Vacuum Circuit Breakers

Vacuum Circuit breakers are a very popular type of circuit breaker used in the industry.  Vacuum Circuit Breakers or VCBs as they are popularly known use vacuum as the quenching medium.

Vacuum Circuit breakers work by opening the contacts in an evacuated chamber.  The vacuum in the chamber ensures that there are very few ionisable molecules which can sustain the arc.  The arc is, thus, unable to sustain itself and is extinguished.

Vacuum circuit breakers have been used up to 36 kV and can interrupt up to 4000A.  When the arc is initiated during contact separation, metallic vapour is produced from the contacts.  If the contacts are of soft metal, very little vapour is produced.  While this ensure quick arc extinction.  The current waveform can get chopped and lead to high voltage transients.  If too soft metals are used in the contacts, the arc would cause erosion of the material.  Hence, the contacts are made of materials which are neither too soft or hard.  Common materials used are alloys Copper-bismuth or copper-chrome.