# DIFFERENTIAL VOLTAGE RELAYS

TY PUSPVDllCPVDllD

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-t-GENERAL@ ELECTRIC

POWER SYSTEMS MANAGEMENT DEPARTMENT

P H I L A D E L P H I A , P A .

, GER-1~70. Differential Voltage Relays Type PM

CASE

OUTER BLOCK J RESET BUTTON

ICONNECTING PLUG

INNER BLOCK

GEH-1770 Differential Voltage Relays Type PVD

Pig. 2 (K-&X795-5 Sn.1) Volt-Ampere Characteristic of Four Series Thyrite Disks in the T?y?e PVDEC Relay

~’ two stacks of four Thyrite disks, connected inparallel. It is intended for application in the fewspecial instances where 10 ampere CTs are neces--Y.

Both operating units have a single circuit-closing contact. These contacts and the contact ofthe seal-in unit are all connected parallel.

SEAL-IN UNIT

A seal-in unit is mounted in the upper leftcorner of the relay (see Fig. 8). This unit has itscoil in series and its contacts in parallel with themain contacts, so that, when the main contactsclose, the unit operates and seals tn. When theseal-in unit picks up, it raises a target into view.The target latches up and remains exposed until itis released by a manual operation of the reset button,which is located at the lower left corner of thecover.

CASE

The case is suitable for either surface orsemi-flush panel mounting and an assortment ofhardware is provided for either mounting. Therover attaches to the case and also carries the resetmechanism when one is required. Each coverscrew has provision for a sealing wire.

The case has studs or screw connections atthe bottom only for the external connections. Theelectrical connections between the relay units and

the case studs are made through spring backed 2 3:contact fingers mounted in stationary molded innerand outer blocks bebreen which nests a removableconnecting plug which completes the circuits. Theouter blocks, attached to the case, have the studsfor the external connections, and the inner blockshave the terminals for the’ internal connections.

The relay mechanism is mounted in a steelframswork called the cradle and is a complete unitwith all leads being terminated at the inner block.This cradle is held firmly in the case with a latchat the top and the bottom and by a guide pin at theback of the case. The cases and cradles are soconstructed that the relay cannot be inserted in thecase upside down. The connecting plug, besidesmaking the electrical connections between the re-spective blocks of the cradle and case, also locksthe latch in place. The cover, which is fastenedto the case by thumbscrews, holds the connectingplug in place.

To draw out the relay unit the cover Is firstremoved, and the plug drawn out. Shorting bars areprovided in the case to short the current trans-former circuits. The latches are then released,and the relay unit can be easily drawn out. Toreplace the relay unit, the reverse order ts followed.

A separate testing plug can be inserted in placeof the connecting plug to test the relay in place onthe panel either from its own source of current andvoltage, or from other sources. Gr, the relay unitcan be drawn out and replaced by another which hasbeen tested in the laboratory.

SHORT F,“GERS

Fig. j (K-6375823-6)- Inted COIYleCtiOI1Sfor type P-&MC Relay (Fmnt V-i-1

INSTALLATION

I&CATION AND MOUNTING

The relay should bs mounted on a verticalsurface in a location reasonable free from ex-cessive heat, moisture, dust and vibration. Therelay case may be grounded, tf desired, Using atleast No. 12 Bk S gage copper wire. The outline andpanel drilling diagram for the Type PVD relays isshown in Fig. 14.

CONNECTIONS

The internal connection diagrams for the TypePVDllC and Type PVDllD relays are shown inFigs. 3 and 4, respectively. The elemenmrydfagramof the external connections for a typical applicationis shown in Fig. 5.

Note in Fig. 5, that when the relay is installed,a connecting jumper should be placed betweenterminals 4 and 5, and that termtnals 5 and 6 arethen connected across the differential junctionpointsA and B of the several CTs. A shorting bar isprovided between 5 and 6 so that if the connection~~~ui~w,~no~~~p~e~i~~a~, the differential

The midpoint between the Thyrtteg,cLyiunit 878 is connected to terminal 3.

Fig. 4 (K-637582i2) IIerw.l Cjmectio;lsfor Type PVDllD Reley (Front Vie.J)

it possible to test or calibrate unit 87Ff without thenecessity of passing high current through the Thyrib+It also makes it possible to short-out the 87R tollif its operation is not desired.

