C. F. BOYCENONMEMBER AlEE

Open-Wire Carrier Systems in South

AFrica

been analyzed for which simple circuitcomponents perform with Lper-cent ac­curacy.3. Freedom from drift and accuracy isimproved by the extensive use of the on-off,digital type of operation for vacuum tubesin the computer components. The com­puter accuracy will remain within ± 2.5per cent for a period of time greater thanthe cycle of operation required in producingthe color-corrected plates of a subject.

4. Experimental use for a period of about5 years has proved its usefulness because

TH E UNION of South Africa and theneighboring mandated territory of

South-West Africa occupy an area ofabout 800,000 square miles at thesouthern tip of Africa, about a quarterthe area of the United States. It isa land of vast mineral wealth, and theworld's chief source of gold and diamonds.Exceptionally rapid industrial develop­ment has characterized its cities duringthe past 20 years.

About a quarter of the total populationof 14,000,000 live in the major cities andadjoining areas, shown in black in Fig. 1.The balance of the population is some­what unevenly distributed throughoutthe country, and in towns, none of whichhave a greater population than 50,000.About one-fifth of the total populationis of European descent.

The climate is in general a temperateone. A long range of mountains roughlyparallel to the coast divides the coastalstrip from the high plateau which formsa large portion of the total area of thecountry. Much of this plateau has a dryclimate not unlike that of SouthernCalifornia and New Mexico. The meanannual rainfall for South Africa is 25inches, varying from 5 inches in the westto about 70 inches in parts of the easterncoastal zone.

There are nearly 600,000 telephones inSouth Africa, and of these just overt wo-thirds are dial telephones. All ofthe latter are in the nine major cities.The balance of the telephones are mainlyof the local battery type employing 17­cycle-per -second (cps) signaling.

Within the areas shown in black in

the parameters of the color input informa­tion system and the final processing stepsof the plates required to make a printedproof have been considered.

5. Fig. 5 indicates the significant contribu­tion of this system.

References

1. THE THEORY OF THREE-COLOR REPRODUCTION,A. C. Hardy, F. L. Wurzburg, Jr. Journal, OpticalSociety of America, New York, N. Y., vol. 27, no.7, July 1937, pp. 227-40.

Fig. 1, telephone circuits are providedby underground cables, including carrier­on-cable and coaxial cable systems. 1,2

Outside of these areas all toll circuitsare provided by open-wire routes andcarrier systems on these routes. Thispaper deals with these open-wire tolltelephone circuits and associated broad­cast and telegraph circuits.

Growth and Present Size of CarrierNetwork

About 30 years ago, the first carriersystems, which were of the single- and3-channel types, were installed in SouthAfrica. 3,4 The subsequent growth incarrier telephone circuit mileage is il­lustrated in Fig. 2. The growth incarrier telegraph circuit mileage, i.e.,voice frequency telegraphs employingcarrier channels as bearer circuits, andcarrier telegraphs (generally known ashigh-frequency or HF telegraphs) in thefrequency range of 6 to 30 kc per secondis shown in Fig. 3. There are about 600carrier systems of various types in service,varying in length from 15 to 1,100 miles.The numbers of carrier toll telephonecircuits on the main backbone routesonly are shown in Fig. 1. In additionto these, carriers are extensively usedon secondary routes.

The Scope of Open-Wire CarrierSystems

The minimum line length for whichcarrier exploitation appears to be eco­nomical on open wires depends on several

2. DIE THEORETISCHEN GRUNDLAGEN DES MEHR­FACHFARBENBUCHDRUCKS, H. E. J. Neugebauer.Zeitschrift fur Wissenschaftliche Photograohie, Photo­phydk und Photochemie, Leipzig, Germany, vol. 36,no. 4, 1937, pp. 73-89. .

3. COLOR CORRECTION IN COLOR PRINTING, A. C.Hardy, F. L. Wurzburg, Jr. Journal, OpticalSociety of America, New York, N. Y., vol. 38, no.4, April 1948, pp. 300-07.

4. AN ELECTRONIC METHOD FOR SOLVING SIMUL­TANEOUS EQUATIONS, A. C. Hardy, E. C. Deneh.tua., pp. 308-1l.

5. THE RCA-lNTERCHEMICAL ALL ELECTRONICCOLOR CORRECTION SVSTEM, Harold E. Haynes.Penrose A nnual, London, England, vol. 46, 1952,pp. 83-86.

interrelated factors such as the numberof channels, the relative costs of theterminal equipment, the line, accommo­dation, availability of power and main­tenance staff, etc. In some cases thisminimum length may be as low as 10 to15 miles. Frequently when a routecannot accommodate additional wires,the use of carrier systems is the onlypracticable method of increasing the cir­cuit capacity. In general, for distancesgreater than 30 to 40 miles, and wherethe circuit growth is such that not morethan about 200 circuits are likely to berequired at the end of 20 or 25 years, anopen-wire route with carrier systemsmay well prove to be the most economicalsolution, on a "present value of annualcharge" basis.

The maximum number of telephonecircuits at present provided on a singleopen-wire route is between 200 and 240.I t is proposed to increase this figure inthe near future, for distances up to 150or 200 miles, to about 350 circuits, withthe aid of compandors. This aspect isdealt with later in more detail.

Equipment Features

The principal features of open-wirecarrier systems, many of which were firstdeveloped in the Bell System, are wellknown. Only those features which maydiffer from those employed elsewhere willbe mentioned. The line and principalmodulating frequencies of the systemsused in South Africa are shown in Fig. 4.Several of these systems have been

Paper 55-231, recommended by the AlEE WireCommunications Systems Committee and ap­proved by the AlEE Committee on TechnicalOperations for presentation at the AlEE WinterGeneral Meeting, New York, N. Y., January 31­February 4, 1955. Manuscript submitted October20,1954; made available for printing December 1,1954.

C. F. BOVCE is with the Government Post Office,Pretoria, South Africa.

The author is indebted to the chief engineer of theGeneral Post Office, Pretoria, South Africa, andto Standard Telephones and Cables Ltd., London,England, for permission to publish the informationin this paper.

