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HIGH-TENSIONUNDERGROUND ELECTRICCABLESA PRACTICAL TREATISE FOR ENGINEERS

HENRY FLOY, M. A., M. E.Co?isulting Engitieer

MEMBER OFAmerican Institute of Electrical Engineers

Illuminating Engineering SocietyNew York Electrical SocietyNational Jury of Awards, Louisiana Purchase Exposition, 1904

AUTHOR OF'THE COLORADO SPRINGS LIGHTING CONTROVERSYEtc.

FIRST EDITION, 1909

Price, $2.00, prepaid.

NEW YORK :ELECTRICAL PUBLISHING COMPANY

165 BROADWAY


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Copyright, 1909, byHENRY FLOY


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PREFACEAs one of the experts in a recent and important con-

troversy regarding" the necessity of putting under-ground certain high-tension aerial wires in the largestcity in America, the author was made to realize thegeneral lack of information with reference to the pos-sibilities and advantages of subsurface electric trans-mission. This led to the writing of a series of paperson high-tension cables, which were published in theElectrical World, during the fall of 1908. The interestdisplayed at this time, in these matters, has led theauthor to conclude that re-writing, expanding and add-ing to these papers so as to compile a summary of thehigh-tension cable situation as it exists to-day, wouldbe a valuable and a helpful contribution to the up-to-date literature of electrical engineering.

In the following pages are contained summaries ofexperience, facts and figures, which have been gatheredfrom almost innumerable sources, so that the wholemay be said to fairly represent the concensus of presentopinion of a majority of engineers acquainted with andpracticed in the use of high-tension subsurface trans-mission.

After a brief explanation of the development of un-derground transmission, the verbatim opinions of ex-perts using such method of operation are given ; also


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6 PREFACE.records made by various companies with a list of themore interesting high-tension cable installations, in-cluding potentials employed, thickness of insulations,sheaths and other data. The advantages of under-ground compared with aerial construction are broughtout, followed by a discussion of the dielectrics em-ployed and the present practical voltage limits attain-able with electric underground transmission. Curves,tables and data are presented relating to the heatingand testing of cables, as well as formulae to be used inelectrical calculations. The book concludes with achapter on the costs of underground installations withparticular reference to the prices of cables.The author desires to extend his thanks to those whohave co-operated in his efforts to compile up-to-dateknowledge and practice, and trusts that this littlevolume will prove of assistance to those who desire toacquaint themselves either with what is being done orwhat are the present possibilities of high-tension un-derground electric transmission.

HENRY FLOY.City Investing Building,New York, February 1, 1909.


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TABLE OF CONTENTS,

PageFrontispiece, St. Paul Cables.Preface 5Table of Contents 7

CHAPTER I, INTRODUCTORYGeneral 11Historical 14Expert Opinions 15

CHAPTER II, CABLE RECORDSOperating Data 18Other Installations 23Exhibits 26Cables in Use 26Table I, List of Installations 28

CHAPTER III, ADVANTAGES OF UNDER-GROUND CABLES

Existing Conditions 33First, Lightning 34Second, Breakdowns 34Third, Interference.. 36


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8 TABLE OP CONTENTS.PageFourth, Accidents 38

Fifth, Interruptions 38Comparative Example 39

CHAPTER IV, CABLE INSULATION.Dielectrics Employed 41Rubber Insulation 43Paper Insulation 51Cambric Insulation 54Dielectric Stresses 57Graded Insulation 59Composite Construction 61Thickness of Commercial Insulations 63Table II, Thickness of Cambric Insulation.. 65Table III, Thickness of Rubber and Paper

Insulation 66Table IV. Thickness of Paper Insulation. ... 67Table V, Thickness of Rubber and Paper

Insulation 67Joints 68Specifications 71Practical Commercial Potentials 73

CHAPTER A;, METAL IN CABLES.Copper 77Table VI. Commercial Bare Copper Solid

AYires 78Table VII. Commercial Bare Copper

Stranded Wires , 79


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TABLE OF CONTENTS. 9Page

Table \ III, Approximate Outside Diametersof Three-Conductor Copper Cables 80Aluminum 81

Table IX, Comparative Diameters * of BareCopper and Aluminum Stranded Wireshaving' the same Conductivity 82Tin and Lead . . 83

CHAPTER VI, HEATING OF CABLES.Cables, versus Wires 88A. C. vs. D. C 90Carrying' Capacity 92Fig. 1, Curves of heating of single-conductor

cables 94Fig. 2. Curves of heating of concentric cables 95Fig. 3. Curves of heating of three-conductor

cable \ 96Table X. Recommended Current Carrying

Capacities for Cables and Watts Lost perFoot 97

Table XI. Equivalent Conductor Areas 98Table XII. Recommended Power Carrying-

Capacity in Kilowatts 99Table XIII. Current Carrying Capacity ofThree-Conductor Cables 102

Temporary Loads 103Ducts 104


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io TABLE OF CONTENTS.PageCHAPTER VII, ELECTRICAL FORMULAEFOR CABLES.

Resistance 106Inductance 107Capacity 108Table XIV, Relative Specific Inductive

Capacities IllTable XV. Insulation Resistance and Elec-

trostatic Capacity Temperature Coefficients 112Table XVI. Capacity of Three-Conductor

Cables 115Table XVII. Cable Capacity Measurements. 116Reactance 118Impedance 119Table XVIII. Approximate Ohmic Resistanceand Impedance 120

Skin Effect 120

CHAPTER VIII. TESTING OF CABLES.Summary 122Ohmic and Puncture Tests 122

CHAPTER IX. COSTS.Total Costs 129Cable Costs 130Fisr. 4, Curves of Cable Costs . . 135


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CHAPTER I.INTRODUCTORY

General. The use of aerial lines for transmis-sion and distribution systems was logically to be ex-pected in the early stages of electrical development onaccount of the simplicity and low cost of construction.With the development of the industry and the necessityof putting wires underground, continuously insulatedconductors were undertaken, which, like many otherinnovations, proved in some cases unreliable and un-satisfactory at the beginning; but with the develop-ment of improved processes and greater perfectednessin manufacture, subsurface cables, even for high volt-ages, have come to be regarded as reliable as almostany other appliance employed in the electrical art.

In spite of the many instances of successful installa-tion and operation of high-tension cables, both underground and under water, there exists a general- lack ofinformation, and to some extent, a general prejudice,which prevents their wider use and installation.The importance to the engineer of knowing the high-

est practical voltage at which subsurface cables can besuccessfully operated, the minimum insulation safelyallowable for a given potential, and the cost of suchcables completely installed, is not fully realized.


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12 INTRODUCTORY.Knowledge, or lack of knowledge, of this subject onthe part of the engineer in charge, may determinewhether an alternating or a direct-current system ofhigh or low voltage shall be selected for a given instal-lation with consequent large or small expenditure intransformer plant, elaborate switchboard, enlargedbuildings, or unnecessarily heavy insulation. The lackof general information regarding recent improvementsmade both in the manufacture of cables and in the solu-tion of the peculiar troubles like!}' to arise in the opera-tion of high-tension cables, probably accounts in a largemeasure for their comparatively limited use. Therehas existed no particularly urgent incentive for investi-gation as to the possibilities and advantages to begained in the construction and use of high-tensioncables. The operating companies, in order to avoid in-vestment, have usually been opposed to, and in conse-quence have developed and used every argumentagainst, underground construction. The cable manu-facturers themselves, conceding the good work doneby some of them, have been working along commercialrather than scientific lines, and the properties of theseveral dielectrics available for insulation have beenconsidered from a commercial rather than an engineer-ing standpoint. The manufacturers have hardly paidsufficient attention to the electric phenomena of dielec-trics, a scientific study of which would doubtless haveproved both interesting and remunerative. By way ofillustration it may be said that few insulated wire man-ufacturers appreciate the difference, in their effect on


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INTRODUCTORY. 13insulation, of alternating- waves of various forms, or ofthe various processes used in the production of resinoil that result in oils, varying widely in value, for useas dielectrics. Recent research along these lines givespromise of far-reaching results that mark a decided ad-vance in the use and the permanency of undergroundcables. It is only just recently for example, that onehas been able to purchase paper cables, the flexibilityof which remains practically unchanged at zero tem-perature.

