Trip Circuit of a Circuit Breaker

Posted on Feb 6, 2014

Consider a simplified circuit of a typical relay as shown in the Fig. 1 usually the relay circuit is a three phase circuit and the contact circuit of relays is very much complicated. The Fig. 1 shows a 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

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connected with its primary in series with the line to be protected. The secondary of C. T. 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 as shown in the Fig. 1, Then current through the line connected to A increases to a very high value. 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. The relay with make type contact requires auxiliary d. c. supply with its operation while the relay with break type contact uses the energy from the main supply source for its operation. Let us see the details of these two types of schemes. As mentioned earlier, a separate supply is necessary for the relay operation. The relays are connected in star while the relay contacts are connected in parallel....

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