• Electrical Device Numbers ANSI/IEEE

    Device numbers are specified in ANSI/IEEE Standard C37.2 and are used to identify the functions of a device shown on a schematic diagram.

    Contents: Device Numbers |  Auxilliary Devices |  Notes |  Printable Sheets
    Important ANSI device numbers to remember: 21 |  25 |  27 |  32 |  33 |  43 |  46 |  47 |  50 |  51 |  52 |  59 |  62 |  67 |  79 |  81 |  83 |  85 |  86 |  87 |  55 |  23 |  26 |  29 |  30 |  37 |  49 |  63 |  68 |  69 |  72 |  78
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    1. Master Element

    Initiating device, such as a control switch, that operates either directly or through other permissive devices to place equipment in or out of service.

    2. Time Delay Starting or Closing Relay

    Functions to give a desired amount of time delay before or after any point of operation in a switching sequence or protective relay system.

    3. Checking or Interlocking Relay

    Operates in response to the position of other devices in an equipment, to allow an operating sequence to proceed, or to stop.

    4. Master Contactor

    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.

    5. Stopping Device

    Used to shut down equipment and hold it out of service, excluding the function of electrical lockout (device 86) on abnormal conditions.

    6. Starting Circuit Breaker

    Connects a machine to its source of starting voltage.

    7. Anode Circuit Breaker

    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.

    8. Control Power Disconnecting Device

    Knife switch, circuit breaker, or pull-out fuse block, used for the purpose of connecting and disconnecting the source of control voltage to and from a control bus or piece of equipment, including auxiliary power feeding small motors and heaters.

    9. Reversing Device

    Used for the purpose of reversing a machine field or for performing any other reversing functions.

    10. Unit Sequence Switch

    Device used to change the sequence in which units may be placed in and out of service in multiple-unit configurations.

    11. Multifunction Device

    Performs three or more comparatively important functions that could only be designated by combining several of these device function numbers. All of the functions performed by device 11 shall be defined in the drawing legend or device function definition list.

    12. Over-Speed Device

    Typically a direct-connected speed switch which functions on machine over-speed.

    13. Synchronous-Speed Device

    Any type of device that operates at approximately the synchronous speed of a machine, such as a centrifugal switch, a slip-frequency relay, a voltage relay, and undercurrent relay.

    14. Under-Speed Device

    Functions when the speed of a machine fall below a predetermined value.

    15. Speed or Frequency Matching Device

    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.

    16. Data Communications Device

    For device 16, the suffix letters further define the device: the first suffix letter is 'S' for serial or 'E' for Ethernet. The subsequent letters are: 'C' security processing function (e.g. VPN, encryption), 'F' firewall or message filter, 'M' network managed function, 'R' router, 'S' switch and 'T' telephone component. Thus a managed Ethernet switch would be 16ESM.

    17. Shunting or Discharge Switch

    Serves to open or to close a shunting circuit around any piece of apparatus, excluding devices that perform such shunting operations as may be necessary in the process of starting a machine.

    18. Accelerating or Decelerating Device

    Closes or causes the closing of circuits which are used to increase or decrease the speed of a machine.

    19. Starting-to-Running Transition Contactor

    Device that operates to initiate or cause the automatic transfer of a machine from the starting to the running power connection.

    20. Valve

    Electrically operated valve used in a vacuum, air, gas, oil, or similar line.

    21. Distance Relay

    Functions when the circuit admittance, impedance or reactance increases or decreases beyond predetermined limits.

    22. Equalizer Circuit Breaker

    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.

    23. Temperature Control Device

    Functions 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. Think of a thermostat that switches on a space heater in a switchgear assembly.

    24. Volts per Hertz Relay

    A relay with an instantaneous or a time characteristic that functions when the ratio of voltage to frequency exceeds a preset value.

    25. Synchronizing or Synchronism-Check Device

    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.

    26. Apparatus Thermal Device

    Functions when the temperature of a equipment, 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.

    27. Undervoltage Relay

    Functions when a given voltage value falls below a predetermined value.

    28. Flame Detector

    Device that monitors the presence of the pilot or main flame of such apparatus as a gas turbine or a steam boiler.

    29. Isolating Contactor

    Used expressly for disconnecting one circuit from another for the purposes of emergency operation, maintenance, or test.

