Vacuum Circuit Breaker Construction and Working Principle

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What is a Vacuum Circuit Breaker?

A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in a vacuum medium. The operation of switching on and closing of current carrying contacts and interrelated arc interruption takes place in a vacuum chamber in the breaker which is called a vacuum interrupter.

The maintenance of vacuum pressure within a vacuum interrupter can be done at 10– 6 bar. The vacuum circuit breaker performance mainly depends on the material used for current-carrying contacts like Cu/Cr.

Construction of Vacuum Circuit Breaker

  • It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak.

  • A glass vessel or ceramic vessel is used as the outer insulating body.

  • The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover.

vacuum circuit breaker
vacuum circuit breaker

Working Principle of Vacuum Circuit Breaker

  • When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts.

  • The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields.

  • Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation (say 0.625 cm). Vacuum circuit breakers are employed for outdoor applications ranging from 22kV to 66kV.

Advantages of Vacuum Circuit Breaker

  • Simple construction.

  • Self contained i.e., no need of periodic refilling of gas or oil.

  • Compact size.

  • Low power requirement for making and breaking operations.

  • Pollution free

  • Long life

  • Non-explosive.

  • Suitable for repeated operating duty.

  • High speed of dielectric recovery.

  • Silent operation.

  • Low maintenance.

  • Capable of interrupting highly inductive and capacitive currents without restricting.

Disadvantages of Vacuum Circuit Breaker

  • Installation cost of vcb is very high.

  • The main working component, the vacuum interrupter is unrepairable and need to be replaced with a few unit.

  • (a vacuum interrupteris a switching devicethat uses electrical contacts in a vacuum. It is the core component of medium-voltage switches.)

  • Generally , current chopping takes place at very low values of current and current chopping leads to overvoltage.(Current Chopping in circuit breaker is defined as a phenomena in which current is forcibly interrupted before the natural current zero. Current Chopping is mainly observed in Vacuum Circuit Breaker)

Applications of Vacuum Circuit Breaker

  • Capacitor bank switching.

  • Reactor switching.

  • Transformer switching.

  • Line dropping.

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