Abstract: A system (20) automatically selects electrical circuits (22) of an electrical unit (24) to be tested, inserts any of a plurality of types of electrical fault signals into the selected circuits and controls the inserted fault signals in accordance with a set of preprogrammed instructions. The fault signals are inserted through an electrical connection adapter (36) into the unit's circuits by a programmed controller. The programmed controller includes a host computer (32) for storing data corresponding to the types of faults to be inserted and a microprocessor (52) for controlling the insertion of the fault signals into the unit under test. A threshold detection circuit (100, 102) monitors the voltage of the inserted fault signals and records an error if the signals are outside preselected limits. Selector means (96, 98) are provided for selecting any of a plurality of fault signal current levels. A timer circuit (60) removes all fault signals after a preselected time interval.