A strip-shaped fusible member is disposed in a slit formed in an electrically insulating member to be not greater than 1 mm in width. Further a granulated electrically insulating material may be charged around the electrically insulating member within an enclosed housing. Alternatively, a plurality of electrically insulating members may alternate in intimate contact relationship fusible member to form a stack. Each fusible member is connected at both ends to two terminal blocks located on the opposite sides of the stack on those portions higher in level than that portion thereof sandwiched between the insulating members while its end portions are slackened.
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