. The external connections in Fig. 5 indicate thatthe differential junction, points A and B, should belocated in the switchyard. For outdoor installationswhere the distance between the breaker and relaypanel may be great, this may be important smce fheresistance through the fault CT loop may otherwtsebe too large. It is satisfactory tolocate the junctionpoints at the panel providing that .the necessaryrelay setting gives the desired sensibvity.

There should be only one ground connection inthe secondary circuit. When the junction points arelocated in the switchyard, the ground connectionshould be made there rather than at the panel.

‘The voltage ltmittng Thyrite is short-time ratedand to protect it the contacts of the auxiliary relay 88short circuits the differential circuit.

GRR~1770 Differentfal Voltage Relays ‘Qpe PVD

AAlUSTblRNTS

87L UNIT

The’ 8’lL unit can be adjusted at any voltagewithin the 4/l range shown on its calibration plate.Four specific calibration values are shown on theplate, which correspond with four n~kings on thecalibration tube. These markings indicate thepostion of the lower edge of the armature in itsdeenergtaed position, to give the specified pick uppoints.

The vertical position of the armature can bechanged by turning it on the threaded plunger rod.The armature is held in position by an internallocking spring which requires no adjustment. The87L unit, unless otherwise specified on the re-quisition, will be set at the factory tc operate atits minimum pick up voltage. If the unit is to beset at some other point, as determined from Table Ior by calculation, the calibratton marks should beused as a guide in making a rough adjustment andthe test circuit shown in Fig. 6 should then be usedto make an exact setting.

Note in Fig. 6, that when the test plug is ln-serted in the relay, the current transformer sec-ondaries are shorted by means of the link between

fhe outer terminals 5 and 6. The adjustable test!tage is applied across terminals 5 and 6 of the

<lay, that is, across the resonant circuit whichincludes the 67L unit. Since the continuous voltagerating of the resonant circuit is only 150 volts, it is

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Fig. 6 (K-6507935-5) Test COMectionSfor Type ?vDLC and Dye ?v-rLX Relays

recommended that a hand-reset lockout relay beused in the test set-up, if the desired 87L setting isto be above this figure.

The following procedure should be followed inchecking pickup of 8’7L unit. Startwith a test voltageconsiderably higher than the expected operatingpoint. Lower the test voltage by successive smallincrements, closing the test switch at each point.The lockout relay will operate each time, protectingthe resonant circuit. Eventually, a point will bereached where the 8’7L unit will just fail to operate.The preceding voltage value can be taken as the pick-up value of the 8’7L unit with reasonable accuracy.At the point where the 8’7L unit fails to pick up,the test voltage must be removed at once to preventdamage to the relay.

If the 87L unit setting is to be less than the150 volt continuous rating, it will not be necessaryto use the lockout relay. The volt-meter used musthave high internal impedance.

87B UNlT

The operating coil of the 87R unit is tap&$to provide three separate pick up ranges.desired range can be selected by means of links onthe molded base at each side of the coil. Thecalibration plate indicates the correct position oftbe links for each range. For each link arrangc-ment, the pick up current can be varied over a 4/ 1range by means of the armature. The right-handlink is shown-in Fig. 8.

. @EEL1’770 Differential Voltage Relays Type PVD

Assuming that ZR is an infinite impedance,consider the effect of an external fault on one of thefeeders. Each of the CTs in the source circuits wiilproduce the secondary voltage necessary to driveits secondary current, as dictated by the ratio,through its winding and leads, while the CT in thefaulted feeder will produce the secondary voltagenecessary to force the total secondary fault currentthrough its winding and leads. Neglecting the effectof load current, the CTs in unfaulted feeders willproduce no secondary voltage. If all CTs performas desired, there will be zero voltage between thejunction points c and d.

Unfortunately, during fault conditions, CTs donot always perform ideally, since core saturationcan cause a breakdown in CT ratio. Such coresaturation usuallv results from a d-c transient in theprimary fault &rent, and may be aggravated byresidual flux left in the core by a previous fault.