It is now recognized that many dielectrics whenfreshly produced, make an excellent showing, but in thecourse of a few months or years, undergo physical ormechanical change which greatly depreciates theirvalue or renders them worthless. Permanency hasbeen generally admitted as the sine qua non of cablesuccess, and this is now being obtained as a result ofexperiment and test. The only really valid objectionthat can to-day be urged against the use of under-ground cables is their relatively high cost as comparedwith aerial lines, but this objection decreases in almostdirect proportion to the increase in the number of cir-cuits installed. Attempts to meet this objection ofinitial expense have been made in several ways, pri-marily and most successfully by substituting a cheapermaterial, such as paper or cambric, for rubber insula-tion ; and secondarily, by the construction of cheaperforms of conduits in which the cables are drawn or bythe abandonment of conduits altogether, simply lay-ing the cables in the ground as is being done at pres-


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14 INTRODUCTORY.ent ; for example, by the New York Edison Companyin city parks or in some cases, where low potentialsare used, embedding the bare conductor, in situ, in acheap insulating material, usually a bitumen com-pound.The practicability and reliability of cables for 110,

220, or even 500-volt service, is usually admitted ; butwhen cables for higher potentials are considered it isoften asserted that they are unreliable. Contradictionof such statements is best made by an examination ofthe records made by high-voltage underground sys-tems, and the conclusion with regard thereto submittedby those having practical experience with such systemsrather than by the consideration of statements of meretheorists or those not practically engaged in the trans-mission of electrical energy at high voltages under-ground, or those endeavoring to operate such systemswho have not the education or experience qualifyingthem to do so successfully.Historical. It is not perhaps generally appreciatedthat 26 years ago, underground cables laid in a trenchfilled with "Bitite," a vulcanized bitumen, were givingsatisfactory service for low-voltage distribution in Eng-land, and that 25 years ago, Eastbourne, England, waslighted from the comparatively high-voltage circuits ofthe Brush Company which were contained in an under-ground system of iron pipes through which the con-ductors were drawn. Twenty years ago, 2000-volt un-derground cables were in use in Rome, Tivoli, Turinand Milan, while Berlin early had a reputation for its


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INTRODUCTORY. 15underground system, and Paris began its subsurfacedistribution by installing copper bars supported onporcelain knobs in its sewers. The well-known 10,000-volt concentric cables of Ferranti were installed in Lon-don over 18 years ago and early proved the success ofhigh-tension underground transmission. Cables withrubber insulation 4/32 inch thick covered with a leadsheath, operating on 7,500-volt arc light circuits, in-stalled in Buffalo 17 years ago, are still in use. In1889, New York City had many miles of low-tensionunderground cables and the city authorities, resortingto police methods, were cutting down aerial lines toforce the companies underground. Since those days,marked advance has been made both in details of con-duit construction and methods of cable manufacture.Expert Opinions. The present status of high-ten-

sion underground distribution can be best learned froma consideration of the expressed opinions of some ofthe well-known members of the engineering professionwho have attained and still hold their high positions,largely by reason of their successful operation of suchhigh-tension systems. Mr. L. A. Ferguson, presidentof the American Institute of Electrical Engineers andvice-president of the Commonwealth Edison Company,Chicago, which company has a station generatingcapacity of 120,000 kw and operates both aerial andconduit systems including nearly 400 miles of 9,000 and20,000-volt paper-insulated underground cable, in ad-dition to much low-tension cable, succintly states the


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1 6 INTRODUCTORY.superiority of subsurface conductors over aerial con-struction as follows :*

"It is generally conceded that when the busi-ness wr ill warrant the investment, electrical linesare much better underground than overhead.''

An ex-president of the same Institute, l\Ir. H. G.Stott, chief engineer of the Interborough Rapid Tran-sit Company having 95,600-kw station capacity dis-tributed wholly through 375 miles of 11,000-volt cables,some submarine says :f

"1 think it dwells in the minds of many ableengineers that high-tension lines are very dan-gerous. I differ from that. 1 think the high-tension underground cable is the safest thing wehave a great deal safer than low-tension."

Mr. J. W. Lieb, Jr., also ex-president of the AmericanInstitute of Electrical Engineers and general managerof the Xew York Edison Company having 150,000 kwrated generating station capacity, or including storagebatteries, 200,000 kw capacity which company, in ad-dition to many miles of low-tension cable, is operatingover 200 miles of 6,600-volt cables, stated to the writerwith reference to the high-tension cables, that,

"There is no question as to the practicabilityand reliability of underground cables whetherfor lowr-tension or high-tension service, whencompared with aerial conductors."

* Paper presented at the International Congress, St. Louis, 1904,entitled, " Underground Electrical Construction."

f Proceedings A. I. E. E., vol., XXI., page 443.


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INTRODUCTORY. 17Warren Partridge, Engineer for the Public Service

Corporation of New Jersey, says*"In spite of all difficulties experienced******

cable systems are fully as reliable .as other ele-ments in the electric power system. Ourrecords for a period of three years show thatcable breakdowns caused but 7 per cent of all in-terruptions to service and that the duration oftime of cable interruptions was no longer thanthe average interruptions from other causes."

Many other less well-known but equally enthusiasticbelievers in the use of subsurface conductors, could becited, if further argument were necessary.* Proceedings A. I. E. E., vol. XXVIII., page 106.


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CHAPTER II.CABLE RECORDS

Operating Data. Reference to the records of break-clowns on high-tension cables in actual use, substan-tiates the claim to reliability for high-tension cables.

Mr. Peter Junkersfeld, referring to the Chicago sys-tem, says their cable troublesf

"during the last three years have averagedonly two cases per hundred miles per year.This includes all troubles on 9,000-volt cablesfrom known or unknown causes, except thosedue to external injury to the lead sheaths."

In a more recent paperj Mr. Junkersfeld shows thatduring the preceding five and a half years, their 9,000and 20,000-volt cables, aggregating 275 miles, had atotal of only 48 breakdowns, of which 26 were due toexternal causes ; or, ignoring damage from externalcauses, there was only one break-down per year per 15miles of cable installed. Of the total number of burn-outs, but a small percentage caused any serious shut-downs, and the company is now engaged in extendingits underground system by adding 68 miles of 9,000-volt, 250,000-cm, three-conductor cable, and 44 miles of

f Proceedings A. I. E. E., vol. XX VI.. page 1614. Part II.\ Proceedings A. I. E. E., vol. XXVI I., page i^/o.


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CABLH RECORDS. 19additional 20,000-volt, No. 00, B. & S. three-conductorcable. The 9,000-volt cables are insulated with 6/32inch paper about each conductor and a jacket or beltof 4/32 inch paper; the 20,000-volt cables with 9/32inch paper around each conductor and a belt of 6/32inch over all.The Interborough Rapid Transit Company, after

several years of operation, found it averaged only onebreakdown per year per 62^ miles of cable, includingthe larger number of interruptions liable to incur onnew installations.* Their Chief Engineer recentlysaid :f

"The number of burnouts per 100 miles ofcable per annum, has fallen during the last twoyears to 0.28, or practically one fault per 400miles of cable per year. That is a reassuringrecord ; when our overhead transmission linescan show anything like it, we can look forwardto reliable long-distance transmission."

The New York Edison Company has never had acomplete shutdown of its system from any cause duringthe past 15 years, which, of course, includes its 200miles of 6,600-volt system. Despite the difficulties en-countered in making installations in the streets of NewYork and the early period at which much of its under-ground system has been installed, this company hashad only 66 cable breakdowns of all kinds during the

* Proceedings A. T. E. E., vol. XXVI.. page 1641. Part II.f Proceedings A. 1. E. E., vol. XXVI 1 1. , page 96.


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20 CABLE RECORDS.nine years of high-tension operation. Of these break-downs, 32 developed during operation and 34 werefound either by periodic insulation tests or by inspec-tions of the cables. Of the 32 that developed duringoperation, only 18 were caused by other than mechani-cal injuries, which, based on 200 miles, makes a recordof one breakdown per year per 100 miles of cable oper-ated.*

Mr. Charles E. Phelps, chief engineer of the Electri-cal Commission of the City of Baltimore, Md., showsthat the breakdowns of all the various cables includ-ing telephone, fire and police service amounting in1906 to nearly 300 miles, operating in Baltimore undervarious potentials and as high as 13,000 volts, were 148for a period covering seven years ; or, omitting theyears 1903-4-5, when the breakdowns were abnormallyhigh owing to street improvements consequent uponthe fire and electrolytic action, there is an average ofabout one breakdown per year per 40 miles of cable ofall kinds.

In Buffalo, where for years they operated 11,000-voltcables with commercial satisfaction and published rec-ords on two of their 9/32 rubber-insulated, three-con-ductor No. 000, B. & S. with no over-all jacket, lead-covered cables, each about 6 miles long show only twobreak-downs, these from mechanical injury, from 1900to 19064 The Public Service Corporation of New Jer-sey, has about 90 miles of underground and 65 miles* Proceedings A. I. E. E., vol. XX VI., page 1615, Part II.J Proceedings A. I. E. E., vol XXV., page 209.


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CABLE RECORDS. 2 1of overhead cables operating at 13,200 volts, most ofthem being- No. 00, B. & S. All these cables arethree-conductor, paper-insulated 7/32 inch over eachconductor, 7/32 inch over all with 1/8 inch lead sheath.The breakdowns from January 1, 1905 t to October 1,1908, 3.75 years, were, 11 in joints, 16 from externalcauses, 25 in cables, a total of 52. Thus the break-downs, excluding external causes and defective in-stallation, are 10 miles of cable per breakdown peryear. Half the total number of cables had no troublewhatever ; 5 cables had 1 each ; 2 cables had 2 each ;and 4 cables had 16 breakdowns, the latter being tie-lines not straight feeder-lines.

