REPORT OK INDUCTIVE INTERFERENCE
PART THREE.
REVISED RULES RECOMMENDED BY COMMITTEE.
Reasons for Revising Rules.
Since submitting its preliminary report, dated July 7, 1914, recom-
mending provisional rules for the prevention or mitigation of inductive
interference which were later embodied in General Order No. 39, this
Committee has greatly extended its investigations of some important
branches of the subject. Considerable experience in the practical appli-
cation of the rules has also been gained. In the light of the additional
information thus made available and with due consideration of criti-
cisms and suggestions which have been offered by others, the Committee
has formulated, and herewith presents, revised rules which it
recommends be embodied in a new order of the Railroad Commission
to supersede General Order No. 39.
In formulating these revised rules, the Committee has endeavored to
utilize the information obtained since its former report so that the
rules may be, so far as practicable, definite and authoritative in respect
to the specific limitations. The general arrangement of the rules hn^
been modified in order to better meet the requirements of practical use.
A detailed discussion of such of the provisions as seem to require it is
given in a section immediately following the rules.
Text of revised rules.
RULES GOVERNING THE CONSTRUCTION AND OPERATION OF
POWER AND COMMUNICATION LINES FOR THE PREVEN-
TION OR MITIGATION OF INDUCTIVE INTERFERENCE.
I. GENERAL PROVISIONS.
(a) Applicability of rules.
These rules, except as otherwise provided in I (e) shall apply and be
effective as follows :
1. Rules limited to lines involved in a parallel,* or to apparatus con-
nected to such lines, shall apply only in case of parallels created
hereafter; except that rules relating to operation or maintenance shall
apply to all such lines and apparatus, both existing and new.
2. Rules not limited to lines involved in a parallel, or to apparatus
connected to such lines, shall apply to new construction only, including,
however, existing lines and apparatus when such are generally recon-
structed or renewed.
(b) Co-operation.
Any party contemplating new construction which may create a
parallel shall confer with the other party or parties concerned and they
shall co-operate with a view of avoiding the parallel, or, if this be
impracticable, of minimizing the resulting interference. Failure to
comply with this requirement will receive consideration by this Com-
mission in any subsequent issue involving such construction.
*For definition of "parallel" see page 32.
FINAL REPORT ON 1NI>r<'TlYK INTERFERENCE 3.1
(c) Principle of least cost.
When there are two or more different practicable methods of avoiding
or mitigating interference, the method which involves the least total
cost shall in general be adopted irrespective of whether the necessary
changes are made in the plant of the party creating the parallel or in
the plant of the other party ; provided, however, that preference shall
In- uiven to methods of avoiding a parallel over methods of mitigating
interference; and provided, further, that as between different methods
of mitigation having different degrees of effectiveness, the most effective
method, the cost of which can be justified, shall be adopted. In esti-
mating such costs, all factors of expense to both parties shall be taken
into account.
(d) Existing parallels.
Parties operating power or communication lines shall exercise due
diligence in applying measures, in general accordance with the prin-
ciples of these rules, for mitigating inductive interference due to exist-
ing parallels. Any such parallels which now or hereafter cause
excessive interference shall be attended to promptly.
When lines involved in existing parallels are added to, extended or
generally reconstructed, or when additional apparatus is connected to
such lines, or when apparatus now connected to such lines is renewed
or rearranged, the new or changed plant shall thereafter conform to the
provisions of these rules.
(e) Saving clause.
The Commission reserves the right to ijiodify any of the provisions
of these rules in specific cases, when in the Commission's opinion, public,
interest would be served by so doing.
II. DEFINITIONS.
Certain technical terms are employed herein in the senses set forth
in the following definitions:
(a) Class H power circuit.
The term "Class H Power Circuit" means any overhead open-wire
constant-potential alternating-current power transmission or distribu-
tion circuit or electrically connected network which has 5,000 volts or
more between any two conductors or 2,900 volts or more between any
conductor and ground; except railway trolley circuits and feeders
electrically connected therewith.
(b) Electrically connected.
The term "Electrically Connected" means connected by a conduct-
ing path or through a condenser, as distinguished from connection
merely through magnetic induction.
(c) Signal circuit.
The term "Signal Circuit" means any telephone, telegraph, mes-
senger call, clock, fire, police alarm, or other circuit of similar nature
used exclusively for the transmission of signals or intelligence, which
32 FINAL REPORT ON INDUCTIVE INTERFERENCE
operates at less than 400 volts to ground, or 750 volts between any two
points of the circuit, provided that if the voltage exceeds 150, the power
transmitted shall not exceed 150 watts.
(d) Communication circuit.
The term "Communication Circuit" means any overhead open-wire
signal circuit, except that, if such circuit be a telephone circuit, it is
limited to inter-exchange metallic telephone circuits and to metallic
telephone circuits operated by a railroad or other company for dispatch-
ing purposes, or for public use between separate communities.
(e) Line.
The term "Line" means any circuit or aggregation of circuits car-
ried on poles or towers, and includes the supporting elements.
(f) Parallel.
The term "Parallel" means a condition where a Class H Power cir-
cuit and a communication circuit follow substantially the same course
or are otherwise in proximity for a sufficient distance so that the pow r er
circuit is liable to create inductive interference in the communication
circuit.
With some parallels interference occurs only at times of abnormal
conditions on the power circuit in which case such of these rules as
affect induction only under normal operating conditions do not apply.
When the application of any rule is thus restricted, the condition under
which the rule applies is referred to as a "normal" parallel.
(g) Configuration.
The term "Configuration" means the geometrical arrangement of a
circuit or circuits, including the size of the wires, and their relative?
positions with respect to one another and earth.
(h) Transposition.
The term "Transposition" denotes an interchange of position of the
conductors of a circuit between successive lengths thereof.
(i) Barrel.
The term "Barrel" means an arrangement of a section of power cir-
cuit of uniform configuration within which each conductor occupies
each of the conductor positions for equal distances.
(j) Discontinuity.
The term "Discontinuity" means any abrupt change in the relative
positions of a power and a communication circuit, or any abrupt change
in configuration, line impedance or load along either such circuit
(including such changes due to connected circuits, transformers, cables,
loading coils or other apparatus) which materially affects the magni-
tude or phase of the induced voltages or currents per unit length or
the capacitances of either circuit. Transpositions, however, are not
considered to be discontinuities.
FINAL RKl'ORT ON INDUCTIVE INTERFERENCE 33
(k) Co-ordination.
The term "Co-ordination" as applied to transposition systems menus
that the transpositions in power and communication circuits involved
in a parallel are efficiently located, with respect to each other and to
the discontinuities, for reducing the inductive effects on the communi-
cation circuits.
(I) Balanced and residual voltages.
The voltages to tiround of the several wires of a power circuit arc
divided for convenience into two classes of components, "balanced"
and "residual."
The "balanced voltages" are those components which are equal in
magnitude and have such phase relations that their algebraic sum is
zero at ever}- instant.