272 Boyce-open-Wire Carrier Systems in South Africa MAY 1955

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Single- and 3-channel systems employinductance-capacitance oscillators indi­vidual to each channel. In practice theseare found to be sufficiently stable tomaintain channel synchronization towithin 2 or 3 cps over periods ofseveral months, particularly the moremodern temperature compensated types.Individual channel oscillators renderequipment more flexible, but in the caseof a 12-channel system there are economicand technical advantages in employing acommon crystal-controlled master oscil­lator in conjunction with harmonic

of the facility to switch 12-channel groupswithout demodulating to voice fre­quencies.

FREQUENCY TRANSLATION

Up to 1940 all systems employed modu­lation methods which translated indi­vidual channels direct to the requiredline frequency. Most modern systemsemploy one or more stages of groupmodulation, the advantages of which arewell known. In the case of the SOJ 12­channel system, which is very similar tothe J system of the American Telephoneand Telegraph Company, the basic groupof 12 channels is translated to the fre­quency range of 60 to 108 kc per secondin one stage, employing crystal channelfilters." This base group is common tocable systems and much use is made

described in various technical publica­tions.v"

MAY 1955 Boyce-Open-Wire Carrier Systems in South Africa 273

Fig. 3. Growth of telegraph circuit mileagesin South Africa

generators. Where there are more thantwo or three 12-channel terminals, anappreciable saving in cost may beeffected by employing a carrier supplysystem common to several terminals.In this event a duplicate carrier supplyset with automatic changeover facilitiesis desirable. The provision of a pilotfrequency to keep the two terminals of asystem in synchronism has not provedto be necessary.

The earlier voice frequency telegraphsystems employ multifrequency genera­tors. These require skilled maintenanceand a duplicate machine in instantreadiness, since the speed of such ma­chines must be maintained within verynarrow limits. Separate oscillators foreach channel have proved to be a moresatisfactory solution.

AUTOMATIC GAIN REGULATION

With the possible exception of single­channel carrier systems, the provisionof one or more regulating pilots is mostessential. The operation of channels ator near zero loss equivalents, and thepossibility of switching several channelsin tandem, demand a close control ofline and equipment variations. Thewidespread and increasing use of voicefrequency signaling? and dialing alsodemands closer tolerances in variationsof channel equivalents. The pilot pro­vides a visual and, when necessary, anaudible indication of the state of a system.In the case of a single-channel system,the line frequency is lower, the circuitis generally short and only one channelis involved. For these reasons automaticgain regulation has not been employed.There is some evidence, however, thatin longer systems channel equivalentsdo vary more than is desirable and theprovision of a pilot would also provide asystem failure alarm.

CARRIER TELEGRAPH SYSTEMS

The voice frequency telegraph systemsare based on a 120-cps spacing andprovide 24 channels capable of trans­mitting at a speed of 50 bauds with lowdistortion. The so-called HF telegraphsystems provide up to six or eight tele­graph channels in the 3-channelcarrier range; see Fig. 4. This systemhas proved most useful in providing smallnumbers of telegraph circuits to smalltowns in conjunction with 3-channelcarrier telephone systems. The earlierTOB system is essentially separate fromthe 3-channel telephone system, whereasthe later STO type employs some equip­ment which is common to the STOtelephone system.

All the systems, which are of theamplitude-modulated type, use rectifiermodulators at the transmitting end andtelegraph relays at the receiving end.Trials on frequency-modulated telegraphsystems indicate that they are con­siderably less affected by short suddenchanges of level. Frequency-modulatedsystems are, however, considerably morecostly, bulkier and somewhat morecomplicated, and their use on stablehigh-quality bearer channels has not,up to the present, been justified.

EQUIPMENT PRACTICES

The design of carrier equipment inregard to the quality of components,methods of mounting and wiring, long­term stability, ease of maintenance, andthe saving of space has for many years

of the single channel system (SOC system)em ploys an inert terminal requiring nopower, a most useful feature in manyremote areas. In this system the activeterminal transmits the. carrier which isvoice modulated at the inert terminal toprovide the return path. A 2-channelversion of the same system has recentlybeen introduced. The range of theinert system is limited and the high levelof carrier transmitted normally limitsits application to one system on a route.It is hoped that the transistor will makeit possible in the near future to providea rural system in which the remote termi­nal is supplied from a small dry batteryor fed with power over the line. Such asystem will probably be arranged sothat channels may be added one at atime with the channels from the exchangeterminating at different points. Thesystem will probably employ 6 or 8 kcper second spacing between the channels.If the cost proves to be sufficiently lowsuch a system may be used for providingcustomer as well as toll lines.

BROADCAST CARRIER SYSTEMS

There are three broadcast programsin South Africa and for each of these a2-way carrier circuit is provided betweeneach two of the principal cities shown inFig. 1, with the exception of Windhoek.Forty carrier systems provide 9,500 milesof 2-way program circuit. The networkis essentially a "ring main," since eachstudio is able to receive on each of thethree programs from each of two differentdirections. The line frequencies corre­spond to the 3-channel spectrum. Thechannel. equivalent is maintained con­stant within 1 decibel (db) over the fre­quency range of 30 to 10,000 cps. Thesuppression of the unwanted side band isachieved by combining the output of twomodulators, in one of which the phase ofboth the program input and the carrierhas been displaced by 90 degrees relativeto the other modulator.! Both the noiseand the distortion products of the channelare of a very low order. At intermediatestudios which normally accept, and onlyoccasionally originate, programs, theprogram is received "in tee" from arepeater to avoid introducing the effectsof local cables. Facilities are providedso that the studio can at any time convertthe repeater into two terminals and sooriginate a program.

On 3-channel and broadcast carriersystems a single pilot in each directionis provided. On the more modernsystems, this pilot provides a measure ofslope as well as flat loss adjustment, bymeans of a thermistor in an equalizer.On the 12-channel system, the greaterfrequency spread requires the use of twopilots at or near the ends of the trans­mission band.

On systems with more than two orthree repeaters, it is necessary to ensure,by careful design, that the regulatorsdo not "hunt." On long systems it isalso desirable that repeaters should re­store to their correct gain after a breakin the transmission path without unduedelay, since restoration delays at suc­cessive repeaters tend to be cumultative,and therefore extend the duration ofshort breaks.

