PROTECTIVE DEVICES AND THEIR FUNCTIONS

The devices in switching equipment are referred to by numbers, with appropriate suffix letters when necessary, according to the functions they perform. These numbers are based on a system adopted as the standard for automatic switchgear by IEEE, and incorporated in American Standard C37.2-1970. This system is used in connection diagrams, instruction books, and in specifications.

1. MASTER ELEMENT is the initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly, or through such permissive devices as a protective relay system, except as specifically provided by device functions 48, 62, and 79.

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

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

4. MASTER CONTACTOR is a device, generally controlled by device No.1 or equivalent, and the required permissive and protective devices that serve to make and break the necessary control circuits to place equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions.

5. STOPPING DEVICE is a control device used primarily to shut down equipment and hold it out of operation. This device may be manually or electrically actuated but excludes the function of the electrical lockout on abnormal conditions.
6. STARTING CIRCUIT BREAKER is a device whose principal function is to connect a machine to its source of starting voltage.

7. ANODE CIRCUIT BREAKER is one 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 is a disconnective device such as a knife switch, circuit breaker, or pull-out fuse block, used for the purpose of connecting and disconnecting the source of control power to and from the control bus or equipment.

Note: Control power is considered to include auxiliary power which supplies such apparatus as small motors and heaters.

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

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

11. Reserved for future application.

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

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

14. UNDER-SPEED DEVICE functions when the speed of a machine falls below a predetermined value.

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

16. Reserved for future application.

17. SHUNTING OR DISCHARGE SWITCH serves to open or 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. Note: This excludes devices that perform such shunting operations as may be necessary for the process of starting a machine by devices 6 or 42, or their equivalent, and also excludes device 73 function which serves for the switching of resistors.

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

19. 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.

20. ELECTRICALLY OPERATED VALVE is an electrically operated, controlled, or monitored valve in a fluid line. Note: The function of the valve may be indicated by the use of the suffixes.

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

22. EQUALIZER CIRCUIT BREAKER 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.

23. TEMPERATURE CONTROL DEVICE functions to raise or lower the temperature of a machine or other apparatus, or of any medium, where its temperature falls below, or rises above, a predetermined value. Note: An example is a thermostat that switches on a space heater in a switchgear assembly when the temperature falls to the desired value as distinguished from a device that is used to provide automatic temperature regulation between close limits and would be designated as 90T.

24. Reserved for future application.

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

26. APPARATUS THERMAL DEVICE functions when the temperature of the shunt field or the armortisseur 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 decreases below a predetermined value.

27. UNDER VOLTAGE RELAY is a device that functions on a given value of under voltage.

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

29. ISOLATING CONTACTOR is used expressly for disconnecting one circuit from another for the purposes of emergency operation, maintenance, or testing.

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

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

32. DIRECTIONAL POWER RELAY is one which functions on a desired value of power flow in a given direction, or upon reverse power resulting from arc back 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 is a device such as a motor-operated multi-contact switch, or the equivalent, or a programming device, such as a computer, that establishes or determines the operating sequence of the major devices in equipment during starting and stopping or during other sequential switching operations.

35. BRUSH-OPERATING, OR SLIP-RING-SHORT-CIRCUITING, DEVICE is used 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.

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

37. UNDERCURRENT OR UNDER POWER RELAY functions when the current or power flow decreases below a predetermined value.

38. BEARING PROTECTIVE DEVICE functions when the current or power flow decreases below a pre-determined value.

39. MECHANICAL CONDITION MONITOR is a device that functions upon the occurrence of an abnormal mechanical condition, 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 machine field current, or on an excessive value of the reactive component of armature current in an AC machine indicating abnormally low field excitation.

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

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

43. MANUAL TRANSFER OR SELECTOR DEVICE transfers the control circuits so as to modify the plan of operation of the switching equipment of some of the devices.

44. UNIT-SEQUENCE STARTING RELAY is a device that functions to start the next available unit in multiple-unit equipment on the failure or on the non-availability of the normally preceding unit.

45. ATMOSPHERIC CONDITION MONITOR is a device that 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 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.

47. PHASE-SEQUENCE VOLTAGE RELAY functions upon a predetermined value of poly-phase voltage in the desired phase sequence.

48. INCOMPLETE SEQUENCE RELAY 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).

49. MACHINE, OR TRANSFORMER, THERMAL RELAY 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.

50. INSTANTANEOUS OVERCURRENT OR RATE-OF-RISE RELAY 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.

51. AC TIME OVERCURRENT RELAY is a relay with either a definite or inverse time characteristic that functions when the current in an AC circuit exceeds a predetermined value.

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

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

54. Reserved for future application.

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

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

57. SHORT-CIRCUITING OR GROUNDING DEVICE is 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 is a device that functions if one or more anodes of a power rectifier fail to fire, or to detect an arc-back or failure of a diode to conduct or block properly.

59. OVERVOLTAGE RELAY is a relay that functions on a given value of overvoltage.

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

61. Reserved for future application.

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

63. PRESSURE SWITCH is a switch that operates on given values or on a given rate of change of pressure.

64. GROUND PROTECTIVE RELAY is a relay that functions on the failure of the insulation of a machine, transformer, or of other apparatus to ground, or on flashover of a de machine to the ground. Note: 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 a 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 or secondary neutral of a current transformer, or in the secondary neutral of current transformer connected in the power circuit of a normally grounded system.

65. GOVERNOR is the assembly of fluid, electrical, or mechanical control equipment used for regulating the flow of water, steam, or another medium to the prime mover for such purposes as 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 also functions to energize a circuit periodically or for fractions of specified time intervals or is used to permit intermittent acceleration or jogging of a machine at low speeds for mechanical positioning.

67. AC DIRECTIONAL OVERCURRENT RELAY is a relay that functions on a desired value of ac overcurrent flowing in a predetermined direction.

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

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

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

71. LEVEL SWITCH is a switch that operates on given values, or on a given rate of change, of level.

72. DC CIRCUIT BREAKER is used to close and interrupt a de-power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

73. LOAD-RESISTOR CONTACTOR 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 the circuit, or to switch light, or regenerative load resistor of a power rectifier or other machine in and out of the circuit.

74. ALARM RELAY is a device other than an annunciator, which is used to operate, or to operate in connections with, a visual or audible alarm.

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

76. DC OVERCURRENT RELAY is a relay that functions when the current in a dc circuit exceeds a given value.

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

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

79. AC RECLOSING RELAY is a relay that controls the automatic reclosing and locking out of an AC circuit interrupter.

80. FLOW SWITCH is a switch that operates on given values, or on a given rate of change, of flow.

81. FREQUENCY RELAY is a relay that responds to the frequency of an alternating electrical input quantity.

82. DC RECLOSING RELAY is a relay that controls the automatic closing and reclosing of a DC circuit interrupter, generally in response to load circuit conditions.

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

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

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

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

87. DIFFERENTIAL PROTECTIVE RELAY is an electrically operated, hand or electrically reset, relay that functions to shut down and hold equipment out of service on the occurrence of abnormal conditions.

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

89. LINE SWITCH is used as a disconnecting load-interrupter, or isolating switch in an AC or DC 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, at a 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 is a relay that operates when the voltage across an open circuit break or contactor exceeds a given value in a given direction.

92. VOLTAGE AND POWER DIRECTIONAL RELAY is a relay that permits or causes the connection of two circuits when the voltage difference between them exceeds a given value in a pre-determined 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 reclosure of a circuit interrupter, in case it should open automatically even though its closing circuit is maintained closed.