The remaining components of the voltages to ground, which exist
under conditions other than perfect balance, are termed residual. They
are equivalent to a single-phase voltage impressed between the power
wires in multiple and ground. The sum of the residual components is
termed the "residual voltage 1 " of the circuit. In case of a three-phase
circuit it is three times the equivalent single-phase voltage above
mentioned.
Mathematically expressed, the residual voltage is the vector sum of
the voltages to ground of the several wires of a power circuit, while
the balanced voltages aro those components whose vector sum is zero.
(m) Balanced and residual currents.
The currents in the several wires of a power circuit are divided for
convenience into two class of components, "balanced" and "residual."
The "balanced currents" are those wholly confined to the wires of the
circuit. Hence their algebraic sum is zero at every instant.
The remaining components of the currents in the several wires, which
exist under conditions other than perfect balance, are termed residual.
The sum of the residual components is the "residual current" of the
circuit. It is equivalent to a single-phase current in a circuit having
the power wires in multiple as one side, and ground as the other side.
Mathematically expressed, the residual current is the vector sum of
the currents in the several power wires while the balanced currents are
those components whose vector sum is zero.
III. LOCATION OF LINES.
(a) Avoidance of parallels.
Every reasonable effort shall be made to avoid creating parallels. If
the parties concerned can agree upon a plan for providing an adequate
separation of the two classes of lines so as to avoid interference, such
plan shall be put into effect. In no case shall a parallel be created
unless the cost of avoidance by separation is greater than the cost of
the remedial measures required by these rules.
(b) Notice of intention.
The party proposing to build a new Class H power or a communica-
tion line which will create a parallel, or generally to reconstruct or
34 FINAL REPORT ON INDUCTIVE INTERFERENCE
change the operating conditions of an existing line involved in a par-
allel, shall give due notice (at least sixty days where practicable but in
any event not less than twenty days in advance of construction, except
for minor extensions, for which notice shall be given immediately after
the work is authorized) of such intention to the other party including
full information as to the location within the parallel and such other
features of the proposed line as would affect induction.
(c) Distance between lines.
Class II power lines and communication lines shall be kept as far
apart as practicable. Their separation should be at least equal to the
height above ground of the power wires, except when closer proximity
is unavoidable.
If, in any case of inductive interference, it should be found imprac-
ticable to obtain a proper degree of relief by means of the remedial
measures set forth in these rules or by other measures of a remedial
nature, the parties concerned shall agree upon and put into effect a
plan for increasing the separation of the lines within the parallel.
To promote the effective application of transpositions, both parties
shall endeavor to maintain a uniform .separation of the two lines
throughout each normal parallel. However, in general, when it is
feasible to secure more than a 20 per cent increase. in separation, for a
distance in excess of one mile, this shall be done.
(d) Length of parallels.
Parallels shall be made as short as practicable.
(e) Discontinuities.
In the location, construction and general reconstruction of lines
within normal parallels every reasonable effort shall be made to avoid
discontinuities (except those due to increases in separation as provided
for in (c) above) which would interfere with the application of effective
and economical co-ordinated transposition systems in the power and
communication lines.
In the location and construction of the first line along a public high-
way, special effort shall be made to avoid crossing the highway and
also to avoid other features which would result in unnecessary discon-
tinuities in the event of the construction of another line along the same
highway.
IV. DESIGN AND CONSTRUCTION OF LINES.
(a) General requirements.
The quality of material, workmanship, methods and grade of con-
struction shall be in accordance with approved modern practice with
special regard to the prevention of failures and the avoidance of
features, such, for example, as inferior insulation, which would tend to
cause or promote inductive interference.
(b) Arrangement and spacing of power conductors.
In the design for construction or general reconstruction of Class H
power lines, consideration shall be given to the configuration of the lines
FINAL REPORT ON INDUCTIVE INTERFERENCE 35
with a view to minimizing (1) throughout the entire length of the line
inequalities among the capacitances to earth of the conductors; and (2)
within normal parallels the intensity of the inductive effects. When
two or more circuits are carried on one line the phase relations among
the conductors of the different circuits should he chosen with the same
purposes in view. The configurations to be preferred for three-phase
lines under different conditions are discussed in the Exhibit attached
hereto.
Excessive spacing of conductors should be avoided.
Two-wire branches electrically connected to a three-phase Class 1 1
power circuit should be avoided except those so short that they do not
materially unbalance the three-phase circuit. Where such branches are
employed they should be so distributed as to cause minimum unbalance.
No single-wire grounded Class H power circuits or branches of multi-
wire Class H power circuits shall be employed.
(c) Transpositions General.
All Class II power circuits and metallic communication circuits, or
extensions of such circuits, hereafter constructed or generally recon-
structed, shall be transposed throughout their entire lengths in such
manner as to balance, as nearly as practicable, the capacitances to earth
of their conductors. For single-circuit three-phase lines the maximum
length of barrel for this purpose shall be twelve miles for circuits of
triangular* configuration and six miles for other configurations. For
twin-circuit three-phase lines the maximum length of barrel shall be six
miles ; except that for circuits of the vertical type (including cases
with the middle conductors displaced slightly outward) and the
equilateral triangular type with vertices upward, nine-mile barrels
may be used when the circuits are interconnected for minimum unbal-
ances. The accompanying Exhibit contains information concerning the
methods of interconnection giving minimum unbalances.
Exceptions. Power lines, located principally on private rights of way and not
electrically connected to other lines, are exempt from this rule if separated from
existing communication lines, and from highways required for the future construc-
tion of communication lines, by distances not less than those given below, except for
crossings at angles over 30 degrees and other sections of unavoidable closer proximity
not exceeding one mile in total length in each ten consecutive miles of line, pro-
vided, however, that such sections of closer proximity to any one such communica-
tion line or highway shall not exceed one mile in each thirty consecutive miles of line.
Voltage between power conductors
Minimum
separation
from high-
ways and
communica-
tion lines
Below 50,000 600 feet
50,000- 75,000 _J i 750 feet
75,000-100,000 ! 850 feet
100,000-1.50,000 1,000 feet
150,000-200,000 _ 1,200 feet
For power lines meeting all these conditions for exemption except that they are
electrically connected to other lines through autotransformers. the maximum lengths
of barrel may be twice those specified above.
*A triangular configuration as here used means one in which the altitude of the
triangle exceeds one-half the length of the longest side as base.
36 FINAL REPORT ON INDUCTIVE INTERFERENCE
The question of whether highways that may be involved will be required for future
communication lines shall be settled by agreement between the power company
contemplating construction and the communication companies operating within the
territory to be traversed. In the event of disagreement or if there is no such
communication company, the matter shall be referred to this Commission. In cases
where the proposed use of a particular highway by a communication company would
be the determining factor in deciding whether a given power line must be transposed,
such communication company shall make an effort to locate its proposed line else-
where and the decision shall be made in accordance with the principle of least cost
laid down in I (c).
