A device includes a first system conductor supplying a first load from a first power source, a second system conductor supplying a second load from a second power source, and an inter-system conductor supplies the first power source to the second load or the second power source to the first load. First and second series-connected switches share a common connection terminal coupled to ground through a resistive element. A circuit measures first, second, and third voltages of the first system conductor, the second system conductor, and the common connection terminal, respectively, to ground; determines a conduction fault of at least one switch based on whether the first and/or second voltage differs from the third voltage under switch on/off states; and determines a cutoff fault based on whether the third voltage is at least a threshold value.