    30. Annunciator Relay

    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.

    31. Separate Excitation Device

    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.

    32. Directional Power Relay

    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.

    33. Position Switch

    Makes or breaks contact when the main device or piece of apparatus which has no device function number reaches a given position.

    34. Master Sequence Device

    Establishes or determines the operating sequence of the major devices in a equipment during starting and stopping or during other sequential switch operations, such as a motor-operated multi-contact switch, or programming device, such as a computer.

    35. Brush-Operating or Slipping Short-Circuiting Device

    Used to raise, lower, or shift the brushes of a machine, or for short-circuiting its slip rings, or for engaging or disengaging the contacts of a mechanical rectifier.

    36. Polarity or Polarizing Voltage

    Permits the operation of another device only at a predetermined polarity, or verifies the presence of a polarizing voltage in an equipment.

    37. Undercurrent or Underpower Relay

    Functions when the current or power flow decreases below a predetermined value.

    38. Bearing Protective Device

    Functions on excessive bearing temperature, or on another abnormal mechanical conditions associated with the bearing that may eventually result in excessive bearing temperature.

    39. Mechanical Condition Monitor

    Functions upon the occurrence of an abnormal mechanical condition not covered under device function 38, such as excessive vibration, eccentricity, expansion shock, tilting, or seal failure.

    40. Field Relay

    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.

    41. Field Circuit Breaker

    Operates to apply or remove the field excitation of a machine.

    42. Running Circuit Breaker

    Functions 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.

    43. Manual Transfer or Selector Device

    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.

    44. Unit Sequence Starting Relay

    Functions to start the next available unit in a multiple-unit equipment upon the failure or non-availability of the normally preceding unit.

    45. Atmospheric Condition Monitor

    Functions upon the occurrence of an abnormal atmospheric condition, such as damaging fumes, explosive mixtures, smoke or fire.

    46. Reverse Phase or Phase Balance Current Relay

    Functions when polyphase currents are of reverse-phase sequence, or when polyphase currents are unbalanced or contain negative phase-sequence components above a given amount.

    47. Phase-Sequence Voltage Relay

    Functions on a predetermined value of polyphase voltage in the desired phase sequence.

    48. Incomplete Sequence Relay

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

    49. Machine or Transformer Thermal Relay

    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.

    50. Instantaneous Overcurrent or Rate -of-Rise Relay

    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.

    51. A-C Time Overcurrent Relay

    A relay with either a definite or inverse time characteristic that functions when the current in an a-c circuit exceed a predetermined value.

    52. A-C Circuit Breaker

    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.

    53. Exciter or D-C Generator Relay

    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.

    54. High-Speed D-C Circuit Breaker

    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.

    55. Power Factor Relay

    Operates when the power factor in an a-c circuit rises above or falls below a predetermined value.

    56. Field Application Relay

    Automatically controls the application of the field excitation to an a-c motor at some predetermined point in the slip cycle.

    57. Short-Circuiting or Grounding Device

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

    58. Rectification Failure Relay

    Functions if one or more 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.

    59. Overvoltage Relay

    Functions on a given value of over-voltage.

    60. Voltage or Current Balance Relay

    Operates on a given difference in voltage, or current input or output, or two circuits.

    61. Density switch or sensor

    Operates on a given value, or a given rate of change, of gas density.

    62. Time-Delay Stopping or Opening Relay

    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.

    63. Pressure Switch

    Operates on given values of liquid or gas pressure or on given rates of change of these values.

    64. Ground Detector Relay

    Operates upon failure of machine or other apparatus insulation to ground. This function is assigned only to a relay that detects the flow of current from the frame of a machine or enclosing case or structure of piece of apparatus to ground, or detects a ground on a normally ungrounded winding or circuit. It is not applied to a device connected in the secondary circuit of current transformer, in the secondary neutral of current transformers, connected in the power circuit of a normally grounded system.

    65. Governor

    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.

    66. Notching or Jogging Device

    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.

    67. A-C Directional Overcurrent Relay

    Functions on a desired value of a-c over-current flowing in a predetermined direction.

    68. Blocking Relay

    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.

    69. Permissive Control Device

    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.

    70. Rheostat

    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.

    71. Liquid or Gas-Level Relay

    Operates on given values of liquid or gas level or on given rates of change of these values.