Existing Class H power circuits and those exempted under the pre-
ceding paragraph, which hereafter become involved in normal parallels,
shall be transposed so as to balance their capacitances to earth, when
necessary for limiting residual voltages and currents to amounts which
can be tolerated. The location and number of transpositions for this
purpose shall be determined by agreement of the parties concerned.
In the location and spacing of the transpositions due regard shall be
paid to discontinuities which affect the capacitances of the circuit.
Sections of circuit between such points of discontinuity should be
treated independently.
In general, transpositions should be omitted at the junction points of
successive barrels.
Metallic communication circuits, and single-phase and two-phase
"Class II power circuits, shall be transposed at intervals not exceeding
four miles.
Power circuits less than three miles in length are not required to be
transposed outside of parallels, except when the absence of transposi-
tions would materially impair the balance of other circuits to which
they are electrically connected.
Power circuits with grounded neutrals, having a voltage of less than
12,500 volts, between conductors, are not required to be transposed out-
side of parallels, except where the lack of such transpositions in any
specific case is the cause of interference.
Within normal parallels the transpositions in the two classes of cir-
cuits shall be as provided in (d) below. When the transpositions
required in a parallel impair the general transposition system of either
line outside the limits of the parallel, the necessary readjustment of
transpositions shall be made in the sections of line adjacent to the
parallel, as a part of the remedial measures therefor.
(d) Transpositions Inside limits of parallels.
Within each normal parallel an adequate scheme of transpositions, to
neutralize, so far as practicable, the inductive effects, shall be installed
in the power circuits, and also in the communication circuits, provided
the latter are metallic. The transposition systems in the two classes of
circuits shall be properly co-ordinated. The parties concerned shall
co-operate to determine upon the transposition scheme to be employed.
The transpositions required in the line last constructed shall be installed
before it is placed in service.
In applying the foregoing, the following rules shall, in general, be
observed :
FINAL REPORT ON INDUCTIVE INTERFERENCE 37
1. For cadi normal parallel at least one barrel shall be installed in
the power circuit. This applies also to a section of parallel where it is
not practicable to obtain a balance by combining it with another sec-
tion. In applying this rule it is not intended ordinarily to change the
span lengths required for other purposes.
2. In long uniform parallels or sections of parallel, involving a tele-
phone line at highway separation from the power line, the barrels shall
be three miles in length, subject to such variation as may be necessary
for co-ordination with the transpositions required in the telephone
circuits. Transpositions should in general be omitted at the junction
points of successive barrels.
3. Except as modified by (1) above, the number of transpositions
required in power circuits paralleling telephone circuits shall be sub-
ject to the following limitations expressed in terms of the average
distance between successive transpositions :
For power circuits of 50,000 volts or more between conductors,
not less than one mile.
For power circuits of less than 50,000 volts between conductors,
not less than one-sixth mile.*
4. In case of a parallel between a power line and a telegraph line or
other grounded communication line, the transpositions in the power cir-
cuit shall be located with due regard to the limits of the parallels and
to discontinuities, in order to form as nearly as practicable a balanced
system, subject to the condition that the transpositions in the power
circuit are not required to be less than one mile apart, except as
modified by (1) above. In lonu- uniform sections of parallel, barrels
six miles in length should be .sufficient. Transpositions should be
omitted at the junction points of successive barrels.
5. The question of the most economical scheme to accomplish the pur-
pose shall always be considered. Effort shall be made to utilize as many
as practicable of the existing transpositions.
It is suggested that in case of a short section of new line, not sufficient
of itself to require transpositions, but which is likely to be extended
later so that transpositions would then be necessary, consideration be
given to the advisability of installing one or more suitably located trans-
positions in the new section of line while it is being constructed in order
to avoid interrupting the service by adding transpositions afterwards.
K.i'<'ci>tionx. Cases of parallelism may occur where the interference is due almost
wholly to residual voltages and currents in which event transpositions in the power
circuit are not required, except as provided in IV (c).
V. DESIGN, CONSTRUCTION AND ARRANGEMENT OF APPARATUS.
(a) Quality and suitability.
In designing, specifying, or otherwise determining the quality or suit-
ability of apparatus to be connected to Class II power or communication
*While barrels of approximately three miles, as provided in 2 above, are generally
to be employed, the shorter barrels specified in 3 are sometimes necessary in short
parallels and in short sections of parallels, in order to co-ordinate with the discon-
tinuities and obtain a proper degree of balance.
38 FINAL REPORT ON INDUCTIVE INTERFERENCE
circuits, and in arranging such apparatus for use, effort shall be made to
avoid, so far as is reasonably practicable, all features which would tend
to create or promote inductive interference under either normal or
abnormal conditions. As instances in applying the foregoing, the fol-
lowing rules shall be observed.
(b) Rotating machinery.
In order to improve conditions generally, companies operating Class
II power circuits shall make every effort to minimize the high frequency
components of voltages and currents caused by rotating machinery. All
new rotating machinery shall have as nearly as practicable a pure sine
wave of voltage and shall not, in any case, deviate therefrom to exceed
the limit set forth in the present standardization rules of the American
Institute of Electrical Engineers.
No ground connection shall be used on the armature winding of an
alternating-current generator or motor electrically connected to a power
circuit involved in a normal parallel unless means are employed to
avoid unbalancing the circuit and to reduce triple-harmonic residuals as
far as may be necessary and practicable.
(c) Transformers and their connections.
In order that the wave-shape of voltage and current may be distorted
as little as practicable by transformers, all new transformers on Class
II power circuits should have an exciting current as low as is consistent
with good practice, and which shall not, at rated voltage, exceed 10
per cent of the full load current ; except that for transformers without
neutral ground connections on the line side, the exciting current at
rated voltage need not be less than 0.2 ampere.
Where three-pha.se transformers are employed with grounded neutrals
the core type is preferable to the shell type.
Transformers or transformer banks shall not be grounded at such
points of their windings as to unbalance a connected circuit involved in
a normal parallel. As important cases under this rule, no grounded
single-phase, grounded three-wire two-phase, or grounded open-star
three-phase connection shall be so employed.
No star-connected transformers or autotransformers shall be em-
ployed with a grounded neutral on the side connected to a three-phase
power circuit involved in a normal parallel, unless low-impedance
delta-connected secondary or tertiary windings or other equivalent
means are used for suppressing the triple harmonic components of the
residual voltages and currents introduced by the transformers.
Care shall be taken that the individual units in each grounded-
neutral bank of transformers, connected to a circuit involved in a
normal parallel, are alike as to type and rating, including all electrical
characteristics, and that they are similarly connected, so as not to
unbalance the circuit.
Closed-delta connections shall be used wherever practicable in prefer-
ence to open-delta connections on three-phase power circuits involved in
normal parallels. When open-delta connections are employed, an effort
shall be made to distribute such connections equally among the three
phases.
J'MNAIj K'KI'ORT ON" IN'WCTI VK INTMKKKKHNCE ')!)
Where triple harmonic residual voltages and currents due to star-
connected transformer hanks exist in amounts which can not be
tolerated, and it is inexpedient to isolate the transformer neutrals, such
residuals shall be limited by operating the transformers at reduced
magnetic density or by other available means.
