A. ELEKASSOCIATE MEMBER AlEE

Proving the Adequacy of StationGrounds

1. No shielding or correction winding,131llV

2. With correction winding but noshielding, 2 mv

3. With correction winding and shield­ing, <0.005 mv

ReFerences

1. CURRENT TRANSFORMER RATIO AND PHASEERROR BY TEST RING METHOD, H. S. Baker.AlEE Proceedings, vol. XXXVI, 1917, PP. 1173-83.

2. A METHOD OF TESTING CURRENT TRANS­FORMERS, W. E. Bruges. Journal, Institution ofElectrical Engineers, London, England, vol. 68,1930, pp. 305-07.

3. TESTING OF CURRENT TRANSFORMERS WITH ACURRENT COMPARATOR AND AN AUXILIARY ELEC­TRICAL SYSTEM (in German), I. Obradovic, P.Mi1janic, S. Spiridonovic. Elektrotechnische Zeit­schrift, Wuppertal-Elberfeld, Germany, pt. A, vol.78,1957,pp.699-701.

4. THE CURRENT COMPARATOR AND ITS ApPLICA­TION TO THE ABSOLUTE CALIBRATION OF CURRENTTRANSFORMERS, N. L. Kusters, W. t. M. Moore.AIEE Transactions, pt. III (Power Apparatus andSystems), vol. 80, Apr. 1961, pp. 94-104.

5. DIELECTRIC ADMITTANCES IN CURRENT TRANS­FORMERS, A. H. M. Arnold. Proceedings, Institu­tion of Electrical Engineers, vol. 97, pt. 2, 1950, pp.727-34.

6. THE EFFECT OF CAPACITANCE ON THE DESIGNOF TOROIDAL CURRENT-TRANSFORMERS, A. H. M.Arnold. Ibid., pp. 797-808.

7. THE EFFECT OF WINDING POTENTIALS ONCURRENT TRANSFORMER ERRORS, N. L. Kusters,W. s. M. Moore. AlEE Transactions, pt. I(Communication and Electronics), vol. 81, July 1962,pp.186-91.

8. EFFECTS OF MAGNETIC LEAKAGE IN CURRENTTRANSFORMERS, H. W. Price, C. K. Duff. Bulletin,University of Toronto, Toronto, Ont., Canada,vol. 2, 1921, p. 167.

9. LEAKAGE PHENOMENAIN RING-TYPE CURRENTTRANSFORMERS, A. H. M. Arnold. Journal, In­stitution of Electrical Engineers, vol. 74, 1934, PP.413-23.

Summary: The definition of ground re­sistance and ground impedance in the case ofalternating currents is discussed. Groundresistance measurements with earth testersare compared with power frequency meth­ods and the errors associated with the loca­tion of auxiliary grounds are described.It is emphasized that the ultimate purposeof testing station grounds is to determinetouch voltages and potentials arising oncommunication circuits. Methods are pre­sented to evaluate the adequacy of stationgrounds, including very large stations.

TH E GROUNDING of a station isadequate if unwanted ground po­

tentials in the station area do not ex­ceed a specified limit during power systemfaults. In order to decide whether or not

DiscussionA. F. Dunn (Division of Applied Physics,National Research Council, Ottawa, Ont.,Canada): The current comparator as apassive magnetic current-ratio device of highstability and accuracy, which was previouslydescribed by two of the three authors, hasnow been extended in range to the pointwhere measurements of 2,000:5 amp arereadily available for 60-cps power-linefrequencies. The authors are to be con­gratulated on the extension they have made,particularly since they have also improvedthe accuracy of their current ratio com­parators to the point where it looks asthough the phrase "parts in 1OS" is theconvenient phrase to eliminate use of smallfractions of a part in a million.

I t may be suggested that accuracy of thisdegree is not needed, and it may not be ifuse of the comparator is restricted to cali­bration of current transformers as we knowthem now. However, it is now possibleto examine current transformers in greaterdetail and with greater reliability and, ifnecessary, improvements in design may beachieved.

The current comparator is not restrictedto this single use; in fact, its greatest valueprobably will be realized as a current-ratiodevice for comparing other types of im­pedances. Ratios which are accurate to afew' parts in lOS are not easy to come by inthis day of increasing demands for ac­curacy and reliability, and the current com­parator may be one of the devices for whichthe precision electrical measurements fieldis waiting.

I would hope that these extended ratiocomparators will prove as successful as theirpredecessors when the operating frequencyis increased.

Have the authors any information on

a station ground is adequate, it is neces­sary to predict first the magnitude of thehighest potential differences which mayoccur, and then to compare these with thetolerable maximum values determined bysafety rules or by other requirements.

The following potential differenceshave to be determined:

1. The expected highest touch voltagewithin or outside the station.2. The expected highest voltage arisingbetween communication and other low­voltage circuits and the station ground.

For most stations both values can bederived from a single quantity: the groundresistance of the station. The groundresistance can be measured using one of

available high er, or lower, frequency use ofthese comparators?

P. N. Miljanic, N. L. Kusters, and W. ]. M.Moore: Dr. Dunn's comment regardingthe use of the current comparator for im­pedance ratio measurement is most im­portant, and we are grateful to him formentioning it here. The current compara­tor possesses many features which we feelare of advantage in this type of measure­ment. It has good long-term stability, isrelatively independent of its operating en­vironment, and its errors are usually smallenough to be negligible in most measure­ments.

The current comparators described inthis and the previous paper were designedfor use with fairly large currents. Theratings of the windings were of the order of400 to 2,000 amp-turns and sensitivity wasno problem. In impedance ratio measure­ments, however, somewhat lower values ofcurrent are usually employed, and the reali­zation of adequate sensitivity in the presenceof proportionally higher external magneticfields is more difficult. Fortunately, withproper magnetic shielding, this difficulty canbe overcome to an appreciable extent, andcurrent comparators rated at 0.040 amp­turns and having errors less than 1 part inlQ6 have been built.

The extension of the current comparatortechnique to higher and lower frequencies ispresently being investigated in our labora­tory. The sensitivity is, of course, pro­portional to frequency, and the existingarrangement for detecting flux in the corebecomes more and more unsuitable as thefrequency is decreased. Other methods,such as those used in magnetometers, maybe used, however, and a d-e comparatorwhich employs a second-harmonic type ofdetector has been operated successfully.

the following methods, all of which aredescribed in this paper.

