Trip Circuit of a Circuit Breaker

Trip Circuit of a Circuit Breaker
       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 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.
Fig. 1  Typical relay circuit

1. Tripping Schemes in Circuit Breaker
        Two schemes are very popularly used for tripping in circuit breakers which areو
1. Relay with make type contact       2. Relay with break time contact
       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.
1.1 Relays With Make Type Contact
       The schematic diagram representing the arrangement of various elements in a relay with make type contact is shown in the Fig.2.
Fig. 3  Relay with make type contact

       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. The entire relay contact unit is connected in series with the auxiliary switch, trip coil and the battery. Relay contacts are open in normal position.
       Operation : When the fault occurs, the current through relay coils increases to a very high value. Due to this, the normally open relay contacts C1, Cand Cget closed. This activates the trip coil of a circuit breaker. The auxiliary switch is initially closed along with the circuit breaker. So when contacts C1, Cand Care closed, the current flows through trip coil of circuit breaker. This activates the trip coil which opens the circuit breaker. As auxiliary switch is mechanically coupled with the circuit breaker, it also gets opened. This interrupts the current through trip coil. Thus supply to fault part gets interrupted and trip coil also gets de-energized. This brings the relay contacts back to normal position. Due to auxiliary switch, arcing across relay contacts gets avoided. As relay contacts are normally open and they 'make' the circuit to open the circuit breaker hence called make type contact relay.
1.2 Relay With Break Type Contact
       The schematic arrangement of various elements in a relay with break type contact is shown in the Fig. 3.
Fig. Relay with break type contact (using C.T.s

       This type of relay does not require external battery supply for the tripping. The current transformers (C.T.s) or potential transformer (P.T.s) are used to derive the energy required for the relay from the main supply source. The relay using C.T.s to derive operating energy from the supply is shown in the Fig. 3.
        In this scheme, the relay coil and trip coil of each are connected in series. The three phases are then connected in star. Under normal working, the relay contacts C1, C2 and C3 are closed. The energy for relay coils is derived from supply using C.T.s. The trip coils of circuit breaker are de-energized under normal condition. When the fault occurs, heavy current flows through relay coils due to which relay contacts C1, C2 and C3 break. Thus current flows through trip coils of circuit breaker due to which circuit breaker gets open.
       The Fig. 4 shows the break type contact relay using P.T. to derive energy to keep relay coils energized.
Fig. 4  Relay with break type contact (using P.t

        In this type, in addition to normal trip coils of circuit breaker, an additional undervoltage trip coil is used. All the relay contacts are in series with the undervoltage trip coil. Through potential transformer, for normal voltage, the undervoltage trip coil is kept energized. When the voltage becomes less than the normal value, the magnetic effect produced by undervoltage trip coil reduced which is responsible for the opening of the circuit breaker. When fault occurs, the normal trip coils of circuit breaker comes into the picture and are responsible for the opening of the circuit breaker.
        In both the types, relay contacts 'break' to cause the circuit breaker operation hence the relay is called break type contact relay.

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