Trip Circuit Supervision and IRF

Back in the olden days, on large industrial power systems circuit breakers, the trip circuit was supervised by a 55 volt lamp wired in parallel with the normally open protective relay mechanically actuated trip contacts. On the 125 Vdc station battery supplied control circuit, the lamp would illuminate as long as the circuit breaker trip coil circuit was complete.

If an electromechanical protective relay operated, the full voltage is applied to the trip coil, releasing the opening springs and opening the breaker primary contacts. After operation, the trip coil circuit is interrupted, as continuous full voltage on the coil would overheat the coil, designed for ultrafast operation (mS) during a trip event.

The status of the lamps was observed on some periodic basis. If the lamp was illuminated, then one was reasonably assured that if a protective relay operated, the breaker would open.

There is no undervoltage release on this sort of application, and no automated action upon the detection of trip circuit failure. In power system design, there is always a backup protection scheme. The possibility of increased damage due to failure of the primary circuit protection is accepted, based on the cost of a circuit interruption.

Without the full story of the application, including financial and operational consequences, it is difficult to justify a given reaction scheme in response to a failure, any given sort of failure.