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Protection System ElementsIEEE Standard C37.2-2008: Protective Relay Numbers

DEVICENUMBER FUNCTION DEFINITION TYPICAL USES

1 Master Element

The initiating device, such as a control switch, etc., thatserves, either directly or through such permissive devices asprotective and time-delay relays, to place equipment in or outof operation.

2 Time-delay Starting orClosing Relay

A device which functions to give a desired amount of timedelay before or after any point of operation in a switchingsequence or protective relay system, except as specificallyprovided by device functions 48, 62 and 79 described later.

Used for providing a time-delay forre-transfer back to the normalsource in an automatic transferscheme.

A relay that operates in response to the position ofa number of other devices (or to a number of predetermined

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3 Checking or InterlockingRelay

conditions) in equipment to allow an operating sequence toproceed, stop, or provide a check of the position of these

devices or conditions forany purpose.

4 Master contactor

A device, generally controlled by device function 1 or theequivalent and the required permissive and protective

devices, that serves to make and break the necessary controlcircuits to place equipment into operation under the desiredconditions and to take it out of operation under abnormal

conditions.

5 Stopping Device

A control device used primarily to shut down equipment andhold it out of operation. (This device may be manually or

electrically actuated, but itexcludes the function of electricallockout [see device function 86] on abnormal conditions.)

6 Starting Circuit Breaker A device whose principal function is to connect a machineto its source of starting voltage.

7 Rate of Change Relay A relay that functions on an excessive rate-of-rise of current.

8 Control PowerDisconnecting Device

A disconnecting device, such as a knife switch, circuitbreaker, or pull-out fuse block, used for the purpose of

respectively connecting and disconnecting the source ofcontrol power to and from the control bus or equipment.NOTE: Control power is considered to include auxiliarypower that supplies such apparatus as small motors and

heaters.

9 Reversing DeviceA device that is used for the purpose of reversing a machine

field or for performing any other reversingfunctions.

10 Unit Sequence SwitchA switch that is used to change the sequence in which

units may be placed in and out of service in multiple-unitequipment.

11 Multifunction Device

A device that performs three or more comparativelyimportant functions that could only be designated by

combining several of these device function numbers. All ofthe functions performed by device 11 shall be defined in the

drawing legend or device function definition list.NOTE: If only two relatively important functions are

performed by the device, it is preferred that both functionnumbers be used.

12 Overspeed Device Usually, a direct-connected speed switch that functions onmachine overspeed.

13 Synchronous-speedDevice

A device such as a centrifugal-speed switch, a slip frequencyrelay, a voltage relay, an undercurrent relay, or any other type

of device that operates at approximately the synchronousspeed of a machine.

14 Underspeed Device A device that functions when the speed of a machine fallsbelow a pre-determined value.

15 Speed or Frequency-Matching Device

A device that functions to match and hold the speed orfrequency of a machine or a system equal to, or

approximately equal to, that of another machine, source, orsystem.

16 Data CommunicationsDevice For communication system application.

17

TopShunting or DischargeSwitch.

A switch that serves to open or close a shunting circuitaround any piece of apparatus (except a resistor), such as a

machine field, a machine armature, a capacitor, ora reactor.

NOTE: This excludes devices that perform such shuntingoperations as may be necessary in the process of starting amachine by devices 6 or 42 (or their equivalent) and also

excludes device function 73 that serves for the switching ofresistors.

18 Accelerating orDecelerating Device.

A device that is used to close or cause the closing of circuitsthat are used to increase or decrease the speed of a machine.

Starting to Running A device which operates to initiate or cause the automatic Used to transfer a reduced voltage

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19 Transition Timer transfer of a machine from the starting to the running powerconnection.

starter from starting to running.

20 Electrically OperatedValve (solenoid valve)

An electrically operated, controlled, or monitored valve usedin a fluid, air, gas, or vacuum line.

21 Distance Relay

A device which functions when the circuit admittance,impedance or reactance increases or decreases beyond

predetermined limits. It has three (3) zones of protection,Zone-1, Zone-2 and Zone-3.

For transmission line protection

22 Equalizer Circuit Breaker

A breaker that serves to control or make and break theequalizer or the current-balancing connections for a machinefield, or for regulating equipment, in a multiple-unitinstallation.