(d) Rectifiers.
Rectifiers and other apparatus lending to distort the alternating cur-
rent wave when installed on power lines involved in normal parallels,
shall, if necessary, be equipped with suitable auxiliary apparatus to
prevent harmful distortion of the wave-form of power-circuit voltage
or current.
(e) Switches.
Each oil-break switch in a power-circuit involved in a parallel, located
between the source or sources of energy and the parallel, and used for
energizing or de-energizing the circuit, shall have all poles mechanically
interconnected for simultaneous action. There shall be at least one
such switch so located as to control the supply of energy to each
power circuit involved in a parallel, and, except at stations where
an operator is constantly on duty, such switch shall be made automatic
for short circuits, grounds, and in case of grounded neutral circuits,
for abnormal neutral grounds.
Careful consideration shall be given to means of minimizing transient
disturbances caused by switching operations on Class H power circuits,
which would cause inductive interference. Wherever practicable pro-
vision shall be made for switching on the station-side rather than on the
line-side of transformer banks.
Oil-break switches, having their poles mechanically interconnected for
simultaneous action, shall be provided wherever the use of air switches
or noninterconnected single-pole oil switches would cause harmful
transient disturbances in parallel communication circuits.
(f) Fuses.
Switches shall be used instead of main line fuses wherever practicable!
in a power circuit involved in a parallel.
(g) Electrolytic lightning arresters.
AVhen electrolytic lightning arresters are employed on a power circuit
involved in a parallel they shall be equipped with auxiliary charging
resistances and contacts so arranged that the horn gaps are short-
circuited at the time of charging, to avoid, as far as possible, the
production of arcs.
(h) Special instruments.
Reliable indicating devices shall be installed at the source of supply
of power circuits involved in parallels, to inform the operators imme-
diately of abnormal conditions, such as grounds, and wherever possible,
epen circuits, which have not operated automatic switches.
Whenever a neutral Around connection is employed on a circuit
involved in a parallel, an ammeter, suitable for measuring the current
in the neutral under normal operating conditions, shall be installed in
40 FINAL REPORT ON INDUCTIVE INTERFERENCE
each neutral connection to ground at the main generating and main
attended substations on the power system electrically connected to the
circuit involved in the parallel.
(i) Communication apparatus.
All apparatus electrically connected to metallic communication cir-
cuits involved in parallels shall be designed and constructed so as to
secure as nearly as practicable an accurate balance of the series
impedances and the admittances to earth of the two sides of the cir-
cuits in order to minimize the detrimental effects of induction from
parallel power circuits.
VI. OPERATION AND MAINTENANCE.
(a) General requirements.
Power and communication companies shall use all reasonable means
to operate and maintain circuits involved in parallels in such a manner
as to minimize interference under conditions of normal operation, and
to avoid transient disturbances.
(b) Balance.
Jn the maintenance of both power and communication circuits
involved in parallels special care shall be given to the prevention of
mechanical and electrical failures which would cause or promote
transient disturbances or unbalances such as those due to tree-grounds,
defective or dirty insulators or other faults.
The voltages and currents of power circuits involved in parallels shall
be kept balanced as closely as practicable and accidental unbalances
shall be promptly corrected.
(c) Record of neutral current.
At all points on grounded neutral systems equipped as required in V
(h), the power company shall observe and record daily the approximate
maximum neutral current.
(d) Transformers.
No transformers connected to power circuits involved in normal
parallels shall be operated at more than 10 per cent above their rated
voltage. Wherever practicable in case of existing equipment and in all
cases of new equipment, transformer banks with grounded neutrals on
the side which is connected to a power circuit involved in a normal
parallel shall not be operated at more than 5 per cent above their rated
voltage.
(e) Switching.
In all switching operations care shall be taken to avoid, so far as
possible, the production of harmful transient disturbances.
(f) Charging electrolytic lightning arresters.
When, notwithstanding compliance with V (
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(f/) Trxi/ions Inside Limits of Parallels.
Transposition systems in both power and communication circuits
when properly co-ordinated offe?- the most reliable and effective means
of preventing interference from the balanced voltages and currents of
the power circuits.
Tn contrast with the corresponding rule of General Order No. 39,
which allowed the communication company to specify the number and
location of the transpositions in the power circuit, this revised rule
provides that both parties shall co-operate to decide upon the trans-
position scheme to be employed. In the past most of the work of
designing transposition schemes for parallels has been done by the
communication companies, but so far as this Committee is aware the
negotiations relating thereto have been generally conducted in a spirit
of co-operation.
Three-mile barrels in three-phase power circuits co-ordinate satisfac-
torily with telephone transposition systems now available (designed
particularly for circuits involved in parallels) and are short enough
so that the effect of phase-change* along the line, in impairing the
efficiency of the transposition scheme, is small. The omission of
transpositions at the junction points of barrels does not usually impair
the co-ordination of the transposition systems and is advantageous in
reducing the effect of phase-change besides reducing the number of
power transpositions. It is impossible in a rule to specify exactly the
spacings of transpositions within parallels either in power or telephone
lines since the spaeings required vary with the circumstances being
considerably influenced by the length of the parallel and by the dis-
continuities in both classes of line.
Barrels less than three miles in length are often very useful for secur-
ing economical and satisfactory schemes of transpositions, but in the
higher voltage lines the difficulty and expense of installation are such
that they are justified only in exceptional cases. For lines of lower
voltages, parallels are more numerous and shorter barrels are practi-
cable, also the greater liability of discontinuities (such as branch
loads) in low-voltage lines renders necessary the greater flexibility of
shorter barrels. The rules accordingly make distinction between lines
above 50,000 volts and those below 50,000 volts in specifying minimum
lengths of barrel. The limitations are identical with those of General
Order No. 39.
Several schemes of co-ordinated transpositions can be designed for
any given parallel depending upon the utilization of existing trans-
positions, length of barrel, type of telephone transposition system and
discontinuities. Obviously, that scheme involving minimum total cost
.should usually be chosen. Generally in a new parallel it is economical
to transpose the new line to co-ordinate in part, at least, with the exist-
ing transpositions of the prior line.
V. Design, construction and arrangement of apparatus.
(a) Qiuilih/ and Suitability.
The same considerations that are mentioned under IV (a) which
demand care in the design and construction of power and communica-
*The effect of phase-change is discussed in Technical Report No. 66.
54 FINAL REPORT ON" INDUCTIVE INTERFERENCE
tion lines to prevent inductive interference and to secure continuity of
service, apply also to the apparatus connected to such lines.
(&) Rotating Machinery.
The elimination of higher harmonics from the wave form of rotating
machinery strikes at the source of disturbance to telephone circuits by
removing an underlying cause. It is obviously a matter which can best
be cared for in specifications for new machinery.
In providing that the deviation from a pure sine wave shall not
exceed the limit set forth in the present standardization rules of the
American Institute of Electrical Engineers, it is recognized that this
limit is unsatisfactory. This Committee has' been in correspondence
with a committee of the American Institute of Electrical Engineers
which has under consideration the revision of the Institute's present
rule on the subject. It is expected that the revised standardization rule
will be more satisfactory, and it is recommended that this revised rule,
when issued, be recognized by the Commission.
