Dielectric strength of a vacuum interrupter contact gap after making current operations

Experimental results concerning the breakdown voltage and prebreakdown current of a vacuum interrupter contact gap, after making current operations, are presented. The dielectric strength of the contact gap is lowest for a switching sequence during which a making of high current is followed by a no-load contact opening. The breakdown voltage is much lower than after making operation, followed by interruption of a sufficiently high current. The basic process of breakdown initiation appears to be field electron emission from a specific structure on the contact surface, created by weld rupture.