MECHANICAL SHOCKS AS CAUSE OF LATE DISCHARGES IN VACUUM CIRCUIT-BREAKERS

HV circuit breakers equipped with vacuum tubes occasionally show late discharges (reignition) after successful interruption of short circuit currents. The time dependence of late discharges in vacuum circuit breakers after current interruption has been measured as a function of the mechanical and electrical stress exerted on the vacuum circuit breaker bottle. A correlation between the occurrence of strong, shock-like acceleration peaks of > 10(4) m/s2 and the increase of the probability of late discharges has been found. A reduction of the shock wave amplitudes by simple mechanical damping results in a reduction of the probability of late strikes by as much as 50%. As a result, we conclude that most of the late discharges observed in these experiments are caused by particles which adhere loosely to the contact and vapor shield surfaces, and which are released by the strong mechanical shocks.