RURAL CARRIER SYSTEMS

Apart from the main routes, carriersystems have played a vital role inproviding toll circuits in rural areas.Although farms tend to be large and thepopulation density low, most areasrequire telephone service at intervals ofthe order of 10 miles or less. Single­and 3-channel systems are extensivelyused to provide toll lines. One version

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received close attention and the latestequipment reflects considerable advancesin all of these respects.P-P The newequipment, which has been standard forthe past 6 years, features light frame­works of welded folded steel sections.The slide-in equipment panels are locatedin the framework by means of die castingsattached to each side. 'These castingsmount terminal blocks for the bay wiringwhich fall opposite similar blocks in thepanels, to which connection is made bysuitable links. Part of a bayside is shownin Fig. 5.

The most striking change in the newerequipment is the great reduction in size,frequently to a third or a quarter of theprevious figure. The limits to progressin this direction are set by maintenanceconsiderations and heat dissipation. Thereduced height has been of considerablebenefit in installing the equipment inbuildings not originally intended forequipment. The weight per foot runof the new bayside is some 30 per centhigher than that of the old, but thishas not proved to be of any consequence.Shipment is made with the frameworkand panels packed separately in one case.This makes for great ease in handlingsince the heaviest component, the bayframework, weighs only about 100pounds.

Installation testing and adjustmentas well as maintenance procedures havebeen simplified. Panels can be quicklyremoved for inspection or replacement.All units requiring ad justment are ar­ranged on the front of the panel. Inaddition to testing points on the bay­panel links, at which both terminatedand level measurements may be made,important points are brought out toauxiliary terminal strips on the face ofthe panel, thus giving access for measure­ment without unsoldering wires. A cen­tralized valve failure alarm panel enablesspace currents to be measured withfacility and also provides a continuousmonitor on space currents. Should 'anyvalve space current fall below a presetvalue, an alarm is operated.

Components may be sealed eitherindividually and then mounted in a dustproof can, or alternatively assembledwithout individual seals, in hermeticallysealed cans filled with dry air. Thelatter procedure is frequently cheaperin manufacture but does necessitatearrangements at central depots for theopening and repair of cans under skilledsupervision. Experience has shown that

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* The fractional sections are spaced at 40 poles per mile. The J2A sections are 4.57 miles long.

Main line A 3-channel and a 12-channel system onevery carrier pair 16 12 40 .. ; J 3

Secondary ..... A 3-channel system on every pair and a12-channel system on half the carrierpairs 16 12 28* , J2

Trans­positionSystem

Number ofPoles per

Mile,Nominal

Value

Maximum Number ofPairs

VoiceCarrier Frequency

Transposed Transposed

(weighted noise over a band of 4kc per second) is assumed. This fig­ure may increase to -70 dbm for apoorly transposed route. The minimumlevel on the open wires on a well-trans­posed repeater section should not be lessthan about -15 dbr if there are severalrepeater sections involved. If there areno repeaters in the system this may beallowed to drop to about -20 dbr.Toll entrance cable is usually involvedat one or both ends of a repeater section.The level in the cable may be allowed tofall to about -40 dbr. If voice fre­quency telegraphs are to be operatedover a system in a lightning area, it isdesirable that the foregoing minimumlevels be some 10 db higher.

Above 10 kc per second, ·line noiseappears to be substantially independentof frequency and it is usual to base theminimum line levels for single- and 3-

Fig. 5 (left). New equipment practice bayside

Fig. 6 (above). Typical carrier route

Table I. Details of Open-Wire Routes

Extent of CarrierTelephone Exploitation

at PresentType ofRoute

add on a root-sum-square basis, it ispossible, although somewhat arbitrarily,to assign limits to noise levels arisingfrom each of these sources. I t is usualto assume that the contribution of linenoise alone for the whole system does notexceed 0.3 millivolt. Assuming againthat the noise contribution of each re­peater section adds on a root-sum-squarebasis, it is possible to plan a tentativerepeater section layout and check whetherthis limit is met. This involves aknowledge of representative line noiseduring thunderstorm seasons. For awell-transposed route a noise valueof -85 db below 1 milliwatt (dbm)

Since single- and 3-channel systemstoday rarely involve a repeater, thesiting of repeater stations is mainlyassociated with 12-channel repeaters.The maximum line-transmitting level perchannel is +17 db relative to theswitchboard test level (dbr). The mini­mum level is set by noise considerations.A maximum noise voltage of 1 millivoltas measured on a European InternationalConsultative Committee on Telephonypsophometer and telephone weightingnetwork measured at the 2-wire end ofthe circuit is permitted. This noiseis made up of equipment noise, crosstalkfrom other systems on the route, andatmospheric or static noise. On theassum ption that the various noise levels

The Siting of Carrier RepeaterStations

the percentage of faulty cans is low, andfaults occur mainly during installation.In general, only cans which containcritical components likely to be affectedby humidity variations should be sealed.All equipment is wired either with poly­ethylene-insulated wire or where per­missible polyvinyl-chloride-insulated wire.

276 Boyce-Open-Wire Carrier Systems in South Africa MAY 1955

Table III. Fine Weather Copper-Wire Attenuation per Mile for 8-lnch Spacing, Db

0.137 300 0.070 0.120 0.208 O. 271 O. 340.112 200 O. 087 0.144 O. 245 O. 317 O. 40

*Pairs on either side of the pole are spaced 24, 23, and 23 inches respectively.t The voice frequency arms are always below the carrier arms and 24 inches from the nearest carrier arm.

Carrier pairs, to 1953 4 8 23 24Carrier pairs on 40-pole-per-mile routes after 1953 4 6 26 24Voice-frequency pairs 6 6 13 12t

Percentageof Fault TimeCause of Fault

* Items such as condensers, rectifiers, etc., i.e.,other than valves and fuses.

