ANSI codes for protection relays

Protection Relay - ANSI Standards

In the design of electrical power systems, the ANSI Standard Device Numbers 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.

ANSI numbers are used to identify the functions of medium voltage microprocessor devices.

ANSI facilitates the development of American National Standards (ANS) by accrediting the procedures of standards developing organizations (SDOs). These groups work cooperatively to develop voluntary national consensus standards.

Accreditation by ANSI signifies that the procedures used by the standards body in connection with the development of American National Standards meet the Institute’s essential requirements for openness, balance, consensus and due process.

ANSI standards (protection) – index

Current protection functions

ANSI 50/51 – Phase overcurrent

Three-phase protection against overloads and phase-to-phase short-circuits.

ANSI 50N/51N or 50G/51G – Earth fault

Earth fault protection based on measured or calculated residual current values:

  • ANSI 50N/51N: residual current calculated or measured by 3 phase current sensors
  • ANSI 50G/51G: residual current measured directly by a specific sensor
ANSI 50BF – Breaker failure

If a breaker fails to be triggered by a tripping order, as detected by the non-extinction of the fault current, this backup protection sends a tripping order to the upstream or adjacent breakers.

ANSI 46 – Negative sequence / unbalance

Protection against phase unbalance, detected by the measurement of negative sequence current:

  • Sensitive protection to detect 2-phase faults at the ends of long lines
  • Protection of equipment against temperature build-up, caused by an unbalanced power supply, phase inversion or loss of phase, and against phase current unbalance
ANSI 49RMS – Thermal overload

Protection against thermal damage caused by overloads on machines (transformers, motors or generators). The thermal capacity used is calculated according to a mathematical model which takes into account:

  • Current RMS values
  • Ambient temperature
  • Negative sequence current, a cause of motor rotor temperature rise

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Automation device used to limit down time after tripping due to transient or semipermanent faults on overhead lines. The recloser orders automatic reclosing of the breaking device after the time delay required to restore the insulation has elapsed. Recloser operation is easy to adapt for different operating modes by parameter setting.

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Directional current protection

ANSI 67 – Directional phase overcurrent

Phase-to-phase short-circuit protection, with selective tripping according to fault current direction. It comprises a phase overcurrent function associated with direction detection, and picks up if the phase overcurrent function in the chosen direction (line or busbar) is activated for at least one of the 3 phases.

ANSI 67N/67NC – Directional earth fault

Earth fault protection, with selective tripping according to fault current direction. 3 types of operation:

  • type 1: the protection function uses the projection of the I0 vector
  • type 2: the protection function uses the I0 vector magnitude with half-plane tripping zone
  • type 3: the protection function uses the I0 vector magnitude with angular sector tripping zone
ANSI 67N/67NC type 1

Directional earth fault protection for impedant, isolated or compensated neutralsystems, based on the projection of measured residual current.

ANSI 67N/67NC type 2

Directional overcurrent protection for impedance and solidly earthed systems, based on measured or calculated residual current. It comprises an earth fault function associated with direction detection, and picks up if the earth fault function in the chosen direction (line or busbar) is activated.

ANSI 67N/67NC type 3

Directional overcurrent protection for distribution networks in which the neutral earthing system varies according to the operating mode, based on measured residual current. It comprises an earth fault function associated with direction detection (angular sector tripping zone defined by 2 adjustable angles), and picks up if the earth fault function in the chosen direction (line or busbar) is activated.

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Directional power protection functions

ANSI 32P – Directional active overpower

Two-way protection based on calculated active power, for the following applications:

  • active overpower protection to detect overloads and allow load shedding
  • reverse active power protection:
    • against generators running like motors when the generators consume active power
    • against motors running like generators when the motors supply active power
ANSI 32Q/40 – Directional reactive overpower

Two-way protection based on calculated reactive power to detect field loss on synchronous machines:

  • reactive overpower protection for motors which consume more reactive power with field loss
  • reverse reactive overpower protection for generators which consume reactive power with field loss.

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Machine protection functions

ANSI 37 – Phase undercurrent

Protection of pumps against the consequences of a loss of priming by the detection of motor no-load operation. It is sensitive to a minimum of current in phase 1, remains stable during breaker tripping and may be inhibited by a logic input.

