A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in 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 vacuum interrupter. Vacuum Circuit Breaker

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Vacuum Circuit Breaker
(VCB) - Principle,
Construction and Working

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In vacuum circuit breakers, vacuum is used as the
arc quenching medium.
Vacuum offers the highest insulating strength. So
it has far superior arc quenching properties than
any other medium.

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For example, when contacts of a breaker are
opened in vacuum, the interruption occurs at first
current zero with dielectric strength between the
contacts building up at a rate thousands of times
higher than that obtained with other circuit
breakers.

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The technology is suitable for mainly
medium voltage application. For higher
voltage vacuum technology has been
developed but not commercially viable.

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Principle of Vacuum
Circuit Breaker

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The production of arc in a vacuum circuit
breaker and its extinction can be
explained as follows :

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•The salient feature of vacuum as an arc quenching
medium is that as soon as the arc is produced in
vacuum, it is quickly extinguished due to the fast rate
of recovery of dielectric strength in vacuum.

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Construction of Vacuum
Circuit Breaker

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•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.

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

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•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.

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Working of Vacuum
Circuit Breaker

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•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 ionisation of metal ions and depends very much
upon the material of contacts.

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•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.

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Vacuum Circuit Breaker

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