The improvement of wave-form in rotating machinery is essentially
a problem for the manufacturer and all new machines should be
designed with this in view. This Committee understands that sub-
stantial improvement in this direction can be effected at relatively
small cost, hence this is one of the general precautionary rules made
applicable to all new construction. For the improvement of conditions
with respect to machines already manufactured and installed, the use
of devices or "networks" external to the machine, designed to shunt
the troublesome high-frequency components from the line, probably
offers the most economical solution. The use of such devices may even
prove economical with new machines.
The provision against grounding armature windings is designed to
avoid a source of large harmonic residuals.
(c) Transformer^ and Their Connections.
If high magnetic densities be employed in transformers, the exciting
Current is large, and large higher harmonic components are introduced.
The 10 per cent limit on exciting current here provided serves to guard
against extreme designs but does not restrict present standard practice.
It is sometimes economical to design small units for a greater exciting
current. Banks of such small units, operated without neutral ground
connections on the line side, do relatively little harm, hence the
exception. Low exciting current is much more important for trans-
formers of grounded neutral banks than for transformers operated
without metallic connection to ground.
Core-type three-phase transformers tend to suppress their triple-
harmonic exciting currents by mutual interaction in the cores. This
is an important advantage over shell-type transformers, if there is a
grounded neutral, as it helps to suppress triple harmonic residuals.
The grounding of transformer banks at such points of their windings
as to unbalance power circuits involved in parallels is prohibited on
account of the residuals thereby caused.
Star-connected transformers inherently tend to cause triple-
harmonic residuals in connected transmission lines if the neutral be
grounded. These residuals can be greatly reduced by providing a
FIN 7 AT. REPORT ON INDUCTIVE INTERFERENCE 55
shunt path for the triple-harmonic currents in the form of del la-
connected windings, but such delta windings must be of relatively low
impedance in order to be effective. Other means, viz, operation at
low magnetic density, interconnected-star arrangements, or provision
of star-delta connected transformers as shunts for the triple harmonics
of star-star banks, may prove adequate in some cases.
Dissimilarities of electrical characteristics or connections among the
different transformers of a grounded-neutral bank cause the power
circuit to be unbalanced.
Open-delta connections cause triple-harmonic currents in transmission
lines whereas in closed-delta transformer banks, such currents are
locally confined.
The triple-harmonic residuals, which occur in grounded star-connected
systems, practically disappear when the neutrals are isolated. How-
ever, it is not always desirable to isolate all neutrals. By reduction in
operating voltage or by changes of transformer taps which reduce the
magnetic density of connected transformers relatively large reduction
in triple-harmonic residuals may be effected. This may in some cases
prove the simplest remedy for excessive residuals.
(e) Switches.
The operation of switches is sometimes the cause of severe transient
disturbances in parallel communication circuits. This is due in part
to nonsimultaneous operation of the several poles of the switch which
gives rise momentarily to large residuals. Oil-break switches usually
complete the opening or closing of a circuit within a few cycles while air
switches require a much longer time.
In order to prevent the continuance of abnormal conditions on power
circuits with the consequent disturbances, lines involved in parallels
should be disconnected immediately upon the occurrence of failures,
and accordingly the rule provides that there shall be at least one
automatic switch if prompt manual control be not available.
The transient disturbances are less severe when switching is per-
formed on the station side of transformer banks, hence this procedure is
recommended when practicable.
(/) Fuses.
The same consideration of avoiding the large residuals incident to
nonsimultaneous opening of the several phases of a power circuit
(referred to under "Switches"), dictates the preference for switches
instead of fuses.
(g) Electrolytic Lightning Arresters.
Electrolytic lightning arresters not equipped a? specified in this rule
ean.se harmonic residuals, due to arcing, which may result in severe
disturbances in parallel communication circuits. The use of charging
contacts and auxiliary charging resistances largely prevents the arcing
and lessens the rush of current so that such disturbances are greatly
reduced.
56 FINAL REPORT ON INDUCTIVE INTERFERENCE
(h) Special Instruments.
The object of this rule is to provide for immediate relief from the
abnormal disturbances occasioned by grounds, open circuits, etc., which
cau.se large residuals on power circuits, by giving notice of the existence
of such unbalanced conditions.
Ammeters in the neutrals of transformer banks serve to indicate the
degree of balance of the power circuit. By observing such meters,
unbalances smaller than those which would operate circuit breakers
can be detected. It is not necessary that such ammeters be constantly
in circuit and suitable provision should be made for their protection
under abnormal conditions.
(i) Communication Apparatus.
It is necessary to balance as accurately as practicable metallic com-
munication circuits and electrically connected apparatus in order to
minimize the disturbing effects of parallel power circuits. This
requires that such apparatus be so designed and connected as not to
introduce irregularities either in the series impedances of the two sides
of the circuit or in their admittances to ground. This applies to all
such apparatus, including that used for superposed grounded signalling
systems (such as telegraph on telephone), although such balancing has
no effect on interference with the superposed grounded system itself.
VI. Operation and maintenance.
(6) Balance.
It is to be expected that by careful inspection and maintenance
incipient causes of failures can be detected and corrected before they
develop serious consequences. In this way interruption to service can
be avoided as well as severe disturbances to parallel communication
circuits.
(d) Transformers.
This rule is designed to limit the introduction of harmonics due to
excessive magnetization of transformers, which occur when they are
operated at voltages above normal. Such overvoltage operation is
particularly undesirable in the case of grounded-neutral banks of
transformers.
(e) Switching.
Examples of compliance with this rule are, the use of oil-break
switches with poles mechanically interconnected for simultaneous
action instead of air-break switches, switching on station side of
transformer banks, and exercise of special care in synchronizing.
(/) Charging Electrolytic Lightning Arresters.
In cases where noticeable interference is occasioned by charging
electrolytic lightning arresters equipped as required in V (g), the
disturbance can be made less troublesome by charging them during the
early morning hours when telephone circuits are little used.
FINAL REPORT ON INDUCTIVE INTERFERENCE 57
(g) Abnormal Conditions.
In General Order No. 39, a more specific procedure is outlined than
is here given. While the general intent of the present rule is the same,
more latitude is given, with the requirement that operating rules shall
be developed with a view of minimizing the disturbing effects upon
parallel communication circuits.
It is recognized that cases will arise where continuance of service
over a faulty power circuit may be of greater importance than the
interruption to communication service occasioned thereby and exception
is made accordingly to the general rule that faulty power circuits shall
not be re-energized for operation until the fault is remedied. It is, of
course, contemplated in such cases, that the fault shall be remedied at
the first opportunity. .
The necessity is evident for having accurate records of abnormal
occurrences and the switching incident thereto, for use in investigations
to remedy such conditions.
Exhibit.
