PURPOSE: To immediately extinguish an arc occurring because of short-circuit fault within a box for an apparatus and prevent the subsequent occurrence of an arc.

CONSTITUTION: This system has a spring for biasing a short-circuiting conductor between electrodes having different phases are short-circuited, and a pressure receiving plate 114 for unlocking a stop member 111 having a stop pin 111a arranged in such a way as to lock the conductor in a unshorted position. The pressure receiving plate 114 is deformed by the flow of gas caused by short- circuit within a box for an apparatus, and the plate 114 is then disengaged from the stop member 111. As a result of this, the plate causes the conductor to move to a short-circuit fixed contact 103 attached to the electrode.