Drop-outA relay drops out when it moves from the energizedposition to the un-energized position

Drop-out/pick-up ratioThe ratio of the iimiting values of the characteristicquantity at which the relay resets and operates. Thisvalue is sometimes called the differential of therelay

Earth fault protective systemA protective system which is designed to respondonly to faults to earth.

Earthing transformerA three-phase transformer intended essentially toprovide a neutral point to a power system for thepurpose of earthing

Effective rangeThe range of values of the characteristic quantity orquantities, or of the energizing quantities to whichthe relay will respond and satisfy the requirementsconcerning it, in particular those concerning pre-crsion.

Effective settingThe 'setting' of a protective system including theeffects of current transformers. The effective settingcan be expressed In terms of primary current orsecondary current from the current transformersand IS so designated as appropriate.

Electrical relayA device designed to produce sudden predeterminedchanges in one or more electrical circuits after theappearance of certain conditions in the electricalcircuit or circuits controlling it

NOTE: The term 'relay' includes all the ancillaryequipment calibrated with the device.

Energizing quantityThe electrical quantity, either current or voltage,which alone or in combination with other energizingquantities, must be applied to the relay to cause itto function.

Independent time delay relayA time delay relay in which the time delay IS indepen-dent of the energizing quantity

Instantaneous relayA relay which operates and resets with no intentionaltime delay

NOTE: All relays require some time to operate; it ispossible, within the above definition, to discuss theoperating time characteristics of an instantaneousrelay.

Inverse time delay relayA dependent time delay relay having an operatingtime which is an inverse function of the electricalcharacteristic quantity.

Inverse time delay relay with definite minimum(rD.M. T.)A relay in which the time delay varies inversely withthe characteristic quantity up to a certain value,after which the time delay becomes substantiallyindependent.

Knee-point e.m.f.That sinusoidal e.m.f applied to the secondaryterminals of a current transformer, which, whenincreased by 10%, causes the exciting current toincrease by 50%.

Main protectionThe protective system which is normally expectedto operate in response to a fault in the protectedzone.

Measuring relayA relay intended to operate with a specified accuracyat one or more values of its characteristic quantity

Notching relayA relay which switches In response to a specificnumber of applied impulses.

Operating timeWith a relay de-energized and in Its initial condition,the time which elapses between the application ofa characteristic quantity and the instant when therelay operates.

Operating time characteristicThe curve depicting the relationship between differentvalues of the characteristic quantity applied to arelay and the corresponding values of operatingtime.

Operating valueThe limiting value of the characteristic quantity atwhich the relay actually operates.

Overshoot timeThe extent to which the condition that leads tofinal operation is advanced after the removal of theenergizing quantity, expressed as time at the rate ofprogress of the said condition appropriate to thevalue of the energizing quantity that was initiallyapplied. '

Pick-upA relay is said to 'pick-up' when it changes from theun - energ ized positio n to the energ ized position.

Pilot channelA means of interconnection between relaying pointsfor the purpose of protection.

Protected zoneThe portion of a power system protected by agiven protective system or a part of that protectivesystem.

7

Protective gearThe apparatus, including protective relays, trans-formers and ancillary equipment, for use in aprotective system.

Protective reievA relay designed to initiate disconnection of a partof an electrical installation or to operate a warningsignal, in the case of a fault or other abnormalcondition in the installation.

A protective relay may include more than one unitelectrical relay and accessories.

Protective schemeThe co-ordinated arrangements for the protectionof one or more elements of a power system.

A protective scheme may comprise several protectivesystems.

Protective systemA combination of protective gear designed tosecure, under predetermined conditions, usuallyabnormal, the disconnection of an element of apower system, or to give an alarm signal, or both.

RatingThe nominal value of an energizing quantity whichappears in the designation of a relay The nominalvalue usually corresponds to the CT and VT second-ary ratings.

Resetting valueThe limiting value of the characteristic quantity atwhich the relay returns to its initial position

Residual currentThe algebraic sum, in a multi-phase system, of allthe line currents.

Residual voltageThe algebraic sum, In a multi-phase system, of allthe line-to-earth voltages.

SettingThe limiting value of a 'characteristic' or 'energizing'quantity at which the relay is designed to operateunder specified conditions.

Such values are usually marked on the relay andmay be expressed as direct values, percentages ofrated values, or multiples.

