At normally working via fuselink tightened the fuse tube is fixfed to form up of close position, In case system occure faults, fault current result in fuse melt immediately and take piece electric are, which let arc-extinguishing tube being heated and fexplode a lotofgas, Th-os will produce high pressure and below of the are along with tube, After fuselink melt moving contact has no tightened strength again, mecganismis locked and fuse tube drop out, Cutoutnow is in open position, When itneeds toswitch off during cutout loading, operat-or shall via insulating operating bar pull the moving contact, atits beginning main contact and auxiliary static contactis contactedst-ill, Whiling pulling the auxiliary contact is separated between auxiliary contacts there occur electric arc and the arc will be lengthene-d in arc-extinguishing enclosure gap and meanwhile arc-exting explode gas to blow off the arc during current passing zero