Abstract: A circuit interrupter with a frangible conductor which breaks to interrupt a circuit in response to an increase of fluid pressure above a predetermined level. In one version, the conductor is on a frangible member exposed to the pressure on one side and sealed by a cover and container on the side. In another version, a spring urged piston tensions a wire conductor and fluid pressure on the piston above the predetermined pressure breaks the wire. In another variation a diaphragm exposed to the pressure moves an element which ruptures the conductor when the pressure on the diaphragm exceeds the predetermined value.