PURPOSE: To reduce electromagnetic mechanical power working to a lead wire on a core by arranging a shield consisting of a nonmagnetic material between the lead wire on the core and the core, and each working repulsive powers by a cover shield and repulsive power by the core to the lead wire actuates in the opposite directions respectively.

CONSTITUTION: The nonmagnetic shield 10 is set up among the core 2 and the lead wires 1, while the shield 10 is connected electrically to one side of a core clamping fitting 3, and the other side is insulated by an insulator 11. Consequently, when short-circuit currents flow through the lead wires, electromagnetic mechanical power K2 works to the lead wires as repulsive power by the cover shield 7, and electromagnetic mechanical power k1 works to the lead wires as repulsive power by the shield on the core side. Since the k1 and k2 work mutually in the opposite directions, electromagnetic mechanical power working to the lead wires becomes only a difference component between k1 and k2, and can be reduced largely more than conventional devices.