Detection system for detecting, in a DC electrical apparatus, a fault in a wiring harness, referred to as the main wiring harness, of cross section S1, the main wiring harness being installed so that it undergoes variations in ambient temperature over its length, the detection system comprising: - another wiring harness, referred to as the control wiring harness, of cross section S2 which is less than the cross section S1, intended to be placed both ways along the main wiring harness; - a monitoring device; - a return cable bringing the electrical potential at the output of the main wiring harness back to the monitoring device; the monitoring device comprising: - a controllable current generator arranged to inject, at the input of the control electrical cable, a current I2 which is equal to a current I1 attenuated by an attenuation factor G1, the current I1 flowing through the main wiring harness, the controllable current generator being connected between the input and the output of the control wiring harness, the attenuation factor G1 being such that the currents I1 and I2 cause the same temperature rise in the main wiring harness and in the control wiring harness, respectively; and - electronic circuitry comprising a first differential amplifier produced by virtue of an operational amplifier connected by a first input to the input of the main wiring harness and by a second input to the output of the main wiring harness, via the return cable, the electronic circuitry being arranged to determine a voltage difference between the output of the first differential amplifier and a matching by virtue of a gain G2 in the voltage between the input and the output of the control wiring harness, with G2 = S2 / (2.G1.S1), and to detect a fault in the main wiring harness when said voltage difference exceeds a predefined margin. - another wiring harness, referred to as the control wiring harness, of cross section S2 which is less than the cross section S1, intended to be placed both ways along the main wiring harness; - a monitoring device; - a return cable bringing the electrical potential at the output of the main wiring harness back to the monitoring device; - a controllable current generator arranged to inject, at the input of the control electrical cable, a current I2 which is equal to a current I1 attenuated by an attenuation factor G1, the current I1 flowing through the main wiring harness, the controllable current generator being connected between the input and the output of the control wiring harness, the attenuation factor G1 being such that the currents I1 and I2 cause the same temperature rise in the main wiring harness and in the control wiring harness, respectively; and - electronic circuitry comprising a first differential amplifier produced by virtue of an operational amplifier connected by a first input to the input of the main wiring harness and by a second input to the output of the main wiring harness, via the return cable, the electronic circuitry being arranged to determine a voltage difference between the output of the first differential amplifier and a matching by virtue of a gain G2 in the voltage between the input and the output of the control wiring harness, with G2 = S2 / (2.G1.S1), and to detect a fault in the main wiring harness when said voltage difference exceeds a predefined margin.