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When the gap length of a vacuum switch at current zero is too small, successive reignitions and extinctions may occur under certain circuit conditions leading to overvoltages. This process of "virtual chopping" is more likely to happen when the contacts accidentally open shortly before current zero. In order to determine the values of the overvoltage and its probability when opening the contacts randomly, a computer program was developed which simulates the transient recovery voltage (TRV) in a single-phase circuit and determines the highest voltage for each switching operation. The moment of contact opening is simulated by pseudorandom numbers. Input data of the calculation are the circuit parameters, the reignition voltage of the gap versus time after current zero, and its capability of quenching the high-frequency current oscillation occurring after a reignition. These recovery data of the opening gap were determined experimentally for some contact materials using a special switching device in a synthetic test circuit with current injection.