ANSI 48/51LR/14 – Locked rotor / excessive starting time

Protection of motors against overheating caused by:

  • excessive motor starting time due to overloads (e.g. conveyor) or insufficient supply voltage. The reacceleration of a motor that is not shut down, indicated by a logic input, may be considered as starting.
  • locked rotor due to motor load (e.g. crusher):
    • in normal operation, after a normal start
    • directly upon starting, before the detection of excessive starting time, with detection of locked rotor by a zero speed detector connected to a logic input, or by the underspeed function.
ANSI 66 – Starts per hour

Protection against motor overheating caused by:

  • too frequent starts: motor energizing is inhibited when the maximum allowable number of starts is reached, after counting of:
    • starts per hour (or adjustable period)
    • consecutive motor hot or cold starts (reacceleration of a motor that is not shut down, indicated by a logic input, may be counted as a start)
  • starts too close together in time: motor re-energizing after a shutdown is only allowed after an adjustable waiting time.
ANSI 50V/51V – Voltage-restrained overcurrent

Phase-to-phase short-circuit protection, for generators. The current tripping set point is voltage-adjusted in order to be sensitive to faults close to the generator which cause voltage drops and lowers the short-circuit current.

ANSI 26/63 – Thermostat/Buchholz

Protection of transformers against temperature rise and internal faults via logic inputs linked to devices integrated in the transformer.

ANSI 38/49T – Temperature monitoring

Protection that detects abnormal temperature build-up by measuring the temperature inside equipment fitted with sensors:

  • transformer: protection of primary and secondary windings
  • motor and generator: protection of stator windings and bearings.

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Voltage protection functions

ANSI 27D – Positive sequence undervoltage

Protection of motors against faulty operation due to insufficient or unbalanced network voltage, and detection of reverse rotation direction.

ANSI 27R – Remanent undervoltage

Protection used to check that remanent voltage sustained by rotating machines has been cleared before allowing the busbar supplying the machines to be re-energized, to avoid electrical and mechanical transients.

ANSI 27 – Undervoltage

Protection of motors against voltage sags or detection of abnormally low network voltage to trigger automatic load shedding or source transfer. Works with phase-to-phase voltage.

ANSI 59 – Overvoltage

Detection of abnormally high network voltage or checking for sufficient voltage to enable source transfer. Works with phase-to-phase or phase-to-neutral voltage, each voltage being monitored separately.

ANSI 59N – Neutral voltage displacement

Detection of insulation faults by measuring residual voltage in isolated neutral systems.

ANSI 47 – Negative sequence overvoltage

Protection against phase unbalance resulting from phase inversion, unbalanced supply or distant fault, detected by the measurement of negative sequence voltage.

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Frequency protection functions

ANSI 81H – Overfrequency

Detection of abnormally high frequency compared to the rated frequency, to monitor power supply quality.

ANSI 81L – Underfrequency

Detection of abnormally low frequency compared to the rated frequency, to monitor power supply quality. The protection may be used for overall tripping or load shedding. Protection stability is ensured in the event of the loss of the main source and presence of remanent voltage by a restraint in the event of a continuous decrease of the frequency, which is activated by parameter setting.

ANSI 81R – Rate of change of frequency

Protection function used for fast disconnection of a generator or load shedding control. Based on the calculation of the frequency variation, it is insensitive to transient voltage disturbances and therefore more stable than a phase-shift protection function.

Disconnection In installations with autonomous production means connected to a utility, the “rate of change of frequency” protection function is used to detect loss of the main system in view of opening the incoming circuit breaker to:

  • protect the generators from a reconnection without checking synchronization
  • avoid supplying loads outside the installation.

Load shedding The “rate of change of frequency” protection function is used for load shedding in combination with the underfrequency protection to:

  • either accelerate shedding in the event of a large overload
  • or inhibit shedding following a sudden drop in frequency due to a problem that should not be solved by shedding.

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About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on


  1. Mohammed kani
    Oct 28, 2014

    What is CVT selection relay? kindly help me out to select the relay from the reputed relay manufacturers.

  2. […] characteristics to coordinate with other protective devices and to protect specific equipment. Instantaneous overcurrent relays have no inherent time delay and are used for fast short-circuit […]

  3. hafizi
    Sep 26, 2014

    this is a very good website for reference in protection system..

  4. Shajeer
    Sep 09, 2014

    Thanks.and please mention ANSI 64 for REF

    Jun 28, 2014

    What is the ANSI codes of Overload protection Relay, Bus CVT selector switch and CVT/PT selection Relay.
    Help urgently

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