1. Measurement with a portable groundtester. Ground testers generate their owntest current, the frequency of which differsfrom that of the system.2. The voltmeter-ammeter method. Thismethod uses a test current of system fre­quency.3. The wattmeter-ammeter method. Thisreplaces the voltmeter-ammeter methodwhere mutual induction exists between thetest leads, but cannot be used to measuregrounds with reactance.

I t is convenient to use earth testerswherever this is possible. I t will beshown that earth testers cannot be usedin the following cases:

1. For very large 'stations where the im-

Paper 62-206, recommended by the AI EE SpecialInstruments and Auxiliary Apparatus Committeeand approved by the AlEE Technical OperationsDepartment for presentation at the AlEE WinterGeneral Meeting, New York, N. Y., January 28­February 2, 1962. Manuscript submitted Novem­ber 24, 1961; made available for printing June 1,1962.

A. ELEK is with the Hydro-Electric Power Com­mission of Ontario, Toronto, Ont., Canada.

368 Elek-Proving the Adequacy of Station Grounds NOVEMBER 1962

pedance of the station ground contains aconsiderable reactive component.2. Where stray power frequency voltagescause excessive vibration of the movementin the indicating instrument of the earthtester.3. Where the ground resistance to bemeasured does not exceed 10% of the lowestrange of the available ground tester.4. Where probe distances are large andthe ground resistance to be measured is low,so that the resistance of the ground pathremote from the field of the station forms asignificant part of the measured resistance.

In order to obtain the total potentialrise of the station ground with respect toa remote ground during faults, the meas­ured ground resistance must be multipliedby the magnitude of the fault currentflowing through the station ground. Asexplained later, this value of current is,in most cases, considerably smaller thanthe total fault current obtained fromconventional calculations or networkanalyzer studies.

In stations with uniform soil the highesttouch voltage can be expected to be be­between 20 and 30% of the calculatedtotal potential rise of the station ground.The highest voltage between communica­tion circuits and the station ground canbe obtained by adding vectorially the volt­age caused by induction from parallel linesto the total potential rise of the stationground.

In the company the author is associatedwith, the highest touch voltage normallypermitted is between 1,000 and 1,500volts. Hence) the total potential rise ofthe station ground will, in most cases,be less than 5,000 volts. (A limit of 3,000volts is recommended wherever it iseconomically feasible.) The reasons forchoosing these values will be explainedlater in the paper. There is no fixed limitto the voltage permitted between com­munication circuits and the stationground; it is, however, usually necessaryto know the magnitude of this voltage be­cause the protection of the communica­tion circuits will depend on its value.

There are certain cases in which it iseither not possible or not worthwhile tomeasure the ground resistance of thestation. Such cases are the following:

1. Where the station ground is connectedto an extensive underground network ofconductors, such as waterpipes, cablesheaths, counterpoise wires, etc., whichcannot be separated from the station ground(for example, stations in the built-up areasof cities).

2. Where the station ground is very largeand is interconnected with other stationgrounds in the vicinity (for example, largegenerating stations and adjacent transformeror switching stations).

3. Stations built on nonuniform soil.

In cases 1 and 2 proper ground resist­ance measurements are either impossibleor costly. Furthermore, the value of theconclusions which could be drawn fromthe results is doubtful, since for verylarge ground grids the assumption that thehighest touch voltage will be between 20and 30% of the total ground potentialrise, may not be valid. This is certainlytrue in case 3, where a general relationbetween the highest touch voltage and thestation ground resistance does not exist.

In the cases mentioned, it is more prac­tical either to measure touch voltagesdirectly at all locations where they aresuspected to be high, or to omit measure­ments altogether and rely on conservativeestimates. Also, the voltage appearingon communication circuits can be eithermeasured directly in such cases or simplyestimated; a highly conservative rating ofthe protection may often be cheaper thana test.

Some extraneous conductors, such asskywires or neutral conductors, can beeasily separated from the station ground,in which case the measurement can becarried out on the station grid alone andthe effect of the extraneous conductorscan be taken into account by calculation.

In addition to the aspects just dis­cussed, two further requirements of sta­tion grounds should be mentioned. First,in a system with effectively groundedneutral, where a reduced insulation levelis used, the zero-sequence resistance mustbe smaller than the positive-sequencereactance. It will be shown that if theground resistance of the major stations ina system does not exceed 2 ohms, which ispractically always the case, this require­ment is satisfied. Second, adequacy ofthe station ground must be insured fromthe point of view of lightning. Exceptfor very small stations or for special re­search studies, the adequacy with respectto lightning is never checked by measure­ments and is assumed to be satisfactory.

Definition of Ground Resistance andGround Impedance

The AlEE Proposed Guide for Meas­uring Ground Resistance defines theground resistance of a ground electrodeas the ohmic resistance between it and aremote grounding electrode of zero resist­ance. In this case, "remote" is meant"at a distance such that the mutual re­sistance of the two electrodes is essen­tially zero."1

This definition needs some amplificationin the case of large stations and alternating

currents. The voltage rise of very largeground grids (such as the ground of largegenerating stations interconnected withthe grounds of adjacent stations, or sta­tions in urban areas connected to citywater mains) is not in phase with thealternating current causing the voltagerise. I t is more accurate, therefore, toreplace the term "ground resistance"in such cases with the more precise term"ground iinpedance," which includes areactive component in addition to theresistance. Since the impedance deter­mines the adequacy of the station ground,this quantity should be specified andmeasured. The ground impedance de­pends, of course, on frequency, and, there­fore, station grounds with a considerablereactance can only be tested at power fre­quency.

In the case of alternating currents thedistance of the remote ground electrodewill also affect the value of ground resist­ance or ground impedance, as definedearlier. The impedance between thestation ground and 'a remote electrodewill be composed of two parts: the groundresistance or impedance of the stationground itself and the impedance of thecurrent path between the two electrodes,outside the field of the station, as ex­pressed by Carson's formula. 2 This im­pedance has a resistive and a reactive com­ponent, which are both functions of fre­quency and distance They are bothequal to zero at direct current.

