Influence of switching on dielectric properties of vacuuminterrupters

This paper deals with the influence of switching operations `closing without current/breaking without current', `closing without current/breaking with rated current', and `closing without current/breaking of rated short-circuit current' on vacuum interrupter dielectric strength. Two types of commercially available vacuum interrupter with transverse magnetic field CuCr contacts were tested. Breakdown voltages and emission currents, which appear before and after switching operations, were determined experimentally. The corresponding characteristics were determined after conditioning. The voltage shapes, dc, lightning impulse voltage (1.2/50 μs), and 50 Hz ac were applied. The obtained results were statistically treated. After taking into account theoretical considerations, it was concluded which physical mechanisms determine the influence of switching operations on the dielectric strength. It was found that in interrupters with poor contact material, the dielectric strength can deteriorate below the limits required by standards     

Mechanism of electrical breakdown left of Paschen minimum

The breakdown mechanism of a gas at small PD values is considered. Comparing experimental results and theoretical analysis, the limits of different breakdown mechanisms are established. For SF₆ and Ar gases the conditions for streamer and those for the Townsend breakdown mechanism are found, as well as those for vacuum breakdown. The pressure values characteristic for avalanche and emission mechanisms of vacuum breakdown are obtained. Left from the Paschen minimum in the vicinity of the minimum point, edge-type breakdown is dominating, which currently is misinterpreted as an anomalous Paschen effect