StabilityThe quality whereby a protective system remainsinoperative under all conditions other than thosefor which it is specifically designed to operate.

Stability limitsThe r.rn.s. value of the symmetrical component ofthe through fault current up to which the protectivesystem remains stable.

8

Starting relayA unit relay which responds to abnormal conditionsand initiates the operation of other elements of theprotective system.

System impedance ratio (S./.R.)The ratio of the power system source impedanceto the impedance of the protected zone.

Through fault currentThe current flowing through a protected zone to afault beyond that zone.

Time delayA delay intentionally introduced into the operationof a relay system.

Time delay relayA relay having an intentional delaying device.

Unit electrical relayA single relay which can be used alone or in com-binations with others.

Unit protectionA protection system which IS designed to operateonly for abnormal conditions within a clearlydefined zone of the power system.

Unrestricted protectionA protection system which has no clearly definedzone of operation and which achieves selectiveoperation only by time grading.

The above IS a summary of principal relay terms anddefinitions In current British and internationalpractice It is not complete and further referenceshould be made to the following standards.

BS. 142 1966 Electrical Protective RelaysB.S 3950.1965 Electrical Protective Systems

for A.C. Plant.Oil Circuit Breakers for Alter-nating Current SystemsCurrent Transformers.Voltage Transformers.Guide to Terms used in A.CPower System Studies.Relay Terminology.

B S. 1161952

B.S. 3938.1973B.S.39411965B.S. 2658.1956

BS 4727. Part 1Group 031971BS 3939.1966/72 Graphical Symbols for E'lectric(11

Equipment.American National Standard forRelays and Relay Systems

Internationa I Electrotechn icalCommission Standards forElectrical Relays

C37.90-1971 }Std.313-1971 .

255-1 .1967 }255-2.1969255-3.1971

1.11LIST OF SYMBOLSA

AA

ACC

ACCR

Alarm

Alarm acceptance

Acceleration

Acceleration receive

ACCS

ACCS/R

AN

AUX

AVC

AVR

AVS

BBAT E

BBBLK

BV

CA

CACLCAC¢;

CC

CK

oOAR

DEIT

DIF

DIFB

DIFPB

DOCIT

DOCITI

EEIT

FE

FFR

FGFP

FPFM

FPSM

FPBU

FPM

HSAR

HSASC

HSOC

I

lOB

IP

INT

INTR

INTS

INT SIR

INTSPR

INTSPS

LDC

LE

LFA

LFT

Acceleration send

Acceleration send/receive

Annunciator

Auxiliary

Automatic voltage control.

Automatic voltage regulator

Automatic voltage selection

Buchholz

Battery earth fault

Busbar

Blocking

Balanced voltage

Check alarm or alarm cancellation

Carrier current, direction-comparison

Carrier current, phase-comparison

Circulating current

Check

o iscri mi nati ng

Delayed auto-reclose

Directional earth fault. inverse time

Differential

Biased differential

Plain balance differential

Directional overcurrent. inverse time

Directional overcurrent, Inverse time,

inhibited

Earth fault, instantaneous

Earth fault, inverse time

Frame earth fault

Fuse failure

Flag

Feeder protection

Feeder protection, first main

Feeder protection, second main

Feeder protection, back - u p

Feeder protection, main

High speed auto-reclose

High speed ammeter shorting

H ig h set overcu rrent

Current

Current out-of-balance

Interposing

Intertrip

I ntertri p receive

Intertrip send

Intertrip send/receive

Intertrip, surge proof, receive

Intertrip, surge proof, send

Line drop compensator

Loss excitation

Low frequency alarm

Low frequency trip

LO

LP

LS

MCP

MHO

MHO INT

NEFA

NEF CKN¢;

NV

NVD

OC

OCCK

OCDT

OCINT

OCIT

OF

OL

OLA

OV

PCF

POP

PP

PPH

PR

PS

PSF

PSS

REF

ROT

RP

RPH

SBE

ST

SY

SYN

SYN AUTO

SYN CK

SYN SYS

SYS BU

T

TC

TCS

TO

TE

T E/H

TE RESET

TH

THOC

TM

TS

Lock-out

Low power

Loss of synchronism

Mesh corner protection

High speed distance (mho)

High speed distance (interlocked mho)

