What Is Vacuum Interrupter ? How Vacuum Interrupters Work in Circuit Breaker ?

Vacuum Interrupter (VI) are vacuum sealed-off devices incorporated in Vacuum Circuit Breakers (VCB) to cause interruption of fault currents in medium voltage electrical distribution systems. The contacts of the VI are pressed against each other under normal circuit conditions. 

In the event of a fault current, the VCB mechanism effects withdrawal of the movable contact from the fixed contact.

A vacuum interrupter uses a high vacuum to extinguish the arc between a pair of contacts. As the contacts move apart, current flows through a smaller area. 

The shape of the contacts produces magnetic forces which move the arc spot over the surface of the contacts, so the arc does not remain in one place for very long.

Vacuum interrupter technology is used for repetitive switching, motor inrush current interruption, fault protection, overcurrent, and short-circuit protection. The vacuum interrupter is a pair of separable contacts (primary contacts), enclosed in a vacuum-tight envelope.

The arc which is established during the withdrawal comprises a plasma of vapours, drawn from the hotspots present on the contact surfaces. 

As the arc continues to burn and carry the fault current in the intercontact region, the metal vapour from the arc continues to condense on the contact surfaces and also on the surrounding metal shield. 

This safeguards the ceramic from the deposition of metal vapours which can otherwise make the ceramic surface electrically conducting.

Production of the vapours ceases as the current zero is approached, giving rise to the original vacuum conditions. This also enhances the dielectric strength of the VI and the fault current is interrupted.

When the contact gap is fully opened, the circuit voltage is isolated by the vacuum gap internally and the ceramic envelope externally.

The assembly of the interrupter is made in a dust-free clean room. The assemblies are further processed in high vacuum, high-temperature vacuum brazing furnaces using special brazing alloys. 

The VI to be thoroughly tested for their dimensions, mechanical integrity, vacuum and dielectric strength before dispatch, to ensure a reliable product to the customer. The VI was manufactured to be type tested to IEC standards on VCB.

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