Power supply system for a motor vehicle with a low-voltage electrical system and a higher-voltage onboard power supply

Power supply system for a motor vehicle having a low-voltage onboard network (6, 7, 10) and with a higher-voltage electrical system (1, 2, 3, 4, 9), wherein each of the minus terminal (9) or the plus terminal (2) at least the high-power consumers (4) of the higher-voltage onboard network via a central manifold (16) are respectively connected to the negative terminal (19) or to the positive terminal (10) of the low-voltage onboard network, wherein the central manifold (16) has a controllable switch (17), wherein said means (8; 18) (for measuring the voltage U1) of the low-voltage onboard network are provided and wherein said controllable switch (17) is opened when the voltage (U1) of the low-voltage onboard network has exceeded a predetermined voltage threshold.