23 Temperature ControlDevice

A device which functions to raise or to lower the temperatureof a machine or other apparatus, or of any medium, when itstemperature falls below or rises above, a predetermined level.

Used as a thermostat to control space heaters in outdoor equipment.

24 Volts per Hertz RelayA relay that functions when the ratio of voltage to frequencyexceeds a preset value. The relay may have an instantaneousor a time characteristic.

25 Synchronizing orsynchronism check device

A device which operates when two ac circuits are within thedesired limits of frequency, phase angle or voltage, to permitor cause the paralleling of these two circuits.

In a closed transition breakertransfer, a 25 relay is used to ensuretwo-sources are synchronized beforeparalleling. Use for feederprotection.

26 Apparatus ThermalDevice

A device that functions when the temperature of theprotected apparatus (other than the load carrying windings ofmachines and transformers as covered by device functionnumber 49) or of a liquid or other medium exceeds apredetermined value; or when the temperature of theprotected apparatus or of any medium decreasesbelow a predetermined value.

27 Under Voltage Relay A device which functions on a given value of undervoltage.Used to initiate an automatictransfer when a primary source ofpower is lost.

28 Flame DetectorA device that monitors the presence of the pilot or main flamein such apparatus as a gas turbine or a steamboiler.

29 Isolating ContactorA device that is used expressly for disconnecting one circuitfrom another for the purposes of emergencyoperation, maintenance, or test.

30 Annuciator Relay

A non-automatically reset device that gives a number ofseparate visual indications upon the functioning of protectivedevices, and which may also be arranged to perform a lockoutfunction.

Used to remotely indicate that aprotective relay has functioned, orthat a circuit breaker has tripped. Typically, a mechanical “drop”type annunciator panel is used.

31 Separate ExcitationDevice

A device that connects a circuit, such as the shunt fieldof a synchronous converter, to a source of separate excitationduring the starting sequence.

32 Directional Power RelayA relay which functions on a desired value of power flow ina given direction, or upon reverse power resulting from arcback in the anode or cathode circuits of a power rectifier.

Used to prevent reverse power fromfeeding an upstream fault. Oftenused when primary backupgeneration is utilized in a facility..

33 Position SwitchA device which makes or breaks contact when the maindevice or piece of apparatus, which has no device functionnumber, reaches a given point.

Used to indicate the position of adrawout circuit breaker (TOCswitch).

34

Top Master Sequence Device

A device such as a motor-operated multicontact switch,or the equivalent, or a programming device, such as acomputer, that establishes or determines the operatingsequence of the major devices in equipment during startingand stopping or during other sequential switching operations.

35Brush-operating or Slip-ring Short-circuitingDevice

A device for raising, lowering, or shifting the brushes of amachine; short-circuiting its slip rings; or engaging ordisengaging the contacts of a mechanical rectifier.

Polarity or Polarizing A device that operates, or permits the operation of, another

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36 Voltage Device. device on a predetermined polarity only or that verifies thepresence of a polarizing voltage in equipment.

37Undercurrent orUnderpower Relay

A relay which functions when the current or power flowdecreases below a predetermined value. Motor protection relay

38 Bearing Protective Device

A device which functions on excessive bearing temperature,or on other abnormal mechanical conditions, such as unduewear, which may eventually result in excessive bearingtemperature.

Motor or generator protection relay

39 Mechanical ConditionMonitor

A device that functions upon the occurrence of anabnormal mechanical condition (except that associated withbearings as covered under device function 38), such asexcessive vibration, eccentricity, expansion, shock, tilting, orseal failure.

40 Field Relay

A relay that functions on a given or abnormally low value orfailure of machine field current, or on an excessivevalue of the reactive component of armature current in an acmachine indicating abnormally low field excitation.

41 Field Circuit Breaker A device which functions to apply, or to remove, the fieldexcitation of a machine. Generator applications

42 Running Circuit Breaker

A device whose principal function is to connect a machineto its source of running or operating voltage. This functionmay also be used for a device, such as a contactor, that isused in series with a circuit breaker or other fault protectingmeans, primarily for frequentopening and closing of the circuit.

43 Manual Transfer orSelector Device

A manually operated device that transfers the control circuitsin order to modify the plan of operation of the switchingequipment or of some of the devices.

