Vacuum Circuit Breaker (VCB), Construction


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What is Vacuum Circuit Breaker?

A circuit breaker is a device that interrupts an electric circuit to prevent excess current during a short circuit. A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in the vacuum. This technology is suitable for medium voltage applications.

The opening and closing of the current carrying contact are dependent on the arc interruption in the arc chamber, which is called vacuum interrupter.

Construction of VCB:

There are two contacts in the VCB as in other: Fixed and movable contacts. The contacts are constructed inside the vacuum chamber.

When the moving contacts start to open an electric arc is formed and flow through the evaporated surface particle from the cathode.

Operation of VCB:

The main objective of any circuit breaker is to turn off the arc during zero current crossing, establishing a high dielectric resistance between the contacts so that the arc reset after the zero current is impossible. This high dielectric strength allows extinguishing a vacuum arc within a very small contact gap. For a short contact space, a low contact mass and no compression of the medium, the inverter power is required in the vacuum circuit breaker is minimum.

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When two face to face contact areas is just being separated to each other, they are not done so instantly. Instead, as the contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. This happens in a fraction of seconds.

At this time, the current is concentrated at that last contact point on the contact surface and creates a hot spot there. The means is to suck the metal to the contact it vaporizes easily. Therefore, the material constituting the arc is very small and, therefore, a weak arc is established even with a high interruption of the current. The arc will start and continue until the beginning of the next current zero.

In the zero current, this vacuum arc is extinguished and the conductive metal vapour condenses again on the contact surface. At this point, the contacts are already separated, so there is no question of re-vaporization of the contact surface, for the next current cycle. That means that the arc cannot be reset again. In this way, the vacuum circuit breaker prevents the restoration of the arc by producing a high dielectric strength in the contact space after the zero current.

Advantages of VCB:

  • They are compact, reliable and have a longer life

  • There are no fire hazards

  • They can interrupt any fault current

  • Low inertia and hence required small power for control operations

  • No noise is produced while operating

  • Higher dielectric strength