If the ground impedance is defined asthe impedance between the station groundand a remote electrode, the impedance ofthe current path will obviously create un­certainty. Even where the ground im­pedance of a station is a pure resistanceand reactive components need not beconsidered, the resistive component ofCarson's formula would still' cause asignificant error in the case of largedistance between the electrodes,

It may be possible to find an un­ambiguous definition for the groundimpedance of stations in the case ofalternating currents. This is, however,not necessary since the value of theground impedance itself is not useddirectly for the evaluation of stationgrounds, but rather the magnitude of thehighest potential difference in the stationarea. The measurement of "groundresistance" or "ground impedance" maybe regarded as only an intermediate stepin the derivation of the highest potentialdifference. For this purpose, ground re­sistance or ground impedance measure­ments are sufficient in most cases. It isimportant, however, to know what exactlyis being measured.

NOVEMBER 1962 Elek-Proving the Adequacy of Station Grounds. 369

Fig. 1. Potential field of ground grid, cempesed of four concentric rings, comparedwith the field of an equivalent hemisp'herical electrode

GROUND TESTERS

Ground testers produce a test current

grid. The potential field close to thegrid is different from that around theequivalent hemisphere, but at a distancegreater than approximately five times theradius of the hemisphere the two fieldsare nearly identical; see Fig. 1.

2. The auxiliary grounds should belocated remote from any extensivegrounded conductor which may providea shunt path for the test current. Con­ductors in telephone cables can be usedas test leads between the auxiliarygrounds and the station ground onlyif their metallic sheaths (or shields) arenot grounded in the station area withina distance at least equal to lOr fromthe center. This sometimes requires iso­lation of the sheath from the stationground and from other grounds alongits route.

3. If possible, the two auxiliarygrounds should be so located that theangle between the two lines, drawn fromthe center of the station to the auxiliarygrounds, exceeds 90 degrees. If this can­not be achieved, the auxiliary groundsmav be located in the same direction, butin this case the mutual impedance be­tween the test leads must be taken intoaccount. Ground testers are not signifi­cantly affected by the inductive compon­ent of the mutual impedance, but they areaffected by the resistive component, whichis a function of frequency. If the leadsare parallel, the wattmeter-ammeter andnot the voltmeter-ammeter method mustbe used.

4. If one circuit of a double-circuitline is used for the connection between anyone of the auxiliary grounds and thestation, circulating currents in the secondcircuit may cause errors if the secondcircuit is in service and, consequently,connected to grounded transformers atboth ends. For this reason the live cir­cuit should be several times longer thanthe test circuit.

5. Induction from external circuitsshould be avoided, if possible, irrespectiveof the test method used. For this reasonthe direction of the potential probeshould be such that power lines carryinglarge currents do not cause excessive inter­ference. Induction in the loop betweenthe station and the current probe does notaffect the measurement. Consequently,if the possibility of .a choice exists, thelead subject to less induction shouldalways be the potential lead and the onesubject to higher induction should be thecurrent lead.

(1)

lor9rsr

( 1 +~) - (~+~)Xab Xsa xs~

where r is the "equivalent radius" of thestation, X sa is the distance between thecenter of the station and the first auxiliaryground (a)'; XSb is the distance betweenthe center of the station and the secondauxiliary ground (b); Xab is the distancebetween the two auxiliary grounds. Theexplanation of these recommendations isgiven in Appendix I. It is shown thatthe ground resistance of a circular gridcomposed of a mesh of buried conductorsis very nearly equal to the ground resist­ance of a hemispherical electrode with aradius equal to one half of that of the

1

tion to the test result. The followingprocedure is recommended:(a). Determine the approximate area of thestation, and determine the radius of a circlehaving the same area. One half of thisradius is defined as the "equivalent radius"of the station r. The distance of the auxil­iary grounds from the center of the stationand from each other should not be less than5r.(b). To correct the error caused by in­sufficient probe spacing, the measuredground resistance should be multiplied bythe following factor:

r ar 3 r 4 r 5 r 6 r 7 rDISTANCE FROM CENTRE OF STATION

or2r

" lJl tI I \ "It

I :<> :1--'- d= .h.: : 50I I

1[8]

I

GROUND GRID ANDEQUIVALENT HEMISPHERE

I,,' <, , I

l 1,/ ':I \

: r'< },\ zr /~, /I ' <, ---",,/ !II

I II I

I I

I II :II I

I

I' -1- -- -- - -I \1 II I[Al~~

n ~ ~ 1/\ ~ ~ POTENTIAL AROUND GROUND GRIDjAND ~t--

II~ \l\J r-, / \~\ V!\ AROUND EQUIVALENT HEMISPHERE

/ I \\, \

7 ,\ \ CURVE I,,

V- GROUND GRID, ,.>, , i\..~,.

....."'-<,~'-- CURVE 2

....~

~EQUIVALENT HEMISPHERE~

I I I I I I Ir-__~~~I='=:t:==-:.,~

I I I I I I Io

100

(fJI-.J0>~

.J 50-ci=zLLl

bQ.

LOCATION OF AUXILIARY GROUNDS

1. The auxiliary grounds should belocated at sufficiently large distancesfrom the station ground and from eachother so as not to cause excessive errorsthrough mutual coupling. In the case oflarge stations the required distances be­come very large, making it often morepractical to locate the auxiliary groundsat a smaller distance and apply a correc-

h =-ll.I I I __ L L_ I I I.- 300~',;;m)}}~

Measurement of Ground Impedance

The most suitable method of measuringthe ground impedance of a station in­corporates the use of two auxiliarygrounds. This method consists of cir­culating a current between the stationground to be tested and one auxiliaryground (the current probe), and of meas­uring the voltage rise caused by this cur­rent, between the station ground and theother auxiliary ground (the potentialprobe). The quotient of the voltage riseand .of the ground current is then found,either directly, as in the case of earthtesters, or by calculation) as in the caseof the voltmeter-ammeter or wattmeter­ammeter methods. In both cases theproper location of the auxiliary groundsrequires special attention.

370 Elek-Proving the Adequacy of Station Grounds NOVEMBER 1962

60 CYCLEPOWER SUPPLY

~ 60 CYCLE= POWER SUPPLY

CURRENTPROBE

VOLTMETER

STATION GROUNDPOTENTIAL

PROBE CURRENTPROBE

VOLTMETER

POTENTIALPROBE

Fig. 2. Method of ground impedance measurement with a-volt­meter and an ammeter

Fig. 3. Method of ground resistance measurement with a wattmeterand an ammeter. (A voltmeter is convenient but not strictly necessary)

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the frequency of which differs from thesystem frequency. They cannot be usedtherefore to measure the impedance of astation ground which has a significantreactive component. Also, they onlyindicate the resistive component of theimpedance, which has no significancewithout the reactive component. Theindication of ground testers having ahand-cranked generator changes withcranking speed when measuring groundswith reactance.