44 Unit Sequence StartingRelay

A relay that functions to start the next available unit inmultiple-unit equipment upon the failure or non-availabilityof the normally preceding unit.

45 Atmospheric ConditionMonitor.

A device that functions upon the occurrence of an abnormalatmospheric condition, such as damaging fumes, explosivemixtures, smoke, or fire.

46 Reverse-phase, or PhaseBalance, Current Relay

A relay which functions when the polyphase currents are ofreverse-phase sequence, or when the polyphase currents areunbalanced or contain the negative phase-sequencecomponents above a given amount.

Motor protection relay

47 Phase-sequence VoltageRelay

A relay which functions upon a predetermined value ofpolyphase voltage in the desired phase sequence. Feeder protection

48 Incomplete SequenceRelay

A relay that generally returns the equipment to the normal, oroff, position and locks it out of the normal starting, oroperating or stopping sequence is not properly completedwithin a predetermined amount of time. If the device is usedfor alarm purposes only, it should preferably be designated as 48A (alarm).

49 Machine, or Transformer,Thermal Relay

A relay that functions when the temperature of a machinearmature, or other load carrying winding or element of amachine, or the temperature of a power rectifier or powertransformer (including a power rectifier transformer) exceedsa predetermined value.

Motor or transformer thermal relayapplication

50InstantaneousOvercurrent, or Rate-of-Rise Relay

A relay that functions instantaneously on an excessive valueof current, or an excessive rate of current rise, thus indicatinga fault in the apparatus of the circuit being protected.

Use for tripping a circuit breakersinstantaneously during a high levelshort circuit. Can trip on phase-phase (50), phase-neutral (50N),phase-ground (50G) faults.

51

TopAC Time OvercurrentRelay

A relay with either a definite or inverse time characteristicthat functions when the current in an ac circuit exceeds apredetermined value

Used for tripping a circuit breakerafter a time delay during a sustainedovercurrent. Used for tripping acircuit breaker instantaneouslyduring a high level short circuit.Can trip on phase (51), neutral

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(51N)or ground (51G) overcurrents.

52 AC Circuit BreakerA device that is used to close and interrupt an ac powercircuit under normal conditions or to interrupt this circuitunder fault or emergency conditions.

A term applied typically to mediumvoltage circuit breakers, or lowvoltage power circuit breakers.

53 Exciter or DC GeneratorRelay

A relay that forces the dc machine field excitationto build up during starting or that functionswhen the machine voltage has builtup to a given value.

54 Turning Gear EngagingDevice

An electrically operated, controlled, or monitoreddevice that functions to cause the turning gear to engage (ordisengage) the machine shaft.

55 Power Factor Relay A relay that operates when the power factor in an AC circuitrises above or below a predetermined value.

Under or over frequency applicationused in generating plants.

56 Field Application RelayA relay that automatically controls the application of thefeld excitation to an ac motor at some predeterminedpoint in the slip cycle.

57 Short-circuiting orGrounding Device.

A primary circuit switching device that functions to short-circuit or ground a circuit in response to automatic or manualmeans.

58 Rectification FailureRelay

A device that functions if a power rectifier fails to conduct orblock properly.

59 Over Voltage Relay A relay that functions on a given value of overvoltage.Used to trip a circuit breaker,protecting downstream equipmentfrom sustained overvoltages.

60 Voltage or CurrentBalance Relay

A relay that operates on a given difference in voltage, orcurrent input or output of two circuits.

61 Density Switch or Sensor A device that operates on a given value, or a given rateof change, of gas density.

62 Time-delay Stopping orOpening Relay

A time-delay relay that serves in conjunction with the devicethat initiates the shutdown, stopping, or opening operation inan automatic sequence.

Used in conjunction with a 27device to delay tripping of a circuitbreaker during a brief loss ofprimary voltage, to prevent nuisancetripping.

63 Pressure Switch A switch which operates on given values or on a given rate ofchange of pressure.

Used to protect a transformer duringa rapid pressure rise during a shortcircuit. This device will typically actto open the protective devices aboveand below the transformer.Typically used with a 63-Xauxiliary relay to trip the circuitbreaker.

