Standby Earth Fault Relay Operation 51N

What is Standby Earth Fault Relay:

In electrical power system, the fault occurs due to the various factor such as natural disturbance, lightning on the transmission line, falling trees on the transmission lines, animal fall on the insulator, insulation failure, electrical equipment failure, etc. In such fault is classified into two categories on is symmetrical fault (L-L-L-G) and another one is unsymmetrical fault (L-G, L-L, L-L-G, L-L-L, and L-L-L-G). In this, one of the relay which is used to protect our system against L-G, L-L-G, L-L-L-G faults. The relay is called Standby Earth fault relay.

It is used to protect the system if the neutral current or unbalance current reaches the predetermined value, then the standby earth fault relay operates.

Why it is called standby earth fault relay?

It is a backup protection for restricted earth fault relay. This relay operates when REF delayed operation, Heavy earth fault outside of the REF protective Zone and all other earth faults. Simply we can say, it is a standby protection of all other earth faults.

Standby Earth fault Relay Construction:

In field side, one current transformer will be installed at a neutral side of the transformer or alternator. The current reference will be taken from that CT. In panel side, an over current element will be directly connected with the CT reference.


Note: To get better operating performance from Standby earth fault relay, we should provide current reference to the relay from individual CT or separate core output (if we use multi core CT). The CT reference should not be shared with any other equipment.

For alternator we should install the current transformer separately. But power transformer or distribution transformer comes with NCT (Neutral current transformer). We can take the current reference from the NCT.

Note: If you do not use the NCT output, then the CT secondary should be in short condition.

Standby Earth fault Relay Working Function:

Consider the Current in the three phases are I1, I2 , and I3 the natural current is IN. During Normal condition, there is zero current flow in the neutral. Apply Kirchhoff’s current law..

The relay does not operate the trip circuit associated with the relay.

During abnormal condition, consider there is a fault in phase R, hence the fault current goes to the Neutral. The current transformer reads the fault current and if the fault current exceeds the preset value, the relay operates.

Note: In practical, always some amount of current flows in the Neutral typically 1-15 Amps depending upon rating of the equipment i.e for 25MW alternator IN= 10 -15 amps at full load. Because of all the phases are exactly equal in magnitude and phase.

What happen if standby earth fault relay operates?

Relay 51N operates, Master trip relay 96 operates and the circuit breaker 52 trips.


Please enter your comment!
Please enter your name here