EXTENSION OF FREQUENCY RANGE

BEYOND 143 Kc PER SECOND

The formation of ice on open-wireconductors is a rare occurrence in SouthAfrica, and then only in very restrictedareas. Thus the possibility of usingfrequencies appreciably above the SOJrange appeared to be restricted by cross­talk considerations. Crosstalk could bereduced further only by shorter trans­position intervals, since crosstalk coeffi­cients and irregularity crosstalk had beenreduced as far as practicable. I t is noteconomical to reduce span lengths further;at 40 poles per mile most routes arealready stronger than they need be.The use of suspended transpositions,while valuable for river crossings, doesinvolve a number of construction andmaintenance problems.

Measurements indicate that six pairsout of the eight on the first two arms ofa J3 40-pole-per-mile route have satis­factory attenuation characteristics up toat least 250 kc per second, while theworst far-end crosstalk, Table IV,has deteriorated by not more than 20db. The availability of a compact ana

Table V. One-Year Analysis of 12-ChannelNetwork Outage

Outage per 100 Channel Miles per Annum,21 Hours 24 Minutes

Line failures 41.0Equipment failures

Consumable items 7.7N onconsumable* items. . . . . . . . . . .. . .. 1.2Adjustments. . . . . . . . . . . . . . . . . . . . . . .. 5. 7

Power plant failures 18.2Tests made outside of routine times 13. 1Faults which either came clear while

being localized or where no faultswere found 13 . 1

a 12-channel system on at least halfthe carrier pairs for three or four repeatersections. This would meet the require­ments for most secondary routes. Theuse of 6-inch spaced pairs on 28-pole-per­mile routes for carrier working is notfavored on account of the possibility ofwind contacts. The use of copper-steelwire on such routes would enable the6-inch spaced ann to be used. The highattenuation of such wire at voice fre­quencies limits its use to short lengthsunless a satisfactory and inexpensivesolution to the problem of providinggain in physical circuits is forthcoming.The possibilities of the transistorizednegative impedance repeater in thisconnection are being explored.

270 Kc

DistanceBetweenArms,Inches

150 Xc

DistanceBetween

Centers ofHorizontally

AdjacentPairs,*Inches

90 Kc

SpacingBetweenWires ofa Pair,Inches

30 Kc

Pairsper

Arm

SECONDARY CARRIER ROUTES

Secondary routes are spaced at 28poles per mile for reasons of economy.The use of the J2 system is, however,not entirely satisfactory since fractionaltransposition sections must be spacedat 40 poles per mile. Consideration isat present being given to the use of theLJ transposition system (a variant ofthe J3 system) at 28 poles per mile forall types of section. This involves a9.14-mile full section. It is probablethat such a route would accommodate

lings are some 6 db worse than in Ameri­can practice. This has been compen­sated on main lines by the use of theJ3 transposition system instead of theJ2 system. By keeping wire sag differ­ences and irregularities in span lengthsdown to small values, it is possible tooperate an SOJ system on every carriertransposed pair for distances of 1,000miles or more. 8

In most cases it is necessary to ac­commodate voice frequency pairs, suchas rural toll or multiparty fann lines oncarrier routes. It is costly to constructthese pairs to J3 standards with fourpairs per ann. These are now erectedon 12-wire arms and transposed to simplevoice frequency patterns. These pairsdo not degrade the performance of carrierpairs on the higher arms, To standardizeon one type of ann for both carrier andvoice frequency circuits it was decidedin 1953 to use this 12-wire voice fre­quency arm as an 8-wire ann on 40-pole­per-mile routes by omitting the centerpair on each side of the ann. Thisinvolves a pair spacing of 6 inches and animprovement of about 4 to 6 db in cross­talk. The attenuation is increasedslightly.

10 Xc

150 to 250 Kc

Types of Pairs

10 to 150 Xc

Table II. Wire Configuration, Open-Wire Main-Line and Secondary Routes

Type

Diameter, Inches Pounds per Mile

Characteristics of Open-Wire Routes

The principal physical and electricaldetails of standard South African open­wire routes are given in Tables I, II,III, and IV.4,8,u Their design owesmuch to the Bell System-! but thereare certain differences arising out oflocal circumstances.

Since supplies of suitable local timberare not yet adequate, most carrier routesare constructed with steel or concretepoles with tubular steel crossarms. Tomaintain the wire spacing accurately,the anus are braced with steel straps ;see Fig. 6. The ann (7 feet) is shorterthan the standard American arm andthe distance between arms is also lessin order to reduce the height of the pole.This has meant that the crosstalk coup-

Far-end 55 to 70 35 to 45Near-end 40 to 65 30 to 45

Worst Crosstalk Between Carrier-TransposedPairs per Repeater Section Over the Frequency

Range Indicated, Db

Table IV. Maximum Crosstalk

channel systems on the same figures asgiven. The separation between open­wire telephone and power routes is basedon limiting the noise induction into thephysical or audio circuit to 1 millivolt(weighted valuer'" and under theseconditions the noise at carrier frequenciesdue to power lines is negligible. Theincreasing use of power line carriersystems has not up to the present causedany interference to telephone carriersystems, but the possibility of its oc­curring should not be overlooked.I!

MAY 1955 Boyce-Open-Wire Carrier Systems in South Africa 277

inexpensive compandor, gtvlng a noiseadvantage of some 20 db or more, hasthus made the application of another12-channel group above the presentSOJ systems on at least half the pairs apractical possibility. The top frequencyof the new system is expected to beabout 270 kc per second. The problemsassociated with such a project, e.g.,method of frequency translation, pilotregulation, interaction crosstalk at re­peaters, matching of entrance cables,etc., have been examined and it is pro­posed to install a number of systems inthe near future. The same repeatersections will be employed since thecompandor will be able to cope with theincreased noise. I t is proposed to em­ploy these systems initially over not morethan two or three repeater sections.

THE ApPLICATION OF CARRIER SYSTEMS

TO EARLY TYPES OF ROUTE

It is possible to operate one carriersystem, even a 12-channel system, overalmost any type of route, by suitablytransposing the pair which will ac­commodat.e the carrier. The operationof more than one carrier system mayrequire the insertion of a few polingtranspositions or extensive retranspos­ing." I t is also necessary to remove anysections of covered wire or cable inter­posed in the route.

