retriggerable monostable component 14 for example a component known under the standardized name of 74hc4538 the logic output of this component alternates with a frequency which is that of the current circulating in the power conductor 3 this monostable component 14 is linked to a state detector 15 for example a latch d which enables a suspension of the tripping of the monostable 14 to be detected the suspension of such tripping corresponds to the suspension of the oscillation of the current which means that there is no longer any current circulating in the power conductor 3 or that such current is direct the state detector 15 in this case supplies a signal error passage into a logic state 1 for example such signal being applied to the triggering means 9 by an or logic gate 16 the triggering means 9 thus cause the circuit breaking means 2 to open the state detector may be made in the form of a specialized electronic component for example a component known under the standardized name of 74hc74 it may simply be constituted by a software module incorporated into a computer such module being programmed so as to detect the suspension of the oscillations at the monostable 39 s 14 output such a computer may furthermore be programmed so as to monitor the shapes of more complex currents the triggering means 9 may for example by constituted by a control circuit to open the circuit breaking means 2 if the circuit breaking means 2 are constituted by a static switch such as a thyristor the triggering means 9 may for example be the electronic circuit controlling the trigger of said thyristor.