Protection Relay ANSI Codes - Electrical Maintenance Engineers (2023)

In any typical electrical power system, different types of protection devices are required such as overload, short circuit, under-voltage, etc. these protection devices protect the electrical equipment against any faults that may occur and trip the circuit before any damage can occur to the equipment.

To differentiate between these different types of protection devices, the ANSI/IEEE C37.2 Standard assigned a specific ANSI code number dedicated to each specific protection function. These ANSI codes are used on the schematic diagrams so that electrical engineers can recognize and easily understand the schematics.

In the same manner, The IEC standard assigns a specific symbol to each protection function as well. As a reminder, ANSI standards are typically used in the USA while IEC standards are typically used in Europe.

Here we shall explain in brief some of the most common protection ANSI codes that are utilized to protect different electrical equipment such as generators, transformers, busbars, motors, etc.

Over Speed (ANSI code 12)

Overspeed protection is mainly found on power plant generators where Overspeed occurs due to a temporary loss of a load or a fault condition.

Note that the generator will trip if the fault is in the generator or the step-up transformer. If the turbine Overspeed problem causes the bus frequency to become too high, a generator trip signal will be initiated.

The rotation speed is calculated by measuring the time between pulses transmitted by a proximity sensor when it is passed by one or two cams driven by the motor or the generator shaft.

Under Speed (ANSI code 14)

If the motor or the generator speed drops below a certain preset value, a trip signal will be initiated. One of the methods of detecting a locked rotor is by sensing a zero speed signal from the speed sensor.

Synchro-Check (ANSI code 25)

The Synchro-check is mainly used in the case of parallel operation of two incomers.

As an example, assuming medium voltage switchgear with two incomers and a bus coupler between them. Before the parallel operation and closing the bus coupler circuit breaker, the Synchro-check relay ensures that the differences in the voltage magnitude (ΔU), frequency (Δf) and phase shift (Δθ) of both the incomers are within set limits, otherwise, the bus coupler will never close.

If there is no Synchronization between the two incomers, a circulating current will pass between them causing unnecessary heating effects.

Undervoltage (ANSI code 27)

This function is used to protect against phase to neutral or phase to phase voltage dips, accordingly, the proper action is made according to the installed power system configuration such as initiating an automatic load shedding or transfer the loads to another source.

This function has a voltage threshold and a time threshold (time delay), in case of temporary Undervoltage, the relay will not be triggered until the time threshold is reached to avoid nuisance tripping.

Positive Sequence Undervoltage and phase rotation direction check (ANSI code 27B)

This function mainly protects the motors from the unbalanced supply voltage and the reversal of the phase rotation. It is based on the measurement of the positive sequence voltage.

Directional active overpower (ANSI code 32P)

This function is used for protection against active overpower consumption (overloading) in the direction of the normal flow of power or the reverse direction, that’s why it is called directional.

It is based on the two or three-wattmeter method. Normally, generators supply power to the load while motors consume power from the supply, this is the normal flow of power.

If active overpower consumption is detected, load shedding can be started. A reverse overpower occurs if a generator starts running like a motor consuming power or a motor starts running like a generator supply power in the reverse direction.

Directional reactive overpower (ANSI code 32Q)

This function picks up when the reactive power consumed passes the Q set point. It is mainly used for the protection against field loss on the synchronous machines.

Reactive overpower protection for motors that consume more reactive power following field loss and for protecting generators that consume more reactive power following field loss.

Temperature monitoring (ANSI code 38/49T)

This function detects any abnormal temperature rise in the equipment by measuring the temperature directly using temperature sensors embedded inside the equipment.

It is normally used to monitor the temperature of the transformers (primary winding, a secondary winding, transformer oil), motors, and generators (stator winding and bearing).

Usually, RTDs are used as temperature sensors because of their high accuracies such as PT100 or NI100. To know more about RTDs, check out What are RTDs?.Normally two set points are utilized, an alarm and a trip setpoint.

Negative sequence / Unbalance (ANSI code 46)

This function protects the equipment against phase current unbalances by measuring the current negative sequence component. If the negative sequence component value passes a preset value, a trip signal will be initiated.

Negative sequence overvoltage (ANSI code 47)

This function protects the equipment against phase voltage unbalances or reversed two phases. This is done by measuring the voltage negative sequence component. The function will pick up if the negative sequence component value passes a preset value.

Excessive starting time / Locked rotor (ANSI code 48/51LR)

This function protects the motors windings from overheating effects that may be caused due to one of the following reasons:

• Excessive motor starting time due to overloads (e.g. conveyor) or insufficient supply voltage.
• Locked rotor condition due to motor load (e.g. crusher) during normal operation (after a normal start) or directly at motor start, before the detection of an excessive starting time.

Thermal overload (ANSI code 49RMS)

As the name implies, this function provides overload or long-time protection for the equipment. It calculates the equipment thermal capacity by measuring the currents of two or three phases using the C.Ts installed on each phase.

Usually, two set points are defined, an alarm setpoint and a trip setpoint.

