PURPOSE: To avoid rupture of a base insulator by a simple structure by defining the distance from the upper surface of the base insulator to the front end of a lead wire no more than half the distance from the upper surface of the base insulator to the upper end inner surface of an insulating cap.

CONSTITUTION: The distance L1 from the upper surface of a base insulator to the front end of a lead wire is determined as no more than half a distance L2 which is from the upper surface of the base insulator to the upper end inner surface of an insulating cap, and the distance L2 is one to three times as long as the interval L3 of the lead wire. Regarding the point of providing a fuse element 3 in the lead wires 2, 2, the point is not limited to the front end of the lead wire but can be the intermediate position of the lead wire. Since the distance L from the front end of the lead wire and the upper end inner surface of the insulating cap is defined long, i.e., no less than the height L2 on the inside of the insulating cap, contact of arc to the upper end inner surface of the insulating cap can be prevented. It is inhibited that the insulating cap receives thermal shock, and rupture of the insulating cap can thus be prevented effectively.