The detection system includes a bi-stable relay (4) having a movable change of state part (5) designed to be activated by at least two windings (9,10) and whose exterior connection terminals (6,7,8) are connected to a transistor (11) which delivers an electrical pulse to the winding at the moment of fall in voltage on the line. This voltage fall causes the relay (4) to move into a first state. The system has another transistor (12) which delivers an electrical pulse to the winding when the line is cut, the pulse is produced by a capacitor (13) which is charged by the line so that the relay (4) is moved into a second state. The system has means for delivering a pulse to the second winding (10) which includes a capacitor (21) used to delay for a pre-determined time the electrical pulse with respect to the start of the line cut off and to only deliver the pulse if the line cut off persists for a time at least equal to the delay.
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