Dielectric strength of the metal vapour

In vacuum circuit breakers (VCB) the presence of the metal vapour in the switching gap, following the interruption of high currents, is a source of limitation of dielectric performance during the recovery period. The temperature of the contacts is still high after current extinction, and metal vapour continues to evaporate from the contacts. Breakdown can initiate for metal vapour density at the condition below the Paschen breakdown limit. The Paschen curve of copper is only known from calculation for a small region around the minimum; because experimental data for copper vapour is not available. This paper deals with the dielectric strength corresponding to copper metal vapour. A special setup was devised for this purpose. This experimental device has allowed us to separate the pure dielectric effect of the metal vapour from that of the post arc plasma sheath and its dynamic movement, under the influence of the transient recovery voltage, TRV. The results allow an improved understanding of the impact of the metal vapour density on the breakdown process relative to the failure of vacuum circuit breaker.