- Trip Circuit Supervision Defined: Trip circuit supervision relay is an essential system in electrical circuit breakers that monitors and ensures the readiness and health of the trip circuit.
- Components: Essential parts of a supervision circuit include NO and NC contacts, relays, lamps, and resistors, all of which contribute to maintaining circuit integrity.
- Supervision Types: Pre and post-close supervision schemes ensure the trip circuit’s functionality before and after the breaker has closed.
- Visual Indicators: The use of lamps in the supervision circuit allows easy monitoring of the system’s status, indicating circuit readiness for operation.
- Alarm and Safety Features: Dedicated alarm circuits, separate from the trip power supply, enhance safety by ensuring system alerts remain active even if the primary trip circuit fails.
There are different contacts connected in series along a trip circuit of an electrical circuit breaker. Certain conditions, like low gas pressure in SF6 circuit breaker or low air pressure in pneumatic circuit breakers, require that the circuit breaker not trip, even if a fault current is detected. In these cases, the trip coil must not be energized. So there must be NO contacts associated with gas pressure and air pressure relays, connected in series with breaker trip coil. Another scheme of trip coil is that it should not be re energized once the circuit breaker is opened. That is done by providing one NO contact of breaker auxiliary switch in series with trip coil. In addition to that the trip circuit of a CB has to pass through considerable numbers of intermediate terminal contacts in relay, control panel and circuit breaker kiosk.
So if any of the intermediate contacts is detached, the circuit breaker fails to trip. Not only that, if DC supply to the trip circuit fails, the CB will not trip. To overcome this abnormal situation, trip circuit supervision becomes very necessary.
The illustration below depicts the simplest form of a healthy trip circuit scheme. It includes a series connection of a lamp, a push button, and a resistor across the protective relay contact. In a healthy state, all contacts, except for the protective relay contact, are closed. Now if push bottom (PB) is pressed, the trip circuit supervision network is completed and lamp glows indicating that the breaker is ready for tripping.
The described scheme provides supervision while the circuit breaker is closed, known as post-close supervision. Another important scheme is known as pre and post-close supervision, which monitors the circuit both before and after closure.
This trip circuit supervision scheme is also quite simple. The only difference is that here in this scheme, one NC contact of same auxiliary switch is connected across the auxiliary NO contact of the trip circuit. The auxiliary NO contact is closed when CB is closed and auxiliary NC contact is closed when CB is open and vice versa. Hence, as shown in the figure below when the circuit breaker is closed the trip circuit supervision network is completed via auxiliary NO contact but when the circuit breaker is open the same supervision network is completed via NC contact. The resistor is used series with the lamp for preventing unwanted tripping of circuit breaker due to internal short circuit caused by failure of the lamp.
Thus far, our discussion has focused on locally controlled installations; however, for distance-controlled installations, a relay system is essential. The figure below shows the trip circuit supervision scheme wherever a remote signal is required.
When the trip circuit is healthy and the circuit breaker is closed, relay A energizes, closing the NO contact A1 and subsequently energizing relay C. Relay C, once energized, keeps the NC contact C1 open. If the circuit breaker opens, relay B energizes and closes NO contact B1, keeping relay C energized and NC contact C1 open.
While CB is closed, if there is any discontinuity in the trip circuit relay A is de-energized which opens contact A1 and consequently relay C is de-energized and which make the NC contact C1 in close position and hence alarm circuit is actuated. Trip circuit supervision is experienced by relay B with the circuit breaker is open in a similar manner as relay A with the circuit breaker is closed. Relays A and C are time-delayed by copper slugs to prevent spurious alarms during tripping or closing operations. The resistors are mounted separately from the relays and their values are chosen such that if any one component is inadvertently short-circuited, a tripping operation will not take place.
The alarm circuit supply should be separated from main trip supply so that the alarm can be actuated even the trip supply failed.
