Vacuum circuit breakers, which are widely operating in medium voltage distribution systems, have a high life expectancy. According to different application of vacuum circuit breakers, they have to interrupt load current up to several kilo amperes and short circuit current up to several tens of kilo amperes. During the breaking operation of the vacuum circuit breaker, the current flows through the vacuum arc from the contacts opening moment at least to the next current zero crossing. Because of the contact material's evaporation during the flow of the vacuum arc, the contact surface may have some erosion after each operation, and this may lead to the reduction of insulation properties of the vacuum interrupter during its service life. In this work we investigate the insulation properties of vacuum interrupters after short circuit current interruption. With the help of field emission current measurement we investigate the effect of short circuit current interruption on the contact surface from the microscopical point of view and subsequently on the breakdown voltage of the interrupter.
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