Where the ground impedance to bemeasured is a pure resistance, groundtesters can usually be used, but a seriouserror may be introduced in the case of low­resistance grounds associated with largeprobe distances, especially if the testleads are parallel. This error is caused bythe frequency-dependent resistance of thecurrent path in the ground remote fromthe field of the station, as expressed by thereal component of the Carson formula. 2

Where the leads are parallel this com­ponent is equal to 0.0016.f ohm per mile(where! is the frequency), or to 0.1 ohmper mile at 60 cycles, but is unknown atthe frequencies produced by groundtesters. Where the leads run in oppositedirections, the error is smaller anddifficult to calculate. The possible effectof this type of error should be consideredwhere large probe distances are used inconnection with ground testers.

In ground testers each of the two main(potential and current) circuits has twoterminals; sometimes all four are accessi­ble, sometimes two terminals are inter­connected within the instrument and thetester has only three external terminals.In the latter case, the common terminalis connected to the station ground with asingle connection and the resistance ofthis connection is measured together withthe station ground resistance. Wherethe station ground resistance is in theorder of 0.1 ohm, neglect of the resistanceof the connecting lead may result in aserious error. In 4-terminal instruments,

this error can be prevented by connectingthe potential and current circuits to thestation ground at different points.

It is sometimes impossible to use aground tester, either because the resist­ance to be measured is too low comparedwith the range of the instrument, or be­cause interfering 60-cycle stray potentialscause excessive vibration, preventing anyacceptable reading.

THE VOLTMETER-AMMETER METHOD

The voltmeter-ammeter method canbe used for ground impedance measure­ments only if the angle between the gen­eral direction of the test leads exceeds90 degrees. The circuit diagram of thismethod is shown in Fig. 2. The prin­ciple is essentially the same as that oftests performed with a ground tester.The test current is circulated through anammeter, through the station ground, andthrough the current probe, while the volt­age rise of the station is measured with avoltmeter between the station ground andthe potential probe. The ground resist­ance or ground impedance (R) of thestation is equal to the measured voltagerise (E t ) divided by the test current (It)and multiplied by the correction factorwhich compensates for the error causedby the insufficient distance of the auxil­iary grounds (equation 1).

E tR=K1 -It

Provisions have to be made to supply thetest current from a 60-cycle low-voltagesource, either directly or through a trans­former.

The measurement of ground impedancewith the voltmeter-ammeter method isaffected by external 60-cycle currentsand voltages (such as stray currentsflowing through the station ground andvoltages caused by induction from parallellines). While in the case of ground testersthese stray effects may only cause ex­cessive vibration, they can cause serious

errors when the voltmeter-ammetermethod is used. It is, however, possibleto eliminate this error by the followingprocedure:

1. Connect the current lead to the stationground through the ammeter. Connect thepotential lead to the station ground throughthe voltmeter. Take a reading on theammeter 10 and on the voltmeter Eo beforeapplying the test current (position 0 in Fig.2).2. Insert the 60-cycle current source in thecurrent lead, as shown in Fig. 2 (position 1).Apply the test current and take a reading onthe ammeter and on the voltmeter (Ii andE 1 respectively).

3. Reverse the polarity of the test current(position 2) and take a third reading on theammeter and on the voltmeter (12 and E 2respectively).

The magnitude of the test current It canbe calculated from the following formula:

~I12+I22

1 t = ----102

2

The voltage rise E, caused by the testcurrent can be calculated from the follow­ing formula:

~E12+E22

E t = ----Eo!2

The ground resistance of the station canbe found by substituting It and E, inequation 2. The proof for equations 3and 4 is given in Appendix II.

THE WATTMETER-AMMETER METHOD

If the leads between the station and thetwo auxiliary grounds are, partly orthroughout their full lengths, runningparallel to each other, a modified versionof the voltmeter-ammeter method may beused. This consists of replacing the volt­meter with a wattmeter as shown in Fig.3. The purpose of the wattmeter is toresolve the potential rise of the stationground into two components: the one inphase with the test current and the onein quadrature with the test current. If

NOVEMBER 1962 Elek-Proving the Adequacy of Station Grounds 371

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HIGH PROBE RESISTANCE

If the ground resistance of the potentialprobe is relatively high compared withthe resistance of the voltmeter (or of thepotential coil of the wattmeter or ohm­meter), all readings except current read­ings have to be multiplied by the followingcorrection factor:

ungrounded single-phase 60-cycle powersupply since both terminals must begrounded alternately in the station. Ifsuch power supply is not available: agrounded 3-phase power supply can beused instead. In this case instead ofthree readings, as described in the pre­vious sections: four readings are takenand, instead of reversing the current be­tween steps 2 and 3, the test current isdrawn in turn from the three phases.

Thus, if lo, Eo, and Po are the readingsbefore applying the test current, there willnow be three readings instead of two foreach quantity measured with the testcurrent flowing: It, 12, 13 ; EI, E2, E3 ; andPI, P2, P3• Equations 3, 4, and 7 will bemodified as follows:

~I) 2+122+13 2

I t = 102

3

HIGHER HARMONICS

The voltages and currents caused byextraneous effects (induction and straycurrents) during ground resistance tests,and measured before or after the applica­tion of the test current, very often containharmonics, especially third harmonics. inconsiderable amounts. Tests performedwith a ground tester are not affected bythese. In the case of the voltmeter­ammeter or wattmeter-ammeter methodit can be shown that if the source supply­ing the test current produces a reasonablysinusoidal 60-cycle voltage Ei, the har­monic content. of which is not higber thannormally encountered in any power sys­tem, no error of any significance will becaused. If the instruments are of theelectrodynamic type, the results w ill berigorously correct) even if Is, Eo or Po arepure higher harmonics. If the voltmeterand the ammeter is a moving-coil rectifier­type instrument, some error will be intro­duced where the harmonics dominate.

