CIRCUIT BREAKER THEORY

CIRCUIT BREAKER

The main difference between a fuse and circuit breaker is that under fault condition the fuse melts and it is to be replaced whereas the circuit breaker can close or breaker the circuit without replacement.

Requirements of circuit breakers:-

The power association with the circuit breakers is large and it forms the link between the consumers and supplies and suppliers. The necessary requirements of circuit breakers are as follows

    1.The normal working current and the short circuit current must be safely interrupted by the circuit breaker

    2.The faulty section of the system must be isolated by circuit breaker as quickly as possible keeping minimum delay

    3.It should not operate with flow of over current during healthy conditions. 

   4. The faulty circuit only must be isolated without affecting the healthy one.

BASIC ACTION OF A CIRCUIT BREAKER

As shown in fig the elementary diagram of a circuit breaker. It consists of two contacts a fixed contact and a moving contact . A handle  is attached at the end of the moving contacts. It can be operated manually or automatically. The automatic operation needs a separate mechanism which consists of a tip coil. The trip coil is energized by secondary of the current transformer. The terminals of circuit breaker are brought to power supply.

Under normal condition working conditions the emf produced in the secondary winding of the transformer is insufficient to energize the trip coil completey for its operation. Thus the contacts remain in closed position carrying the normal working current. The contacts can be opened manually also by handle.

       Under abnormal or faulty conditions high current in the primary winding of the current transformer induced sufficiently high emf in the secondary winding so the trip coil is energized. This will start opening motion of the contacts. The contacts are moved towards right away from fixed contacts.

As we have seen already the separation of contacts will not lead to breaking or interruption of circuit as an arc is struck between the contacts. The production of arc delays the current interruption and in addition to this it becomes large amount of heat which may damage the system or the breaker. Thus it becomes necessary to extinguish the arc as early as possible in minimum time, so that heat produced will lie within the allowable limit. This will also ensure that the mechanical stresses produced on the parts of circuit breaker are less.

The time interval which is passed in between the energization of the trip coil to the instant of contact separation is called the opening time. It is dependent on fault current level.

The time interval from the contact separation to the extinction of arc is called arcing time. It depends not only on fault current but also on availability of voltage for maintenance of arc and mechanism used for extinction of arc.