Dielectric investigations on micro discharge currents and conditioning behaviour of vacuum gaps

The dielectric "management" in vacuum circuit breakers influences more and more design, development and production strategies. In this regard lightning impulse voltage is the most decisive parameter. Measurements and evaluation of dielectric parameters in open position are thus a meaningful task for vacuum circuit-breaker design and development. Usually contact systems are conditioned to get a reliable electric strength. Many conditioning methods are known. In experimental investigations with lightning impulse voltage, as described in this paper, the impulse voltage test itself can be used to condition the contact systems. For this purpose, plate-plate electrodes from different materials (stainless steel, brass, copper an copper-chromium) are stressed and conditioned with lightning impulse voltage (1.2/50mus). The decreasing micro discharge current level while conditioning routine makes it possible to set a limit for the current value. Conditioning is finished by falling below this limit. The breakdown strength is influenced by the conditioning procedure and micro discharge currents as well