Abstract: A fast acting solid state AC circuit breaker includes a power transistor module in series with a load. Load current is continuously monitored and compared with threshold current values which define maximum current for equipment turn-on and steady state conditions. The steady state conditions may be briefly exceeded to allow normal transient overloads to exist on the line for periods up to 25 milliseconds. A maximum current level greater than the normal transient overload can exist on the line for 10 milliseconds after the circuit breaker turns on. The circuit breaker can be turned off in less than 10 microseconds.