1 - Restricted earth fault relay:

Restricted earth fault relay is used for protection of an earth fault for the restricted zone as per your requirement but mainly is used for ground fault in transformer HT side.

It is a very highly sensitive relay as 2-8% and another differential relay works as sensing 20-40%. It works as fast due to the simple measuring process with the help of one winding. It protects a zone between two measuring points.

2 - Working principle of restricted earth fault relay.

Restricted earth fault relay is installed restricted zone like as transformer primary winding to primary CT in HT panel or secondary winding to neutral.

All three-phase CT current and neutral CT current is monitored by restricted earth fault relay (REF) relay. As per REF relay function, the phase current transformer (CT) current and neutral CT current should be equal. It means total current should be zero. So, this type situation the REF relay will not be activated.

Any fault condition the both side CT current will become unbalance and the unbalance current will flow in REF coil. If the unbalance current value crosses above preset value then the REF relay will be activated and supply breaker will trip.

3 - Connection diagram of restricted earth fault relay.

4 - Why restricted earth fault relay is used?

A restricted earth fault relay is a very sensitive relay. It works minimum leakage current as 5% but another earth fault relay operates on more than 20% leakage current. if the earth’s fault current is less than 20% then it will be very difficult to protect the system. So, 100% earth fault protection is not possible by other normal earth fault relays.

So, remove for uncertainty situation and keep in mind for 100% internal earth fault protection, we used very sensitively restricted earth fault relay.

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