ANSI code

Name of function

Definition

12

Overspeed

Detection of rotating machine overspeed

14

Under speed

Detection of rotating machine under speed

21

Distance protection

Impedance measurement detection

21B

Underimpedance

Back-up phase-to-phase short-circuit protection for
generators

24

Flux control

Over-fluxing check

25

Synchro-check

Check before paralleling two parts of the power system

26

Thermostat

Protection against overloads

27

Under-voltage

Protection for control of voltage sags

27D

Positive sequence
under-voltage

Protection of motors against operation with insufficient
voltage

27R

Remnants under-voltage

Check on the disappearance of voltage sustained by
rotating machines after the power supply is disconnected

27TN

Third harmonic
under-voltage

Detection of stator winding insulation earth faults
(impedant neutral)

32P

Directional active
overpower

Protection against active overpower transfer

32Q

Directional reactive
overpower

Protection against reactive overpower transfer

37

Phase undercurrent

3-phase protection against undercurrent

37P

Directional active
underpower

Protection against active underpower transfer

37Q

Directional reactive
underpower

Protection against reactive underpower transfer

38

Bearing temperature
monitoring

Protection against overheating of rotating machine
bearings

40

Field loss

Protection of synchronous machines against faults or
field loss

46

Negative sequence /
unbalance

Protection against unbalanced phase current

47

Negative sequence
overvoltage

Negative sequence voltage protection and detection of
reverse rotation of rotating machines

48 – 51LR

Excessive starting time and locked rotor

Protection of motors against starting with overloads or
reduced voltage, and for loads

that can block

49

Thermal overload

Protection against overloads

49T

RTDs

Protection against overheating of machine windings

50

Instantaneous phase overcurrent

3-phase protection against short-circuits

50BF

Breaker failure

Checking and protection if the circuit breaker fails to
trip after a tripping order

50N or 50G

Instantaneous earth fault

Protection against earth faults:

50N: residual current calculated or measured by 3 CTs

50G: residual current measured directly by a single
sensor (CT or core balance CT)

50V

Instantaneous voltage-restrained phase

overcurrent

3-phase protection against short-circuits with
voltage-dependent threshold

50/27

Inadvertent generator energization

Detection of inadvertent generator energization

51

Delayed phase overcurrent

3-phase protection against overloads and short-circuits

51N or 51G

Delayed earth fault

Protection against earth faults:

51N: residual current calculated or measured by 3 CTs

51G: residual current measured directly by a single
sensor (CT or core balance CT)

51V

Delayed voltage-restrained phase overcurrent

3-phase protection against short-circuits with
voltage-dependent threshold

59

Overvoltage

Protection against excessive voltage or sufficient
voltage detection

59N

Neutral voltage displacement

Insulation fault protection

63

Pressure

Detection of transformer internal faults (gas, pressure)

64REF

Restricted earth fault differential

Earth fault protection for star-connected 3-phase
windings with earthed neutral

64G

100% generator stator earth fault

Detection of stator winding insulation earth faults
(impedant neutral power systems)

66

Successive starts

Protection function that monitors the number of motor
starts

67

Directional phase overcurrent

3-phase short-circuit protection according to current
flow direction

67N/67NC

Directional earth fault

Earth fault protection depending on current flow
direction (NC: Neutral compensated)

78

Vector shift

Vector shift disconnection protection

78PS

Pole slip

Detection of loss of synchronization of synchronous
machines

79

Recloser

Automated device that recloses the circuit breaker after
transient line fault tripping

81H

Over-frequency

Protection against abnormally high frequency

81L

Under-frequency

Protection against abnormally low frequency

81R

Rate of change of frequency (ROCOF)

Protection for fast disconnection of two parts of the
power system

87B

Busbar differential

3-phase protection against busbar internal faults

87G

Generator differential

3-phase protection against internal faults in AC
generators

87L

Line differential

3-phase protection against line internal faults

87M

Motor differential

3-phase protection against internal faults in motors

87T

Transformer differential

3-phase protection against internal faults in
transformers