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. <EOS>