Abstract: A power supply system, which supplies a number of different A.C. and D.C. voltage levels to a plurality of loads, is monitored at a number of points for undervoltage, overvoltage and/or overcurrent fault conditions. A diagnostics logic circuit, which is formed from relatively inexpensive, readily available logic units, generates a BCD code signal indicating a fault in response to the occurrence of a fault condition at one of the monitored points. The system responds to this BCD fault indicating code signal by, on the one hand, protecting the power system by actuating a main relay to open the A.C. power line to the system and generating a shutdown signal applied to a pulse width modulator of a converter in the system and, on the other hand, by driving a seven-segment display which displays a numeral to indicate the location of the fault condition.