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Electrical Power System Device Numbers and Acronyms By Alphabetical ANSI/IEEE


In the design of electrical power systems, the ANSI Standard Device Numbers ANSI/IEEE Standard C37.2 denote what features a protective device supports such as a relay or circuit breaker. These types of devices protect electrical systems and components from damage when an unwanted event occurs, such as an electrical fault. Device numbers are used to identify the functions of devices shown on a schematic diagram. Function descriptions are given in the standard.
ANSI/IEEE C37.2-2008 is one of a continuing series of revisions of the standard, which originated in 1928.

List of device numbers and acronyms BY Alphabetical

ALARM RELAY 74
Is a relay other than an annunciator, as covered under device function 30, that is used to operate, or to operate in connection with, a visual or audible alarm.

ACCELERATING OR DECELERATING DEVICE - 18
Is a device that is used to close or to cause the closing of circuits which are used to increase or decrease the speed of a machine.

CIRCUIT BREAKER - 52
Is a device that is used to close and interrupt an a-c power circuit under normal conditions or to interrupt this circuit under fault of emergency conditions.

ATMOSPHERIC CONDITION MONITOR - 45
Is a device, that functions upon the occurrence of an abnormal atmospheric condition, such as damaging fumes, explosive mixtures, smoke or fire.


ANNUNCIATOR RELAY - 30
is a non-automatically reset device that gives a number of separate visual indications of the functions of protective devices, and which may also be arranged to perform a lockout function.

AUTOMATIC SELECTIVE CONTROL OR TRANSFER RELAY - 83
Is a relay that operates to select automatically between certain sources or conditions in a equipment, or performs a transfer operation automatically.

AUXILIARY MOTOR OR MOTOR GENERATOR - 88
Is one used for operating auxiliary equipment, such as pumps, blowers, exciters, rotating magnetic amplifiers, etc.

A-C RECLOSING RELAY - 79
Is a relay that controls the automatic reclosing and locking out of an a-c circuit interrupter.

APPARATUS THERMAL DEVICE - 26
is a device that functions when the temperature of the shunt field or the amortisseur winding of a machine, or that of a load limiting or load shifting resistor or of a liquid or other medium, exceeds a predetermined value: or if the temperature of the protected apparatus, such as a power rectifier, or of any medium decrease below a predetermined value.

ANODE CIRCUIT BREAKER - 7
Is a device used in the anode circuits of a power rectifier for the primary purpose of interrupting the rectifier circuit if an arc-back should occur.

A-C DIRECTIONAL OVERCURRENT RELAY - 67
Is a relay that functions on a desired value of a-c over-current flowing in a predetermined direction.

BLOCKING RELAY - 68
Is a relay that initiates a pilot signal for blocking of tripping on external faults in a transmission line or in other apparatus under predetermined condition, or cooperates with other devices to block tripping or to block re-closing on an out-of-step condition or on power savings.

BRUSH-OPERATING OR SLIPPING SHORT-CIRCUITING DEVICE - 35
Is a device for raising, lowering, or shifting the brushes of a machine, or for short-circuiting its slip rings, or for engaging or disengaging the contacts of a mechanical rectifier.

BEARING PROTECTIVE DEVICE - 38
Is a device that functions on excessive bearing temperature, or on another abnormal mechanical conditions associated with the bearing, such as undue wear, which may eventually result in excessive bearing temperature.

CARRIER OR PILOT-WIRE RECEIVER RELAY - 85
is a relay that is operated or restrained by a signal used in connection with carrier-current or d-c pilot-wire fault directional relaying.

CHECKING OR INTERLOCKING RELAY - 3
is a relay that operates in response to the position of a number of other devices (or to a number of predetermined conditions) in an equipment, to allow an operating sequence to proceed, or to stop, or to provide a check of the position of these devices or of these conditions for any purpose.

CONTROL POWER DISCONNECTING DEVICE - 8
is a disconnecting device, such as a knife switch, circuit breaker, or pull-out fuse block, used for the purpose of respectively connecting and disconnecting the source of control power to and from the control bus or equipment.


D-C CIRCUIT BREAKER - 72
is a circuit breaker that is used to close and interrupt a d-c power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.


DIFFERENTIAL PROTECTIVE RELAY - 87
is a protective relay that functions on a percentage or phase angle or other quantitative difference of two currents or of some other electrical quantities.

