1. By R. Vijay Krishna

2. CONTENTS  INTRODUCTION  WORKING PRINCIPLE OF C.B  TYPES OF CIRCUIT BREAKERS  SF6 CIRCUIT BREAKER

3. INTRODUCTION  A circuit breaker is an equipment designed to protect an electrical apparatus from damage caused by overload or short circuit.  Unlike a fuse, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

4. WORKING PRINCIPLE OF CIRCUIT BREAKER  The basic circuit breaker consists of a simple switch, connected to either a bimetallic strip or an electromagnet. electromagnet  The electricity magnetizes the electromagnet When the current jumps to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage.  The entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit. The electricity shuts off.

6. TYPES OF CIRCUIT BREAKERS  OIL CIRCUIT BREAKERS  AIR CIRCUIT BREAKERS  VACUUM CIRCUIT BREAKERS  SF6 CIRCUIT BREAKERS

7. SF6 CIRCUIT BREAKER  SF6 has excellent insulating strength  When a free electron collides neutral gas molecule a negative ion is formed and the electron is absorbed by neutral gas molecule SF6 + e SF6 -  Negative ions formed are relatively higher compared to free electrons

8. FEATURES OF SF6  SF6 possess high dielectric strength  SF6 has property of fast recombination after source energizing spark is removed.  SF6 has excellent heat transfer properties because its high molecular weight together with its low gaseous viscosity enabling it to transfer heat by convection.

9. CONSTRUCTION OF SF6 CIRCUIT BREAKER SF6 circuit breaker consists of  Interrupter unit  Gas system

10. INTERRUPTER UNIT  Consists of fixed contacts with set of current carrying fingers and an arcing probe.  Fingers make contact with the moving contact when the breaker is in close position.  The contacts are surrounded by interrupting nozzles and a blast shield

11.  The moving contact is in the form of hollow nozzle sliding in a second set of spring loaded fingers.  As soon as the moving contact is with drawn from the fixed finger contacts an arc is drawn between the moving nozzle and the arcing probe.  As the contacts move further apart, the arc is extended and attenuated and finally extinguished by the gas flow from high pressure to low pressure.

13. GAS SYSTEM  Perfect sealing to prevent leakage of gas as it is maintained at high pressure.  Low and high pressure system are fitted with low pressure alarms which gives warning the moment gas pressure drops below certain value.  If danger limit is reached the safety devices immobilize the breaker.

14. ADVANTAGES OF SF6 C.B.  Current chopping is minimized at low pressure and low velocity.  Eliminates moisture problem.  Gives noiseless operation.  No carbon particle is formed during arcing.  Atmospheric conditions does not affect Circuit breaker performance.