Circuit breaker failure (ANSI code 50BF)

This function acts as backup protection, if an overcurrent occurs and the downstream breaker failed to trip, a signal is sent to the upstream circuit breaker to trip the circuit after a preset time delay.

Phase overcurrent (50/51)

This function protects against overloads or short circuit currents. The tripping curve can be Definite Time (DT) or Inverse Definite Minimum Time (IDMT).

ANSI code 50 indicates an instantaneous overcurrent protection while ANSI code 51 indicates a time-delayed overcurrent protection.

Current and time settings are set to ensure proper coordination between the downstream circuit breaker and the upstream one.

Earth fault (ANSI code 50N/51N or 50G/51G)

This function protects against earth leakage currents or phase short circuit to ground currents.

It detects the earth fault current by either calculating the summation of the 3 phase currents from the C.Ts installed on each phase (50N/51N) or by directly measuring the summation of the 3 phase currents through a toroid C.T installed on the 3 phases (50G/51G).

Voltage-restrained overcurrent (ANSI code 50V/51V)

This function is mainly used to protect generators from close short circuits that cause a voltage dip to the generator voltage. This function measures both the current and voltage of the generator.

Overvoltage (ANSI code 59)

This function protects against phase to neutral overvoltage or phase to phase overvoltage.

If the overvoltage persists after a preset time delay, a source transfer is started (if possible) to ensure continuity of supply to the loads and the faulty supply will be tripped.

Restricted earth fault (ANSI code 64REF)

This function is mainly used to protect generators or power transformers against internal earth faults.

It is implemented by calculating the earth fault current (If1) through the summation of the phase currents C.Ts in addition to measuring the earth fault current (If2) through the toroid C.T of the earthed neutral point of the generator or the transformer, then calculate the difference between them (Id = If1 – If2).

In case of an external fault, If1 will cancel If2 and the function won’t be initiated but in case of an internal faulty, only If2 will sense the fault and the function will pick up.

Starts per hour (ANSI code 66)

This function is mainly used to protect motors from successive multiple starts that might cause overheating to the motor windings, so a certain number of starts is defined to each motor which mainly depends on the current temperature of the motor and the cooling time constant of the motor.

If the preset successive number of starts of the motor is exceeded, a start inhibition signal will be initiated for a certain amount of time till the motor is cooled down.

The number of starts is usually defined as the successive number of starts per hour. Usually, there is a cold number of starts and a hot number of starts.

A Cold start means that the motor is started while the motor is cold before reaching its operational temperature. A hot start means that the motor is started while the motor is hot after reaching its operational temperature.

Directional phase overcurrent (ANSI code 67)

This function is just like the phase overcurrent function (ANSI code 50/51) in addition to the detection of the fault current direction.

This function is mainly used in the case of parallel operation of two feeders. If a fault occurred at the output of the first feeder, a fault current will pass from the first feeder in the normal direction and another fault current will pass from the second feeder through the busbar in the reverse direction feeding the fault.

The function will pick up if the current in the reverse direction passes a preset value.

Recloser (ANSI code 79)

Some electrical faults in certain parts of the power system are temporary so after the circuit breaker tripping, an automatic reclosing of the circuit breaker is initiated to ensure the continuity of the power supply to the loads.

This function is mainly used in overhead transmission and distribution lines because usually, the faults are temporary and self-cleared (ex. falling of a tree branch), on the other side, this function isn’t used in underground networks as faults tend to be permanent.

Overfrequency (ANSI code 81H)

This function is mainly used in power generators. In a typical power system, the prime mover mechanical (generated) power is kept equal to the generator electrical (output) power, and accordingly, the system voltage frequency is kept constant in the normal operating conditions.

Any frequency variations that occurred in the system will be a result of a mismatch between the generation (mechanical) power and the load (electrical) power.

Overfrequency occurs due to the disconnection of large loads, this load disconnection increases the prime mover speed which in turn increases the generator output frequency.

The prime mover Overspeed must be limited to avoid high vibrations or even equipment damage so the function will pick up if the frequency passes the set point.

Underfrequency (ANSI code 81L)

Underfrequency occurs due to a system overloading. Due to system overloading, the prime mover speed decreases, and accordingly, the voltage frequency decreases.

Underfrequency is a big issue especially to generators because as the prime mover speed decreases, the generator ventilation decreases and the flux density (V/HZ) increases. A trip signal will be initiated if the frequency goes below the set point.

Differential protection (ANSI code 87)

This function protects the equipment against short circuits, it requires two sets of C.Ts, it only protects the zone between the C.Ts, any external fault occurs outside this zone can’t be seen.

This function depends on the difference in current value (Id) between CT1 and CT2 in the same phase as shown in the figure below. The function will pick up if the difference in current (Id = I1 – I2) passes the Id set point.

Differential protection implementation is very expensive, that’s why it is installed only for critical equipment such as generators, power transformers, and large medium voltage motors.

References

[1] IEEE Std C37.2-2008 – Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.

[2] Easergy Sepam Series 80, “Protection, metering, and control functions” User Manual 07/2017.