Trip circuit plays a very important role in protection system. A Trip circuit Supervision Relay is the watch dog to the trip circuit of any electrical system. If by some means the trip circuit is not functioning-it will send out alarm or whatever means is assigned to.
Trip circuit – what its functions
Why the trip circuit is required to be live always:
As the trip circuit has some important purpose the functionality or health of it must be ensured. What happens if a trip circuit is open or damaged? In this case the circuit will be totally unprotected. The relay/ PLC will send out signal after happening a fault but as trip circuit is dead , the breaker will not open or disconnect.
In a protection system the tripping of a circuit breaker is crucial. If an interruption occurs in the trip circuit, the possible network fault will not be disconnected and the fault would have to be cleared by another protection upstream in the power system. The supervision function is particularly important when there is only one tripping coil and CB tripping is vital. (For instance, in the case of generator circuit breakers or any other important circuit breaker in distribution networks.)
Functions of a typical trip circuit supervision relay
[alvaa]Below is trip circuit supervision relay function described based on a specific model- the settings and data might varied to models but the core principle is same.
The supervision function is based on a low-level (~ 3 mA) current injection principle. The injected current is sensed by two opto-couplers. The supervision function in three steady states of circuit breaker-trip circuit is below image.In normal condition the indicator LED glows green and output relays are in ‘picked-up’ condition. In the event of a fault, the measuring current goes below the operating value of the relay (0.3 – 0.7 mA) or completely stops flowing.
The supervision relay operates (drops-off) after a delay of 0.6 sec and the indicator LED turns red. The supervision relay, for its functioning requires an auxiliary voltage (AC or DC) of a rated value . This voltage can be the same as that of the supervised circuit or it could be a separate source with same magnitude (AC or DC). Should a fault occur in the auxiliary voltage supply, the LED does not glow and the output relay drops off. Relays with differing rated voltage for supervision circuit and auxiliary supply can be supplied as a special execution.