Breakdown probability distribution before and after no-load switching was investigated for a vacuum interrupter made from copper-chromium alloy. Since the multi-break vacuum circuit breaker was considered as a method for realizing a high-voltage vacuum circuit breaker, a double-break vacuum circuit breaker was investigated for breakdown probability distribution. Breakdown probability distribution after no-load switching can be represented by a Weibull distribution in the same manner as before switching. The scatter of breakdown voltage increases when no-load switching is carried out. The shape parameter becomes constant, from 6.0 to 8.5 irrespective of the gap length. If the vacuum circuit breaker uses a double-break, breakdown probability at low voltage becomes lower than single-break probability. Although the double-break vacuum circuit breaker is inequality of potential distribution, its insulation reliability is better than that of the single-break vacuum interrupter even when the inequality of the vacuum interrupter's sharing voltage is taken into account.
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