In the example of Fig. ‘7, the worst conditionpossible would be realized if the CT in the faultedfeeder saturated completely, losing all its abilityto produce a voltage in its secondary. The sourceCTs would then have to produce enough voltage toforce their currents through their own windings andthrough the winding and leads of the fault CT, un-assisted. As a result, avoltagewculdappear acrossthe junction points c and d. Since the fault CT isassumed to be completely saturated, this voltage isequal. to the total of the secondary currents from

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Fig. 9 (so00510) T??Je ?rnEC Relay unit in cradle(ZeEr new)

w.the source CTs multiplied by the resistance of thefault CT secondary including leads. This relativelysmall voltage (on the order of 100 volts) is, for thefault magnitude in question, the maximum voltagewhich could possibly appear between points c and dunder any condition. Obviously, the fault CT willnot lose all its ability toproduceanassisting voltage,and the source CTs will tend to saturate. Hence, inpractice, the junction point voltage will be some-thing less than the IR drop mentioned above.

Consider the effect of an internal fault. Asbefore the source CTs will attempt to producesufficient voltage to drive their currents throughthe secondaries of the feeder CTs. In this case,however, the CTs in effect have open-circuitedprimaries and their secondary impedances areaccordingly very high. The voltage appearing across 4c and d will therefore be high (on the order of 1000volts), its RMS value being limited by saturationof the source CTs. Even for a very moderate in-ternal fault, this voltage will be well in excess of thepessimistic maximum external-fault voltage pre-viously mentioned.

Thus, if a voltage relay having a high coil im-pedance were connected across junction points cand d, it could be calibrated so that it would not

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pick up on the highest possible external fault voitage,but would pick up at a voltage less than that causedby the minimum internal fault. Such a relay would

&tbe selective between internal faults and externalfaults or load current.

The preceding paragraphs describe in simplifiedform, the operating principles of the Type PVDhigh-impedance differential-voltage relay. fn prac-tice the relay circuit has been further refined to

secure better operating characteristics. For ex-ample, it is desirable that the differential circuitbe insensitive to the d-c component and to fre-quencies other than the fundamental in the differentialvoltage. Consequently, the high-impedance operatingcoil is fed through a series-resonant circuit tunedto the fundamental frequency. It is desirable thatsuch a relay should have’s wide adjustment range.The resulting change in inductive reactance in theunit itself would disturb the tuning of the resonantcircuit. This is prevented if the operating coil ofthe adjustable high-impedance unit is isolated by afull wave rectifier.

During an internal fault saturation of the sourceCTs, as previously noted, limits the RMS value ofthe secondary voltage. However, crest voltagessufficiently high to overstress insulation may occurif some form of voltage limiter is not provided.The Thyrite resistor stack connected in parallelwith the high-impedance circuit serves thispurpose.

The introduction of the resonant circuit inseries with the high-impedance operating coil slightlyincreases the operating time of this unit. Althoughthis increases (to about 3 to 6 cycles, dependingupon severity of the fault) is not objectionable formoderate faults, it is desirable to shorten the timeon severe faults. This is accomplished by theaddition of an instantaneous overcurrent unit inseries with the Thyrite stack, which is set tooperate on the Thyrite current which flows onsevere faults.

APPLICATION CONSIDERATIONS

GENERALThe Type PM relay can be applied for bus

protection in most cases where bushing-type cur-rent transformers are in use, and in metal-cladequipment where current transformers with toridallywound cores having their windings completely dis-tributed are employed. The following points shouldbe considered or should be known on any proposedType PVD relay application:

1. All CTs in the bus differential circuit musthave the same ratio. Small auxiliary currenttransformers for ratio matching cannotbeused.

2. flel CTs must be operated on their full winding;do not use tap connections unless the

&ding section between the taps used is fullydistributed.

3. Determine both maximum phase fault and maxi-mum ground fault.

4. Determine minimum ground fault.5. Lead resistance from the differential junction

points to the most distant CT.6. Secondary excitation curves of the CTs in-

volved.

Differential Voltage Relays Type PVD GER-17’70

RELAY TYPE

III applications involving bushing CTs, the TypePVDllC relay is used in the great majority of cases.There are two operating ranges available for the8’7L unit, 75/3OOV and 100/4OOV. The 75/3OOVrange is satisfactory for the vast majority of ap-plications; whereas,, the 100/4OOV range is avail-able for the few mstances where the 87L unitsetting, as determined by calculation, is greaterthan 3oov.