In a paper read before the Pittsburg Branch of theAmerican Institute of Electrical Engineers, May 8,1907, Mr. Charles W. Davis, reported figures relatingto operating breakdowns on 1,462,000 feet of three-con-ductor lead-covered underground cable with potentialsof from 11,000 to 16,000 volts installed on 14 different"construction jobs." The number of breakdowns ofall kinds were 15, or one breakdown in joint for every324 made ; one breakdown in bends in manholes forevery 340,000 feet of cable, and one breakdown forevery 227,000 feet of cable lying wholly within ducts.Taking into consideration the four years covered bythe breakdowns, there were from all causes whatsoeverone breakdown per year per 390,000 feet (74 miles) ofcable. Mr. Davis concludes, his paper with the state-ment that the figures indicate that


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22 CABLE RECORDS."practically all the defects or faults existing in

a system will be weeded out by the initial high-voltage tests, the remaining few, if such still ex-ist, being developed by the first few months ofregular service. This conclusion is confirmedby observations on many other installations notcovered by these remarks."

Among the examples of less extensive installationsthan those referred to above, may be mentioned theTwin City Rapid Transit Company, of Minneapolis,Minn., which has, at present, some 60 miles ofthree-conductor 13,000-volt, paper-insulated cable,much of which has been operating since 1897, andduring' the last three years it has had a totalof only six breakdowns due to other than mechan-ical injury or poor workmanship. Two three-conductor cables, one with paper insulation andthe other with rubber insulation, were installed inSt. Paul, Minn., in 1890, for 25,000-volt service and havebeen giving satisfactory results under rather exactingconditions, Although the first underground installa-tion made for operating potentials anything like as highas 25,000 volts, at a time when there was considerablyless knowledge and experience with high-tension work,these St. Paul cables have a total record of but 37 fail-ures from all causes in nearly eight years of con-tinuous service, and 33 per cent of all the failuresoccurred in one year, due mainly to special difficulties.The cables are connected to the end of a 24-mile aerial


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CABLE RECORDS. 23transmission line and are possibly therefore particu-larly subject to lightning.At Montreal, Canada, four three-conductor cables are

operated, each about 4,500 feet long, at 25,000 volts.These cables were installed in 1902, and during the sixyears intervening to date only eight breakdowns in allhave been reported, although for part of their lengththey are installed in ducts under a canal.There is a second installation under the St. Lawrence

River at Montreal, of three-conductor and single-con-ductor rubber-insulated cables operating at 25,000volts, part of which was made in 1906, and althoughconnected to aerial lines and operating under water,only a total of one breakdown from all causes has beenreported to date. The same company is also operatingsatisfactorily several 12,500-volt submarine cables.

Other Installations. At York Haven, Pa., in 1906,were installed two three-conductor rubber-insulatedarmored cables, each 3,280 feet in length, which havebeen operating continuously at 25,000 volts. These ca-bles, between the generating station and one end of anaerial transmission line, are laid under water across theSusquehanna River. Philadelphia has about 100 milesof three-conductor lead-covered cables and Baltimoreover 125 miles of No. 000 B. & S. three-conductorpaper-insulated cables, all being operated at 13,200volts under ground. In New Orleans, where the ductsare more or less continuously filled with water, thereare about 8 miles, and in Boston and Washington, D.C., many miles of 6,600-volt cable. In Portland, Ore.,


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24 CABLE RECORDS.an ll,COO-volt submarine cable is in use. San Fran-cisco, Cal., has been using 11,000-volt, three-conductorcable with "graded" insulation, about 10 years. InNew York City, the passenger service of all railroadsis operated entirely therein by electricity supplied at11,000 volts through three-conductor lead-coveredcables that are partly submarine and partly under-ground, or in iron pipes; in the Borough of Queens, an-other railroad system depends for the operation of alarge part of its service upon 11,000-volt undergroundcables.Under the Hudson River at Poughkeepsie, N. Y.,

there are two three-conductor rubber cables operatingat 12,000 volts, and at Houghton, Mich., similar sub-marine cables are operated at the same voltage.Across Great Bay, at Portsmouth, X. II. , there are twothree-conductor rubber submarine cables, 5,000 feet inlength, operating at 13,000 volts ; and at Norfolk, Va.,4,000 feet of three-conductor submarine cable operatingat 11,000 volts. Berlin is using three-conductor, steeltaped 10,000-volt cables. In both Toronto and Quebec,Canada, and Providence, R. I., 12,000-volt undergroundcables are in use. Detroit, Michigan, is operating twoNo. 2, B. & S. three-conductor cables each 7.5 mileslong, at a potential of 23,000 volts, and insulated with2/32 inch rubber plus 6/32 inch varnished cambricabout each conductor with a jacket of 3/32 inch cambricand a 3/32 inch lead sheath. In Rio Janeiro, Brazil, 13,-000-volt cables have recently been installed, and about ayear ago, there were put in operation in Durham and


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CABLE RECORDS. 25Northumberland Counties, England, nearly 100 milesof 20,000-volt, three-conductor cable, with a consider-able additional mileage of 12,000 and 6,000-volt cables,all of which, at last reports, \vere operating satisfac-torily. 9

Spain has installed some cables operating at 15,000volts, while in Italy they are using 10,000-volt cablesat Naples, 12,000-volt cables at Genoa and 16,000-voltcables at Milan, and at the end of aerial lines some 20,-000, 25,000 and even 30,000-volt underground cables.The Moutiers-Lyons, France, continuous current,

60,000-volt transmission line feeds into two substa-tions at Lyons, which are about 2 T/ miles apart, andconnected in series through single-conductor under-ground cables. The cables have a section of 75 sq.m. m., and after being insulated, are protected withboth a lead covering and steel armoring.The above mentioned installations, although only

a partial list, indicate, to some extent, the present-daywide use and exacting requirements made of high-ten-sion cables. The successful employment of high-ten-sion cables under water is particularly interesting, be-cause of a popular belief that the use of high-tensioncables under such conditions is almost impossible.Furthermore, such cables are often installed at the endor in the middle of a high-tension line, so that they areparticularly subject to damage by lightning or the pil-ing up of potential due to a change in the constants ofthe transmission line. Although the installations citedabove refer only to constant potential circuits, it is


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26 CABLE RECORDS.well known that there are miles of underground seriesarc circuits in most large American cities nightly carry-ing potentials of from 6,000 to 10,000 volts.

Exhibits. At the Louisiana Purchase Exposition atSt. Louis, 1904, samples of cables designed for 50,000volts (effective) and tested to 100,000 volts without per-foration, were exhibited. Similar cables were shown atthe Milan Exposition in 1906, which, being tested forbreakdown point, in about 15-feet lengths, gave way atslightly above 200,000 volts.The cable manufacturers in America and abroad are

prepared to furnish what may fairly be termed high-tension cables. Some makers are prepared to supplyand guarantee cable for 40,000 to 50,000-volt service,while one reliable manufacturer has submitted thewriter a bona fide proposition for a client, to furnishsingle-conductor cables, lead-sheathed, for 75,000-voltservice, pieces cut off to withstand a test of 150,000volts, the price being comparable with that of a cabledesigned for more moderate voltages. One Americanmanufacturer reports the production of a small amountof cable for commercial service, which satisfactorilywithstood time tests of 150,000 volts and requiredabout 240,000 volts to break down.

Cables in Use. Below is given a table showing someof the most interesting high-voltage underground andsubmarine cable installations in America, together withinformation as to the method of operating the character


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CABLE RECORDS. 27and thickness of insulation, insulation per thousandvolts, etc. It will be noted that the total thickness ofinsulation per thousand volts between conductorsvaries from over 4/6-i inch in 6,600-volt cables, down toabout 1/64 inch in 25,000-volt cable. This differ-ence is due to four different causes :

First. Difference in their value as dielectrics, of thematerials employed as insulation.

Second. Ignorance as to the minimum insulationthat is safe for a given potential.

Third. Difference of opinion among engineers as tothe proper factor of safety to use in the design of highpotential cables.Fourth. The higher the operating voltage probablythe less the proportionate increase in temporary poten-tial strains due to surges, etc. ; and hence, the less thenecessity for a high factor of safety.The list of 35 distinct installations hereafter given,

aggregating over 2,300 miles of high-tension cableoperated under so many diverse circumstances as tovoltage, current carrying capacity, character of insula-tion, outside metal protection, with widely varyingelectrical and climatic conditions demonstrates thatsubsurface electrical transmission at high voltages can-not be considered in any sense experimental.


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CHAPTER III.ADVANTAGES OF UNDERGROUND

CABLES.

