TYPES OF CIRCUIT BREAKERS

1. Air-Break Circuit Breakers (ACB)
2. Oil Circuit Breakers
3. Vacuum Circuit Breakers (VCBs)
4. Air-Blast Circuit Breakers (ABCB)
5. SF₆ Circuit Breakers

PROTECTIVE RELAYS

Whenever an abnormal condition occurs in an electrical power system, some action is required to isolate the abnormal condition instantaneously or after a predetermined time delay (in some cases) and leave the remainder in normal service. The protection provided by the fuse is limited to low voltage circuits. For higher voltages from 33 kV and above, the protection is provided using protective relay. When a fault occurs the relay operates to complete the trip coil circuit which results in the opening of the CB and therefore, faulty section is isolated from the rest of the system.

Some of the important features of a good protective relays/gear are as follows :

• Selectivity: The property of discrimination is an absolute requirement of a protective gear, i.e., the protective system must select correctly the faulty part of the system and disconnect the same without disturbing the rest part of the system.
• Sensitivity: The protective system must be able to operate even for a low value of fault current i.e. it should be as sensitive as possible.
• Reliability: The protective system must operate definitely under predetermined condition.
• Quickness: The protective system must be capable to respond quickly in order to improve quality of service, increase safety of life and equipment and increase stability of operation.
• Non-interference with future extension: The initial installation of the protective system should be such that future extension can be carried out if required without interfering the original one.
• Stability: A protective system should remain stable even when a large current is flowing through its protective zone due to external fault, which does not lie in its zone.