Many of the earlier routes in SouthAfrica embodied "revolved wires," i.e.,a quad of four wires twisted or revolvedmaking a complete twist in each fourspans. Like all routes which employspecial configurations to reduce mutualcoupling, some pair combinations givegood results while others are very poor.By a careful choice of pairs and theinsertion of poling transpositions it ispossible to operate several carrier systemsover such routes. In one case 60 carrierchannels were operated over such a routefor 56 miles, although with somewhatrelaxed crosstalk standards. 8 There islittle doubt, however, that the flat carriertransposed route is superior to all othertypes of construction for carrier systemoperation.

Lead-in Arrangements

The SOJ carrier system with its ex­tended frequency range introduces prob­lems of matching between lead-in cablesand the open wires not present withsystems operating up to 30 kc persecond. 7,8, 15,16 The open-wire pairs areconnected to the SOJ line filters bymeans of low-capacity star-quad cablehaving a characteristic impedance of

240 ohms. Variable load units arefitted to one or both ends of this cableand its impedance is built out to matchthe 575-ohm line, with a reflection coeffi­cient of less than 5 per cent over thefrequency range of 10 to 150 kc persecond. In the frequency range of over150 kc per second, it will be possible toperm-it a somewhat higher reflectioncoefficient, since the near-end crosstalkhas not deteriorated as much as the far­end, the relative improvement being ofthe order of 10 db.

This arrangement is satisfactory fordistances between the line and the equip­ment up to about 200 yards. For greaterlengths it would be possible, but un­economical, to use intermediate loading.In these cases the 12-channel line filtersare divorced from the equipment andplaced in a hut near the end of the route.The filters are connected to the terminalor repeater equipment by a paper­insulated star-quad 46-pair 16-gauge(40 pounds per mile) toll entrance cable.The high-pass side is brought in over anunloaded pair, giving a loss of 3.6 dbper mile at 140 kc per second. The low­pass side is brought in over a pair loadedat intervals of 250 yards with 3-mi1li­henry coils, giving a loss of about 1 dbper mile at 30 kc per second. If thecable is short, the B-millihenry loadingmay be dispensed with and autotrans­formers used at both ends of the pairfor matching purposes. This latter ar­rangement does, however, introduce aconsiderable loss into the circuit and itis not favored for cables longer than about500 yards.

In cases where only a limited numberof 12-channel systems is proposed, andit is therefore possible to relax theimpedance matching requirements,matching transformers covering the fre­quency range 0.5 to 150 kc per secondare available. This transformer matchesthe open-wire line to the cable andeliminates the need for the filters at theterminal of the open-wire route.

Crosstalk in toll entrance cables isreduced to satisfactory values by suitably"doping" the joints on a basis of capacityun balance and also by fi tting crosstalkbalancing condensers at one end of thecable.

Power Plant

Equipment installed up to about 1945is operated from 24-volt and 130-voltd-e supplies. At most stations this isprovided by one or more selenium recti­fier units. The voltage regulation isachieved by tap-changing transformers

controlled by what is essentially amoving-coil contact voltmeter. In theevent of a failure of the public mains, orof the rectifier units themselves, the 24­and the 130-volt supplies are obtainedfrom two d-e generators driven by anautomatic-starting diesel engine. Thebreak of about 10 seconds during whichthe engine is running up to speed iscovered by very small batteries floated

. across the output of the rectifier units.On restoration of the' mains supplythe stand-by set recharges its starterbattery. 6,8

During the past 10 years, most carrierequipment has been operated from smallpower panels associated with each bay­side. Filaments are heated from a6.3-volt a-c supply, while the platevoltage is 220 volts. This new arrange­ment has a great deal to recommend it.'though there are two important com­plications. In the first place, the mainsvoltage must be regulated to within1 per cent and surges on the mains mustbe reduced as much as possible since theyare applied directly to the equipment.Secondly, the problem of supplying ano-break stand-by power supply is noteasily solved.

At less important stations, or wherevoice frequency telegraphs or broadcastcarriers are not involved, automatic­starting Diesel-alternator sets are pro­vided. In these cases there is a 5- or 10­second break in the supply on a mainsfailure. At all other stations a con­tinuous a-c supply is provided by eitherof the following types of machine:

1. A synchronous motor running off themains drives a large flywheel. On mainsfailure this motor supplies the load as analternator driven by the flywheel and, atthe same time, the flywheel starts theDiesel engine through a magnetic clutch.

2. The station is supplied from an in­duction-motor-alternator set running off the.mains. This set drives a large flywheel.On mains failure the flywheel continues tosupply the station by driving the set whilethe engine is brought up to near full speedbefore being magnetically coupled to therest of the set.

There are several advantages ill thesecond set described, e.g., mains voltagefluctuations tend to be smoothed out,the control circuits are simpler, and nosynchronization of the machine with therestored mains supply is required.

The control equipment has proved tobe the most vulnerable part of stand-bysets. It is essential that all relays and

_contactors be of the most reliable typeand that all connections be as secure aspossible. The sets should also be fittedwith adequate and reliable alarms to

278 Boyce-Open-Wire Carrier Systems in South Africa MAY 1955

cover a failure of any of the essentialfunctions of the set.

Maintenance

Although maintenance standards oncarrier systems have always been high,the tolerances of fault incidence and thestability requirements of toll circuitsare even more stringent today, becauseof the following:

1. The automatization of the toll networkin the form of operator dialing and in some'measure customer dialing. 16

2. The operation of circuits at or nearzero loss equivalents, and the connectionof several of these circuits in tandem.3. The fact that a fault may affect a largenumber of channels.

EQUIPMENT PRACTICES

The effect of equipment practices onmaintenance has already been men­tioned. The more important pointsare:

1. Jack-in panels give ready access tocomponents and wiring, and facilitate therepair and replacement of faulty panels.

2. The miniaturization of equipmentshould not be carried too far. The cost offloor space is low in relation to the equip­ment, and there are diminishing returns inminiaturization which may be absorbedby higher maintenance costs. I t is, ofcourse, also true that in some cases floorspace is acutely limited, and in these casesvery compact equipment may be essential.

