Protective Relay Protection Element Tests

Note: This summary is provided for reference only. Consult your specific relay manufacturer’s instructions for complete testing guidelines. Further information on testing and maintenance procedures can be found in the Relay Testing and Maintenance Guide.

2/62 Timing 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. Serves in conjunction with the device that initiates the shutdown, stopping, or opening operation in an automatic sequence or protective relay system.

  1. Determine time delay.
  2. Verify operation of instantaneous contacts.

21 Distance Relay

Functions when the circuit admittance, impedance or reactance increases or decreases beyond predetermined limits. Distance relays respond to the voltage and current (impedance) at the relay location.

  1. Determine maximum reach.
  2. Determine maximum torque angle and directional characteristic.
  3. Determine offset.
  4. Plot impedance circle.

24 Volts per Hertz Relay

A relay with an instantaneous or a time characteristic that functions when the ratio of voltage to frequency (V/Hz) exceeds a preset value. Used to protect generator and step-up transformer from damage due to excessive magnetic flux resulting from low frequency and/or overvoltage.

  1. Determine pickup frequency at rated voltage.
  2. Determine pickup frequency at a second voltage level.
  3. Determine time delay.

25 Sync Check Relay

A Synchronizing or Synchronism-Check Device operates when two AC circuits are within the desired limits of frequency, phase angle, or voltage, enabling or causing the paralleling of these circuits.

The primary application of this relay is in situations requiring verification of synchronism before closing a circuit breaker. These scenarios encompass paralleling a generator to a system, reestablishing an interconnection between two segments of a power system, and supervising fast transfer schemes where rapid pickup and dropout of the phase measuring circuit are necessary.

  1. Determine closing zone at rated voltage.
  2. Determine maximum voltage differential that permits closing at zero degrees.
  3. Determine live line, live bus, dead line, and dead bus set points.
  4. Determine time delay.
  5. Determine advanced closing angle.
  6. Verify dead bus/live line, dead line/live bus and dead bus/dead line control functions.

Related: Sync Check Relay (25) Fundamentals and Testing Explained


27 Undervoltage Relay

Functions when a given voltage value falls below a predetermined value. The typical uses for this relay function include bus undervoltage protection, source transfer schemes, permissive functions, backup functions and timing applications.

  1. Determine dropout voltage.
  2. Determine time delay.
  3. Determine time delay at a second point on the timing curve for inverse time relays.

32 Directional Power Relay

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. Used in installations where a generator runs in parallel with the utility or another generator so as to prevent power from the bus bar or another generator from flowing back to the active generator when its output fails.

  1. Determine minimum pickup at maximum torque angle.
  2. Determine tripping/closing zone.
  3. Determine maximum torque angle.
  4. Determine time delay.
  5. Verify time delay at a second point on the timing curve for inverse time relays.
  6. Plot the operating characteristic.

40 Loss of Field (Impedance) 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. Used for loss of excitation protection of a generator operating in parallel with other system generators.

  1. Determine maximum reach.
  2. Determine maximum torque angle.
  3. Determine offset.
  4. Plot impedance circle.

46 Current Balance 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. Used for the protection of lines and of three- phase machines, especially motors and synchronous converters against damage that is caused by phase- unbalancing and single-phase operation.

  1. Determine pickup of each unit.
  2. Determine percent slope.
  3. Determine time delay.

46N Negative Sequence Current Relay

  1. Determine negative sequence alarm level.
  2. Determine negative sequence minimum trip level.
  3. Determine maximum time delay.
  4. Verify two points on the (I2)2t curve.

47 Phase Sequence or Phase Balance Voltage Relay

Functions on a predetermined value of polyphase voltage in the desired phase sequence. Provides protection for rotating equipment from the damaging effects of excessive negative sequence voltage resulting from phase failure, phase unbalance and reversed phase sequence.

  1. Determine positive sequence voltage to close the normally open contact.
  2. Determine positive sequence voltage to open the normally closed contact (undervoltage trip).
  3. Verify negative sequence trip.
  4. Determine time delay to close the normally open contact with sudden application of 120 percent of pickup.
  5. Determine time delay to close the normally closed contact upon removal of voltage when previously set to rated system voltage.

49R Thermal Replica 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.

  1. Determine time delay at 300 percent of setting.
  2. Determine a second point on the operating curve.
  3. Determine pickup.

