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.
The image shows the internal components of a typical VI.
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 fumaces using special brazing alloys. The VI to be thoroughly tested for their dimensions, mechanical integrity, vacuum and dielectric strength before despatch, to ensure a reliable product to customer. The VI manufactured to be type tested to IEC standards on VCB.