Existing Conditions. In American cities and townsof any considerable size the local regulations usuallyrequire that all wires be put under ground, except inthe more sparsely populated portions. Where such re-quirements exist, the distribution of electrical energyis at present being generally done by means of lead-covered cables threaded through vitrified clay, woodfiber or bituminized paper ducts laid in Portland ce-ment. By far the greater proportion of work installedunder the conditions cited is for low-potential distri-bution, although in the larger cities and as a section ofa transmission line entering such a city district or pass-ing under a river, many high-tension underground in-stallations can be shown. The relative advantages ofhigh-tension underground, as against aerial construc-tions, cannot perhaps be properly considered in suchconnection, because subsurface construction is moreor less compulsory. It has, moreover, rather been thepractice of engineers not to resort to the use of high-tension underground installations except under somesuch compulsory conditions. As the advantages andreliability of high-tension cable construction arerealized, a wider use of such cables is sure to result.


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34 ADVANTAGES OF UNDERGROUND CABLES.Among- the advantages are :First, Lightning. Absolute freedom from interrup-

tion of service and damage to apparatus from lightningdisturbances. It is generally recognized and acknowl-edged that any system of electrical distribution whichis completely underground is immune from atmos-pheric lightning, although, of course, disturbances andundue potentials that arise by reason of surges, arcinggrounds, etc., may occur with underground as withaerial systems.

Second, Breakdowns. Less liability of interruptionof service from breakdowns.

"In Xew York where probably there is morecable than any other city in the United States,or in the world, interruptions of service due tothe breaking down of a cable are almost unheardof."*

Most of the breakdowns occurring in high-tensioncables are the result of "human frailty," which can belargely anticipated and avoided.

"As a rule, more trouble will develop on un-derground cables due to poor work on installa-tion rather than to faults in the cables them-selves,"f

Engineering opinion is practically unanimous in thestatement that the weakest point of cable installation* C. W. Rice, Proceedings A. I. E. E., vol. XXIV., page 416.f I. A. Ferguson " Underground Electrical Construction ", Proceed-

ings International Electrical Congress of St. Louis, 1904.


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ADVANTAGES OF UNDERGROUND GABLES. 35is the joint. It is essential that the insulation at thejoint shall exclude air and moisture, and be as solidand perfect as the balance of the insulation. To indi-cate the perfection of workmanship and material whichmay be attained by due care, it is said with regard tothe method of making cable joints employed by theCommonwealth Edison Company, of Chicago, that*

"the method (see page 70) has been tried forsix years on an installation comprising 420 milesof high-tension cables. (This figure includes4400-volt circuits. Ed.) During the entire termof this test only one failure of a cable jointoccurred on these lines, and this was plainly at-tributable to external causes."

Surprising as it may seem at first thought, experienceshows that the short-circuiting or grounding of a high-tension cable results in as little as or less damage thanin the case of low voltage cables. With low voltagesand large currents, the burning resulting may be seri-ous, and in at least one instance brought to the author'sattention, damaged several miles of cable ; whereas,with high voltages, the arc is so severe as to promptlyextinguish itself or open the station safety deviceswithout burning more than perhaps two to five feetof cable.On account of the increased cost of cables with highfactors of safety, there is a strong tendency to reducethe thickness of insulation and thereby the cost, but at* Electrical World, page 544, Sept. 5, 1908.


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36 ADVANTAGES OF UNDERGROUND CABLES.an increased risk of breakdowns. This is being coun-teracted by some manufacturing companies throughthe adoption of the same business methods of installinghigh-tension underground cables that were employedand still are, to some extent, used in connection withthe installation of storage batteries, namely, furnishing,drawing in and connecting up the cables complete, thenundertaking to maintain them, as against defects inmanufacture or installation, for an annual charge,which in some cases, is as low as one-half of one percent of the total cost of the cables.

Third, Interference. Fewer interruptions of servicefrom extraneous interference. Short circuits andgrounds are more or less continually occurring withaerial lines due to breaking of mechanically weak wiresor insulators, storms of wind and ice, objects fallingacross the wires and short-circuiting them, andmalicious interference. With underground con-ductors, annoyances of the above character are almostentirely done away with, the cables usually being in-stalled in ducts of tough material, enclosed in concreteseveral inches thick, the whole being from one and ahalf feet to three feet below7 the surface of the ground,except at manholes, which are protected by double,heavy iron covers, affording protection against almostanything but dynamite. In case of strikes, it would bemuch easier to patrol and protect lines in conduits thanthose carried on poles, because the latter can be dam-aged from a distance by rifle shots or wires thrown


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ADVANTAGES OF UNDERGROUND GABLES. 37over the lines, whereas underground construction mustbe directly approached before it can be injured.

"In the overhead system (Boston), thetroubles are ten to one in comparison with theunderground cable system, almost all of theseoccuring in cable newly installed."*

Operators of aerial circuits usually do not keep asfull and complete records of interferences caused byfailures of their lines as do those in charge of cableinstallations. Lack of explicit information frequentlyleads those operating overhead lines to the conclusionthat their interruptions are not anything like as fre-quent as are troubles in cable circuits, indicated byrecords that have been published. The followingrecord for the last twelve months furnished by a com-pany maintaining detailed accounts of each shutdownof their aerial line may be taken as indicating results atleast as favorable as the average, because the line islocated too far south to be ever troubled by snow orice, is well built and operated under independent, pro-gressive management. The line is a little over 100miles long and shows one breakdown per year for each6 miles of line.

It must be admitted that a breakdown in a cable ismore serious than in an aerial line, because the lattercan be repaired more quickly and with cheaper laborthan the former ; but breakdowns in cable systems are* Proceedings A. J. E. E., Jan., 1909, page 14.


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38 ADVANTAGES OF UNDERGROUND CABLES.not near as frequent as interruptions of service withaerial lines, despite the fact that many high-tensioncables are operating with rather light insulation for theservice.

Fourth, Accidents. The liability of accident to thepublic with consequent damage suits is almost entirelyremoved. The only likelihood of danger is from ex-plosions of gas that may accumulate in ducts or man-holes, but with proper design and construction thisdanger can be practically eliminated. Frequent injuryor death to individuals coming in contact with brokenaerial conductors are too much of an every-day occur-rence to need any argument to prove the desirability ofunderground construction from the standpoint of safe-ty to the public, regardless of aesthetic considerationsor the inconvenience of poles in streets.The danger to employees is less with underground

than aerial construction for the reason that in repairingor stringing new aerial lines there are usually other livecircuits on the same pole with which the workmen maycome in contact, whereas with underground construc-tion, the live cables are enclosed in lead sheaths whichare grounded, and therefore, harmless.

Fifth, Interruptions. "With aerial circuits, interrup-tions in the continuity of the transmission systemusually occur without any forewarning. In contra-dis-tinction to this the weakening of the insulation ofcables is often determined by tests or by suitably de-signed indicating instruments, sufficiently in advance of


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ADVANTAGES OF UNDERGROUND CABLES. 39the actual breaking- clown of the insulation to allowtransferring to another cable without interruption ofthe service. There has been developed by Messrs.Torchio and Varley, of New York, a device now incommercial use which takes into account the unbalanc-ing of the condenser capacity current, when the insula-tion of a conductor begins to depreciate, and giveswarning of approaching danger sufficiently in advanceof a breakdown to allow the cable to be disconnected.Comparative Example. As illustrative of the rela-

tive cost of aerial and underground constructions, thefollowing figures are given, having been prepared inconnection with plans for an actual installation of un-derground cables for the transmission of 20,000 H. P.,15 miles across country, from a certain hydro-electricstation to a substation in a neighboring city. By theuse of conduits laid in the highways, the cost of expen-sive rights-of-way, real estate, building and loweringtransformers for a substation at the outskirts of thecity and liability of interference with the circuits will beavoided. While the increased cost of the undergroundconstruction seems large compared with aerial lines,the difference will be only a small percntage of the totalcost of the complete system, as noted in the followingtable : COMPARATIVE COSTS OF SYSTEMS.

AERIAL LINES.Private right of way across country $45,000Steel towers with three circuits, complete for

15 miles, , 85,000


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4o ADVANTAGES OF UNDERGROUND CABLES.Three miles of 8-cluct conduit, at $7,000 permile, 21,000Six 10,000-volt paper cables (1 spare), at $1.10

per ft., 105,000Substation complete, with 18,000-kw trans-former capacity 95,000

$351,000UNDERGROUND LINES.Eighteen miles of 4-clnct conduit, at $5,000

a mile $90,000Three 25,000-volt paper cables (1 spare), at

$1 .50 per foot, 428,000$518,000

The above example illustrates, of course, only oneset of conditions. The use of cables designed for othervoltages than those specified would naturally resultin different total costs. Again, when considering thecost of transmission lines in connection with stationapparatus, it might be found advisable to generateat 15,000 volts and transmit at that voltage, therebyavoiding the cost of step-up transformers, included inthe estimates based on 25,000 volts for transmission.The principle point to be brought out by the figuresis that the use of high-tension underground cables isnot limited to city streets ; but, under certain circum-stances, may be advantageously used across country.


