Answer:
The tripping mechanism for most high-voltage circuit breakers is released through the use of a solenoid built into the circuit breaker. The supply to the tripping solenoid is from a DC source located in the substation building. There are various types of protection relay that will energise the tripping circuit -e.g. an overcurrent relay will detect a fault current due to, say, a short circuit in the protected line; after a specified time delay, a pair of contacts inside the relay will close, and energise the trip solenoid, opening the circuit breaker, and disconnecting the faulted line.