K2= Rmeter+Rprobe

Rmeter

In the case of ground testers, in whichsome means of compensation are in-

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The power P e caused by the test currentand by the voltage rise can be calculatedfrom the following formula:

Pl+P2Pt=---Po2

caused by extraneous grounds and/or bymutual induction between the test circuitscannot be distinguished one from theother, parallel test leads should not beused if the ground impedance of thestation is suspected to contain a sig­nificant reactive component.

To eliminate the effects of extraneous60-cycle interference, by induction or bystray currents flowing through the sta­tion ground, the following procedure isrecommended:

1. Connect the current lead to the stationthrough the ammeter and the current coil ofthe wattmeter. Connect the potential leadto the station ground through the potentialcoil of the wattmeter. Take a reading onthe ammeter 10 and on the wattmeter Po be­fore applying the test current (position 0 inFig. 3).

2. Insert the 60-cycle current-source in thecurrent lead as shown in Fig. 3 (position 1).Apply the test current and take a readingon the ammeter and on the wattmeter (IIand PI respectively). Note the sign of thewattmeter reading.

3. Reverse the polarity of the test current(position 2), and take a third reading on theammeter and on the wattmeter (12 and P 2respectively).

The magnitude of the test current Itcan again be calculated from the followingformula:

~I12+122

I t = ----16-2

2

GROUNDED 3-PHASE POWER SUPPLY

It is obvious from Figs. 3 and 4 that themethod of current reversal requires an

The station ground resistance can befound by substituting It and P t in equa­tion 6.

In the procedure described, it may benecessary to change the polarity of thewattmeter between readings. In thiscase, attention must be directed to ensurethat the correct signs appear before PI,P2, and Po in equation 7. The proof forequations 6 to 8 is given in Appendix II.

Where the test leads are in parallel, 0.1ohm must be subtracted from the com­puted resistance for every mile be­tween the station and the current or thepotential probe, whichever is the closer.This is derived from Carson's formula,and is the 60-cycle resistance of theground path parallel to the test leads,and is not part of the ground resistanceof the station.

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REST OF THESYSTEM~

~

REST OF THE

SYSTEM-+-----

IfTOTAL FAULT

CURRENT

~ it ~ ttI gI \,,~~~~~= =--~~~===~.=.-=-~==-----.t:':~--~--~:'~)CURRENT THROUGH STATION GROUND

[AlFAULT AT THE STATION

STATIONTRANSFORM ERS

-+t--;r

STATION"TRANSFORMERS

tt Zt

.t

the test current is It and the power meas­ured by the wattmeter is P t , the in-phasecomponent of the voltage can be foundfrom the following formula:

[B]FAULT REMOTE FROM THE STATION

Fig. 4. Distribution of fault current duringfaults inside and outside a station

~tt !! t! ~ttI' I I ~ I I I'

Ig2 \,:~=====~_::./ \,~====-~-----~-~~=':./CURRENT THROUGHSTATION GROUND

PtEd=­

It

The voltage component in quadraturewith the test current is usually caused bymutual induction between the test leads,and would constitute an error if it werenot eliminated. The in-phase componentof the measured voltage Ed can then beassumed to be caused entirely by the sta­tion ground resistance. The groundresistance of the station R can be ex­pressed from equation 5 as follows:

~ CURRENT SUPPLIED BY THE TRANS­FORMERS IN THE STATION

-+----- CURRENT SUPPLIED BY THE RESTOF THE SYSTEM

K I is the correction factor compensatingfor the error caused by the proximityeffect of the auxiliary grounds (equa­tion 1).

It was mentioned before that, if thestation ground is of large physical size (be­ing, for instance, connected to low-resist­ance extraneous grounds), the measuredvoltage may contain a reactive com­ponent, even in the absence of mutualinduction between the measuring circuits.Reactive voltage components caused byextraneous groundsmust not be elimi­nated, since such grounds do affect thevoltage rise of the station ground duringfaults. Since the reactive components

272 Elek-Proving the Adequacy of Station Grounds NOVEMBER 1962

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other transformers. In this case thecurrent flowingthrough the station groundwill be

MAGNITUDE OF TOUCH VOLTAGES

The magnitude of the highest touchvoltage can be expressed as a percentage

of the total potential rise of the station.This value has been calculated on the basisof reference 3 for some typical groundgrids. It was found that the highesttouch voltage varies between 20 and 30%of the total potential rise, the lower valuebeing valid for large stations, the higherfor small stations. These figures agreevery well with a large number of testresults.

Of course, this percentage figure is onlyvalid for touch voltages within and im­mediately surrounding the station. Ex­traneous conductors may transfer po­tentials to larger distances where thetouch voltage may be higher. This is trueespecially in the case of metallic con­ductors which are, more or less, insulatedfrom ground, such as railway tracks onrock ballast; the potential of such conduc­tors may rise to a value considerably ex­ceeding 20 or 30% of the station potentialrise. It is, however, not practical to basethe general specification of the highesttolerable touch voltage on such speciallocations; abnormally high touch voltagescan be eliminated by local measures,such as for instance isolation of the rail­way tracks, etc.

After estimating the highest touch volt­age in this manner. it must be decidedwhether the value thus obtained is satis­factory or not.

The tolerable limit of touch voltage canbe calculated either on a minimum or on aprobability basis. In the first case allfactors influencing the shock hazard areassumed to be close to their worst possiblevalue. In the second approach, the prob­ability distribution of each factor is deter­mined separately and the probabilities arecompounded.

The company the author is associatedwith will specify a maximum telerabletouch voltage of 1,000 to 1,500 volts inits new grounding guide (otherwise ex­ceptional safety measures would be re­quired) . This corresponds in an averagestation to a maximum total station po­tential rise of 5,000 volts (3,000 volts isrecommended, however, where economi­cally feasible). This specification is basedon a probability study assuming amean body conductance of 1.1 milli­amperes/volt (equivalent to 900 ohms)with a standard deviation of 26%. Thefoot resistance on a 4-inch layer of wetcrushed stone was found from a surveyin a station to vary with equal probabilitybetween 3,000 and 12,000 ohms. Com­pared with this range, the body resistancebecomes almost insignificant. The prob­ability of fault duration was computedfrom a survey of fault records on a 115­and 230-kv system; it was found that

(13)

(14)

(15)

where Zext is the impedance of the ex­traneous grounds, as seen from the station,and Rgr id is the measured ground resist­ance (or ground impedance) of the grid.If the extraneous conductors are multi­grounded conductors (such as skywires orneutral conductors), then

Z _ ~zscriesext-

gshunt

where Zseries is the series .impedance of theconductor in ohms per mile and gshunt is itsshunt conductance in mhos per mile. Insome cases, as for example in small dis­tributing stations, the ground resistanceof the station grid becomes unimportantand the potential rise of the station will bealmost solely determined by the imped­ance of the extraneous grounds (distribu­tion neutrals).