64

TopGround Protective Relay

A relay that functions on a failure of the insulation of amachine, transformer, or of other apparatus to ground, or onflashover of a dc machine to ground.NOTE: This function is not applied to a device connected inthe secondary circuit of current transformers in a normallygrounded power system, where other device numbers with thesuffix G or N should be used; that is, 51N for an ac timeoverurrent relay connected in the secondary neutral of thecurrent transformers.

Used to detect and act on a ground-fault condition. In a pulsing highresistance grounding system, a 64device will initiate the alarm.

65 Governor

The assembly of fluid, electrical, or mechanical controlequipment used for regulating the flow of water, steam, orother media to the prime mover for such purposes as starting,holding speed or load, or stopping.

66 Notching or joggingdevice

A device that functions to allow only a specified number ofoperations of a given device, or equipment, or a specifiednumber of successive operations within a given time of eachother. It also functions to energize a circuit periodically or forfractions of specified time intervals, or that is used to permitintermittent acceleration or jogging of a machine at lowspeeds for mechanical positioning.

Motor protection applications

67 AC DirectionalOvercurrent Relay

A relay that functions on a desired value of AC overcurrentflowing in a predetermined direction.

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68 Blocking Relay

A relay that initiates a pilot signal for blocking of tripping onexternal faults in a transmission line or in other apparatusunder predetermined conditions, or that cooperates with otherdevices to block tripping or to block reclosing on an out-of-step condition or on power swings.

69 Permissive Control Device

A device that is generally a two-position manually operatedswitch that in one position permits the closing of a circuitbreaker, or the placing of equipment into operation, and in theother position prevents the circuit breaker to the equipmentfrom being operated.

Used as a remote-local switch forcircuit breaker control.

70 RheostatA variable resistance device used in an electric circuit whenthe device is electrically operated or has other electricalaccessories, such as auxiliary, position, or limit switches.

71 Level Switch A switch that operates on given values, or on a given rate ofchange of level.

Used to indicate a low liquid levelwithin a transformer tank in order tosave transformers from loss-of-insulation failure. An alarm contactis available as a standard option ona liquid level gauge. It is set toclose before an unsafe conditionactually occurs.

72 DC Circuit BreakerA circuit breaker that is used to close and interrupt a dcpower circuit under normal conditions or to interrupt thiscircuit under fault or emergency conditions.

73 Load-Resistor Contactor

A contactor that is used to shunt or insert a step of loadlimiting, shifting, or indicating resistance in a power circuit;to switch a space heater in circuit; or to switch a light orregenerative load resistor of a power rectifieror other machine in and out of circuit.

74 Alarm RelayA device other than an annunciator, as covered under devicenumber 30, which is used to operate, or to operate inconnection with, a visible or audible alarm.

75 Position ChangingMechanism

A mechanism that is used for moving a main device from oneposition to another in equipment; for example, shifting aremovable circuit breaker unit to and from the connected,disconnected, and test positions.

76 DC Overcurrent RelayA relay that functionswhen the current in a dc circuitexceeds a given value.

77 Telemetering Device

A transmitter usedto generate and transmit to a remote locationan electrical signal representing ameasured quantity, or a receiver used toreceive the electrical signal from a remotetransmitter and convert the signal to representthe original measured quantity.

78Phase-angle Measuring orOut-of-StepProtective Relay

A relay that functions ata predetermined phase angle between twovoltages, between two currents, orbetween voltage and current.

79

Top AC Reclosing Relay A relay that controls the automatic closing and locking out ofan AC circuit interrupter.

Used to automatically reclose acircuit breaker after a trip, assuming the fault has been cleared after thepower was removed from thecircuit. The recloser will lock-outafter a predetermined amount offailed attempts to reclose.

80 Flow Switch A switch that operates on given values, or on a given rate ofchange, of flow.

81 Frequency RelayA relay that functions on a predetermined value of frequencyeither under or over, or on normal system, frequency or rateof change frequency.

Used to trip a generator circuitbreaker in the event the frequencydrifts above or below a given value.

82 DC Load-measuring A relay that controls the automatic closing and reclosing of adc circuit interrupter, generally in response to load circuit

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Reclosing Relay conditions.

83Automatic SelectiveControl or TransferRelay

A relay that operates to select automatically between certainsources or conditions in equipment or that performs a transferoperation automatically.

84 Operating Mechanism

The complete electrical mechanism or servo mechanism,including the operating motor, solenoids, position switches,etc., for a tap changer, induction regulator, or any similarpiece of apparatus that otherwise has no device functionnumher.

