The invention relates to an on-board electrical system supply system (1), having a DC/DC converter (11) with at least one semiconductor rectifier element (4), having two supply connections (9a, 9b) for an on-board electrical system, which supply connections are connected at the output end to the DC/DC converter (11), having an isolating switch (5) which is coupled between one of the supply connections (9a, 9b) and the DC/DC converter (11), and having an on-board electrical system isolating circuit (10) which is designed to determine a reverse voltage (U, Ua, Ub) across at least one of the semiconductor rectifier elements (4), and to actuate the isolating switch (5), in order to decouple the on-board electrical system from the DC/DC converter (11), depending on a value of the determined reverse voltage (U, Ua, Ub).