The trip circuit supervision relay always monitors the healthiness of the trip circuit including a connected trip coil of the CB. Tripping or isolation of CB is the most important part of power system protection. Always 02 nos. trip circuits are used in the CB for system redundancy.
Due to the failure of the trip coil or any discrepancy in the trip circuit, the CB will not operate even after receiving the trip signal from the protection relay. A separate relay is installed in the tripping circuit to monitor or supervise the healthiness of the trip circuit. If the supervision relay is operated or triggered, it should attend immediately by Protection Engineer.
Diagram & Working Principle of Trip Circuit Supervision Relay
As shown in the diagram, the CB trip coil will energize when the trip relay is operated. When the CB is open, the position of the S1 & S2 contacts are NO & NC respectively and when the CB is closed, the position of S1 & S2 contacts are NC & NO.
DC(+Ve) of the trip circuit supervision relay is permanently connected and through the trip coil and S1/S2 contact, DC (-Ve) is extended to the relay. Under CB open condition, DC (-ve) is extended through S2 contact, and under CB close condition, DC(-ve) is extended through S1 contact.
At both the CB open & close position, DC (-ve) is extended to the trip supervision relay and it will not operate. Due to the failure of the trip coil or any other issue, DC (-ve) will not be extended to the supervision relay, and under that condition, the relay will operate.
Why does the supervision relay used?
02 nos. trip circuits are used in circuit breakers to ensure tripping of the CB during fault conditions. In addition to trip circuit supervision relay is also used for continuous monitoring of the healthiness of the circuit.
The supervision relay can monitor the healthiness of the trip coil. After the failure of the trip coil, the supervision relay will alert for the same.
If the DC source failed, under this condition also supervision relay will operate and create an alarm