85 Carrier or Pilot-WireReceiver Relay

A relay that is operated or restrained by a signal used inconnection with carriercurrent or DC Pilot-Wire faultrelaying.

86 Locking-out RelayAn electrically operated hand, or electrically, reset relay thatfunctions to shut down and hold an equipment out of serviceon the occurrence of abnormal conditions.

Used in conjunction with protectiverelays to lock-out a circuit breaker(or multiple circuit breakers) after atrip. Typically required to bemanually reset by an operator beforethe breaker can be reclosed.

87 Differential protectiveRelay

A protective relay that functions on a percentage or phaseangle or other quantitative difference of two currents or ofsome other electrical quantities.

Used to protect static equipment,such as cable, bus, transformers bymeasuring the current differentialbetween two points. Typically theupstream and/or downstream circuitbreaker will be incorporated into the“zone of protection.”

88 Auxiliary Motor or MotorGenerator.

A device used for operating auxiliary equipment, such aspumps, blowers, exciters, rotating magnetic amplifiers, etc.

89 Line Switch

A switch used as a disconnecting, load-interrupter, orisolating switch in an ac or dc power circuit. (This devicefunction number is normally not necessary unless the switchis electrically operated or has electrical accessories, such asan auxiliary switch, a magneticlock, etc.)

90 Regulating Device

A device that functions to regulate a quantity orquantities,such as voltage, current, power, speed,frequency, temperature, and load, at a certain value orbetween certain (generally close) limits for machines, tielines, or other apparatus.

91 Voltage Directional RelayA relay that operates when the voltage across an opencircuit breaker or contactor exceeds a given value in a givendirection.

92 Voltage and PowerDirectional Relay

A relay that permits or causes the connection of two circuitswhen the voltage difference between them exceeds a givenvalue in a predetermined direction andcauses these two circuits to be disconnected from each otherwhen the power flowing between them exceeds a given valuein the opposite direction.

93

Top Field-changing Contactor A contactor that functions to increase or decrease, in one step,the value of field excitation on a machine.

94 Tripping or Trip-freeRelay

A relay that functions to trip a circuit breaker, contactor, orequipment, or to permit immediate tripping by other devices,or to prevent immediate reclosure of a circuit interrupter, incase it should open automatically even though its closingcircuit is maintained closed.

95-99

Used only for specific applications inindividual installations if none of thefunctions assigned to the numbersfrom 1 to 94 is suitable.

B - Bus F - Field G - Ground or generator N - Neutral T - Transformer

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Sources and References:

- IEEE Standard C37.2-2008

- ABB Device Function Numbers according to IEEE C.37.2-1991

- EATON Sheet 0017 Jan, 2005

Protection Relay Considerations

The advent of programmable microprocessor-based relays (also called numeric relays) have provided some flexibility in the use ofprotective relays. However, a large part of protection relays still in use are electro-mechanical/or simply thermal-magnetic relays with noelectronic component at all.

Cost wise, digital relays are much affordable than electro-mechanical relays. Old protection specialists who has trudged the long andrigorous path to bring us the present state of advance protective relaying technique consider employment of affordable digital relay devicesis not the primary concern, but how the "art of protective relaying" is preserved with the entry of programmable micro-processor basedrelays.

Micro-processor based relays is considered a big leap for a great number of protection specialists in big or small engineering firms. It is abig big leap, too, most especially for digital relay manufacturers, since the massive investment in precision equipment needed to buildelectro-mechanical relay components, such as precision balance beams, polarizing coils, induction disks, is no longer a big concern. Allthese complicated components have all been replaced by a simple easy to manufacture printed boards that can be mass produced withminimum time.

The advent of digital relay has one big disadvantage: a big number of engineers now almost always ignore symmetrical component theoryand application.

Probably the most onerous practice today is A/E firms' passing the responsibility of short-circuit calculations and system protection toelectrical contractors or testing specialist freelance which is creating a legal liability environment. Only the protection specialist employedor maintained by A/E firms MUST perform all the complex calculations for the protection system.

Further Reading:

ANSI/IEEE Standard Device Numbers

ABB ANSI Numbers per IEEE C.37.2-1991

References

ANSI/IEEE C37.2 Standard

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