Zo192=-1/Zt

To calculate the highest potential rise,the type of fault which produces thelarger current through the station groundshould be considered. It is obvious fromequations 12 and 13 that if Z t>2Zo, it isthe fault in the station which producesthe larger current through the stationground and if Z t<2Zo} it is the faultoutside.

Skywires carry part of the fault currentand, therefore, I g! will actually be smallerthan calculated. This effect can, how­ever, be considered as a safety margin.

In stations with extraneous grounds thecurrent flowing through the stationground consists of two parts: the partflowing through the station ground gridand the part flowing through the ex­traneous grounds. It is usually recom­mended that the ground impedance of theground grid be measured after discon­nection of all those extraneous groundsthat can be disconnected. Hence, thestation ground current, as calculated fromequations 12 or 13, must be furtherdivided into two parts. To obtain thepotential rise of the station ground, onlythe part flowing through the ground griditself should be multiplied by the meas­ured ground resistance. This part of thecurrent can be calculated from the fol­lowing complex equation:

z.;19 r id = 19

Zext+R.grid

Evaluation of Test Results

When ground resistance measurementsare performed, it should never be £or­g-otten that the ultimate purpose of themeasurements is to determine potentialdifferences, such as touch voltages andovervoltages arising in the communica­tion plant. These potentials have to becalculated, taking into account severalfactors, of which the ground resistance(or ground impedance) is only one.

corporated, their instruction manual hasto be consulted.

MAGNITUDE OF FAULT CURRENTS

The potential rise of the station groundwith respect to a remote ground (thetotal potential rise of the station) is theproduct of the ground impedance and thefault current flowing through the stationground. It is important to emphasizethat the current flowing through the sta­tion ground is very seldom equal to thefull ground fault current. In an inter­connected system there are almost al­ways several connections between trans­former neutrals and the ground, all ofwhich will carry a portion of the total faultcurrent.

Fig. 4 illustrates the difference betweena fault in the station and a fault outsidethe station. In the first case, as shown inFig. 4(A), the current supplied by thegrounded transformers in the station re­turns to the transformer neutrals directlythrough metallic conductors withoutentering the ground. Hence, this portionof the fault current does not cause anypotential rise. In parallel with the sta­tion transformers, however, are all theother grounded transformers in the sys­tem. The portion of the fault currentflowing through these transformers hasto flow through the station ground and is,therefore, the current which has to be con­sidered in the calculation of the potentialrise of the station ground.

If I I is the total fault current, Zo is thezero-sequence impedance of the wholesystem, as seen from the station, and Z t isthe zero-sequence impedance of the sta­tion transformers only, the current flow­ing through the station ground will be

lUI = (1-;:)11

If the fault occurs outside the station,as shown in Fig. 4(B), only the portionsupplied by the station transformers willflow through the station ground: theportion supplied by all the other trans­formers in the system will flow away fromthe fault location in the direction of these

NOVEMBER 1962 Elek-Proving the Adequacy of Station Grounds 373

PROTECTION OF COMMUNICATION

CIRCUITS

The voltage appearing between com­munication circuits and the station groundduring faults can be considered to beapproximately equal to the total potentialrise of the station. Of course, any addi­tional potential caused by induction fromparallellines must be added (vectorially)to the ground potential rise. Theknowledge of the total potential rise isnecessary for the selection of the voltagerating of neutralizing and insulating trans­formers. It is also necessary where noprotection is applied and the insulation ofthe wires is expected to withstand thepotential rise. The value of the potential

67% of the faults last less than 12 cycles,25% between 12 cycles and 1 second, 7%between 1 and 2 seconds, and approxi­mately 1% between 2 and 3 seconds.Compounding all probabilities, it can beshown that the probability of death is 1%in the case of a touch voltage of 1,000volts, and 2.5% in the case of 1,500 volts.This compares unfavorably with theassumption of 0.5% made by the AlEEWorking Group on Voltage Gradients. 3

However, it should be considered that dur­ing the survey the ground was found to bewet only once out of every six cases; thus,the probabilities are actually reduced toone sixth of the given values. It seemsreasonable to allow a higher probabilityin small stations than in large stationsand, hence, due to the higher gradientsin small stations, a value of 5,000 voltsis obtained for the tolerable potentialrise in every case.

I t should be noted that the ratio of thetouch voltage to the total potential riseis sometimes considerably less than 20%.This is particularly true in stratified soilswhere a shallow low-resistivity overburdencovers high-resistivity rock. The groundresistance of such stations is high be­cause of the high-resistance subsoil.The potential gradients are, however,determined by the resistivity of thesurface. A typical case has been cal­culated on the basis of reference 4; thetouch voltage was determined at a dis­tance of 1 meter outside a single buried

ring on a 2-layer soil. The results areshown as a function of the ratio of thetwo resistivities in Fig. 5. It can be seenthat while the ground resistance risessteeply with an increasing resistivity ofthe subsoil, the touch voltage remains al­most constant. Hence, the percentagevalue of the touch voltage, which is 30%in the case of a uniform soil, drops to 6%for a resistivity ratio of 10, and to 1% fora ratio of 100. In nonuniform soils it istherefore often not necessary to lower theground resistance, at great expense, onlyon account of potential gradients: in suchcases, a direct measurement of the gra­dients may show that a total potentialrise exceeding the normal limit could bepermitted.

rise is not important where protector gapsare applied, since these will break: down inany case.

The incremental cost of the protectionof conununication circuits against higherpotentials is usually less than the costof improving the ground. It is good prac­tice, therefore, to determine the require­ments of the station ground on the basis ofprotection against touch voltages andinstall a suitable protection for the com­munication circuits in compliance with thestation potential rise thus obtained.

GROUNDING OF TRANSFORMER NEUTRALS

The ground resistance of the stationmust be sufficiently low to permit effec­tive grounding of systems with operatingvoltages of 115 kv or higher, providedthat the system is designed for a reducedinsulation level, as customary in high­voltage systems.