It is provided in rule IV (&) that in the design and construction of
Class H power lines, consideration shall be given to the configuration
of such lines with a view to minimizing their unbalances and their
inductive effects in neighboring communication circuits. Configura-
tions differ widely with respect to their characteristic unbalances and
intensities of induction, some arrangements, particularly of twin cir-
cuits, giving only 10 per cent or less of the unbalances or induction
given by others. In many cases, at little or no additional cost,
interference may be materially lessened from what it otherwise would
be by giving proper consideration to this matter.
As such information is not always easily available, this exhibit has
been prepared to give a brief summary of the Joint Committee's
technical data bearing on the subject. Doubtless many cases will arise
which will be beyond the scope of the recommendations made in this
Exhibit. In such cases a special study may be made. Methods for
carrying on such studies are described in the Committee's Technical
Imports which are referred to in the Exhibit.
58 FINAL REPORT ON INDUCTIVE INTERFERENCE
APPENDIX I.
INTERFERENCE NOT COVERED BY RECOMMENDED RULES.
EXPLANATORY NOTE.- In the roles recommended, the term "communication cir-
cuit" is used in a restricted sense as defined in II (d) and the power circuits to
which the rules apply are limited to those defined in II (a) as Class H. While
these include the circuits most commonly involved in inductive interference, cases
sometimes occur where either the power circuit causing the induction or the signal
circuit affected by induction is of a type or character excluded under the above
definitions. The Committee has not had an opportunity to make a special study of
such cases. In the following- the attempt is made to state the governing- principles
involved, for the assistance of the Commission in considering cases of this character.
(a) Alternating Current Railways.
Alternating current railway trolley circuits as now generally
operated differ radically from other types of alternating current power
circuits in that one side of the former is grounded throughout so that
they are inherently unbalanced, and moreover, cannot be transposed.
To such circuits the provisions of the foregoing rules in general do not
npply, and are not so intended.
Where railway circuits of this character are operated, it is necessary
to employ special measures in order to prevent inductive interference
with neighboring communication circuits. Other than separating the
two classes of lines where this is practicable, the most important of such
measures can be embodied in the railway construction and should be
included in the design of the electrification after a comprehensive study
of the requirements of the particular case by the parties concerned.
Also, the communication circuits, if metallic, should be properly trans-
posed and otherwise balanced as closely as practicable. The parties
should endeavor to agree as to the responsibilities involved and as to
further measures to be adopted, if any such are necessary. In the event
of failure so to agree the matter should be referred to this Commission.
NOTE. This Committee has undertaken no investigation of cases of interference
due to alternating-current railways, but as the seriousness of the inductive effects of
such railways is recognized, provision is made for co-operation when such cases arise.
(6) Constant-Current Lighting Circuits.
Care should be taken in the location, design, construction, mainte-
nance and operation of constant-current lighting circuits (both direct-
current and alternating-current) to avoid, so far as practicable, inductive
interference with communication circuits. In particular every
reasonable effort should be made to avoid creating new conditions which
would produce such interference, especially where interexchange
telephone lines are affected. In cases where such conditions are
unavoidable, remedial measures should be employed as may be necessary,
FINAL REPORT ON INDUCTIVE INTERFERENCE 59
the details of which should bo agreed upon by the parties concerned in
general accordance with the following provisions:
1. Where necessary, the two sides of the lighting circuit should be
run on one pole line within the section where the interference
is set up and co-ordinated systems of transpositions applied
to the lighting and telephone circuits.
2. Preference should be given to those types of lamps and other
equipment which do not introduce high frequency components
in the lighting current. The use of incandescent lamps
instead of arc lamps is usually advantageous in this respect.
X. Due regard should be given to the insulation and balance of both
1lic lighting and communication circuits. Balance of the
lighting circuit requires equalization of the voltages to ground
of the two sides of the circuit within the section where the
two circuits are in proximity. This necessitates that the cir-
cuit be well insulated and in general that the lamps be similarly
distributed in the two sides of the circuit with equal numbers
of lamps in the two sides between the source of supply and
the section of proximity.
NOTE. It is common practice in city lighting to run single-wire circuits through
many lamps in series, scattered widely, instead of carrying the return conductor on
the same line, or win-re the two conductors are on the" same line, without balancing
their voltnges to ground. Both of these features tend to create residuals and to
cause severe inductive effects in neighboring signal circuits.
It should be practicable, by care in laying out such lighting circuits, and in locating
important telephone lines, such as toll lines which occupy but a few streets, to avoid
close proximity between these classes of circuits. In those cases where proximity is
unavoidable, it is possible, by running both sides of the lighting circuit close jtogether
on the same line, by care in distributing the lamps and by transposing the circuit
within the section of proximity, greatly to reduce the residuals.
A considerable difference exists among the various types of lamps used in that arc
lamps introduce large harmonics into the lighting circuit, while incandescent lamps
produce no appreciable distortion.
The balancing of a lighting circuit can be accomplished in many different ways
depending upon the specific conditions. The simplest general procedure is outlined
above.
(c) Power Circuits of Lower Voltages.
In case of interference with the operation of communication circuits
by constant potential alternating-current circuits of voltage lower than
the limits specified in the definition .of a Class H power circuit, the
parties concerned should agree upon remedial measures in general
accordance with the rules recommended in this report and should
co-operate in applying such measures to the extent that may be neces-
sary, as follows:
1. Where practicable at reasonable expense, the lines should be,
separated sufficiently to avoid interference.
2. Co-ordinated systems of transpositions should be applied to both
classes of circuits within the section where the interference is
set up.
60 FINAL REPORT ON INDUCTIVE INTERFERENCE
3. If practicable, the residual voltage and current of the power
circuit should be reduced.
4. Due regard should be given to the insulation and balance of
metallic communication circuits.
5. Consideration should be given to the reduction of the high-
frequency components of the voltages and currents of the
power circuit.
NOTE. The physical principles upon which the rules as a whole are based apply
in ease of power circuits of all voltages, the differences being only quantitative in
respect to the relative importance of different factors.
(d) Cables.
In case of inductive interference where either the power circuit or
the communication circuit is carried in cable, consideration should be
given to the employment of such remedial measures, included in the
rules recommended or otherwise as may be reasonably applicable.
In such cases, particular features to which attention should be
directed, are: (1) limiting the residual current of the power circuit,
(2) balancing the communication circuits if they are metallic, and (3)
transposing the communication circuits, if they, are metallic and in
open wire.
NOTE. Where cables are used for either power or communication circuits within
sections where these two clases of circuits are in proximity, there is, in general, far
less liability of interference, and many provisions of the recommended rules are
inapplicable. In some cases, however, residual currents in cabled power circuits may
cause interference to either open-wire or cabled communication circuits, and open-
wire power circuits sometimes cause severe disturbance to communication circuits
which are in cable.
(e) Telephone Subscribers' Circuits.
In case of inductive interference by any electric power circuit with
metallic telephone subscribers' circuits (which will usually occur only
when the latter are long open-wire surburban loops) the parties con-
cerned should agree upon a plan for avoiding the interference by
removal of one of the lines or for mitigating the interference by
remedial measures, as the circumstances may require, in general
accordance with the recommended rules.