49T Temperature (RTD) Relay

Used to protect motors and generators which are equipped with a resistance-temperature-detector (RTD) against overtemperature. The relay may be used as a protective device to trip the machine on overtemperature or as an alarm to reduce loading of the machine.

  1. Determine trip resistance.
  2. Determine reset resistance.

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. Instantaneous overcurrent relays have no inherent time delay and are used for fast short-circuit protection.

  1. Determine pickup.
  2. Determine dropout.
  3. Determine time delay.

50BF Breaker Failure

Breaker failure relaying is the use of a current monitoring relay to determine whether or not current continues to flow into a faulted circuit some time after a circuit breaker has been instructed to interrupt the circuit. In the event that current continues to flow into the faulted circuit, then the circuit breaker is considered to have failed.

  1. Determine current supervision pickup.
  2. Determine time delays.
  3. Test all inputs and outputs. Test all used initiate inputs and all used outputs.

51 Time Overcurrent

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

  1. Determine minimum pickup.
  2. Determine time delay at two points on the time current curve.

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

Determine tripping angle.
Determine time delay.


59 Overvoltage Relay

Functions on a given value of over-voltage. Provides reliable protection for generators, motors, and transformers against adverse system voltage conditions.

  1. Determine overvoltage pickup.
  2. Determine time delay to close the contact with sudden application of 120 percent of pickup.

60 Voltage Balance Relay

Operates on a given difference in voltage, or current input or output, or two circuits. Provides high speed protection for power systems equipment and protective systems from misoperation or false tripping in the event of a sudden loss of sensing potential resulting from a blown fuse.

  1. Determine voltage difference to close the contacts with one source at rated voltage.
  2. Plot the operating curve for the relay.

63 Transformer Sudden Pressure Relay

Operates on given values of liquid or gas pressure or on given rates of change of these values. Transformer Sudden Pressure relays respond to the sudden increase in gas pressure in a power transformer which would be caused by an internal arc.

  1. Determine rate-of-rise or the pickup level of suddenly applied pressure in accordance with manufacturer’s published data.
  2. Verify operation of the 63 FPX seal-in circuit.
  3. Verify trip circuit to remote operating device.

64 Ground Detector Relay

Operates upon the failure of machine or other apparatus insulation to ground. This function is assigned solely 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 the 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 a current transformer, in the secondary neutral of current transformers, or connected in the power circuit of a normally grounded system.

  1. Determine maximum impedance to ground causing relay pickup.

67 Directional Overcurrent Relay

Functions on a desired value of a-c over-current flowing in a predetermined direction. Polarization is the method used by the relay to determine the direction of current flow.

  1. Determine directional unit minimum pickup at maximum torque angle.
  2. Determine tripping zone.
  3. Determine maximum torque angle.
  4. Plot operating characteristics.
  5. Determine overcurrent unit pickup.
  6. Determine overcurrent unit time delay at two points on the time current curve.

79 Reclosing Relay

Controls the automatic reclosing and locking out of an AC circuit interrupter after it has been opened by overcurrent or other protective relay action. The relay can be adjusted to provide several reclosures at predetermined time intervals. In the event that the breaker does not remain closed after the first reclosure, additional reclosures will be initiated.

  1. Determine time delay for each programmed reclosing interval.
  2. Verify lockout for unsuccessful reclosing.
  3. Determine reset time.
  4. Determine close pulse duration.
  5. Verify instantaneous overcurrent lockout.

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.

  1. Verify frequency set points.
  2. Determine time delay.
  3. Determine undervoltage cutoff.

85 Pilot Wire Monitor

A relay that is operated or restrained by a signal used in connection with carrier-current or DC pilot-wire fault directional relaying. Pilot relaying is an adaptation of the principles of differential relaying for the protection of transmission-line sections. The term “pilot” implies the existence of an interconnecting channel of some sort between the ends of the transmission line over which information can be conveyed.

  1. Determine overcurrent pickup.
  2. Determine undercurrent pickup.
  3. Determine pilot wire ground pickup level.

87 Differential

Functions on a percentage or phase angle or other quantitative difference of two currents or of some other electrical quantities. Transformer differential relays protect against short-circuits between turns of a winding and between windings that correspond to phase-to-phase or three-phase type short-circuits.

  1. Determine operating unit pickup.
  2. Determine the operation of each restraint unit.
  3. Determine slope.
  4. Determine harmonic restraint.
  5. Determine instantaneous pickup.
  6. Plot operating characteristics for each restraint.

References