The ground resistance of the stationmust not be higher than the leakagereactance of all transformers in the sta­tion. If the station ground resistance isless than 2 ohms, this will assure propergrounding for transformer capacities upto 600 mva at 115 kv and 2,500 mva at230 kv, since the leakage reactance of atransformer bank (assuming a per-unitreactance of 0.1) is 2.2 ohms in the firstcase and 2.1 ohms in the second.

Very Large Station Grounds

Because of the reactance of largeground grids (and of ground grids con­nected to an extensive network of under­ground conductors), their ground imped­ance can only be measured with 60-cyclecurrents. Since large ground grids re­quire large probe distances, 60-cyc1estraypotentials will usually be high. To over­come these, a large test current has to beused, and in order to drive that currentover the high impedance of a large probedistance a relatively large power supply isrequired. In addition, transmission lineshave usually to be taken out of service toprovide test leads of sufficient lengths....'-\11 these requirements make tests of thistype rather expensive.

Where underground pipes and cor.due­tors cannot be separated from the stationground, it is not possible to specify therequired probe distance. Furthermore,even if the ground impedance could bemeasured, this value would be almost use­less, since the assumption that the highesttouch voltage is approximately 20% ofthe total potential rise would probably beno longer valid for stations with verycomplex grounds and with undergroundextensions in several directions.

For these reasons. in such stations it

Fig. 5. Touch volt­ageand potential riseof station on nonuni­form soil. Singlering with dimensionsshown in insert

i-Current densityin amperes per footpt-Resistivity of topsoil in ohm-metersps-Resistivity ofsubsoi lin ohm-meters(To obtain actualvoltages, multiplyordinates by i XPt)

3 4 5 6 8 10 20 30 40 60 100

h RATIO OF SUB-SOIL RESISTIVITY TO TOP-SOIL RESISTIVITYfT

I v,l'/

/VV

~\~v

~~er:¢~ ~ -

~'\~ 3.3' -~~~ : r-

-. X ~o~i '~.~I>< I ~"''' 053"~~"[~~ ,"""'~ ~ ~"",,',,"" ~ f-

/ Clt-~)-::"O(" _~~,~101\1 -iG~

~ -is I~~ ~I I 'v~l ~J?:Af( "~~

~/.s ~AJ

TouCH VOLTAGEe ~~

"- ...........-~ "",<,

<,r-,I ,

300

at' g 200lL -;; 4>1(1)... <J..J ­o l&J> en 100z ~- ..J 80(l) oC(

t= 60w Zt/) W 50

~ ~ 40

~ ~ 30Z i=LaJ oC(... t- 20~ en~ ~i= b1< C)t- ~ 10en ZC l&J 8Z 041( ffi 6

~ ~ 5UJ ~ 4

~ ~... 0..J >o ::E:> Uo a:> ...o...

374 Elek-Proving the Adequacy of Station Grounds NOVEMBER 1962

form tests at all, provided that voltagegradients are not expected to be unusuallyhigh.

(21)pI

V(x)=­21rx

2IoEo*= ItE t* = P t (23)

This is the proof of equation 7. The equa-

P1+ P 2_P O

2(10+I t)( E t*+E,*) +(10 ­

] t)(Eo*-Et*)

The quantity to be determined is

Pt=ItEt*The following equation can be written:

(Eo+ Et)(Eo*+Et*)+(Eo­E t)( Eo*- E t*)

Equation 4 is thus proved. The stray cur­rent can be eliminated in an analogous way,resulting in equation 3.

In the wattmeter method the power ismeasured instead of the voltage. If Po is the(complex) power measured with no testcurrent, PI is the power measured with thetest current II flowing in one direction, P 2 isthe same after current reversal, and I o is thestray current

The stray voltage Eo can be separatedfrom the voltage E, caused by the test cur­rent by performing three measurements:one with no test current and the other twowith test current, but reversing the directionof the current the second time. The volt­age measured with the test current flowingis then E 1 =(Eo+Et); and after current re­versal it is E 2= (Eo- E t ) . All quantities arecomplex numbers. It can therefore bewritten that

2

EoEo*=EtEt*=jEtI2 (22)

Po = loEo*

PI = (Io+It)(Eo+Et)*

P 2 = (10 - I t )( Eo- E t)*

Appendix II. Elimination ofStray Voltages and Currents

At a distance equal to 5r the differencebetween the two curves is less than 4%.Since the configuration of four rings can beregarded as a good approximation of anytypical ground grid, it can be concluded thatat a sufficient distance the field of a stationground can be represented by the field of anequivalent hemisphere, the radius of whichis equal to one half of the average radiusof the grid.

In reference 5 an equation is given tocalculate the error caused by insufficientprobe distances for a hemispherical electrode.The correction factor K 1 used in this paperwas derived from this equation.

as shown in Fig. l(A). The field of therings (curve 1) is plotted according to equa­tion 16, whereas the field of the hemi­sphere (curve 2) is plotted according to thefollowing equation:

(20)

(19)

(18)

(16)P2:4 2rn (2vrnx)V(x)=- in--K--1r rn+x rn+x

n=l

pIV(J=-­

3.15r4

pIVh=­

21rr

Obviously, if Vg = Vh, then r ~r4/2Hence, a hemisphere with a radius equal toone half of the radius of a grid, consistingof four concentric rings, has approximatelythe same ground resistance as the grid.The two potential fields are also very similar

Appendix I. Potential Fields ofConcentric Rings and of an

Equivalent Hemisphere

Similar equations can be written for Veri)V(r2), and Vera). All four potentials mustbe equal to Vg • the potential of the groundgrid, whereas the sum of the currents, dis­sipated from the rings, must be equal tothe total ground fault current I.

where p is in ohm-meters and r4is in meters.On the other hand, the potential (Vh) of ahemispherical electrode with a radius equa1to r can be expressed as follows:

[2:' '. a-, (2V rnx) . 8rn. ]1-n -- K --- +1-4 In --===

rn+x rn+x Vd.hn=l

(1'7)

A. ground grid is assumed to be composedof four concentric rings, the spacing of whichis determined so as to obtain a constantdensity of conductor material. 4 The rela­tive dimensions chosen for the example areshown in Fig. I(B). Deviations from thechosen proportions within practical limitswould, however, not affect the results ap­preciably.