DIRECTIONAL POWER RELAY - 32
is a device that functions on a desired value of power flow in a given direction or upon reverse power resulting from arcback in the anode or cathode circuits of a power rectifier.

D-C RECLOSING RELAY - 82
is a relay thast controls the automatic closing and re-closing of a d-c circuit interrupter, generally in response to load circuit conditions.

D-C OVERCURRENT RELAY - 76
is a relay that function when the current in a d-c circuit exceeds a given value.

DISTANCE RELAY - 21
is a relay that functions when the circuit admittance, impedance, or reactance increases or decreases beyond predetermined limits.

EQUALIZER CIRCUIT BREAKER - 22
is a breaker that serves to control or to make and break the equalizer or the current-balancing connections for a machine field, or for regulating equipment in a multiple -unit installation.

EXCITER OR D-C GENERATOR RELAY - 53
is a relay that forces the d-c machine field excitation to build up during starting or which functions when the machine voltage has been built up to a given value.

FIELD APPLICATION RELAY - 56
is a relay that automatically controls the application of the field excitation to an a-c motor at some predetermined point in the slip cycle.

FIELD-CHANGING CONTACTOR - 93
is a contactor that functions to increase or decrease, in one step, the value of field excitation on a machine.

FREQUENCY RELAY - 81
is a relay that functions on a predetermined value of frequency (either under or over or on normal system frequency) or rate of change of frequency.

FIELD RELAY - 40
is a relay that functions on a given or abnormally low value or failure of a machine field current, or on excessive value of the reactive component of armature current in an a-c machine indicating abnormally low field excitation.


FLAME DETECTOR - 28
is a device that monitors the presence of the pilot or main flame of such apparatus as a gas turbine or a steam boiler.

FIELD CIRCUIT BREAKER - 41
is a device that functions to apply or remove the field excitation of a machine.

GOVERNOR - 65
is the assembly of fluid, electrical, or mechanical control equipment used for regulating the flow of water, steam, or other medium to the prime mover for such purposes a starting, holding speed or load, or stopping.

GROUND PROTECTIVE RELAY - 64
is a relay that functions on failure of the insulation of a machine, transformer, or of other apparatus to ground, or on flashover of a d-c machine to ground.


HIGH-SPEED D-C CIRCUIT BREAKER - 54
is a circuit breaker which starts to reduce the current in the main circuit in 0.01 second or less, after the occurrence of the d-c overcurrent or the excessive rate of current rise.


ISOLATING CONTACTOR - 29
is a device that is used expressly for disconnecting one circuit from another for the purposes of emergency operation, maintenance, or test.


INCOMPLETE SEQUENCE RELAY - 48
is a relay that generally returns the equipment to the normal, or off, position and locks it out if the normal starting, operating, or stopping sequence is not properly completed within a predetermined time. If the device is used for alarm purposes only, it should preferably be designated as 48A (alarm).


INSTANTANEOUS OVERCURRENT OR RATE -OF-RISE RELAY - 50
is a relay that functions instantaneously on an excessive value of current or on an excessive rate of current rise, thus indicating a fault in the apparatus

or circuit being protected.

LIQUID OR GAS FLOW RELAY - 80
is a relay that operates on given values of liquid or gas flow or on given rates of change of these values.


LOCKING-OUT RELAY - 86
is an electrically operated hand, or electrically reset relay or device that functions to shut down or hold an equipment out of service, or both, upon the occurrence of abnormal conditions.

LIQUID OR GAS PRESSURE OR VACUUM RELAY - 63
is a relay that operates on given values of liquid or gas pressure or on given rates of change of these values.

LINE SWITCH - 89
is a switch used as a disconnecting, load-interrupter, or isolating switch in an a-c or d-c power circuit, when this device is electrically operated or has electrical accessories, such as an auxiliary switch, magnetic lock, etc.

LIQUID OR GAS-LEVEL RELAY - 71
is a relay that operates on given values of liquid or gas level or on given rates of change of these values.

LOAD-RESISTOR CONTACTOR - 73
is a contactor that is used to shunt or insert a step of load limiting, shifting, or indicating resistance in a power circuit, or to switch a space heater in circuit, or to switch a light or regenerative load resistor, a power rectifier, or other machine in and out of circuit.

MECHANICAL CONDITION MONITOR - 39
is a device that functions upon the occurrence of an abnormal mechanical condition (except that associated with bearing as covered under device function 38), such as excessive vibration, eccentricity, expansion shock, tilting, or seal failure.


