Abstract: A circuit for detection of a missing phase voltage condition, and certain other malfunction conditions, in the output of a three-phase A.C. alternator or power supply. Three pulse forming circuits are provided with their input ends each connected to receive a different one of the phase voltages. Each pulse forming circuit provides an output pulse signal for a duration corresponding to the excursion of its corresponding phase signal in a predetermined one of the voltage polarities of the power supply. The outputs of the pulse forming circuits are connected to individual inputs of an OR gate circuit. The OR circuit is adapted to clamp the trigger input of a retriggerable monostable multivibrator to ground during presence of a pulse output from any of the pulse forming circuits. The duration of a triggered output state of the multivibrator is chosen to be greater than the period of alternation of the phase voltages.