NOVELTY - Isolation fault detection system (10) comprises fuel cell stack (12), battery (14), system bus, and ground rail (24). Each of the fuel cell stack and battery include two terminals (16, 18, 20, 22). The system measures voltage potentials in the system relative to the isolation resistances and compares the measured voltage potentials to predetermined voltage potentials to determine whether any of the isolation resistances have fallen below a predetermined minimum value. The ground rail with isolation resistances defined in it.    USE - The system is useful as part of a fuel cell hybrid vehicle (claimed). The system is particularly use for detecting isolation faults in the fuel cell system associated with fuel cell hybrid vehicle. The system can also detect the location of the isolation fault.    ADVANTAGE - The system does not rely on an imbalance between voltage potential to indicate fault. The system identifies the location of the isolation fault so that it can be readily corrected.    DETAILED DESCRIPTION - Isolation fault detection system comprises fuel cell stack, battery, system bus, and ground rail. The fuel cell stack and battery include two terminals. The system bus includes two rails. The positive rail is electrically coupled to the positive terminals of the fuel cell stack and battery. The negative rail is electrically coupled to the negative terminals of the fuel cell stack and the battery. The system loads are electrically coupled in parallel to two rails. The ground rail with isolation resistances defined in it. The system measures voltage potentials in the system relative to the isolation resistances and compares the measured voltage potentials to predetermined voltage potentials to determine whether any of the isolation resistances have fallen below a predetermined minimum value.    An INDEPENDENT CLAIM is also included for detecting isolation faults in a fuel cell system. An isolation fault has occurred if the positive fuel cell voltage potential is greater than the stack voltage potential minus a predetermined threshold voltage potential or if the absolute value of the positive fuel cell voltage potential plus the negative fuel cell voltage potential falls below a threshold voltage potential.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of isolation fault detection system.    Isolation fault detection system (10)    Fuel cell stack (12)    Battery (14)    Terminals (16, 18, 20, 22)    Ground rail (24)