Abstract: To avoid component damage due to the presence of an incorrect voltage on the power bus of an electrical system when the system's power supply is turned on, the system carries out a self-testing procedure before power up to detect fault-indicating low impedance paths in the power bus. For this, the system includes a test power supply which delivers to the bus during the self-testing routine test voltages which are much lower than those supplied to the bus by the main supply. In accordance with the self-testing routine, the main power supply is turned on upon completion of the routine only if no low impedance paths are detected in the bus. As the supply voltages are applied to the bus, they are compared with preselected limits. If any voltage is not within its limits, the system provides an indication to that effect and/or shuts down the system.