ONBOARD POWER SUPPLY SYSTEM

A vehicle onboard power supply system, supplying a consumer with electrical energy, includes a first voltage detection device () detecting a voltage level in a line area () between an input terminal () of a first circuit breaker () and a first battery (), a second voltage detection device () detecting the voltage level in a line area () between an input terminal () of a third circuit breaker () and a second battery (), a third voltage detection device () detecting the voltage level in the area of a consumer connection line (). An overvoltage detection device () detects an overvoltage in the area of the consumer connection line based on the voltage level detected by the third voltage detection device and switches the first circuit breaker and the third circuit breaker into open states when overvoltage is detected in the area of the consumer connection line.