Fail safe circuit breaker

A circuit breaker comprising movable contact means, and a mechanism for holding the movable contact means in engagement with the complementary contact means to close an electrical circuit, the mechanism including thermally responsive bimetallic means of relatively high resistance properties disposed in the circuit to be self-heated within a first selected period of time in response to the flow of a selected overload current in the circuit through the bimetallic means to trip the mechanism to move the movable contacts to open the circuit, characterized in that, conductor means are joined by a thermally separable bond and are disposed in the circuit in selected heat-transfer relation to the bimetallic means to be thermally separated after a second period of time by heat transferred thereto from the bimetallic means when said selected overload current flows in the circuit to open the circuit after said second period of time in the event that heating of the bimetallic means is ineffective to move the movable contact means to open the circuit after said first period of time, one end of the thermally responsive bimetallic means is secured in a fixed position relative to the complementary contact means, a pair of conductive metal strips have overlapping ends thereof soldered together to form said thermally separable bond means, one of the strip means extends in a selected direction from said one end of the bimetallic means toward the complementary contact means and the other strip means extends to and is electrically connected to the complementary contact means, and spring means extend along the second strip biasing the second strip means to separate for assuring opening of the circuit on the occurrence of thermal separation of the strip means.