3. There should be an adequate number oftest points to facilitate the speedy locationof faults. Where necessary it should bepossible to make terminated, as opposedto level, measurements.

4. Controls, such as potentiometers, etc.,should be concealed to prevent unwarrantedor accidental operation.5. The equipment should be reasonablydustproof.

RELIABILITY OF COMPONENTS

A system is no more reliable than itsweakest component. Electrolytic con­densers should in general be avoided.The voltage rating of condensers in de­coupling circuits and in line filters mustbe generous. Wire-wound resistors shouldbe used only for low-resistance valuesand where a high accuracy is necessary.Rectifiers in the form of metal rectifiersor germanium diodes are used extensivelyin modern circuitry and it is importantthat they should have stable characteris­tics. Catastrophic and emissive failuresin thermionic tubes have been, and arestill, receiving close and considerablysuccessful investigation by manufacturers.Such special, or "trustworthy," tubes are

of course more expensive than com­mercial tubes, but mean lives of between3 and 10 years may be expected fromthem.

DEFECTIVE CONNECTIONS

As important as the components, arethe connections between them. Carefulcontrol of soldering in both factory andfield is essential. There is much to besaid for soldering in tubes, particularlyif they are reliable ones. Potentiometersemploying contacts sliding over wire orstuds, keys, jacks, blind sockets, rivetedjoints in links, or tags should be avoided.Flat-bladed links in spring sockets aresuperior to the round type. Contactsshould always be of precious metal.The internal connections in such itemsas paper condensers, miniature rectifiers,etc., are possible weaknesses.

I t is now the practice to subject allcomponents and wiring after equipmentinstallation to a vibration or percussiontest.F Briefly, the method involvesthe sending of a test tone, of a suitablefrequency and very low level, throughthe equipment to a demodulator followedby a high gain loudspeaker-amplifier.The disturbance of a bad connectionmodulates the test tone and produces acrackle from the loudspeaker. It isfound that this method is very successfulin revealing defective connections whichappeared perfect under visual inspection.It is necessary to employ conscientiouspersonnel for this type of testing.

TESTING EQUIPMENT AND MAINTENANCE

AIDS

Terminal stations of any appreciablesize and the more important repeaterstations, are equipped with the followingitems of test equipment:

1. A continuously variable frequencyoscillator covering the entire frequencyrange of the equipment, with the exceptionof the group translation stages in the SOJsystems. At larger stations there areadditional oscillators, some covering theaudio-frequency range only.

2. A nonfrequency selective transmissionmeasuring set or sets to cover the entirefrequency range of the equipment and ca­pable of reading down to - 40 dbm or insome cases-55 dbm. Most sets fall intoone of the ranges, 30 cycles to 10 kc persecond, 30 cycles to 150 kc, or 60 to 600kc per second.3. A multirange general-purpose 20,000­ohm-per-volt voltmeter with current andresistance scales.

At stations which have 3-channelterminals only, an 800-cps oscillator, to­gether with a suitable level measuringset, is generally adequate. With some

exceptions, no testing equipment is sup­plied for stations with single channelterminals only, since these are often un­attended and small portable sets are car­ried by visiting maintenance personnel.

At the largest stations special itemssuch as impedance bridges, psophometers,oscilloscopes, valve voltmeters, etc., areheld. Harmonic analyzers for measuringthe harmonic content of line amplifiersare also available at a number of stationsbut these have not proved convenientto use. Most equipment is mounted onsuitable trolleys. This arrangement hasproved most satisfactory since one setserves the maximum amount of equip­ment, the test leads .are kept short, andthe testing officer is in general con­veniently near the equipment he istesting.

Experience has shown that it is im­portant that station alarm systems beadequate, well maintained, and standard­ized throughout the country. The termi­nals and repeaters of important systemsor routes are connected by a privatetelephone circuit, generally a physicalline. On the longer systems a specialcircuit provides an immediate indicationat both terminals of which repeater orrepeater section has failed. The actionof this circuit is quite independent ofthe faulty station or section since it isactuated by pilot alarms at adjacentrepeaters. This device is of particularvalue in locating short breaks which havenot been noticed by repeater attendants,and for calling out the correct attendantwhen faults occur during the night. Itis important that the correct maintenancetools be available, particularly in regardto soldering equipment.

ROUTINE TESTING

Routine tests are not made as fre­quently as they once were. It is gener­ally agreed that the value of such testsin preventing or anticipating faults hasbeen over estimated. Channel equiva­lents at a single frequency are checkedweekly in most systems, but it is probablethat this period will be increased to amonth or more in the case of shortcircuits. System levels at importantpoints, such as the output of the lineamplifiers, carrier supply levels, syn­chronization, etc., are generally checkedat intervals of 2 or 3 months. Theemission currents of tubes are no longermeasured, with the exception of the lineamplifiers on multichannel systems, inwhich case the measurements serve asa check on intermodulation products.The frequency response of channels isat present measured at intervals of 4,

MAY 1955 Boyce-Open-Wire Carrier Systems in South Africa 279

or in some cases 12, months. Considera­tion is being given to the elimination ofthis test. Annual overhauls are nolonger made, since a system with ade­quate maintenance should never de­teriorate to the extent which would makesuch an overhaul necessary. If the per­fonnance of the system as a whole, asindicated by the measurements men­tioned, is satisfactory, there is little tobe gained by measuring its individualparts.

FAULT PROCEDURE

The efficient clearance of faults de­pends on both the technical ability of thepersonnel and the manner in which theyco-operate with each other. To achievethe latter it is necessary that the pro­cedure for receiving and passing oninformation relating to faults and theirclearance be specified in some detail.

In South Africa there is a main ormaster control station which is advisedof important breakdowns and which ifnecessary issues instructions in regardto repairs, authorizes work on importantcircuits, and collates and analyzes faultdata. Several subsidiary control stationsfunction in a similar capacity for theirrespective areas, usually in regard tosecondary routes, and act as clearingcenters for information required by themain control station. The fact thatcertain stations are designated as faultcontrol stations does not lessen the re­sponsibility of personnel, irrespective ofthe status of their station, to exercisetheir initiative, and to act immediatelya fault occurs.

