The system comprises a direct current bus having a positive terminal and a negative terminal. A first switching transistor and a second switching transistor have their respective switched terminals connected in series with respect to each other between the positive terminal and the negative terminal. A primary supply is arranged to generating a test voltage that is lower than an operational voltage of the direct current bus. The test voltage is applied between the positive terminal and the negative terminal of the direct current bus. A primary thermal circuit is associated with the supply for detecting whether the supply exceeds an operational temperature threshold. The thermal circuit provides a signal or logic level state indicative of a detected short circuit in the direct current bus if the primary supply exceeds the operational temperature threshold.