Neutral earth fault alarm

Neutral earth fault check

Negative phase sequence or phase

unbalance

No-volt

Neutral voltage displacement

Overcu rrent. insta nta neous

Overcurrent check

Overcurrent. definite time

Overcurrent. interlocked

Overcurrent. inverse time

Overfrequency

Overload

Overload alarm

Overvoltage

Pilot channel fail

Post Office pilot

Private pilot

Positive phase sequence

Protective relay

Pilot shorting

Pilot supply fail

Protection d.c. supply supervision

Restricted earth fault

Rotor earth fault

Reverse power

Reverse phase

Standby earth fault

Stalling

Selection (svnchronizinq)

Synchronizing

Automatic synchronizing

Check synchronizing

System synchronizing

System back-up

Tripping

Circuit breaker trip coil

Trip circuit supervision

Definite time

Tripping, electrically reset

Tripping, electrically and hand reset

Trip relay reset

Tripping, hand reset

Thermal overcurrent

Time delay or timing relay

Tripping, self reset

9

TSEQ

TSS

UC

UF

UP

UV

UVIT

V

Tripping, sequential

Trip supply supervision

Undercurrent

U nderfreq uency

Underpower

Undervoltage

Undervoltage, inverse time

Voltage

Voltage regulating

Voltage selection

Distance (reactance)

Distance (impedance)

VR

VS

X

Z

1.12< [ 'J

2 Time delay starting or closing relay3 Checking or interlocking relay

21 Distance relay25 Synchronizing or synchronism check relay27 Undervoltage relay30 Annunciator relay32 Directional power relay37 Undercurrent or underpower relay40 Field failure relay46 Reverse phase or phase balance current relay49 Machine or transformer thermal relay50 Insta ntaneous overcurrent or rate- of - rise relay51 Ac. time overcurrent relay52 Ac. circuit breaker52a Circuit breaker auxiliary switch-normally open52b Circuit breaker auxiliary switch-normally

closed55 Power factor relay56 Field application relay59 Overvoltage relay60 Voltage or current balance relay64 Earth fault protective relay67 Ac. directional overcurrent relay68 Blocking relay74 Alarm relay76 D.c. overcurrent relay78 Phase angle measuring or out-of-step protective

relay79 Ac. reclosing relay81 Frequency relay83 Automatic selective control or transfer relay85 Carrier or pilot wire receive relay86 Locking-out relay87 Differential protective relay

1.13

RELAY CONTACT YSTE Sa. Self-resetThe contacts remain operated only while thecontrolling quantity is applied, returning to theiroriginal condition when it is removed.

b. Hand or electrical reset.These contacts remain in the operated position afterthe controlling quantity is removed. They can bereset either by hand or by an auxiliary electro-magnetic element,

10

The majority of protective relay elements have self-reset contact systems, which, if it is so desired, canbe made to give hand reset output contacts by theuse of auxiliary elements.

Hand or electrically reset relays are used when it isnecessary to maintain a signal or a lock-out condition.Contacts are shown on diagrams in the positioncorresponding to the un-operated or de-energizedcondition regardless of the continuous servicecondition of the equipment. For example, a voltagesupervising relay, which is continually picked-up,would still be shown in the de-energized condition,

A 'make' contact is one that closes when the relaypicks up, whereas a 'break' contact is one that isclosed when the relay is un-energized and openswhen the relay picks up, Examples of these conven-tions and variations are shown in Figure 1.6.

--D 0-- -a:::n--- SELF RESET

--D 0----,NORMALLY OPEN

~HANDRESET

NORMALLY CLOSED

TIME DELAY ONPICK-UP

TIME DELAY ONDROP-OFF

Figure 1,6 tnd.ceuon of contects on cftaqrams

A protective relay is usually required to trip a circuitbreaker, the tripping mechanism of which may be asolenoid with a plunger acting directly on themechanism latch or, in the case of air-blast orpneumatically operated breakers, an electricallyoperated valve. The relay may energize the trippingcoil directly, or, according to the coil rating, and thenumber of circuits to be energized, may do :;0through the agency of another multi-channelauxiliary relay

The power required by the trip coil of the circurtbreaker may range from up to 50 watts, for a small'distribution' circuit breaker, to 3000 watts for alarge extra-high-voltage circuit breaker.

