What is a blocking relay and under what circumstances would it block the operation of a circuit breaker?

What is a blocking relay and under what circumstances would it block the operation of a circuit breaker?

• A. A blocking relay prevents tripping unless a permissive condition is met; used to avoid unnecessary trips or coordinate with higher-priority protection schemes.
• B. It prevents any breaker operation under all fault conditions.
• C. It blocks protection actions only when the system is isolated.
• D. It is used to trip breakers faster during faults.

Answer: (A)

Blocking relays provide a permissive path for a breaker to trip, acting as a veto so that the trip signal is allowed only when a higher-priority protection action or a specific condition is met. This keeps the system from tripping unnecessarily and enforces selective coordination between protective devices. For example, if a fault on a feeder can be cleared by a upstream protection scheme, the blocking relay will hold off the local breaker until that upstream device issues a permit to trip. The other scenarios don’t fit because a blocking relay does not prevent trips under all fault conditions, it doesn’t block protection actions merely because the system is isolated, and it is not used to make breakers trip faster; its purpose is to delay or prevent trips until the correct permissive condition exists.

Blocking relays provide a permissive path for a breaker to trip, acting as a veto so that the trip signal is allowed only when a higher-priority protection action or a specific condition is met. This keeps the system from tripping unnecessarily and enforces selective coordination between protective devices. For example, if a fault on a feeder can be cleared by a upstream protection scheme, the blocking relay will hold off the local breaker until that upstream device issues a permit to trip. The other scenarios don’t fit because a blocking relay does not prevent trips under all fault conditions, it doesn’t block protection actions merely because the system is isolated, and it is not used to make breakers trip faster; its purpose is to delay or prevent trips until the correct permissive condition exists.