A trip unit is electrically connected in parallel with a current limiting polymer element in series with circuit breaker contacts to function as a shunt resistance for the polymer element.It becomes energized by transition of the polymer element to its high resistive state for tripping the breaker contacts. The solenoid may be particularly wound with a resistive wire.
Simplified Circuit Diagram Of Trip Circuit Of A Circuit Breaker :
Here a simplified circuit of a typical relay is given.Basically the relay circuit is a three phase circuit and the contact circuit of relays is very much complicated. This diagram is of single phase simplified circuit to explain the basic action of a relay. Let part A is the circuit to be protected. The current transformer(C.T.) is connected with its primary winding in series with the line to be protected. The secondary winding of the current transformer is connected in series with the relay coil. The relay contacts are the part of a trip circuit of a circuit breaker. The trip circuit consists of a trip coil and a battery, in addition to relay contacts. The trip circuit can operate on a.c. or d.c.
If the fault occurs, Then current through the line connected to A increases to a very high value.Then the current transformer senses this current. Accordingly its secondary current increases which is nothing but the current through a relay coil. Thus the relay contacts get closed mechanically under the influence of such a high fault current. Thus the trip circuit of a circuit breaker gets closed and current starts flowing from battery, through trip coil, in a trip circuit. Thus the trip coil of a circuit breaker gets energized. This activates the circuit breaker opening mechanism, making the circuit breaker open. This isolates the faulty part from rest of the healthy system.