Abstract: An electrical wire cutting circuit breaker in which a wire cutting piston within a housing moves from a rest position to a wire cutting position under the impetus of a compression spring. The piston is normally retained in its rest position against the force of the compression spring by a retainer held in the path of the piston by a fusible stop. The retainer is resiliently biased out of the path of the piston so that when the fusible stop melts and flows at a predetermined elevated temperature the retainer will release the piston which is propelled by the compression spring and builds up kinetic energy sufficient to cut an electrical wire.