The Type PVDllD relay is intended for appli-cation in the few special instances where thecharacteristic Es of the CT exceeds 600 volts, orthe maximum secondary fault current exceeds 100amps. The PVDllD relay has two Thyrite stackspermitting 200 amps maximum. For fault currentsin excess of 200 amps it is recommended that suchapplications be referred to the factory.

DISABLING THE RELAY

If tripping must be prevented because of by-passing a breaker, or other known reasons forfalse tripping, the relay may be burned out bysimply opening its contact circuit. This may beavoided by removing the connection plugs; or, ifexternal means are required, by short circuitingstuds #5 and #6 by different contacts of the sameswitch that is provided for the contact circuit.Opening the contact circuit then serves only toprevent false tripping by manual operation of +herelay contacts.

If the generators feeding the protected busare operated at subnormal frequency, as during aWarIll

studs “pperiod, it is recommended that the Thyrite

studs 3-6) be shorted during this period. Besure to remove the short when normal conditionsare restored.

RELAY SETTINGS

When the Type PVD relay is applied with bush-ing CTs regardless of manufacturer, the correctsettings of the 8’7L and 8’lH units can be determinedby a simple calculation.

The formulae and procedures described in thefollowing paragraphs for determining relay settingsassume that the relay is connected to the full windingof the differentially connected bushing CTs, or totan sections which are fullv distributed on the core.G&eral Electric bushing CTs with tap leads taggedXl, x2, X3? X4 and X5 will have fully distrib-uted tap sectrons. The full winding of any bushingCT as produced by a manufacturer can be assumedto be fuily distributed.

SETTING HIGH-IMPEDANCE UNIT 87L

The secondary winding of a bushing CT, whenevenly distributed around the core, has negligtbleleakage reactance. Consequently, if it is assumedthat an external fault causes complete saturation ofthe CT in the faulted feeder, the current forcedthrough this secondary by the source CTs will beimpeded only by the resistance of the winding and

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GEH-1770 Differential Voltage Relays Type PVD

Fig. 10 (~-6507936-i) Curve Used to Obtain thePerfonrznce Factor of tlz Type PW Relay

leads. The resulting IR drop will thus be the maxi-mum possible voltage which can appear across thedifferential junction for that external fault.

The setting of the high-impedance 87L unit forapplication with any bushing CT can thus be ex-pressed as follows: -

VR IF=2(Rs+RLJT (1)Where:

VR = pickup setting of 8’7L unit

RS = d-c resistance of bushing CT secondary,in@iding leads to housing terminal (at75 C).

RL = resistance of leads from junction pointto most distant CT (one-way for phasefaults, two-way for ground faults).

IF = Msximum external fault current, separatevalues for phase and ground faults.

N = Number of secondary turns (ratio ofbushing CT).

Calculate VR, separately for phase fault withme way lead res&ance for ground faults with twoway lead resistance and~use the higher of the iwovalues of VR so obtained.

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The multiplier 2 is used to provide a reasonablesafety factor. In cases where the maximum ex-ternal fault current is not known, the maximuminterrupting rating of the circuit breaker should beused.

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As previously noted (see PRINCIPLES OFOPERATION), the pessimistic value of the junctionpoint voltage determined by the equation (1) is neverrealized in practice. The CT in the faulted feederwill not saturate to the point where it produces noassisting voltage. Furthermore, the conditionwhichcaused the fault CT core to saturate also tends tosaturate the cores of the source CTs, which resultsin a further decrease in the junctionvoltage.

These effects, of course, are not readily cal-culated. However, extensive tests on bushing CTs 3used in General Electric circuit breakers, undersimulated fault conditions, have resulted in theestablishment of a so-called “performance factor”which can be determined for each application. Theeapression for pick-up voltage for the 8lL unit isthen written:

VR = 2K(RS+RL)+-

where:

K = performance factor.

This performance factor K is not a constantfor a given bushing CT, but varies for each in-stallation depending on the value of (Rs + RL)$-. (p+

’It is readily determined from the curve in Fig. 10which is based on test data. The use of this curveis explained in the sample calculation whichfollows.

MINIMUM FAULT TO TRIP 87LAfter the pick-up setting of 87L has beenestab-

lished for an application, a check should be made todetermine the minimum internal fault current whichwill cause the unit to operate. If this value is lessthan the minimum internal fault current eapected,the relay is suitable for the application.