MASTER ELEMENT - 1
is the initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly or through such permissive devices as protective and time-delay relays to place an equipment in or out of operation.

MANUAL TRANSFER OR SELECTOR DEVICE - 43
Is a manually operated device that transfers the control circuits in order to modify the plan of operation of the switching equipment or of some of the devices.

MASTER CONTACTOR - 4
is a device generally controlled by device function 1or the equivalent and the required permissive and protective devices, that serves to make and break the necessary control circuits to place an equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions.

MASTER SEQUENCE DEVICE - 34
is a device such as a motor-operated multi-contact switch, or the equivalent, or programming device, such as a computer, that establishes or determines the operating sequence of the major devices in a equipment during starting and stopping or during other sequential switch operations.

MACHINE OR TRANSFORMER THERMAL RELAY - 49
is a relay that functions when the temperature of a machine armature
or other load-carrying winding or element of a machine or the temperature of a power rectifier or power
transformer (including a power rectifier transformer) exceeds a predetermined value.

NOTCHING OR JOGGING DEVICE - 66
is a device that functions to allow only a specified number of operations of a given device or equipment, or a specified number of successive operations within a given time of each other. It is also a device that functions to energize a circuit periodically or for fractions of specified time intervals, or that is used to permit intermittent acceleration or jogging of a machine at low speeds for mechanical positioning.

OVER-SPEED DEVICE - 12
is usually a direct-connected speed switch which functions on machine over-speed.

POSITION CHANGING MECHANISM - 75
is a mechanism that is used for moving a main device from one position to another in an equipment: as for example, shifting a removable circuit breaker unit to and from the connected, disconnected, and test positions.

PULSE TRANSMITTER - 77
is used to generate and transmit pulses over a telemetering or pilot-wire circuit to the remote indicating or receiving device.

PHASE-ANGLE MEASURING OR OUT-OF-STEP PROTECTIVE RELAY - 78
is a relay that functions at a pre-determined phase angle between two voltages or between two currents or between a voltage and current.

POSITION SWITCH - 33
is a switch that makes or breaks contact when the main device or piece of apparatus which has no device function number reaches a given position.

POWER FACTOR RELAY - 55
is a relay that operates when the power factor in an a-c circuit rises above or falls below a predetermined value.

SHORT-CIRCUITING OR GROUNDING DEVICE - 57
is a primary circuit switching device that functions to short-circuit or to ground a circuit in response to automatic or manual means.

OPERATING MECHANISM - 84
is the complete electrical mechanism or servomechanism, including the operating motor, solenoids, position switches, etc., for a tap changer, induction regulator, or any similar piece of apparatus which otherwise has no device function number.

OVERVOLTAGE RELAY - 59
is a relay that functions on a given value of over-voltage.

POLARITY OR POLARIZING VOLTAGE DEVICE - 36
is a device that operates, or permits the operation of, another device on a predetermined polarity only, or verifies the presence of a polarizing voltage in an equipment.

PERMISSIVE CONTROL DEVICE - 69
is generally a two-position, manually-operated switch that, in one position, permits the closing of a circuit breaker, or the placing of an equipment into operation, an in the other position prevents the circuit breaker or the equipment from being operated.


PHASE-SEQUENCE VOLTAGE RELAY - 47
is a relay that function upon a predetermined value of polyphase voltage in the desired phase sequence.

RHEOSTAT - 70
is a variable resistance device used in an electric circuit, which is electrically operated or has other electrical accessories, such a auxiliary , position, or limit switches.

REGULATING DEVICE - 90
is a device that functions to regulate a quantity, or quantities, such as voltage, current power, speed, frequency, temperature, and load at a certain value or between certain (generally close) limits for machines, tie lines, or other apparatus.


RECTIFICATION FAILURE RELAY - 58
is a device that functions if one or mote anodes of a power rectifier fail to fire, or to detect and arc-back or on failure of a diode to conduct or lock properly.

TIME DELAY STARTING OR CLOSING RELAY - 2
is a device that functions to give a desired amount of time delay before or after any point of operation in switching sequence or protective relay system, except as specifically provided by service function 48, 62, and 79.


STOPPING DEVICE - 5
is a control device used primarily to shut down an equipment and hold it out of operation. (This device may be manually or electrically actuated, but excludes the function of electrical lockout [see device function 86] on abnormal conditions.)

STARTING CIRCUIT BREAKER - 6
is a device whose principal function is to connect a machine to its source of starting voltage.