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CHAPTER IV*CABLE INSULATION

Dielectrics Employed. While it should not bethe duty of the purchaser to attempt to specify thedetails of insulation manufacture and application anymore than he attempts to specify all the details of con-struction of standard apparatus used in the electricalbusiness, nevertheless, in the present state of the artit seems necessary that the engineer be well informedas to properties and limits of cable dielectrics and theirmethods of production and application so as to be ableto control the manufacturer, unless the latter is willingto assume all responsibility for his product, backingthat up with a guarantee covering a long period ofyears.The early attempts in America at operating con-

ductors underground, were made in the larger cities bythe Edison Companies. The original system consistedof iron pipe, usually in 20 feet lengths, containing cop-per rods covered with a light cotton or jute insulationembedded in bituminous compounds. The pipes werecalled tubes, and contained two conductors for feedersand three conductors for the three-wire mains ; theconductors were united at their ends by means of flexi-ble conductors enclosed in cast iron couplings or junc-tion boxes filled with compound similar to the tubes.


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42 CABLE INSULA TION.Even for the low potentials employed by the EdisonCompanies, the type of insulation available, with thelow melting" point of the compound, was soon found in-sufficient and unsatisfactory for subsurface work andresulted in the adoption of rubber and gutta percha forunderground insulation.The lack of flexibility and accessibility in a systemwhere the conductors could only be examined or with-drawn by tearing up the streets, developed nearly 25

year ago, the drawing-in system, namely, the use ofducts united by manholes permitting the drawing inof a thoroughly insulated, usually lead-covered, flexibleconductor. Although the duct system has become al-most universally accepted and adopted in America, thesolid system is still being used with satisfactory resultsabroad, and for lower voltages and special installationsmay, in many cases, be desirable. Recent improve-ments in the quality of the insulating and water-proof-ing compounds with reductions in their price, may yetsee the more extended use in America of the solidsystem, which permits the installation of a bare con-ductor in situ.As increasingly higher potentials were attempted, his-

tory shows that rubber and its compounds were almostexclusively used for insulation ; however, on accountof its high cost substitutes were quickly sought andpaper carefully applied and properly treated was soonfound to be satisfactory, provided dampness and mois-ture could be kept away. This was accomplished byinclosing the insulation in a lead sheath, which, as long


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CABLE INSULA TION. 43as it remains intact, allows paper insulated cables togive very satisfactory service for the highest voltagesyet commercially employed under ground. More re-cently, shellaced cambric has been used, which, al-though more costly than paper is less .expensive thanrubber, and unlike the former does not quickly depreci-ate in the presence of water. The latest developmentis "graded" insulation, which is a combination of differ-ent dielectrics or the use of a nonuniform material.Rubber Insulation. Rubber, the unique vegetable

product, for which no full substitute has ever beenfound, makes an unsurpassed dielectric when properlytreated, by reason of its insulating qualities, extremeflexibility and imperviousness to moisture. Crude rub-ber varies widely in its characteristics and value, de-pending on its age, purity, and to some extent, the lo-cality in which it is produced. It comes on the marketmixed mainly with impurities such as bark, clay andother foreign substances which are removed by wash-ing and manipulation, resulting in a reduction inwreight of from 10 to 50 per cent, the finest Para rubberlosing about 18 per cent.Rubber used as insulation is adulterated or degraded

with various substances, so that the compound containsat most, only 40 per cent of pure rubber, more usuallyabout 30 per cent in the highest grade insulation downto 5 or 10 per cent in the poorer rubber-covered wires,with no rubber in some cheap insulations which arecalled rubber.


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44 CABLE INSULA TION.Pure rubber is valueless for insulating purposes, be-ing too soft, hydroscopic, and readily oxidizable.When proper foreign substances to the extent of about

60 per cent or more, including about 3 per cent of sul-phur, are added and the compound vulcanized by heat-ing to from 250 to 300 degrees Fahr., the rubber ismade stable, tough and durable, its value as adielectric depending upon the details of this treatment.The exact temperature and duration of time necessaryfor vulcanizing depends on the grade of rubber, the in-gredients used, and the percentage of sulphur added tothe compound. The adulterants most commonly usedfor making the better grade rubber compounds are drymineral matter or reclaimed rubber; the composition ofthe particular material used for compounding, may, inmany cases, be left to the discretion of the manu-facturer. Proper vulcanization is as important in pro-ducing high grade insulation as the quality of the rub-ber used or the method of compounding. The amountof free sulphur left in the compound changes with oxi-dization. In no case should the free sulphur exceedabout 1 per cent the amount being determinable fromthe acetone extract as an excess shortens the life ofthe rubber. Sulphur gives an indication of the qualityof the rubber used, because much sulphur is requiredto vulcanize poor rubber and a large amount of com-bined sulphur may be taken as an indication that it wasrequired in order to produce vulcanization.The best rubber comes from South America, and isknown as Para. Weber states that the reason for the


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CABLE INSULA TION. 45inferiority of the African rubber is generally due to thepresence of albuminous substances which are not re-moved by washing" and which result in a brittle insula-tion ; he also states that light will oxidize rubber, themore rapidly the less the degree of. vulcanization.While a compound containing 30 'to 35 per cent "oldup-river" Para rubber is generally accepted as therequirement for insulation to be used in high-tensionwork; as a matter of fact, it is almost impossible forany chemist to ascertain, after vulcanization, just whatthe constituents of the insulation may be, and whileany number of tests have been proposed, it is muchbetter to rely on the standing and integrity of themanufacturer and his guarantees than to do businesswith unreliable firms, expecting to prove from an exam-ination of the product furnished whether or not theyare fulfilling contract specifications. While 30 per centPara seems to insure high grade insulation, it is beyondcontroversy that certain compounds containing lessthan 30 per cent Para give most satisfactory results inservice, although they fail to meet some of the testshereafter indicated, as the requirements for the best in-sulation.High-grade rubber is not only very elastic but

possesses great tensile strength. If over vulcanized itwill break; if under vulcanized it is not elastic, so thatstrength and elasticity are fair indications, of its valueas an insulating material. Those best informed agreethat a new sample of 30 to 35 per cent Para compoundproperly vulcanized, should be capable of withstanding


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46 CABLE INSULATION.a tension of 700 to 800 pounds per square inch beforebreaking, and when stretched from two to three timesits original length should return to within at least 125per cent of its original length, when at a temperatureof about 100 degrees Fahr. Although insulation con-taining appreciably less than 30 per cent of Para, withadditional amounts of cheaper rubber, making a totalof say, 40 to 50 per cent, may pass the tensile and elas-tic tests mentioned above, such test usually indicatesthat the insulation contains only rubber, and noshoddy or bituminous products; but some authoritiesclaim that a compound containing only Para will haveconsiderably greater resistance to puncture than in-sulations containing the same proportion of Para witha proportion of inferior rubber in the materials usedin compounding.For the larger proportion of cables manufactured,

namely, those used for low tension work, say under5,000 volts, 30 per cent pure Para is unnecessary, thecheaper grades, as high grade Ceylon, Malay orAfrican Lapori, for example, giving very satisfactoryresults for this particular class of lo\v-voltage work,while even the rubber produced from the Mexicanguayule is used for the insulation of telephone wires.The quality and life of rubber insulation has here-tofore generally been considered as indicated by the

amount of resinous or extractive matter it contained.A high percentage of resinous matter, say 15 to 20per cent, was taken to indicate a cheaper and poorer


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CABLE INSULA TION. 47grade of rubber, whereas a low percentage, 1 to 2 percent, was assumed to be found only in the best gradeof Para, which come from South America.Owing to some unknowrn cause, the amount of ex-

tractive matter increases largely in the working andmastication of the gum, there being still further in-crease during vulcanization. The amount of resin-ous matter is determined by digesting the gum in ace-tone, which dissolves out the extractive matter.Standard practice specifies that the acetone extractionshall be carried on for five hours in a SoxhletExtractor, as improved by Dr. C. O. Weber. It isimportant that care be taken in making the tests,not only that proper conditions should be observed,but that the duration of the test is, as specified, other-wise the results may vary widely. As an example ofthis, it may be stated that in a given instance, a 40per cent Para rubber compound, such as used by theUnited States Government, if heated for a period ofabout twelve hours, to 105 degrees Cent., in a dryingoven prior to being treated with acetone, resulted inincreasing the weight of the acetone extract from 2per cent to Sj4 per cent. The greater part of this in-crease took place during the first few hours of heat-ing. In the same way, the longer an acetone test iscontinued the larger the percentage of extract ob-tained, although by far the greater proportion is givenoff during the first five hours. From the above, thenecessity of properly conducting and carefully timingthe length of the test will be recognized. The rubber


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48 CABLE INSULATION.to be tested should first be dried over calcium chloridein a vacuum at slightly elevated temperature, andthen treated with acetone in the extractor.

