High-current interruption in vacuum circuit breakers

The aim of this project was to find a correlation between contact gap length and switching behavior of a vacuum circuit breaker. A large number of interruption experiments were executed in a vacuum chamber with butt type contacts made of Cu, CuCr 50/50 and AgWC. The currents to be interrupted varied from 2.5 to 32 kA. The rate of change of current and recovery voltage were kept at a fixed value at current zero. Many re-ignitions of the dielectric type, scattered over a wide range of re-ignition voltages, were observed and only a few of the thermal type. The total amount of energy dissipated in the vacuum chamber appears to be determinative for the type of re-ignition. On Cu severe anode spot melting was found, whereas CuCr and AgWC suffered little anode melting. The wide range of re-ignition voltage values found shows that a straight correlation with the contact gap length can not be defined. At 5 to 10 µs after current zero a `second' post arc current did appea