REVERSING DEVICE - 9
is a device that is used for the purpose of reversing a machine field or for performing any other reversing functions.

RUNNING CIRCUIT BREAKER - 42
is a device whose principal function is to connect a machine to its source of running or operation voltage. This function may also be used for a device, such as a contactor, that is used in series with a circuit breaker or other field protecting means, primarily for frequent opening and closing of the breaker.

RESERVED FOR FUTURE APPLICATION - 16

RESERVED FOR FUTURE APPLICATION - 11

(USBR assigned – Control Power Transformer).

RESERVED FOR FUTURE APPLICATION - 24
(USBR assigned – bus tie circuit breaker, contactor, or switch.)

REVERSE PHASE OR PHASE BALANCE CURRENT RELAY - 46
is a relay that functions when the polyphase currents are of reverse-phase sequence, or when the polyphase currents are unbalanced or contain negative phase-sequence components above a given amount.

RESERVED FOR FUTURE APPLICATION. - 61

STARTING-TO-RUNNING TRANSITION CONTACTOR
is a device that operates to initiate or cause the automatic transfer of a machine from the starting to the running power connection.

SEPARATE EXCITATION DEVICE - 31
is a device that connects a circuit, such as the shunt field of a synchronous converter, to a source of separate excitation during the starting sequence; or one that energizes the excitation and ignition circuits of a power rectifier.

SYNCHRONIZING OR SYNCHRONISM-CHECK DEVICE - 25
is a device that operates when two a-c circuits are within the desired limits of frequency, phase angle, or voltage, to permit or to cause the paralleling of these two circuits

SYNCHRONOUS-SPEED DEVICE - 13
is a device such as a centrifugal switch, a slip-frequency relay, a voltage relay, and undercurrent relay , or any type of device that operates at approximately the synchronous speed of a machine.

SPEED OR FREQUENCY MATCHING DEVICE - 15
is a device that functions to match and hold the speed or frequency of a machine or of a system equal to, or approximately equal to, that of another machine, source, or system.

SHUNTING OR DISCHARGE SWITCH - 17
is a switch that serves to open or to close a shunting circuit around any piece of apparatus (except a resistor, such as a machine field, a machine armature, a capacitor, or a reactor).

TRIPPING OR TRIP-FREE RELAY - 94
is a relay that function to trip a circuit breaker, contactor or equipment, or to permit immediate tripping by other devices; or to prevent immediate re -closure of a circuit interrupter if it should open automatically even though its closing circuit is maintained closed.

TEMPERATURE CONTROL DEVICE - 23
is a device that function to raise or lower the temperature of a machine or other apparatus, or of any medium, when its temperature falls below, or rises
above, a predetermined value.

TIME-DELAY STOPPING OR OPENING RELAY - 62
is a time-delay relay that serves in conjunction with the device that initiates the shutdown, stopping, or opening operation in an automatic sequence or
protective relay system.


TIME OVERCURRENT RELAY - 51
Is a relay with either a definite or inverse time characteristic that functions when the current in an a-c circuit exceed a predetermined value.


UNIT SEQUENCE SWITCH - 10
is a switch that is used to change the sequence in which units may be placed in and out of service in multiple-unit equipments.

UNDERVOLTAGE RELAY - 27
is a relay that functions on a given value of under-voltage.

UNDER-SPEED DEVICE - 14
is a device that functions when the speed of a machine fall below a pre -determined value.

UNDERCURRENT OR UNDERPOWER RELAY - 37
is a relay that function when the current or power flow decreases below a predetermined value.

UNIT SEQUENCE STARTING RELAY - 44
is a relay that function to start the next available unit in a multiple-unitequipment upon the failure or non-availability of the normally preceding unit.

VOLTAGE OR CURRENT BALANCE RELAY
is a relay that operates on a given difference in voltage, or current input or output, or two circuits.

VOLTAGE DIRECTIONAL RELAY - 91
is a device which operates when the voltage across an open circuit breaker or contactor exceeds a given value in a given direction.

VOLTAGE AND POWER DIRECTIONAL RELAY - 92
is a relay that permits or causes the connection of two circuits when the voltage difference between them exceed a given value in a predetermined direction and causes these two circuits to be disconnected from each other when the power flowing between them exceeds a given value in the opposite direction.

VALVE - 20
is one used in a vacuum, air, gas, oil, or similar line, when it is electrically operated or has electrical accessories such as auxiliary switches.

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