It should be understood that the acetone test deter-mines the quality of the rubber compound, so that theusual maximum precentage limit of 5 per cent, forexample, must be raised provided it is intended thatthe manufacturer shall be permitted to introduce othersubstances in the adulterant used for compounding,which in themselves contain extractive matter. If itis clearly stated in the specifications that only Pararubber, to a definite percentage is to be used andthat the remainder of the compound is to be of someother material than rubber, then the 5 per cent shouldnot be exceeded.There exists a wide difference of opinion and prac-

tice as to the proper limit of extractive matter thatshould be permitted in a high-grade compound. Somespecifications* specify that the acetone extract shouldnot exceed 3 l/> per cent by weight, of the gum in thecompound, while the more usual specification andthat issued by the rubber manufacturers, sets theupper limit of extractive matter, as 5 per cent byweight, of the total compound, that is, in a 30 percent Para compound, the weight of extractive mattershall not exceed about 17 per cent of the weight ofthe gum. More recent experience and research hasshown that some African rubber gum may contain as

* Specifications of the Railway Signal Association.


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CABLE INSULATION. 49low as 2 to 3 per cent of extractive matter, whilesome high-grade Para, giving excellent results inservice, will contain over 4 per cent of such matter.With these wide variations and with the knowledgethat by proper treatment the amount *of acetone in acompound can be reduced to even 2 or 3 per cent ifnecessary, the value of the acetone test is being dis-credited and abandoned by many engineers ; for ex-ample, the Specifications for Electric Wires andCables,, issued by the Navy Department,* omit all ace-tone tests whatever, depending upon other tests en-tirely to determine the value of the compound.The introduction into rubber compounds of waxy

ingredients such as paraffine, for the purpose of in-creasing megohm measurements, etc., should be lim-ited ; a small amount, say 3 to 4 per cent of the weightof the rubber gum will not prove injurious.

In America the rubber compound is applied to aconductor in either of two ways :

(a) By passing the conductor through a presssimilar to a lead machine and applying the compoundin a plastic state at relatively high temperature as aseamless tube, as the conductor passes out of the ma-chine. This is called "spewing," and is used moreparticularly with smaller sized conductors.

(b) By applying the insulation in a longitudinalstrip by means of a machine which folds the compound* Dated June 10, 1908.


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50 CABLE INSULATION.around the conductor and unites the edges in a con-tinuous and almost invisible seam. As judged frompractical results, the strip insulation seems about assatisfactory and reliable as the seamless insulation, andit has the further advantage of keeping the conductorproperly centered and having imperfections in onelayer covered by additional layers.

In Europe the insulation of conductors by windingwith rubber tape has been successfully accomplished.This method should be expected to result in a moreuniform dielectric capable of withstanding greaterpotential stress, for a given thickness, than when ap-plied by "spewing" or in strips.With any method of applying the compound, a braid

or tape over all, is employed to better hold the com-pound in position and prevent its swelling and becom-ing porous during vulcanization ; such tape has no par-ticular value as a dielectric.Rubber insulated underground cables are usually

covered with a lead sheath both for mechanical protec-tion and to guard against attacks from oils, acids oroxidization. The substitution of a fibrous coveringserved with a bituminous compound or something ofthat sort, has been attempted in place of the lead sheathand is said to be found satisfactory under some condi-tions, particularly where electrolysis cannot be avoided,although such substitution is ordinarily based pri-marily on considerations of cost.When completed the rubber insulated cable is themost flexible of all, and should be capable of being


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CABLE INSULATION. 51bent on a radius equal to five times its diameter, bentsimilarly in a reverse direction ; have the process re-peated three times and then withstand puncture andohmic tests hereinafter specified.

Paper Insulation. Paper insulation is made by tap-ing paper ribbon about a conductor in successive layersuntil the required thickness is obtained. The cable isthen dessicated by baking, or more satisfactorily bygiving it a preliminary drying in air and placingin a vacuum, and immediately immersed in a bath ofoily insulating resinous compound, at a temperature ofnot less than 120 degrees Cent. (250 degrees Fahr.),until thoroughly saturated ; the whole is then promptlyenclosed with a lead sheath, which is necessary to ex-clude moisture, and at the same time, holds the insulat-ing compound in position. The value of the insulationas a dielectric depends on the quality of the paper andthe compound.The best paper is that made from Manilla fibre, pri-

marily because of its mechanical strength. The papershould show uniform texture when held to the light,be free from coarse or metallic particles, or pin holes,and should show no trace of chlorine or other residualchemicals, or be loaded with low grade material. Stripsof paper five-thousandths of an inch in thickness, andone inch wide, after being impregnated with the in-sulating compound to be used, should sustain withoutbreaking, a load of 40 pounds. The thickness of paperordinarily used is from five to six-thousandths of an


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5 2 CABLE INSULA TION.inch, with a tendency toward thinner papers for thehigher voltages. The width of the paper ribbon em-ployed varies from one to two and a half inches, thewidest ribbon being used on the conductors of largediameter.Rosin oil, which is the diluent and chief ingredient

of the fluids, used for impregnating paper insulation,is obtained from the distillation of rosin gum. Rosincomes from oleo turpentine, which is exuded by thelong-leaf pine or coniferous trees. Rosin producesrosin oil and pitch ; the former is distilled a secondtime producing what is known as "second oil," which,more or less treated or refined, is the impregnatingfluid used as the principal dielectric in paper-insulatedcables. The method of preparing the rosin oil forimpregnating, varies with the different manufacturersin accordance with their particular formulae whichlike those relating to the ingredients of rubber com-pounds, are guarded as "State Secrets" and make thechief difference in the quality of paper insulation.Lack of uniformity in commercial rosin oil, its lia-

bility to contain moisture and deleterious substances,necessitate the greatest care in the proper prepara-tion of rosin oil for insulating purposes. A. Bartoli*gives the relative value of insulating oils, and it isnoteworthy that those which are the more capable ofbeing oxidized are the less valuable as dielectrics,which would indicate a departure from the presentuse of rosin oil in its usual unoxidized condition.* L. L. Nuovocimento, 1890, vol. XXVIII. page 25.


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CABLE INSULATION. 53In the application of rosin oil to paper, the oilabietic anhydride, C44 H62 O4 , seems to soak into

the paper leaving the rosin largely on the outside.The insulation shows the highest puncture tests whenthe pores of the paper are filled with .oil, which maytake many hours or even days, at low temperature,to accomplish, where the impregnation is madethrough many layers, of paper.The use of too viscuous oil results in the absorption

of the diluent by the paper leaving the rosin "highand dry," resulting in a non-flexible and hard cable.Recently, the advantage of using a more fluid oil hasbeen recognized, which, wrhile reducing the megohmmeasurements, results in a cable that will withstandsatisfactorily high puncture tests, and at the same time,make it more flexible and thus largely avoid thedifficulties that have heretofore been encountered inhandling paper cables, namely, their liability to splitor crack when bent, particularly in cold weather. In-vestigation and experiment has recently produced avery much improved quality of rosin oil, which doesnot become viscous even at zero degrees Fahrenheit, sothat one very practicable objection to paper cables,their lack of flexibility, is now likely to be removed.With all cables, however, it is just as well to keep themin a warm room for some hours if they are to beinstalled when the temperature is below freezing.As long as the paper insulation of cables can be kept

intact within their lead sheaths, they are found togive most excellent satisfaction ; but if by reason of


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54 CABLE INSULATION.defects in manufacture, electrolysis or damage, thesheaths are punctured so that water, or even watervapor, can gain access to the dielectric, the breakingdown of the insulation is a question of minutes, or atmost, hours.At the time of manufacture, the lead sheathing of

paper cables is continued so as to completely encloseand protect the ends of the insulated conductors withlead, to keep out moisture. The lead sheath shouldnever be stripped off the ends of the cable until every-thing is prepared for making a prompt and dry joint,or inserting in an "end bell" for making a terminal.The stiffness of paper cables is related to their tem-perature and the quality of the impregnating fluid; butwith the use of the best oils, a cable should be capableof being bent back and forth three times, on a radiusof eight times its diameter, even at a temperature offreezing, and then withstand the regular puncture andohmic tests.On account of their relatively low first cost, paper in-

sulated cables are being more and more used for allservices even submarine and are proving success-ful, despite their inherent limitations. There are moremiles of high tension cables in use insulated with pa-per than with all other insulations combined.

