Restricted earth fault relay is that relay which can be defined as half differential relay. Figure (43) Shows a connection diagram for the restricted earth fault relay.
This definition is due to its connection. This is relay compare the current flow in :
- The neutral path of current transformer connected on the star side of power transformer:
- Current flow in the current transformer connected on the star point of the power transformer.
- For internal faults ( between the two current transformers), There is differential current which is transformed to volt by a very high shunt resister (metrosil) and this value of volt is sufficient for operation of the relay.
- For external faults (as example: outgoing feeders faults), the differential current in this case is zero and the relay will not operate, and no tripping for circuit breakers will occur.