PURPOSE: To enable short circuit protecting working to be performed, and miniaturize a device, and lower a cost by setting a means for determining the level of grounding current from the output of current transformers for respective phases, in the circuit breaker of a multi-phase AC electric-circuit.

CONSTITUTION: In a mold case, a detection controlling circuit 11, an excess current trip 7, and a short circuit trip 8 are arranged. The input of the secondary side output of current transformers 10a, 10b, 10c, to a micro-computer 20 via a current detecting circuit 12 and phase detecting means 13a, 13b, 13c, is provided. From the micro-computer 20, excess current trip command is generated when a load current value comes to a level equal to or more than a specified level, and the excess current trip 7 is driven. Besides, arithmetic is performed on the vector sum of respective-phase load current, and the level of grounding current is determined. When its level exceeds a specified value, then trip command is generated, and a thyristor 16 is turned ON, and the short circuit trip 8 is driven.