Cambric Insulation. A recently developed dielectricfor insulating high-tension cables is varnished muslinor cotton fabric usually called cambric. The muslin iscoated on both sides with several separate films of insu-


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CABLE INSULATION. 55lating varnish, or in some cases, linseed oil com-pounded with some paraffine or ozokerite, or evenrosin. The coated material is then cut into strips mak-ing ribbon which is wound spirally about the conduc-tor in layers to any desired thickness; between thewrappings is applied a thin layer of viscuous adhesivecompound which prevents the unwrapping of the tapewhen cut, largely precludes the absorption of moisture,and increases the flexibility by permitting the layersof cambric to slide upon one another. More usuallya thin layer of pure rubber, or in some cases, treatedpaper or cloth, is first applied to the conductor beforethe cambric insulation is put on, in order to preventthe varnish from attacking the copper, and in the caseof the rubber, to secure a dielectric or high resistancenext the conductor. Asbestos has also been used asa separator, with the idea, among others, of permittinggreater heating, that is, greater carrying capacity,without injury to the varnished cambric.The application of the dielectric by taping, with the

use of a filling compound, as is the case with paperinsulation, should result in avoiding such defects inthe dielectric as the formation of air pockets and decen-tralization of the conductor, that are possible with"spewed" rubber insulation. The splicing of cambriccables is more simple than with paper insulation, asmoisture is not as readily absorbed nor is the cambricattacked by mineral oils, making it particularly con-venient for connecting into apparatus submerged inoil, as switches, transformers, etc. For station wiring,


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56 CABLE INSULATION.varnished cambric can be installed without a metallicsheath and does not require end bells, for which serviceit is usually finished with a tape and asbestos braid.Cambric insulated high-tension cables should not be

continuously operated at higher temperatures thanrubber, preferably not above about 65 degrees Cent.,whereas paper insulated high-tension cables may besafely operated at about 80 degrees Cent. Aside fromsomewhat increased flexibility and less liability of in-jury from moisture in case of injury to the leadsheath, or where it is desired to install cables withouta lead sheath, as in a power station, cambric insulationseems to offer no very especial advantages over paperinsulation, particularly at existing prices, as the papercables are appreciably less expensive than those withcambric insulation. The usual practical advantage ad-vanced for cambric insulation, as against rubber, seemsto be that of cost ; but, on the other hand, charring be-tween the layers of the cambric has been observed, duepossibly to air bubbles ; and the question has also beenraised whether the ageing and drying out of the var-nish will not cause the insulation to become friable anddeteriorate, particularly if operated at relatively hightemperatures.

Shellaced cambric insulation is considerably morepliable than paper and the cable complete should with-stand the puncture tests given on a later page afterbeing bent three times in opposite directions on aradius equal to six times its diameter.


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CABLE INSULATION. 57Dielectric Stresses. The dielectric strength of rub-

ber is much higher than that of treated paper or var-nished cambric, being as a maximum as high as 20,000volts per millimeter of thickness in thin sheets, whereasthe same thickness of treated paper will not withstandmore than from one-half to two-thirds this potential,so that unless some other materials are found, orfurther improvement be made in paper insulation,which seems possible, it is likely that rubber must beused, at least in part, on cables, designed for the highestpotentials, in order that the completed cable may notbecome so great in diameter as to be cumbersome andimpracticable to handle. It was early appreciated thatdoubling or tripling the thickness of a given insulationdid not increase its ability to stand up under appliedelectrical stresses, in anything like the same ratio. Itwas found, with an insulation of homogeneous mate-rial, that the fall of potential through the insulation,from the conductor to the lead sheath, was not uniformbut increased very much more rapidly nearest the con-ductor, being for a certain insulation, for example,5,000 volts per millimeter of insulation next the con-ductor and only 1,000 volts for the same thickness nextthe sheath. Without more fully considering what maybe the fall of potential along the radii, from the sur-face of the conductor to the sheath, or the complexformulae by which these values may be calculated, forvarious dielectrics, it may be said that both theory andexperiment prove the fact; and, furthermore, that therate of fall of potential varies with different materials,


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5 8 CABLE IXSULATIOX.depending upon their various specific inductive capaci-ties. Knowledge of these conditions led an English-man, Mr. M. O'Gorman, and an Italian, Mr. E. Jona,about the same time, to suggest equalizing the fall ofpotential so as to secure a uniform or practicallyuniform ''potential gradient" throughout the insulationeither by impregnating the insulating material todifferent extents depending on its distance from theconductor, or by applying successive layers of insula-tion each made up to have different inductive capacitieswith the layers arranged so that those of material withthe highest capacity should be nearest the conductor.This arrangement of insulating material causes theouter layers to support approximately the same strainsper unit of thickness as the inside layers; and hence,the total stress due to the potential of the conductor issupported by a wall having a total thickness of insula-tion very much less than if homogeneous. Theo-retically, the insulating material should vary graduallyinstead of by layers ; but this, of course, is imprac-ticable, so that the fall of potential from conductor tosheath proceeds by a series of small steps instead of ina smooth curve.

Experiment with high potentials seems to havedemonstrated that the distribution of stress in soliddielectrics, such as paper or rubber, is very similar tothat wrhich we know occurs in air. About conductorsof small diameter air apparently breaks down, resultingin a conducting medium made up of the solid conductorand air, which is considerably larger in diameter than


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CABLE INSULA TION. 5 9the solid material. It is probable that similar actiontakes place with the insulation about conductors ofsmall diameter, so that the dielectric itself, for a smalldistance from the conductor, breaks down and be-comes also a conducting medium. In any case, it isclear that insulation of a given character about a con-ductor of large diameter will sustain a considerablyhigher potential before puncture, than the same insula-tion about a small conductor. As a result of experi-ments made by him, Mr. Jona concludes that bysheathing a copper conductor in lead, thus both in-creasing its diameter and affording an absolutelysmooth and cylindrical exterior, there may be produced"a diminution in the potential gradient in the veryfirst stratum of dielectric of something like 20 to 30 percent or even more," and he has so sheathed with leadhigh-tension cables made under his direction.Graded Insulation. The theory of applying layers

of insulating material having different capacities hasbeen carried out in practice and the value of "graded"cables for high potentials successfully demonstrated.For example, there was shown at the 1906 Milan Ex-hibition, such cable, having a total thickness of insula-tion of only 14.5 m.m., though designed for a normalworking pressure of 100,000 volts, and at present, therearc installed across the Lake of Garda, Italy, single-conductor "graded" cables operating at 13,000 volts.These cables, are insulated by several layers of vulcan-ized india rubber to a total thickness of 5.5 mm. Out-side the rubber is a coating of 1.2 mm. of gutta percha


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60 CABLE INSULA TION.to further insure imperviousness. This is covered with"tanned jute" and armored with No. 18 steel wire3 m.m. in diameter. As three of these cables arerequired for three-phase operation an interesting planwas adopted in order to avoid undue self-induction;each of the steel wires used in armoring was wraptwith tarred hemp before being wound around theinsulated conductor. The result of this experimentseems to be satisfactory, as the drop of pressure due toself-induction is reported to have been reduced to thesame amount as the drop due to the ohmic resistance.Connecting the generating station and transformerhouse of the Ontario Power Company at Niagara Falls,are some high voltage "graded" cables.

Variation in the capacity of rubber used for "grad-ed" cable is obtained by "loading" it with other sub-stances such as talc, zinc, etc., while the capacity ofpaper may be similarly varied by changing the qualityof the 'paper or the process of impregnating. Theprocess used at present for impregnating cables has theeffect of sometimes giving the greater dielectricstrength and capacity where they are not wanted,namely, in the outer layers of the insulating material.This is due to the fact that the liquid used for impreg-nating more easily reaches and solidly fills the outerportions of the dielectric. As often manufactured,rubber cables are subject to the same fault; becausepure rubber, which is of the lowest specific capacity,is placed next the conductor, the tougher, degraded orvulcanized rubber of greater capacity being used for


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CABLE INSULATION. 61the outer layers. While the unequal distribution ofdielectric strength is of little importance in itself, thereis greater danger of a breakdown than if the insulationwere homogeneous throughout, due to the increasedcapacity created in the outer layers.Composite Construction. Not with a view to ob-

taining the results to be secured by "grading" the in-sulation but primarily for the purpose of reducing thecost, cables have recently been made with rubber andpaper, or rubber and cambric insulation combined.By using rubber next to the conductor and paper orcambric outside the rubber, the more expensive andbetter insulation is distributed where its greaterstrength is most advantageously used. Attempts havebeen made to enclose paper insulation with a lightjacket of rubber as a protection against moisture ; butowing to the difficulty of vulcanizing the rubber with-out injuring the paper, such results have met with butlittle success.Where two or three-phase currents are employed for

high-tension work, the several underground conductorsrequired for such a circuit are usually separately in-sulated, laid up with jute and then the whole enclosedin a "jacket" or "belt" of insulating material, whichfurther insulates, to ground, economizes space andinsulation and especially protects mechanically. Forfull working potential between conductors and ground,the "jacket" or "belt" is, particularly with paper andcambric, usually equal in thickness to the insulationabout each conductor; in case of star-connected circuits


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62 CABLE INSULATION.with grounded neutrals, the insulation between con-ductor and ground need be, theoretically, but six-tenths that between conductors, practically, however,it is made somewhat heavier than theory would re-quire.At present there seems to exist a well-founded feel-

ing that too much money has been expended in, andtoo high an electrical value placed on the "jacket'* or"belt," usually employed with high-voltage cables. Inconsidering whether or not it is desirable to use partof the insulation of such a cable in a "jacket" or "belt",or whether the same expenditure for insulation couldbe better made in thickening the insulation abouteach conductor, it should be borne in mind that ifthe "belt" is injured as will usually be the case if abreakdown occurs its value is reduced to little ornothing, as supplementing the insulation about thetwo other conductors, which may be uninjured. Thisreasoning relates to electrical considerations and doesnot include the mechanical advantages obtained bythe application of a second separate and distinct layerof insulation which affords a smooth, even surface forthe application of the lead sheath, and withal makesthe cable more flexible. It would seem as if a lighterbelt and heavier insulation about each conductorwould be more advantageous than the present gen-eral practice of making the belt and the insulationabout each conductor of the same thickness per thou-sand volts of potential stress.

