Abstract: Each phase circuit comprises a static switch of the IGT or GTO type, between the anode and the cathode of which there is arranged in parallel a series mounting of a CTP resistance and of a normally open switch which is closed when a current detector detects a short abnormal intensity lower than six times the rated current; said static switch is locked either immediately when the rate is higher than six, or with a short delay on the contrary. Said device is particularly convenient to be used in industrial installations where it is desired to replace the associations of contactors, thermal relay and circuit breaker by static apparatuses.