While the quantity of equipment inservice has increased considerably, thesupply of skilled maintenance personnelhas not kept pace with this increase.Fortunately modern equipment requiresvery little maintenance. On the otherhand, this means that the maintenanceofficer gets very little practice in clearingfaults. Centralized repair depots andpoints from which technical assistancemay be obtained telephonically are ofconsiderable value.

ANALYSIS OF FAULTS

Control stations receive fault detailsdaily, and in some cases weekly. Theinformation is coded so that the clericalwork involved is reduced to a minimum.The information is then transcribed tofault cards on a system and on a circuitbasis. The control stations examine thefault returns and the fault cards atfrequent intervals and any unusual trendsare followed up. On a short-term basisthis follow-up generally involves a high

fault incidence at or on a particularstation, system, or circuit. On a long­term basis the analysis reveals weak­nesses in particular types of equipment.

An analysis of the outage of the 12­channel network over a period of a yearis given in Table V. The line faultspredominate and the following shows thenumber of faults per 100 miles of open­wire circuit per annum, based on 96,500miles of circuit including a considerableamount on secondary routes and someon old routes:

Broken wires and binders 1 .61Protector faults 1.47Faults due to any external objects

except trees. . . . . . . . . . . . 0 .96Tree faults... . . . . . . . . . . . . . . . . . . .. 0.53Slack wires " 0.38Broken insulators 0.38Wires cut by lightning 0.35Faults due to workmen 0.14Faults due to power contacts or

surges. . . . . . . . . . . . . . . . . . . . . . . . . . .0 .03All other faults 0. 96

Total 6.81

The contribution of line faults has beenreduced very considerably by the pro­vision of stand-by or reserve physicalcircuits for 12-channel systems. Powerplant failures are relatively few innumber but their contribution is highsince one fault often affects a largenumber of circuits. A considerable num­ber of the short breaks which occurappear to be due to unauthorized inter­ference by maintenance staff, and it isimportant that stringent measures toprevent this be applied.

Protection from Lightning Damageand Interference

A considerable amount of lightning isexperienced in South Africa: in severalparts there are over 70 thunderstormdays per year.P

On a long open-wire line, such as atoll line, the crest surge current whichoccurs as a result of a near-by groundstroke rarely exceeds 200 amperes. Thehigh currents associated with a highground resistivity do not arise as theydo on short customer lines. The currentthrough the terminal protectors is ingeneral of an oscillatory nature, partlybecause of successive reflections fromthe ends of the )ine which may increasethe duration of the surge to about1 to 5 milliseconds.

Air-gap metal-electrode protectorssealed in polyvinyl-chloride give verysatisfactory service on toll lines. Gasprotectors are not used in lightning areasbecause when damaged by lightning theygive no indication of failure.

Toll protection at the end of an open­wire pair thus consists of two metal­electrode protectors with a drainage orequalizing coil. This latter coil is veryeffective in reducing the transversevoltage resulting from protector opera­tion. No heat-coils or fuses are em­ployed. It is important to bond theprotector earth to the earth of the equip­ment, e.g., line filters, and also to thesheath of the lead-in cable. In highresistivity soil, where it may be difficultto obtain a low resistance earth con­nection, a horizontal buried wire providesthe most economical solution.!? In spiteof the high lightning incidence and con­siderable amount of equipment installedin South Africa, the damage which occursis negligible.

Apart from the effects of lightningdischarges, protectors may operate dueto changes in the electric gradient nearthe ground during periods of thunder­stonn activity. In these cases the cur­rents which pass through the protectorsare less than a few milliamperes. Theprotector operation does, however, causenoise and this effect may be overcomeby earthing the centerpoint of the linetransformer, or by employing a drainageelement such as silicon-carbide.

The effects of lightning on carriertelephone and voice frequency telegraphsystems have been studied in somedetail. 18 Briefly, the nature of thetransients at the various points aredetermined by the filter characteristics.The frequency of oscillation is at themid-band of a band-pass filter and at thecutoff frequency of a low-pass filter.The duration of the transient is a func­tion of the bandwidth: the narrowerthe bandwidth the longer the duration.There is no appreciable lengthening ofthe transient until the output of thelow-pass filter following the channeldemodulator is reached. The importantlengthening process occurs in the re­ceiving telegraph filter, where the dura­tion is always about 20 milliseconds fora 120-cycle spaced telegraph system.The peak voltage of the transient tendsto increase as the gain of the terminalis increased.

It is found that amplitude-modulatedvoice frequency teletype circuits can beoperated over open-wire carrier telephonechannels with a negligible number ofmutilations even under severe thunder­storm conditions, provided drainage(equalizing) coils are used, and providedthat the system line levels are sufficientlyhigh. The gain-time characteristics ofthe telegraph detector are also of im­portance. Operation over 12-channel

280 Boyce-Open-Wire Carrier Systems in South Africa MAY 1955

systems is superior to operation over 3­channel systems.

Frequency-modulated telegraph sys­tems are superior to amplitude-modulatedsystems as far as lightning mutilationsare concerned but until their cost canbe made comparable to that of amplitude­modulated systems, their use on high­quality carrier telephone channels is notjustified.

References

1. COAXIAL CABLE CARRIER TELEPHONE SYSTEMSIN SOUTH AFRICA, T. W. Elliott, N. J. Paola.Transactions, South African Institute of ElectricalEngineers, Johannesburg, South Africa, vol. 43,Nov. 1952.

2. THE PROGRESS OF SOUTH AFRICAN TELECOM­MUNICATIONS, J. A. F. Michell. Ibid., vol. 41,Jan. 1950.

3. TRUNK TELEPHONE PRACTICES IN THE UNIONOF SOUTH AFRICA, P. Machanik. Ibid., vol. 28,Mar. 1937.

4. THE EVOLUTION OF THE LONG DIST ANCETRUNK SYSTEM IN SOUTH AFRICA, M. Hewitson.Ibid., vol. 43, June 1952.

5. THE CAPE TOWN-JOHANNESBURG CARRIERTELEPHONE AND TELEGRAPH SYSTEM, P. Machanik,E. H. Harwood. Electrical Communications, NewYork, N. Y., July 1936.

