CIRCUIT BREAKER

Instrumental

Technique

Md Bodiuzzaman19-12-15

WHAT IS A CIRCUIT BREAKER?

• A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.

• Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Operating Principle

Two contacts called electrode remains closed under normal operating conditions. When fault occurs on any part of the system, the trip coil of the circuit breaker get energized and contacts are separated.

Arc Phenomenon

• An arc is struck when contacts are separated.

Thus the main duty of a circuit breaker is to

extinguish the arc within the shortest possible

time.

• The arc provides the low resistance path to the

current and the current in the circuit remains

uninterrupted.

Methods of Arc ExtinctionHigh Resistance MethodSeparate the contacts of circuit breaker such that the arc resistance increases to a very high value. The potential difference between the contacts is unable to maintain the arc current. For high voltage circuit breakers this method is impracticable since a separation of many meters will be required.

Low Resistance Method

The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized ,the arc extinction is facilitated .This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.

TYPES OF

CIRCUIT BREAKER

OIL CIRCUIT BREAKER

AIR CIRCUIT BREAKER

SF6 CIRCUIT BREAKER

VACCUMCIRCUIT BREAKER

Air Circuit Breakers

1.In this Circuit Breaker the arc is elongated using arc runners and arc splitters so as to increase the resistance of the arc.

2. This increases the voltage required to maintain the arc and if the available voltage cannot sustain the arc ,the arc gets extinguished.

3.Used for low voltage levels and current levels.

4.Air is used as medium to extinguish the arc which have inferior extinguishing properties compared to SF6.

5. It is used up to 6.6kV with a breaking capacity of 15MVA.

SF6 Circuit Breaker

1. Sulphur Hexafluoride (SF6) gas is used as an arc quenching medium.

2. SF6 is an electro-negative gas.3. It has strong tendency to absorb electrons.4. When contact are opened in a high pressure flow of

SF6 gas, arc produced.5. Free electron in the arc are captured by the gas.6. Which build up enough insulation strength to

extinguish arc.7. It is much effective for high power and high

voltages services,

Sulfur Hexafluoride Circuit Breaker

Advantages Silent operation, compact size

Vary short arcing time

No risk of fire

No reduction in dielectric strength due to operation

Can interrupt larger currents

Suitable for explosive environment due to totally enclosed body