Electrical system for a motor vehicle, said system comprising: - an electrical circuit having at least one phase input point and a neutral input point, a grid voltage from an electrical grid being intended to be applied between each phase input point and the neutral input point so as to supply electrical power to the electrical circuit, - an electrical ground, - a device for evaluating an insulation fault between the electrical circuit and the electrical ground, said device for evaluating an insulation fault comprising: - a device (168; 156) for measuring a first common-mode current (i mc) entering the electrical circuit via its phase input point or points and its neutral point and looping back via the electrical ground, in particular through the insulation fault, the first common-mode current (i mc) being supplied by an electrical generator (164; 102; 502), - a device (174; 302) for measuring a voltage (v cp; v ac; v r) supplied by the electrical generator (164; 102; 502), - a device for multiplying the measurement of the first common-mode current (i mc) by the measurement of the voltage (v cp; v ac; v r) so as to obtain a power (p) that is delivered by the electrical generator (164; 102) so as to supply the first common-mode current (i mc), - a device for determining an active part of the power (p), - a device for determining, from the active part of the power (p), at least one electrical quantity (R; I R_eff) that is representative of the insulation fault, said electrical system further comprising: - at least one phase conductor and a neutral conductor having first ends that are respectively connected to the phase input and neutral input points (120, 122) of the electrical circuit, and second ends that are intended to be respectively connected to at least one phase terminal and a neutral terminal of the electrical grid, and characterized in that said electrical system further comprises: - a device for cancelling out a second common-mode current (i mc ∗) entering the phase and neutral conductors (124, 126) via their second ends, comprising: - a device for measuring the second common-mode current (i mc ∗), - a generator, from the measurement of the second common-mode current (i mc ∗), of a compensation current (i cp) flowing from the electrical ground to the phase and neutral conductors (124, 126), and being substantially equal to the first common-mode current (i mc), in which the generator of the compensation current (i cp) is the electrical generator of the device for evaluating the insulation fault. - an electrical circuit having at least one phase input point and a neutral input point, a grid voltage from an electrical grid being intended to be applied between each phase input point and the neutral input point so as to supply electrical power to the electrical circuit, - an electrical ground, - a device for evaluating an insulation fault between the electrical circuit and the electrical ground, - a device (168; 156) for measuring a first common-mode current (i mc) entering the electrical circuit via its phase input point or points and its neutral point and looping back via the electrical ground, in particular through the insulation fault, the first common-mode current (i mc) being supplied by an electrical generator (164; 102; 502), - a device (174; 302) for measuring a voltage (v cp; v ac; v r) supplied by the electrical generator (164; 102; 502), - a device for multiplying the measurement of the first common-mode current (i mc) by the measurement of the voltage (v cp; v ac; v r) so as to obtain a power (p) that is delivered by the electrical generator (164; 102) so as to supply the first common-mode current (i mc), - a device for determining an active part of the power (p), - a device for determining, from the active part of the power (p), at least one electrical quantity (R; I R_eff) that is representative of the insulation fault, - at least one phase conductor and a neutral conductor having first ends that are respectively connected to the phase input and neutral input points (120, 122) of the electrical circuit, and second ends that are intended to be respectively connected to at least one phase terminal and a neutral terminal of the electrical grid, and - a device for cancelling out a second common-mode current (i mc ∗) entering the phase and neutral conductors (124, 126) via their second ends, comprising: - a device for measuring the second common-mode current (i mc ∗), - a generator, from the measurement of the second common-mode current (i mc ∗), of a compensation current (i cp) flowing from the electrical ground to the phase and neutral conductors (124, 126), and being substantially equal to the first common-mode current (i mc), - a device for measuring the second common-mode current (i mc ∗), - a generator, from the measurement of the second common-mode current (i mc ∗), of a compensation current (i cp) flowing from the electrical ground to the phase and neutral conductors (124, 126), and being substantially equal to the first common-mode current (i mc),