Abstract: A switch is operated by an electrical current to automatically change from a ready condition to a tripped condition. The switch includes a fixed cylindrical contact and a movable plunger contact that are held apart by a thermal holding link acting against the force of a spring. When the switch in the ready condition, an electrical current of sufficient magnitude passing through the thermal holding link will weaken and separate the link and thereby allow the spring to move the plunger contact into electrical contact with the cylindrical contact. In an alternative embodiment, the switch can be configured to be closed in the ready condition, in which case current does flow through the cylindrical contact and the plunger contact, but ceases when the switch is changed to the tripped condition.