The basic trip circuit is simple, being made up of ahand-trip control switch and the contacts of theprotective relays in parallel to energize the trip coilfrom a battery, through a normally open auxiliaryswitch operated by the circuit breaker. This auxiliaryswitch is needed to open the trip circuit when thecircuit breaker opens, since the protective relaycontacts will usually be quite incapable of performingthe interrupting duty, The auxiliary switch will beadjusted to close as early as possible in the closingstroke, to make the protection effective incase thebreaker is being closed on to a fault.

Protective relays are precise measuring devices, thecontacts of which should not be expected toperform large making and breaking duties, Attractedarmature relays, which' combine many of thecharacteristics of measuring devices and contactors,

occupy an intermediate position and according totheir design and consequent closeness to one orother category, may have an appreciable contactcapacity

Most other types of relay develop an effort which isindependent of the position of the moving system.At setting, the electromechanical effort is absorbedby the controlling force, the margin for operatingthe contacts being negligibly small. Not only doesthis limit the 'making' capacity of the contacts, butif more than one contact pair is fitted any slightmisalignment may result in only one contact beingclosed at the minimum operating value, there beinginsufficient force to compress the spring of the firstcontact to make, by the small amount required topermit closure of the second.

For this reason, the provision of multiple contactson such elements is undesirable. Although twocontacts can be fitted, care must be taken in theiralignment, and a small tolerance in the closing valueof operating current may have to be allowedbetween them. These effects can be reduced byprovidinq a small amount of 'run-In' to contactmake In the relay behaviour, by special shaping ofthe active parts.

For the above reasons It is often better to use inter-posing contactor type elements which do not havethe same limitations, although some measuringrelay elements are capable of tripping the smallertypes of circuit breaker directly These may be smallattracted armature type elements fitted in the samecase as the measuring relay

In genera I, stati c relays have discrete measu ri ng andtripping circuits, or modules. The functioning of themeasuring modules will not react on the trippingmodules. Such a relay is equivalent to a sensitiveelectromechanical relay with a tripping contactor,so that the number or rating of outputs has no moresignificance than the fact that they have beenprovided

For larger switchgear installations the tripping powerrequirement of each circuit breaker is considerable,and, further, two or more breakers may have to betripped by one protective system. There may also beremote signalling requirements, interlocking withother functions (for example auto-reclosing arrange-ments), and other control functions to be performed.These various operations are carried out by multi-contact tripping relays, which are energized by theprotection relays and provide the necessary numberof adequately rated output contacts.

1.14OP':" f. T Or' rr~DIC • O!As a guide for power system operation staff,protective systems are invariably provided withindicating devices. In British practice these arecalled 'flags', whereas in America they are known as'targets'. Not every component relay will have one,as indicators are arranged to operate only if a tripoperation is initiated. Indicators, with very fewexceptions, are bi-stable devices, and may be eithermechanically or electrically operated. A mechanicalindicator consists of a small shutter which is

released by the protective relay movement toexpose the indicator pattern, which, on GEeMeasurements relays, consists of a red diagonalstripe on a white backgroond.

Electrical indicators may be simple attracted arma-ture elements either with or without contacts.Operation of the armature releases a shutter toexpose an indicator as above.

An alternative type consists of a small cylindricalpermanent magnet magnetized across a diameter,and lying between the poles of an electromagnet.The magnet, which is free to rotate, lines up itsmagnetic axis with the electromagnet poles, butcan be made to reverse its orientation by theapplication of a field. The edge of the magnet iscoloured to give the indication.

1.15

Auxiliary contactors can be used to supplementprotective relays in a number of ways.

a. Series sealing.b. Shunt reinforcing.c. Shunt reinforcement with sealing.These are illustrated in Figure 1.7.

PR~2::, ~-----=~-~ /'( --~

- IL I

(a) SERIES SEALING

(b) SHUNT REINFORCING

I IL J

(c) SHUNT REINFORCING WITH SERIES SEALING

Figure 1.7 Tvpicel relav [npplng circuits

When such auxiliary elements are fitted, they canconveniently carry the operation indicator. avoidingthe need for indicators on the measuring elements.

Electrically operated indicators avoid imposing anadditional friction load on the measuring element,which would be a serious handicap for certaintypes. Another advantage is that the indicator canoperate only after the main contacts have closed.With indicators operated directly by the measuringelements, care must be taken to line up theiroperation with the closure of the main contacts.The indicator must have operated by the time thecontacts make, but must not have done so morethan marginally earlier. This is to stop indicationoccurring when the tripping operation has notbeen completed.

