a power source circuit for use with an electric discharge machine which machines a workpiece by a discharge produced between an electrode and the work . when an arc forms in a working gap defined between the electrode and the work , the off period of a switching element inserted in the charging path of a capacitor for storing discharge energy is increased so that current flowing across the working gap is cut off for a longer period of time than in the case of a normal operation , thereby quickly restoring the insulating qualities of the gap following arcing .