    72. D-C Circuit Breaker

    Used to close and interrupt a d-c power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

    73. Load-Resistor Contactor

    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.

    74. Alarm Relay

    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.

    75. Position Changing Mechanism

    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.

    76. D-C Overcurrent Relay

    Functions when the current in a d-c circuit exceeds a given value.

    77. Telemetering device

    A transmitter used to generate and transmit to a remote location an electrical signal representing a measured quantity, or a receiver used to receive the electrical signal from a remote transmitter and convert the signal to represent the original measured quantity.

    78. Phase-angle measuring or out-of-step protective relay

    Functions at a predetermined phase angle between two voltages, between two currents, or between voltage and current.

    79. A-C Reclosing Relay

    Controls the automatic reclosing and locking out of an a-c circuit interrupter.

    80. Liquid or Gas Flow Relay

    Operates on given values of liquid or gas flow or on given rates of change of these values.

    81. Frequency Relay

    Functions on a predetermined value of frequency (either under or over or on normal system frequency) or rate of change of frequency.

    82. D-C Reclosing Relay

    Controls the automatic closing and re-closing of a d-c circuit interrupter, generally in response to load circuit conditions.

    83. Automatic Selective Control or Transfer Relay

    Operates to select automatically between certain sources or conditions in a equipment, or performs a transfer operation automatically.

    84. Operating Mechanism

    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.

    85. Carrier or Pilot-Wire Receiver Relay

    A relay that is operated or restrained by a signal used in connection with carrier-current or d-c pilot-wire fault directional relaying.

    86. Locking-Out Relay

    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.

    87. Differential Protective Relay

    Functions on a percentage or phase angle or other quantitative difference of two currents or of some other electrical quantities.

    88. Auxiliary Motor or Motor Generator

    Used for operating auxiliary equipment, such as pumps, blowers, exciters, rotating magnetic amplifiers, etc.

    89. Line Switch

    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.

    90. Regulating Device

    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.

    91. Voltage Directional Relay

    Operates when the voltage across an open circuit breaker or contactor exceeds a given value in a given direction.

    92. Voltage and Power Directional Relay

    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.

    93. Field-Changing Contactor

    Functions to increase or decrease, in one step, the value of field excitation on a machine.

    94. Tripping or Trip-Free Relay

    Functions 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.

    95. For specific applications where other numbers are not suitable

    96. Busbar Trip Lockout relay

    97-99. For specific applications where other numbers are not suitable

    Auxiliary Devices

    These letters denote separate auxiliary devices, such as:

    • C - Closing Relay or Contactor
    • CL - Auxiliary Relay, Closed (energized when main device is in closed position).
    • CS - Control Switch
    • D - “Down” Position Switch or Relay
    • L - Lowering Relay
    • 1. - Opening Relay
    • OP - Auxiliary Relay, Open (energized when main device is in open position).
    • PB - Push Button
    • R - Raising Relay
    • U - “Up” Position Switch or Relay
    • X Auxiliary Relay
    • Y Auxiliary Relay
    • Z Auxiliary Relay


    • Device numbers may be combined if the device provides multiple functions, such as the instantaneous/time-delay AC over current relay denoted as 50/51.
    • A suffix letter or number may be used with the device number. For example, suffix N is used if the device is connected to a Neutral wire (59N in a relay is used for protection against Neutral Displacement).
    • Suffixes X,Y,Z are used for auxiliary devices. Similarly, the "G" suffix can denote a "ground", hence a "51G" is a time overcurrent ground relay. The "G" suffix can also mean "generator", hence an "87G" is a Generator Differential Protective Relay.
    • The "T" suffix can denote a "transformer", hence a "87T" is a Transformer Differential Protective Relay. "F" can denote "field" on a generator or "fuse", as in the protective fuse for a pickup transformer.
    • Suffix numbers are used to distinguish multiple "same" devices in the same equipment such as 51-1, 512.
    • In the control of a circuit breaker with an X-Y Relay Control Scheme, the X relay is the device whose main
    • contacts are used to energized the closing coil or the device which in some other manner, such as by the release of stored energy, causes the breaker to close. The contacts of the Y relay provide the anti-pump feature for the circuit breaker.

    Printable ANSI/IEEE Device Number Reference Sheets

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