PURPOSE: To eliminate malfunction and to improve reliability by providing an elastic wave counter and discriminating unit between an amplifier and display unit and by discriminating the elastic wave due to mechanical switching of a vacuum switch from the elastic wave generated at the time of defective vacuum.
CONSTITUTION: An elastic wave counter 13 and a discriminating unit 14 are provided between an amplifier 11 and a display unit 12. The elastic wave counter 13 integrates the number of elastic waves of within a certain time and the discriminating unit 14 discriminates whether the number of elastic waves integrated by the elastic wave counter 13 exceeds a certain numerical value or not. On the other hand the elastic wave generated at the time of defective vacuum is continuously generated. If, therefore, the time allowance of the elastic wave counter 13 be set up so that the integration number of the elastic wave of this case can become larger enough than the integration number of the elastic wave generated at the time when a vacuum switch is switched mechanically, the elastic wave generated at the time of defective vacuum and the elastic wave generated on switching can be discriminated and malfunction eliminated and reliability improved.
IWASAKI HIDEO
KANBARA SHIZUO
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