A power supply control system (1) for use in a vehicle, the system including: a first power control arrangement (2) incorporating at least two inputs (3, 4) and at least two outputs (5, 6); and a second power control arrangement (7) incorporating at least two inputs (8, 9) and at least two outputs (10, 11), each power control arrangement (2, 7) being configured to operate in an active mode or an isolated mode. In the active mode, the inputs and outputs of the power control arrangement (2, 7) are connected electrically to one another. In the isolated mode, the inputs and outputs of the power control arrangement (2, 7) are electrically isolated from one another. A selector arrangement (37) is configured to select the operating mode of the power control arrangements (2, 7) so that, at any one time, only one power control arrangement (2, 7) operates in the active mode while the other power control arrangement (2, 7) operates in the isolated mode.