A dc circuit from station battery to protection relay trip contact and onwards to high voltage circuit breaker trip coil and back to the station battery is usually called trip circuit for the transmission line.
A continuous supervision of this circuit is necessary which ensures presence of DC supply, healthiness of complete trip circuit. Relay which performs the above functions is a trip circuit supervision relay.
The trip circuit monitor does not operate during system faults. A momentary closed tripping contact does not lead to a failure message. If, however, tripping contacts from other devices operate in parallel in the trip circuit, then the fault annunciation must be delayed. After the malfunction in the trip circuit is cleared, the fault annunciation is reset automatically after the same time period.
In modern digital relays supervision with binary inputs not only detects interruptions in the trip circuit and loss of control voltage, it also supervises the response of the circuit breaker using the position of the circuit breaker auxiliary contacts.
