Wednesday, March 23, 2011

Classification of Circuit Breaker

3. Classification of Circuit Breaker
        The circuit breaker can be classified on the basis of rated voltages: circuit breaker below rated voltage of 1000 V are called low voltage circuit breaker and above 1000V are called high voltage circuit breakers.
          The type of the circuit breaker is usually identified according to the medium of arc extinction. The classification of the circuit breakers based on the medium of arc extinction is as follows:
  1. Air-break circuit breaker. Miniature circuit breaker.
  2. Oil circuit breaker (tank type or bulk oil).
  3. Minimum oil circuit breaker.
  4. Air blast circuit breaker.
  5. Sulphur hexafluoride circuit breaker . (Single Pressure or Double Pressure)
  6. Vacuum circuit breaker.
           Each circuit breaker will be studied thoroughly in the subsequent sections.these circuit breaker employ various technique to extinguish the arc resulting from separation of the current carrying contacts. The mode of arc extinction is either 'high resistance interruption' or 'zero-point interruption'.

High Resistance Interruption: In this process the resistance of the arc is increased by lengthening and cooling it to such an extent that the system voltage is no longer able to maintain the arc and the arc gets extinguished. this technique is employed in air break circuit breakers and d.c. circuit breakers.

Low Resistance or Zero Point Interruption: In this process, the arc gets extinguished at natural current zero of the alternating current wave and is prevented from restriking again by rapid build-up of dielectric strength of the contacts space. The process is employed in almost all a.c. circuit breakers.

             Each leading manufacturer of circuit breaker develops two or more types of circuit breakers for every voltage class. The construction of the circuit breaker depends upon its type (arc-quenching medium), voltage rating and structural form.

Air-break circuit breaker : Utilize air at atmospheric pressure for arc extinction.

Air-blast circuit breakers : Utilize high pressure-compressed air for arc extinction. They need compressed air plant.

Bulk-oil and Minimum-oil circuit breaker : utilize dielectric oil ( Transformer oil ) for arc extinction. In bulk-oil circuit breakers, the contacts are separated inside a steel tank filled with dielectric oil. In minimum oil circuit breakers the contacts are separated in an insulating housing (interrupter) filled with dielectric oil.

SF6 circuit breakers : sulphur-Hexa-Fluoride gas is used for arc extinction. There are two types :
  • Single pressure puffer type SF6 circuit breaker, in which the entire circuit breaker is filled with SF6 gas at single pressure ( 4 to 6 kg/cm2). The pressure and gas flow required for arc extinction is obtained by piston action.
  • Double pressure type SF6 circuit breaker in which the gas from high-pressure system is released into low pressure system over the arc during the arc quenching process. This type has been superseded by puffer type.
Vacuum circuit breaker : In vacuum circuit breaker the fixed and moving contacts are housed inside a permanently sealed vacuum interrupter. The arc is quenched as the contacts are separated in high vacuum.

A brief comparison between the different types of circuit breakers is given in table 1. (see the newer post).

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