Primary relay or primary protection relay is the first line of power system protection whereas backup relay is operated only when primary relay fails to be operated during fault. Hence backup relay is slower in action than primary relay. Any relay may fail to be operated due to any of the following reasons,
1. The protective relay itself is defective.
2. DC Trip voltage supply to the relay is unavailable.
3. Trip lead from relay panel to circuit breaker is disconnected.
4. Trip coil in the circuit breaker is disconnected or defective.
5. Current or voltage signals from CT or PT respectively is unavailable.