Monitoring is done by passing a small current through F74 relay connected with the tripping coil of breaker. The current flowing through this monitoring circuit is normally less than 1.4mA. Therefore, F74 relay have consumes power a very small.
Must be compatible with the tripping time of main protection relay. The tripping time of the circuit breaker must be less than 150ms, otherwise the relay will trip wrong and warning the trip-circuit is broken when main protection relay tripped.