Abstract: An electrical service installation comprises a plurality of circuit breakers protecting against overcurrent hazards in branch load circuits. An acceleration, bypass circuit is provided for each circuit breaker. Current flow through each of the bypass circuits is controlled by a normally non-conductive SCR. Current flow to each branch circuit passes through a sensing resistor that is connected across the gate and cathode of the associated SCR. When the voltage drop across the sensing resistor reflects a current level indicative of an "arc short circuit", the SCR is triggered to a conductive state. Current then flows through the bypass circuit increasing current flow through the associated circuit breaker, to thereby provide a substantially increased current magnitude for magnetically tripping the circuit breaker. Circuit breakers are thus tripped to provide protection against the hazards of "arc short circuit" currents that do not have a magnitude sufficient to initiate magnetic tripping.

Abstract: Protective circuits for preventing fires due to arc short circuits (as opposed to dead short circuits), are described. Current flow in excess of that existing during normal transient overloads is indicative of an arc short circuit. Protection is provided by triacs which are triggered by circuit current flow in excess of that existing during normal transient overloads. When triggered, the triacs, in series with the fuse or circuit breaker, are placed directly across the power source and thus actuated substantially instantaneously to interrupt current flow to the load circuit. By thus interrupting current flow the hazard of a fire is obviated. Means are also provided for giving a visual indicium that the protective circuit has been actuated by triggering of the triacs. Additional means are provided for triggering the triacs in response to an overheated main electrical service connection.