On-board power supply system for a vehicle, comprising: - a first network area with a rechargeable first d.c. voltage source and with a first group (G 1) of electrical energy consumers, - a second network area with a rechargeable second d.c. voltage source and with a second group (G 2) of electrical energy consumers, - a circuit breaker device connected between the first network area and the second network area with a first circuit breaker and with a second circuit breaker, wherein each circuit breaker (32, 34) of the first circuit breaker and second circuit breaker allows the flow of current between its input terminal (E 1, E 2) and its output terminal (A 1, A 2) in both directions in a conductor state and it allows a flow of current in a diode state only from the input terminal (E 1, E 2) to the output terminal (A 1, A 2), wherein the output terminal (A 1) of the first circuit breaker is connected to the first network area, the input terminal (E 1) of the first circuit breaker is connected to the input terminal (E 2) of the second circuit breaker, and the output terminal (A 2) of the second circuit breaker is connected to the second network area, and - a third network area with a third d.c. voltage source and with a third group (G 3) of electrical energy consumers, wherein the input terminal (E 1) of the first circuit breaker and the input terminal (E 2) of the second circuit breaker are connected to the third network area, characterized in that the first circuit breaker is configured to switch into its diode state when the first circuit breaker has been switched into its conductor state when a back current flowing over the first circuit breaker exceeds a first current threshold, or/and that the second circuit breaker is configured to switch into its diode state when the second circuit breaker has been switched into its conductor state when a back current flowing over the second circuit breaker exceeds a second current threshold. - a first network area with a rechargeable first d.c. voltage source and with a first group (G 1) of electrical energy consumers, - a second network area with a rechargeable second d.c. voltage source and with a second group (G 2) of electrical energy consumers, - a circuit breaker device connected between the first network area and the second network area with a first circuit breaker and with a second circuit breaker, wherein each circuit breaker (32, 34) of the first circuit breaker and second circuit breaker allows the flow of current between its input terminal (E 1, E 2) and its output terminal (A 1, A 2) in both directions in a conductor state and it allows a flow of current in a diode state only from the input terminal (E 1, E 2) to the output terminal (A 1, A 2), wherein the output terminal (A 1) of the first circuit breaker is connected to the first network area, the input terminal (E 1) of the first circuit breaker is connected to the input terminal (E 2) of the second circuit breaker, and the output terminal (A 2) of the second circuit breaker is connected to the second network area, and - a third network area with a third d.c. voltage source and with a third group (G 3) of electrical energy consumers, wherein the input terminal (E 1) of the first circuit breaker and the input terminal (E 2) of the second circuit breaker are connected to the third network area,