Circuit protecting sensor

A circuit protecting sensor for protecting an associated circuit from an overcurrent or short-circuit current, comprising a cylinder divided into first and second cylinder bodies coupled axially endwise with a magnetic head interposed between them and fixed thereto, a yoke in which said cylinder is fixed, with said first cylinder body projected partly out of said yoke, a detecting coil wound around the cylinder, a fluid plunger contained together with a viscous fluid in said first cylinder body to a spring-load acting in the direction separating the body to be axially movable toward and away from said magnetic head, a first spring means provided for normally biasing said fluid plunger in a direction away from the magnetic head, an operating plunger contained in said second cylinder body to be axially movable toward and away from the magnetic head in opposing relationship to movements of the fluid plunger, one end of said operating plunger being projected out of the cylinder to be engageable with an actuating part of an external device, and a second spring means provided for normally biasing said operating plunger in a direction away from the magnetic head.