The following expression can be used:

Imin=(xIe+Ir+I1)N (3)

where:

Imin = minimum internal fault to trip 8lL.

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x = number of breakers connected to the bus(i.e., number of CTs per phase).

Ie = secondary excitation current of the CTat a voltage equal to pickup of 87L.

Ir = current in 87L relay unit at pick-upvoltage.

I1 = current in Thyrite limiter circuit atpick-up voltage.

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N = CT secondary turns (i.e., ratio).

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P

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Mfferential Voltage Relays Type.PVD GEE-1770

FL&% 11 (K-6507923-3) Cunre for Obtaining 87~ Unit Settings (21~ (Rs + RL ) Less Than 350 volts)

N

a-I Fig. 12 (K-65W917-2) Curve for Cbtaining 87'd Unit Setthgs ( 2% (R~ + R, ),mcee&

N

350 volts)

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,. GEB-1770 Differential Voltage Relays Type PVD

The value of Ie is obtained from the secondaryexcitation characteristic curve for the current trans-

i former in use. If the curve is not ava’&ble, but thevalues of ES and IE are known for the bushingCT involved, an excitation curve can be plotted by thetemplate method described in GET-1225. It isundesirable for the pick-up voltage of the 87L unitto be beyond the knee of the excitation curve, asthis results in poor sensitivity due to inconsistenciesbetween CTs and causes large variations in 87Lvoltage.

ES is the CT saturation voltage defined as the

Boint on the excitation curve where the slope is 45 osee Fig. 13). le is the excitation current at

voltage ES.The relay current can be determined from the

resistance of the resonant circuit, assumed to beconstant at 2600 ohms. That is:

I, = V,/2600

The current 11, drawn by the Thyrits limiter,can be obtained from the volt-amperecharacteristicof the Thyrite unit as shown in Fig. 2.

SETTING CURRENT UNIT 87H

The purpose of the 87R unit is to provide in-stantaneous operation (less than two cycles) forextremely severe internal faults. It should thus beset for as low a pickup as possible without the rtsk

- of false operation on an external fault.

During an external fault the Thyrite circuit, forthe worst condition, is subjected to the voltage re-sulting from a fully offset current flowing throughthe resistance of the secondary windtng’and leadsof the saturated fault CT. Even though the d-ccomponent of the offset secondary current will bedecaying, the 87H unit is so fast that the effect ofthe first cycle of offset should be considered.

The correct setting of the 87H unit can be de-termined from one of the curves (Fig. 11 or Fig. 12).The curve in Fig. 11 shows the RMS current whichwill flow in the Type PVDllC relay Thyrite stackwhen a fully-offset voltage is applied. For con-venience, the ordinate is plotted in terms of sym-metrical RMS VOLTS. The abscissa shows theRMS value of current which wouldflow inthe Thyriteif the ordinate voltage were fully offset. The voltage,read on the ordinate, should be the value of VRdetermined from equation (1). Under no circum-stances should the performance factor K be usedin determining the 87H unit setting.

The curve in Fig. 11 assumes that the appliedvoltage is a perfect, fully-offset sine wave, resultingfrom the flow of an offset current through a re-sistance. The pulse of current drawn bythe Thyriteof course subtracts from the offset current. Con-sequently, as the magnitude of Thyrite current

- increases, the applied voltage will deviate fromoffset sine wave configuration. The curve in Fig. 11will thus be inaccurate in the pessimistic directionat the higher voltage values. It should not be usedfor values of VR above 350 volts.

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A second set of curves (Fig. 12) is furnishedfor use in cases where VP becomes excessive.This figure shows a famUy’&f curves for variousvalues of (RS + RL), the total resistance of thefault CT 1000. includine wtndine and leadresistance.These cum& show ths RMS Glue of the current atwhich the 87R unit should bs set if a fully-offsetcurrent from the source CTs enter the junction ofthe Thyrite circuit and the fault CT loop. Selectthe curve for the resistance closest to the actualfault CT loop resistance or interpolate betweencurves for greater accuracy. The curves in ,Fig. 12should be used when VR from equation (1) exceeds350 volts.

Note that when the Type PVDllD relay is used,there are two Thyrite stacks in parallel. Hence

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the 87H unit setting, determined by either of theforegoing methods, should be doubled.