Concentric cables consisting of a rod, insulated, and


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CABLE 1XSULA TION. 63inserted in one or two metal tubes, as the second orthird conductor, were early employed, particularlyabroad ; but have hardly demonstrated their claims tosuperiority; their use is being restrained, in Germany,for example, being' prohibited for voltages over 3,000.For low voltage work, concentric cables offer someadvantages which are extending" the use of such cablesin America. On account of the increased thicknessof insulation required with higher potentials, say from50,000 volts upward, single conductor cables will prob-ably be necessary for such potentials, at least whenthey are to be much handled or drawn in ducts.

Thickness of Commercial Insulations. Variousformulae have been suggested by which to determinethe proper thickness of the different insulations to usefor a given potential. Such formulae usually containempirical constants, the value of which largely dependson a personal equation. The errors caused by thepractical difficulties of manufacture, such as eccentricplacing of the insulation about the wire, unevenness ofapplication, imperfections in the dielectric, mechanicalconsiderations of strength, make tables of insulationrequired for different voltages and sizes of conductors,much more valuable and reliable, than formulae, as theformer are based on practical experience, tests andguarantees that manufacturers are willing to standback of.

In determining the thickness of insulation of high-tension cables, whether from the standpoint of theo-retical design or consideration of actual installations, it


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64 - CABLE INSULATION.must be borne in mind that quantity gives no indica-tion of the quality of dielectrics. Furthermore, thenormal voltage at which a cable may be expected to beoperated gives little indication of the monetary or di-electric values of the insulation used ; these values aredetermined rather by the factor of safety employed andthe breakdown or puncture tests which the cables mustpass. The superiority of a given character of insula-tion furnished by one manufacturer as compared withthat of another manufacturer for a given service, ofnecessity compels relegating to a secondary considera-tion the question of mere thickness of a dielectric. Asone manufacturer has expressed it, "puncture testsrather than working voltage, or thickness of insula-tion, is what we want specified." Nevertheless, thefollowing information is here submitted, not as indicat-ing the minimum limiting thickness of the best gradeof insulation for the voltages specified, but as showingin a general way what some representative manu-factures are offering, and as a conservative guide towhat can reasonably be asked and obtained.Mr. H. G. Stott states that from his experience,

paper insulation for 3,000 volts on wires from No. 6 toNo. 00 B. & S., inclusive, should be 5/32 of an inchthick, and for larger sizes up to 300,000 c. m., 6/32 ofan inch thick with an increase of 1/32 inch for each1,000 volts up to 11,000 volts and after that 1/64 inchadditional insulation for each 1,000 volts. For 35per cent Para rubber compound or varnished cambric,he states that it is only necessary to add 1/64 inch


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CABLE INSULATION. 65additional insulation for each 1,000 volts above 3,000until 25,000 volts is. reached.The General Electric Company, Schenectady, N. Y.,

for three-conductor stranded, varnished-cambric insu-lated, leaded cables, recommending the -same thicknessof insulation about each conductor as in the jacket,give the following figures:

TABLE II.THICKNESS OF CAMBRIC INSULATION.

(G. E. CO.)

^General Electric Co. Bulletin Mo. 4591.

The Safety Insulated Wire & Cable Company, NewYork, specify the following thicknesses for rubber (30per cent Para), and paper insulated cables, they do notfurnish varnished-cambric insulation.

It will be noted that no jacket is provided with therubber-insulated cables intended for use at the lower


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66 CABLE INSULATION.potentials, this is due to the fact that a thin rubberjacket will be relatively largely reduced in thickness bythe pressure from the insulated conductors, as it seemsimpossible, practically, to maintain uniform pressure ofthe jute filling 1 and the conductors against the jacket.

TABLE III.THICKNESS OF RUBBER AND PAPER INSULATION.

(S. I. W. & C. CO.)RUBBER INSULATION PAPER INSULATIONNormal

Working

Pirelli and Company, Milan, Italy, usually employimpregnated paper for cables up to 20,000 volts ; forhigher pressures they employ their own special systemof india-rubber and paper insulation. As indicating ina very general way their practice, the following figuresare given :


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CABLE INSULATION.TABLE IV.

THICKNESS OF PAPER INSULATION.(P. & CO.)

NormalWorking Voltage

IO,OOOl6,OOO2O,OOO

Total 'thickness ofInsulation

.27 inch

.38 "50 "

The British Insulated & Helshy Cables, Ltd., Pres-cot, England, gives the same thickness of insulation onthree-conductor cables that is specified by the Engi-neering Standards Committee, as follows, for mediumsize conductors :

TABLE V.THICKNESS OF RUBBER AND PAPER INSULATION.

(B. I. & H. C., LTD.)

Most of the above tables are based on full workingpotentials between conductors or between conductor


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68 CABLE INSULATION.and ground. In case the three conductors are used forstar-connected circuits with grounded neutrals, thethickness of insulation between a conductor andground need be but 6/10 of that between conductors,but in practice it is made somewhat thicker thantheory dictates. This relation of voltage and insula-tion should be borne in mind when testing and alltests on the cable should be properly proportioned tothe thickness of the insulation.

Joints. The purpose of a paragraph on cable jointsis not to teach splicing to those unacquainted with themethods employed, but rather for the purpose of em-phasizing the importance of this part of cable installa-tion. Reference has already been made to the gen-erally admitted fact that joints are the weakest pointsin high-tension cables. This is so not because thejoints necessarily need be \veak, but because properattention and care has not been exercised in makingthem. More breakdowns in cable operation haveprobably resulted from defective joints than from allother internal causes. Careful and competent workin making joints at the time of installation will lateravoid much worry, inconvenience and monetary loss.Except for the largest companies, which can afford tomaintain regularly in their employ high grade, experi-enced cable workmen, it is advisable for all purchasersto include in their contracts with the manufacturers,the drawing-in and jointing of cables complete.Various types of tape and compound are employed


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CABLE INSULATION. 69for making cable joints, depending on the insulationused. For rubber insulated cables usually a layer ofpure rubber is applied and then compounded tape isused, which must be vulcanized, after application,by immersion in a bath of suitable hot compound orby mearfs of a torch used only in the hands of anexpert, as undue heat applied to rubber tape will in-jure it. Paper and cambric-insulated cables have theirjoints wrapped with paper or cotton tape, which maywell be kept in the hot compound, which later will beused for filling the lead sleeve. All joint wrappingmaterial left exposed to the air or moisture deterioratesand should be carefully protected. Even the moisturefrom the hands of the workmen has been known to besufficient to destroy an otherwise perfect splicing job.No acids should be used as a flux in soldering, as it isliable to injure the insulation.The secret in making a perfect joint, provided proper

materials are furnished, is(a) Exclude all moisture.(b) Make the wrappings as tight as possible, to ex

elude air.(c) Have the layers of tape overlap and adhere uni-

formly.(d) Be certain the compound is sufficiently hot, be-

fore pouring.The insulation in a joint should be made somewhatthicker than that about the conductor. A com-mon rule is to make the insulation at the splice 150per cent of that about the conductor.


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70 CABLE INSULA TION.For paper-insulated cables, a special paper tube hasbeen brought out large enough to slip over the insula-

tion about a conductor, so that after the conductors arespliced and taped the tube is drawn over the splice andfurther tape added ; and from the records made, thismethod of insulating a joint seems to give very satis-factory results, the advantage of having part of theinsulation about the joint made up in a uniform andperfect manner in a factory, will be recognized.The method of making joints, which has resulted in

the excellent record of paper cables, referred to on page35, may be interesting, as no special tools or particu-larly skilled labor are required. The outside paper beltof the cable is first removed and the jute filler turnedback and tied down out of the way. The ends of theconductors are bared and joined by metal sleevesproperly sweated on. An insulating compound, manu-factured in Chicago, heated to about 150 degrees Cent.(300 degrees Fahr.), is then poured over the conduct-ors to drive out all mois

50593663 HV Underground Cables

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