Surface migration as a possible cause for late breakdowns [in vacuum interrupters]

With a certain probability, in vacuum interrupters the recovery voltage leads to breakdown with a delay of many milliseconds or even seconds to current zero when the plasma or metal vapour has already completely disappeared. A conventional explanation of these late breakdowns are microparticles released by mechanical shocks from the electrodes or inner walls. The present paper reports on a model experiment. Metal vapour plasma is injected from a vacuum arc into a neighbouring high voltage gap where field electron emission is used as an indicator for the voltage strength. It has been found that this plasma enhances the field emission, thus reducing the voltage strength. The effect takes place some milliseconds up to several seconds after the plasma injection, and the time constants of the increase of the emission amount to 1-1000 ms. Mechanical shocks have similar effects and time constants. From this behaviour, it is concluded that surface migration with long time constants can influence the voltage strength of arced surfaces.