6. COMMUNICATION NETWORK OF THE UNION OFSOUTH AFRICA, D. P. J. Retief. Transactions,South African Institute of Electrical Engineers,johannesburg, South Africa, vol. 38, Mar. 1947.

7. SOJ-12 OPEN-WIRE CARRIER SYSTEMS INSOUTH AFRICA, D. P. J. Retief, H. J. Barker.Electrical Communications, New York, N. Y.,Sept. 1947.

8. TWELVE CHANNEL CARRIER TELEPHONE SYS­TEMS IN SOUTH AFRICA, N. ]. Paola, C. F. Boyce,I. C. Ramsay. Transactions, South AfricanInstitute of Electrical Engineers, Johannesburg,South Africa, vol. 39, Oct. 1948.

9. AUTOMATIC TRUNK EXCHANGES AND Two­VOICE-FREQUENCY SIGNALLING IN SOUTH AFRICA,D. W. Corlett. Ibid.., vol. 42, Nov. 1951.

10. TRENDS IN THE DESIGN' OF LINE TRANSMIS­SION EQUIPMENT, R. J. Halsey. Paper 200,Institution of Post Office Electrical Engineers,London, England, Nov. 1949.

11. IMPROVED EQUIPMENT PRACTICE REDUCESSIZE OF TELEPHONE TRANSMISSION SYSTEMS, F.Fairley, R. J. M. Andrews, A. C. Delamare. Elec­trical Communications, New York, N. Y., Mar.1950.

12. THE ACTIVITIES OF THE POWER AND COM­MUNICATION SYSTEMS CO-ORDINATING COMMITTEEDURING THE PERIOD MAY 1941-JuLY 1952, C.

F. Boyce. Transactions, South African Instituteof Electrical Engineers, Johannesburg, SouthAfrica, vol. 44, July 1953.

13. CROSSTALK IN SOUTH AFRICAN TELEPHONELINES, C. F. Boyce, N. J. Paola. Ibid., vol. 33,Sept. 1942.

14. LINE DEVELOPMENT PROBLEMS IN THEDEVELOPMENT OF THE 12-CHANNEL OPEN-WIRECARRIER SYSTEM, L. M. Ilgenfritz, R. N. Hunter,A. L. Whitman. Bell System Technical Journal,New York, N. Y., Apr. 1939.

15. A 12-CHANNEL CARRIER TELEPHONE SYSTEM,B. W. Kendall, H. A. Affel. ius., Jan. 1939.

16. SOME ApPLICATIONS OF THE TYPE J CARRIERSYSTEMS, L. C. Starbird, J. D. Mathis. Ibid.,Apr. 1939.

17. MAINTENANCE OF MULTI-CHANNEL CARRIERTELEPHONE SYSTEMS, F. O. Morrell. Paper 194,Institution of Post Office Electrical Engineers,London, England, April 1948.

18. THE PROTECTION OF OPEN-WIRE COMMUNICA­TION SYSTEMS FROM LIGHTNING DAMAGE ANDI NTERFERENCE WITH PARTICULAR REFERENCE TOSOUTH AFRICA, C. F. Boyce, I. C. Ramsay, D. P.J. Retief. Monograph, South African Institute ofElectrical Engineers, Johannesburg, South Africa,Apr. 1955.

19. THE EARTHING OF TELEPHONE SYSTEMS WITHP ARTICULAR REFERENCE TO SOUTH AFRICA, C.F. Boyce. Transactions, South African Instituteof Electrical Engineers, Johannesburg, South Africa,vol. 43, Dec. 1952.

Regulator Curves and Transient Currents

of Double-Way and Double-W ye

Rectifiers

Transient Analyzer Components

~Zl

OSCILLO­SCOPE

OSC;ILI.-O·SCOPE

SI SYNCHRONOUS ROTATING SWITCH

1 . Double-way and double-wyerectifier circuits

Fig.

Germanium cells with the characteris­tics shown in Fig. 2 were used as therectifying devices. Voltage and im­pedance magnitudes were chosen to makethe voltage drop of the germanium cellsnegligible. The"arc-drop" of the cellswas less than 0.3 per cent (%) of EdO forthe double-way circuit. The reactanceelements had resistance-reactance (R/X)ratios in the order of 0.025. Thus,

:3 (6 60 rv SINE

WAVE GENERATOR

c. E. RETTIGASSOCIATE MEMBER AlEE

The 6-phase double-way and 6-phasedouble-wye circuits used in the miniaturestudy are shown in Fig. 1. The equiva­lence between double-way and double­wye rectifiers has been developed ana­lytically in previous papers's" and wasverified by the use of the transientanalyzer. Most of the data in thispaper were taken with the double-waycircuit.

Rectifier Circuits

of time and effort over purely analyticaltreatment.

An extensive study of typical circuitconditions has been made with thetransient network analyzer." This studyhas resulted in a fairly complete coverageof regulation curves and currenttransients during d-e faults for mostpower rectifier operating conditions; anda simplified method of obtaining regula­tion and transient currents for typicalpower rectifiers.

Paper 55-45, recommended by the AlEE ElectronicPower Converters Committee and approved bythe AlEE Committee on Technical Operations forpresentation at the AlEE Winter General Meeting,New York, N. Y., January 31-February 4, 1955.Manuscript submitted October 21, 1954; madeavailable for printing November 29, 1954.

L. E. JENSEN and C. E. RETTIG are with the GeneralElectric Company, Schenectady, N. Y.

L. E. JENSENASSOCIATE MEMBER AlEE

CALCULAT ION of instantaneousand sustained current resulting from

d-e faults on a power rectifier is ex­tremely cumbersome. Certain idealizedcases have been treated analytically,an important factor in establishing andunderstanding the circuit action. Theanalytical treatment also provides anecessary check on results obtained byother methods.

One effective way to reduce the amountof cumbersome calculation in such prob­lems is to set up a model or miniature ofthe full-scale equipment which duplicatesknown conditions. This model or minia­ture can then be used to extend investiga­tions through a great variety of circuitconditions with a considerable saving

MAY 1955 Jensen, Rettig-Double-Way and Double-Wye Rectifiers 281