SAMPLE CALCULATION

The various steps in checking a Type PVD relayapplication will be better understood if a typicalexample is given. Referring to the one-line diagramin Fig. 5, assume that the breakers are TypeFK-439, 60 KV, 1500 MVA., 1200 amperes, with amaximum tnterrupting ratmg of 14,500 amperes.The excitation curve for the bushing CT in thesebreakers is shown in Fig. 13. Similar data can beobtained from the Handbook and Instruction Bookon other current transformers.

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yk

VOLTAGE UNIT 87LThe 87L unit setting will first be determined

by means of equation (l), neglectingtheperformancefactor K. The value of R from Fig. 13 is 0.80 ohm.t”,““ouFoe ,E the measured value of lead resistance

. Substituting the various quantities inequation (1) we have:

VR = 2 ( 0.80 + 0.90 ) =$ = 205 volts

The relay sensitivity using this setting of 87Lwould be obtained from squation (3) as follows:

Imin = (5 x 0.05 + G + 0.06) 240 = 93 amps

If the installation is checked on the basis ofequation (2) using the performance factor the resultswould be as follows:

(Rs + RL) IF/N+

14500 (0.80 + 0.90) = o 34

% 300(240)

U this value is checked on the ordinate of thecurve in Fig. 10, the value of K is found to beapproximately 0.77. Using this value in equation (2),the setting of the 87L unit is found to be:

VR = 2 (0.77) (0.80 + 0.90) % = 158 volts

The sensitivity wtth this setting would be:

Imin=(5X0.04 + -$$ + 0.02) 240 = 67 amps

It ts obvious that in this example very little isgained by using the performance factor K. On manyapplications especially those where ground faultcurrent is limited, it may be necessary to make useof the performance factor in order to realize. thedesired sensitivity. Also, ln many cases it will befound that the value of Vequation (l), will be above %h,““,,EE~~~~the bushing CT in use; whereas if equation (2) isused, involving the performance factor, the valueof vR will be below the knee of the curve andhence the application will be feasible.

The performance factor curve in Fig. 10 wasof course determined for the bushing CTs used inGeneral Electric circuit breakers. When the TypePM rela is applied with CTs of other manufacture,equation 71) should be used in determtning the 87Lunit setting. If the resulting value of VR is too high,it is suggested that the application be referred tothe factory.

Mfferential Voltage Relays Type PVD GEH-1770

CURRENT UNIT 87H

h this elgmpk, the value of VR from equation(1) is 205 volts, andhencethecurve in Fig. 11 shouldbe used in setting 87H. This curve indicates thata 1.3 ampere setting of the 87H unit would be safe.Since the minimum setting of the 87H unit is 2amperes, the unit would be set at this pickup.

If the value of VR had exceeded 350 volts inthis example, the curves in Fig. 12 would have beenused. Since (Rs + R L) is 1.70 the 1.5 ohm curvewould be used, or an interpolation made between the1.5 and 2.0 ohm curves.

MAINTENANCE

The relays are adjusted at the factory and itis advisable not to disturb the adjustments. If forany reason they have been disturbed, the sectioniBJMSTMENTS should be followed in restoring

INSPECTION

A mechanical inspection of the relay should bemade at least once every six months.

CONTACT CLEANING

For cleaning fine silver contacts, a flexibleburnishing tool should be used. This consists ofa flexible strip of metal with an etched roughenedsurface, resembling in effect a superfine file. Thepolishing action is so delicate that no scratches areleft, yet corroded material will be removed rapidlyand thoroughly. The flexibility of the tool insuresthe cleaning of the actual points of contact. Some-times an ordinary file cannot reach the actualpointsof contact because of some obstruction from someother part of the relay.

Fine silver contacts should not be cleaned withknives, files or abrasive paper or cloth. Knivesor fUes may leave scratches which increase arcingand deterioration of the contacts. Abrasive paperor cloth may leave minute particles of insulatingabrasive material in the contacts and thus preventclosing.

The burnishing tool described above can beobtained from the factory.

RENEWAL PARTS

It is recommended that sufficient quantities ofrenewal parts be’ carried in stock to enable the~~~~~lacement of any that are worn, broken,

When ordering renewal parts,. address thenearest Sales Office of the General Electric Com-pany, specifying the quantity requiredanddescribingthe parts by catalogue numbers as shown in Re-newal Parts Bulletin No. GEF-3798.

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