Dual-voltage onboard power supply system with excess voltage protection

The system has a 12-V subnet provided with a first voltage source (10) and a first set of consumers (12), and a 48-V subnet provided with a second voltage source (20) and a second set of consumers (16, 22). A connection between the subnets exists over a switching element (14) that is actuated from the 12-V subnet in order to switch current in the 48-V subnet. A control terminal (St) of the switching element is connected with the 12-V subnet over a potential divider that prevents that high voltage of the 48-V subnet passes-through over the 12-V subnet in a case of failure. The switching element comprises a power semiconductor such as MOSFET. The voltage sources are formed as batteries.