Switching of capacitive currents and the correlation of restrike and pre-ignition behavior

The new IEC 62271-100 requires an extensive proof of the capability of capacitive switching for a breaker under test. For vacuum circuit breakers, dielectric properties are mainly determined by the condition and topology of the contact surfaces, which are modified by in-rush currents as well as load-breaking currents and other effects. A synthetic single-phase test device has been erected in order to simulate three-phase network conditions and to collect more data on the statistical properties of the relevant processes. The distribution of pre-ignition field strengths is evaluated for different contact strokes and surface conditions, when discharging a capacitor through the closing interrupter. On the other side, the probability of restrikes for a given switching condition defined by full contact gap d₀ and peak recovery voltage U_reˆ is measured and compared with the cumulative probability of pre-ignition just at the field strength E=U_reˆ/d₀. A correlation between pre-ignition and restrike probabilities suggests a breakdown mechanism being field-emission dominated. In addition a rather strong conditioning effect has been observed at smaller contact gaps smoothening the contact surfaces.