11

a. Series sealingThe coil of the series contactor carries the tripcurrent initiated by the protective relay, and thecontactor closes a contact in parallel with the pro-tective relay contact. This closure relieves theprotective relay contact of further duty and keeps thetripping circuit secureiy ciosed, even if chatteroccurs at the main contact. Nothing is added to thetotal tripping time, and the indicator does notoperate until current is actually flowing through thetrip coil.

The main disadvantage of this method is that suchseries elements must have their coils matched withthe trip circuit with which they are associated.

The coils of these contactors must be of lowimpedance, with about 5% of the trip supply voltagebeing dropped across them.

When used in association with high speed triprelays, which usually interrupt their own coilcurrent, the auxiliary elements must be fast enoughto operate and release the flag before their coilcurrent is cut off. This may pose a problem indesign If a variable number of auxiliary elements(for different phases and so on) may be required tooperate In parallel to energize a common trippingrelay

b. Shunt reinforcingHere the sensitive contacts are arranged to trip thecircuit breaker and simultaneously to energize theauxiliary unit, which then reinforces the contactwhich is energizing the trip coil

It should be noted that two contacts are requiredon the protective relay, since it IS not permissible toenergize the trip coil and the reinforcing contactorin parallel If this were done, and more than oneprotective relay were connected to trip the samecircuit breaker, all the auxiliary relays would beenergized In parallel for each relay operation andthe indication would be confused.

The duplicate main contacts are frequently provided

PR 52~~

~~~~~"~

(a) SUPERVISIONWHILECIRCUITBREAKERIS CLOSED

PR~~r-~-D o-~~r--o o-~~

(b) SUPERVISIONWHILECIRCUITBREAKERIS OPENORCLOSED

PR.~~T-~~ O-~~r--O o-~~TC~-a~~

!~------------~~i~ALARM

(e) SUPERVISIONWITH CIRCUITBREAKEROPENORCLOSEDWITH REMOTEALARM

Figure 1.8 Examples of trip circuit supervision

12

as a three point arrangement to reduce the numberof contact fingers.c. Shunt reinforcement with sealing.This is a development of the shuntreinforcing circuitto make it applicable to relays with low torque move-ments or where there is a possibility of contactbounce for any other reason.

Using the shunt reinforcing system under thesecircumstances would result in chattering on theauxiliary unit, and the possible burning out of thecontacts not only of the sensitive element but alsoof the auxiliary unit. The chattering would only endwhen the circuit breaker had finally tripped

It will be seen that the effect of bounce is counteredby means of a further contact on the auxiliary unitconnected as a retaining contact.

This means that provision must be made for releasingthe sealing circuit when tripping is complete, this isa disadvantage, because it IS sometimes In-convenient to find a suitable contact to use for thispurpose.

1.16

The trip circuit extends beyond the relay enclosureand passes through more components, such asfuses, links, relay contacts, auxiliary switch contactsand so on, and in some cases through a considerableamount of circuit wiring with intermediate terminalboards. These complications, coupled With theimportance of the circuit. have directed attention toits supervision

The simplest arrangement contains a healthy triplamp, as shown in Figure 1.8(a).

The resistance in series with the lamp prevents thebreaker being tripped by an internal short circuitcaused by failure of the lamp This provides super-vision while the circuit breaker is closed; a simpleextension gives pre-closing supervision.

Figure 1.8(b) shows how, by the addition of anormally closed auxiliary switch and a resistanceunit, supervision can be obtained while the breakerIS both open and closed

In either case, the addition of a normally open push-button contact in series with the lamp will make thesupervision indication available only when required

Schemes using a lamp to indicate continuity aresuitable for locally controlled Installations, but whencontrol is exercised from a distance it is necessarvto use a relay system. Figure 1.8(c) illustrates such ascheme, which is applicable wherever a remotesignal is required.

With the circuit healthy either or both of relays Aand 8 are operated and energize relay C. Both Aand 8 must reset to allow C to drop-off. RelaysA and C are time-delayed by copper slugs toprevent spurious alarms during tripping or closingoperations. The resistors are mounted separatelyfrom the relays and their values are chosen such thatif anyone component is inadvertently short-circuited,a tripping operation will not take place.

The alarm supply should be independent of thetripping supply so that indication will be obtained inthe event of the failure of the tripping battery.