According to reference 4, the potentialVex) in volts at a distance x from the centerof the grid can be expressed with good ap­proximation as

This results in five unknowns and fiveequations, from which i I .... . i4 and Vgcan be determined. After substituting thedimensions shown in Fig. I( B) the followingexpression is obtained for the potential ofthe ground grid:

where p is the soil resistivity in ohm-meters,in is the current dissipated per-unit lengthfrom ring "n' in amperes per meter, and r« isthe radius of ring "n:" K(k) is the completeelliptic integral of the first kind of a variablek.

The potential of the ground grid itselfcan be expressed, for example, by expressingthe potential V(r4) on the surface of ring4.

is more convenient either not to performtests at all, or if these are believed to benecessary, to measure touch voltagesdirectly. Locations where high touchvoltages can be suspected are the follow­ing:

1. The periphery of the station groundgrid, especially at the corners.

2. Exposed metal parts which arerelatively far from buried ground busses.

3. Isolated structures far from denselybuilt-up parts of the station, but electri­cally connected to the ground grid (suchas nearby transmission towers, railwaytracks, fire hydrants, etc.).

The method of potential gradient(touch voltage) measurements is essen­tially the same as that of ground resist­ance measurements with the voltmeter­ammeter method. A current is circulatedbetween the station ground and a remoteprobe. The touch voltage is then meas­ured at various locations by connecting avoltmeter between a structure and a smallpotential probe at a distance of approxi­mately 3 feet from the structure. Sincethe probe must be small, its resistance willbe high. It is expedient, therefore, to usea vacuum tube voltmeter. Measure­ments with an earth tester may lead tolarge errors, due both to reactive com­ponents in the ground impedance and tothe low internal resistance of the instru­ment as compared with the probe resist­ance. The requirements of the currentprobe are the same as before.

The measurements may be performedin a similar way as the ground resistancemeasurements described before. Equa­tions 3 and 4 can be used without change,except that E t should be interpreted astouch voltage.

To find the potential difference betweenthe station ground and communicationcircuits, direct measurements can be per­formed in the same way as for touch volt­ages. The voltmeter should be connectedbetween the station ground and the tele­phone wire, the remote end of which isgrounded (usually in an exchange or inanother station). It is convenient to dothe measurement on the same circuit forwhich protection is required. Whereneutralizing or insulating transformers areconsidered, the voltmeter can be insertedin the circuit at the same location wherethe transformer is to be installed.

Regarding the protection of communi­cation circuits, it is practical to estimatein advance the maximum possible savingwhich could be expected from a test.Sometimes, the incremental cost of anover-conservative design may be lesscostly than the test itself; in such cases.it may be more economical not to per-

NOVEMBER 1962 Elek-s-Prooing the Adequacy of Station Grounds 375

tions for a 3-phase power supply (equa­tions 8, 9, and 10) can be proved in a similarmanner.

References

1. PROPOSED RECOMMENDED GUIDE FOR MEASUR­ING GROUND RESISTANCE AND POTENTIAL GRA­DIENTS IN THE EARTH. AIEE Special PublicationNo. 81, July, 1960.

2. WAVE PROPAGATION IN OVERHEAD WIRES WITHGROUND RETURN, J. R. Carson. Bell System Tech­nical Journal, vol. 5, Oct. 1926, pp. 539-55.

3. VOLTAGE GRADIENTS THROUGH THE GROUNDUNDER FAULT CONDITIONS, AlEE Committee Re­port. AlEE Transactions, pt. III (Power Appara­tus and Systems), vol. 78, Oct. 1958, pp. 669-85.

4. EFFICIENCY OF GROUNDING GRIDS WITHNONUNIFORM SOIL, J. Zaborszky. Ibid., vol. 75,Dec. 1955, pp. 1230-33.

5. SOME OF 1 HE FUNDAMENTAL ASPECTS OFGROUND RESISTANCE MEASUREMENTS, E. B. Curdts.Ibid., pt. I (Communication and Electronics), vol.77, Nov. 1958, pp. 760-66.

-----+----

Discussion

C. A. Duke (Tennessee Valley Authority,Chattanooga, Tenn.): The author hasprovided a well-written paper on a timely

subject, which is assuming importance asground fault currents reach higher andhigher values.

We thoroughly agree with all the state­ments in the paper except those on instru­mentation. Particularly close agreement isemphasized on the points of the ultimatepurpose of determining potentials that mayexist and of the ground impedance's havingan appreciable reactive component.

We do differ on some points of instru­mentation but not on theory. We havebeen particularly interested in makingmeasurements! on large station groundswhere the author states that it is "eithernot possible or not worthwhile to measurethe ground resistance of the station."Our experience has been that there is neveran impossible case of measurement. Also,we have found that the impedance of astation ground cannot always be predictedand should be measured by a practicalmethod which gives the information whichthis author shows is of great importance.

REFERENCE

1. THE TECHNIQUE AND INSTRUMENTATION OFLow-IMPEDANCE GROUND MEASUREMENT, C. A.Duke, L. E. Smith. AlEE Transactions, pt I.(Communication and Electronics) vol. 77, Nov.1958, pp. 767-70.

A. Elek: I agree with Mr. Duke that thereis never an impossible case of measure­ment, provided that the quantity to bemeasured is well-defined. Thus, for ex-

ample, the current driven through thestation ground during a test and the poten­tial difference between the station groundand a zero-potential reference point arewell-defined and can be measured. Thedifficulty arises when one attempts toderive a single figure from these, call it"ground resistance," and to use this figure todecide whether the station ground is ade­quate or not. Considering, for example, astation within the limits of a city, inter­connected with several other stations inclose vicinity through a maze of under­ground cables, waterpipes, etc., the testresults will vary to a large extent with thereturn path of the test current and withthe direction of the zero-potential referencepoint, even if both auxiliary electrodes areremote. In such case no single value ofground resistance can be established.

What I really wished to emphasize inthe paper was the fact that since it is thepotential difference between the stationground and certain reference points inwhich we are normally interested, there isno reason to increase our difficulties by in­troducing such a vaguely defined additionalquantity as the ground resistance, whichhas to be derived from the measurementof a potential difference only to be used todetermine another potential difference.In the example mentioned, it is much simplerto measure the potential differences inquestion directly and not to attempt toattach any single value of ground resistanceto the station.

376 Elek-Proving the Adequacy of Station Grounds NOVEMBER 1962