NOTE. Telephone circuits falling within this class are far more numerous than
those included under the definition of communication circuit II (d) . Fortunately,
subscribers' telephone circuits are not as a rule seriously exposed to the influence of
power circuits since they are generally short and are often run partly in cable. When,
however, they are exposed, the disturbing effects are relatively more severe on account
of the closer proximity of the parallel to the subscriber's station. A parallel involv-
ing a subscriber's circuit does not affect so large a portion of the public as a parallel
involving an interexchange circuit .and on such a basis the former is of less impor-
tance. However, this does not justify excluding such circuits from all protection
against interference. Consequently, with due regard to the relative importance of the
service affected, telephone subscribers' circuits should be afforded such protection as
is necessary in general accordance with the principles governing the protection of
other communication circuits.
FINAL RKI'ORT ON" INDUCTIVE INTERFERENCE 61
(/) Direct Current Circuits.
In cases of inductive interference with communication circuits due to
constant potential direct current circuits, usually occurring only where
grounded railway trolley circuits and telephone circuits are in
proximity, adequate remedial measures should be agreed upon and put
into effect by the parties concerned. Where telephone circuits are
involved, in addition to transposing and balancing such circuits, special
consideration should be given: (1) to securing generators and motors
having a voltage as free as practicable from high-frequency waves; and
(2) to the use of special devices external to the generators, motors and
rectifiers which tend to absorb the high-frequency currents and thereby
prevent their appearance in the line.
NOTE. High-frequency components may occur in constant-potential direct-current
circuits and occasionally constitute a source of interference. This is particularly
true of electrified railway circuits which use a large amount of power. It is there-
fore provided that effort be made to secure apparatus as free as possible from such
high-frequency components and that if necessary, suitable shunt paths be provided to
confine these high-frequency components to local circuits.
(g) Other Cases of Interference.
If any case of inductive interference, not otherwise covered by this
report, should be experienced or become imminent, the parties con-
cerned should endeavor to agree upon a procedure for avoiding
preferably, or if avoidance be not feasible, for mitigating the interfer-
ence by applying, to such extent as may be necessary, the measures set
forth in this report, or by other means.
62 FINAL REPORT ON" INDUCTIVE INTERFERENCE
APPENDIX II.
LIST OF TECHNICAL REPORTS.
The following is a list of the technical reports which have been
prepared in connection with the investigation by the Joint Committee on
Inductive Interference. The reports recommended for publication are
indicated by an asterisk (*) after the number.
Technical
report Subject or title.
number.
1* General outline of tests to be made at Salinas on parallels between lines of the
Sierra and San Francisco Power Company, the Western Union Telegraph
Company, the Southern Pacific Company, and The Pacific Telephone and
Telegraph Company. (6 pages.) Dated January 6, 1913.
2* Summary of results of tests at Morgan Hill on parallel between lines of the
Coast Counties Gas and Electric Company and The Pacific Telephone and
Telegraph Company hpjwppn Morgan Hill and Gilrov. (8 pages.) Dated
February 3, 1913.
3* A description of the noise standard in use for measuring noise on telephone
circuits in terms of a standard unit. (3 pages, 1 drawing.) Dated Feb-
urary 24, 1913.
4* A description of the instruments and methods used for the measurement of
effective values of induced voltages and currents. (2 pages.) Dated
February 24, 1913.
5* A description of apparatus and connections used in measuring line and residual
currents and voltages of power circuits. (4 pages, 2 drawings.) Dated
February 24, 1913.
(5 Tests of the effects of opening the secondary delta of the autotransformer bank
at Salinas. (7 pages.) Dated March 31, 1913.
7 Tests of the induction in the block signalling circuits of the Southern Pacific
Company paralleled by the Salinas-King City circuit of the Coast Valleys
Gas and Electric Company. (3 pages, 1 drawing.) Dated March 31, 1913.
S Tests of the induction in the telephone circuits of exposure No. 2 at Salinas
under normal operating conditions of the power system with particular
reference to the effects of grounding and isolating the neutral of the Salinas
autotransformers. (15 pages, 1 drawing.) Dated March 31, 1913.
1) Experimental determination of the coefficients of induction for residual cur-
rents and voltages in exposure No. 2 at Salinas. (4 pages.) Dated March
31, 1913.
10 Measurements of the harmonics of the neutral current at Salinas. (3 pages,
1 drawing.) Dated March 31, 1913.
1 1 Investigation of current transformers, ratios and errors due to the use of cur-
rent transformers under the conditions of the tests. (17 pages, 4 drawings.)
Dated April 7, 1913.
12 Formula? for the computation of electrostatic and electromagnetic induction
from power circuits in neighboring communication circuits. (14 pages, 4
drawings. ) Dated March 31, 1913.
13. An investigation of errors in measurements of residual voltage due to the
potential transformers used and a discussion of the method of measurement
at Salinas. (28 pages, 2 drawings.) Dated September 3, 1913.
14 Comparative tests of the noise in exposed telephone circuits with power on and
off the 55,000- volt power circuit of the Sierra and San Francisco Power Com-
pany between Guadalupe and Salinas. (7 pages, 1 drawing.) Dated
April 30, 1913.
l-'LNATj KKl'OKT ON INDUCTIVE INTERFERENCE 4.
Protection of Telephone and Telegraph Lines near High Tension Linos. It. E.
Hiefwood, Jr. Electrical World and Engineer. May 21, 1904.
The Influence of High Tension Conductors on Telephone Lines. V. Schroltke.
Elek. /oil.. April 11), 1!)()T.
Inductive Disturbances io Telephone Lines. L. Cohen. Trans. A. I. E. E. Vol.
2<5. p. 1 1 .V,. May. I'.IOT.
ElVccfs of High Tension Power Transmission Upon Telephone Lines. O. Brauns.
Elek. Zeiu April 9, 190S.
Indnciivc Interference with Telephone Circuits in Proximity to High Potential
Transmission Lines. Electric Review. New York. June 13, Unix.
Inductive Disturbances. Mirabeli. Journal Telegraphique de Berne. August 2,">
1909.
Telegraph and Telephone Systems as Affected by Alternating Current Lines. J. B.
Taylor. Trans. A. I. E. E. Vol. 28, p. 11G9, October, 1909.
Induct ive Effects on Telephone and Telegraph of High Tension System of Khenish.
Prussia. Arch. Post. & Tel. November, 1909. Science abstracts "B." January
31, 1910.
Telephone Line Protection from High Tension Transmission Lines. R. W. Krauss.
Sib. Journal of Engineering. December, 1909.
Inductive Disturbances. Muller. Journal Telegraphique de Berne. March 2.~.
1910.
Telephone Disturbances from Earl lied Three-Phase Systems. E. Von Holstein
Ralhlou. Elek. Zeit. June 23, 1910.
Influence of High Tension Installations on Telegraph and Telephone Installations.
Electric Review, New York. June 30, 1910.
