Recovery of breakdown strength of a vacuum interrupter after extinction of high currents

Relevant phenomena during the period of the recovery of the dielectric strength of vacuum interrupter (VI) are reviewed. Metal vapor, residual charge and the effects of molten contact surfaces reduce the breakdown strength after the interruption of high currents. Metal droplets seem to play only a secondary role during the recovery phase. Instability of liquid protrusions is suspected to initiate breakdown if the electric field strength exceeds 10 kV/mm at the molten contact surfaces (undisturbed field). Key parameters are surface temperature and vapor density. Breakdown during recovery of the dielectric strength depends not only on the value of the voltage applied but also on the duration of the high voltage stress. A spontaneous triggering event in the presence of metal vapor appears to initiate breakdown at conditions below the Paschen breakdown limit. This type of breakdown events limits the interruption performance of VI´s.