Various contacts are connected in series with the trip circuit of an electrical circuit breaker. There must be some condition when the circuit breaker must not trip even one fault current through the electrical contacts. Such conditions are low gas pressure in SF6 circuit breaker, low air pressure in pneumatic operated circuit breaker etc. In this situation the trip coil of the CB should not be energized to trip the CB. Therefore there should be no contact with the gas pressure and air pressure relays, which are connected in series with the breaker trip coil. Another scheme of the trip coil is that it should not be re-energized after opening the circuit breaker. This is done by providing a NO contact of the auxiliary switch connected to the breaker in series with the trip coil. Also CB’s trip circuit has to pass through a large number of intermediate terminal contacts in relays, control panels and circuit breaker kiosks.
So if any of the intermediate contacts are separated, the circuit breaker fails to trip. Not only this, if the DC supply to the trip circuit fails, the CB will not trip. To overcome this abnormal situation, trip circuit monitoring becomes very important. The figure below shows the simplest form of a trip circuit healthy scheme. Here a series combination of a lamp, a push-bottom and a resistor is connected between the contact of a safety relay as shown. In healthy condition all contacts except safety relay contact are in close position. Now if the push bottom (PB) is pressed, the trip circuit monitoring network is completed and the lamp flashes indicating that the breaker is ready to trip.
What we have discussed so far is only for locally controlled installation but for remote controlled installation, relay system is necessary. The figure below shows the trip circuit monitoring scheme wherever a remote signal is required.
When the trip circuit is healthy and the circuit breaker is closed, relay A is energized which closes the NO contact A1 and hence relay C is energized. Energized relay C keeps the NC contact in the open position. Now if the circuit breaker is open, relay B is energized with no contact B1 is closed so relay C is energized. As C is energized, it keeps the NC contact C1 in the open position. While CB is closed, if there is an interruption in the trip circuit relay A is de-energized which opens A1 and in turn relay C is de-energised and which makes NC contact C1 close position. , and hence the alarm circuit is activated. Trip circuit monitoring is experienced through relay B with the circuit breaker open as well as relay A with the circuit breaker closed. Relays A and C have time delays due to copper slugs to prevent false alarms during tripping or closing operations. The resistors are mounted separately from the relay and their values are chosen such that tripping operation will not occur if one of the components is inadvertently short-circuited.
The alarm circuit supply should be isolated from the main trip supply so that the alarm can be activated even if the trip supply fails.