70 FINAL REPORT ON INDUCTIVE INTERFERENCE
Interference Between Energy Transmission Lines. London Electrician, October 28,
1910.
A Method of Preventing Inductive Troubles in Telegraphy. G. Girousse. Comptes
Rendus, July 10, 1911.
Influence of Heavy Current Lines on Light Current Lines and Apparatus. Karl
Hohage. Elek. Zeit., December 28, 1911.
The Maintenance of Telegraph Lines. C. M. Yorke. Telephone and Telegraph
Age, 1912,
Inductive Disturbance on Telephone Lines. B. Smith. Telephone Engineer, Jan-
uary, 1912.
Interference between Transmission Lines and Telegraph and Telephone Lines.
Electrical World, February 24, 1912.
Protection of Low Tension Lines Against High Tension Lines. M. Girousse.
Bull. Soc. Int. Electriciens, June 12, 1912.
Relation of Transmission Lines to Telephone and Telegraph Lines. H. B. Gear.
Electric Review and Western Electrician, February 8, 1913.
Disturbance of Telephone Circuits Due to 3-Phase Transmission Lines. O. Brauns.
Elek. Zeit, February 13, 1913.
Telephone Disturbances from Three-Phase Lines. O. Brauns. Electrical World,
February 22, 1913.
Disturbance to Telephone Circuits from Power Lines with and without Grounding of
the Generator Neutral Points. Elek. Zeit., May 22, 1913.
Inductive Disturbances as Affecting Telephone and Telegraph Lines. P. J. Howe.
Electrical World, May 31, 1913.
Interference with Power and Telephone Lines. Electric Review and Western
Electrician, June 28, 1913.
Inductive Effects of Traveling Waves on Telephone Lines. K. W. Wagner. Elek.
Zeit., June 4, 1914.
Investigation of Inductive Interference. F. E. Pernot. Journal of Electricity,
Power and Gas, June 27, 1914.
Neutralization of Inductive Interference. Journal of Electricity, Power and Gas,
November 21, 1914.
Telephone Transmission in its Relation to High Tension Distribution. E. S.
Moorer. The London Electrician^ November 27, 1914.
The Successful Operation of a Telephone System Paralleling a High Tension Trans-
mission Line. C. E. Bennett. General Electric Review, December, 1914.
Power Circuit Induction with Telegraph and Telephone. S. C. Bartholomew.
London Electrician, January 29, 1915.
Power Circuit Interference with Telephone Lines. S. C. Bartholomew. Telephone
Engineer, April, 1915.
The Inductive Effects of a 140,000 Volt Transmission Line. R. D. Parker. The
Michigan Technic, October, 1915.
Inductive Interference Between Power Transmission Circuits and Telephone Lines.
Trans. A. I. E. E. Vol. 34, p. 2113, 1915.
How a Transmission Company Prevented Telephone Troubles. F. E. Gillespie.
Electrical World, December 11, 1915.
Desirability of Transpositions in Power Lines. N. E. L. A. Bulletin, April, 1910.
Some Problems of Inductive Interference. A. H. Griswold, L. P. Ferris and R. W.
Mastick. Journal of Electricity, Power and Gas, April 15, 1916. Telephone
Engineer, May, 1916.
Causes of Telephone Noise and Its Elimination. Electrical World, June 17, 1916.
Serving and Safeguarding the Public. F. C. Dunbar, Ohio N. E. L. A. Monthly,
July, 1916.
Inductive Interference as a Practical Problem. A. H. Griswold and R. W. Mastick.
Proc. A. I. E. E., September, 1916.
Eliminating Transmission Line Telephone Troubles. E. P. Peck. Electrical W T orld,
September 9, 1916.
FINAL REPORT ON INDUCTIVE INTERFERENCE 71
Interference UH ween Electric Circuits. Electrical World, December 1'."., 11)10.
Transmission Lines Interference. L. K. Hurtz. Telephony; December 28, 1916.
Inductive Interference Report of Committee on Overhead Lines and Inductive
Interference. N. E. L. A., June, 1917.
Interference from electric railways.
.\eu( ralizing Inductive Disturbances in Telegraph Lines Due to Single-Phase Cur-
rents. Western Electrician, March 21, 1908.
Inductive Interference from Single-Phase Railway Circuits. American Telephone
Journal, June 27, 1908.
Overcoming Disturbances of Telegraph Working Caused by Electrical Traction
System. C. Mirabeli. Post Office Electrical Engineering Journal, January,
1909.
Disturbances in Telephone Lines from Alternating Current Railways. C. Stein.
Elek. Zeit., August 15, 1912.
Telephone Disturbances on Electrical Railway Lines. F. Marguerre. Elek. Zeit.,
November 21, 1912.
Minimizing Induction from Single-Phase Railways. Electrical World, May 2, 1914.
Inductive Interference With Railway Lines. London Electrician, January 29, 1915.
Influence of Alternating Current Railway Lines on Telephone Lines. Telephone
Engineer, November, 1915.
Single-Phase Traction and Feeble Current Lines. G. Girousse. La Lumiere
Electrique, November 29, 1915.
Publications by the Joint Committee on Inductive Interference.
Inductive Interference by High Tension Lines. Journal of Electricity, Power and
Gas, January 25, 1915.
Proposed Investigation for Inductive Interference. Journal of Electricity, Power
and Gas, March 15, 1913. Western Engineering, April, 1913.
General Progress Report of Committee on Inductive Interference. Journal of
Electricity, Power and Gas, October 4, 1913. Western Engineering, Novem-
ber, 1913.
General Progress Report of Joint Committee on Inductive Interference. Journal of
Electricity, Power and Gas, January 31, 1914, Western Engineering, February,
1914.
Report by the Joint Committee on Inductive Interference to the Railroad- Commis-
sion of the State of California. July 7, 1914. Published by the California Rail-
road Commission. Trans. A. I. E. E. Vol. 33, p. 1441, 1914. Journal of
Electricity, Power and Gas, September 12, 1914.
Discussion of Irregular Wave Form. Trans. A. I. E. E. Vol. 34, p. 1171, 1915.
Progress Report. Trans. A. I. E. E. Vol. 34, p. 2113, 1915. Journal of Electricity,
Power and Gas, September 25, 1915.
Discussion of "Characteristics of Admittance Type of Wave-Form Standard."
Bedell. Trans. A. I. E. E. Vol. 35, p. 1711, 1916.
State Public Utility Commissions.
California, General Order No. 39, August 20, 10.14. Construction and ()|M'r:ition of
Power and Communication Circuits which are or are proposed to be so located
as to create a parallel.
Illinois, General Order No. 30, October 12, 1910. Overhead Electrical Construction.
Iowa, Decision and Order, December 30, 1910. Electrical Interference between
\4ransmission, telephone and telegraph lines
Ai'.nKK^hvriONS :
A. I. E. E. American Institute of Electrical Engineers.
N. E. L. A. National Electric Light Association.
Elek. Zeit. Elektrotechnische Zeitschrift.
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