and causes tripping to protect the circuit breaker and the supplied circuits when a strong short circuit occurs involving a signal di dt greater than the threshold of the unit 40 the latter supplies a signal to the input of the and gate 42 in parallel the integrating unit 34 transmits a signal respresentative of the current i to the lower threshold detection unit 38 which transmits a tripping signal to the and gate 42 as soon as the value of the current i exceeds this threshold this overshoot occurs very quickly and enables the circuit breaker to be tripped quickly with a high current limiting effect as soon as the current i exceeds the upper threshold of the detector 36 a second tripping signal is transmitted to the circuit breaker but this second signal has no effect since it occurs after that of the lower threshold detector 38 it is clear that the change of threshold determined by the unit 40 enables tripping to take place more quickly thus favoring short circuit current limiting in case of the connecting conductors 24 26 or the sensor 20 being interrupted the voltage increase at the terminals of the sensor 20 due to this interruption causes a signal i greater than the upper threshold of the detector unit 36 which emits a tripping signal causing the circuit breaker to trip proper connection of the sensor 20 to the trip unit 22 is thus permanently monitored any defect resulting in the circuit breaker tripping tripping for.