Protection relay codes


Numerical codes are sometimes used when defining the protection relay, whereas in other cases symbols are used. The numerical codes refer to the IEEE C37-2 Standard, whereas the symbols refer to the IEC Standards. In the definition of the symbols the IEC Standards have not detailed all the symbols to be used and so in practice, one still uses the codes mentioned in C37-2.

ANSI Protetion Relay Codes:

An extract of the numerical codes is given below, as given in the C37-2 Standard relative to protection systems. The description is a summary of what is given in the Standard:

– 2 starting timer;
– 21 distance relay (impedance);
– 24 overfluxing relay (volts per hertz);
– 25 synchronizer or synchronism verifier;
– 26 apparatus for temperature control;
– 27 undervoltage relay;
– 32 directional power relay;
– 37 undercurrent or under-power relay;
– 40 loss of field relay;
– 46 negative sequence relay or for current balance by
means of current measurement;
– 47 cyclic sequence relay by means of voltage measure-
– 48 incomplete sequence relay;
– 49 thermal relay for transformers or machines;
– 50 instantaneous overcurrent relay;
– 51 overcurrent relay with inverse time;
– 55 power factor control relay;
– 59 overvoltage relay;
– 60 voltage balance relay;
– 62 stop timer;
– 63 pressure sensor;
– 64 relay to identify ground faults (not used for networks
with grounded neutral);
– 66 apparatus which detects a certain number of opera-
– 67 directional overcurrent relay for alternating current;
– 68 locking relay (for example to prevent reclosing after
loss of step);
– 74 alarm relay;
– 76 overcurrent relay for direct current;
– 78 loss of step relay or for measurement of phase angle;
– 79 reclosing relay for alternating current;
– 81 frequency relay;
– 82 reclosing relay for direct current;
– 83 automatic changeover relay of for selective control;

– 85 pilot wire relay;
– 86 look-out relay;
– 87 differential relay;
– 90 regulator device;
– 91 directional voltage relay;
– 92 directional power voltage relay;
– 94 trip relay.

A Little Explanation:

The meaning of the codes used most frequently is given in
more detail since they are often the cause of misinterpretations
and misunderstandings.

code 48: is a little known code which is, however, now commonly used to indicate the protection against prolonged motor starts. Sometimes it is confused with the protection called 51LR (‘locked rotor’ overcurrent). There are two codes to be used to indicate the protections which serve to control motor starting and locked rotor: 48 for the starting phase (prolonged starting) and 51LR for the locked rotor (when the motor is already running);

– code 50: for the Standard this is an overcurrent protection of instantaneous type. The definition of instantaneous relay was valid for the electromechanical, now the various thresholds of the overcurrent relays always have the possibility of introducing a delay. In common practice it is considered to be the overcurrent protection which identifies strong currents typical of short-circuit;

– code 51: for the Standard, this is an overcurrent protection of the dependent (inverse) time type. The definition of a relay with inverse time is typical of American tradition. In common practice code, 51 is used both for overcurrent relay with dependent (inverse) time characteristic and with independent (definite) time characteristic. In general, it is considered the overcurrent protection which identifies weak currents typical of an overload or of short circuits with high fault impedance.

Further clarifications are necessary in defining the numerical codes to be used for the protection against ground faults. The C37-2 Standard only specifies a code to be used for ground faults: 64, but specifies that this code cannot be used for the protections connected to the CT secondary in grounded networks where code 51 must be used with suffixes N or G. In defining the N and G suffixes, the C37-2 Standard is very clear and they are used as follows:

– N when the protection is connected by means of transducers which measure the phase parameters and the vectorial sum of the parameter to be measured (current or voltage) is sent to the relay. This connection is generally called residual connection (Holmgreen);

– G when the protection is connected directly to the secondary of a transducer (CT or VT) which measures the homopolar parameter directly (current or voltage);

Therefore it is correct to use the following definitions for protection against ground fault:

– 51G for the overcurrent protection connected to the secondary of a ring CT which measures the ground current;

– 51G for the homopolar overcurrent protection connected to the secondary of a CT positioned on the grounding of the machine (star point generator or transformer);

– 51N for the homopolar overcurrent protection connected with residual connection to three phase CTs;

– 59N for the homopolar overvoltage protection con- nected on the vectorial sum of the three phase VTs (open delta – residual voltage);

– 59G homopolar overvoltage protection connected to the VT secondary positioned on the machine grounding (star point generator or transformer);

– 64 only applicable in networks with isolated neutral both for overcurrent and overvoltage protection.

Apart from the N and G suffixes, sometimes other suffixes are added to indicate the application of the protection in detail.
For example:

– G generator (for example 87G differential protection for generator);
– T transformer (for example 87T differential protection for transformer);
– M motor (for example 87M differential protection for motor);
– P pilot (for example 87P differential protection with pilot wire);
– S stator (for example 51S overcurrent stator);
– LR motor protection against locked running rotor (51LR);
– BF failed opening circuit-breaker 50 BF (BF = breaker failure);
– R used for different applications:
– reactance (for example 87R differential protection);
– undervoltage to indicate residual voltage (27R);
– rotor of a synchronous machine (64R ground rotor);
– V associated with the overcurrent protection (51) it indicates that there is voltage control or voltage restraint (51V);
– t indicates that the protection is timed (for example 50t protection against overcurrent short-circuit with delay added).

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