Abstract: A protection circuit which senses and responds to an output transistor failure to provide appropriate power interruption to protect an electrical discharge machining apparatus. The circuit receives an input of either standard or reverse polarity from the gap. This signal is passed through a comparator stage where it is compared with a preset voltage level. Further inputs are provided in accordance with the polarity setting and from the multivibrator in such manner that an appropriate phase relationship is maintained depending on the preset polarity. A control signal is then generated to control a transistor switching device with an interrupt signal thus being furnished to turn-off the power supply. Also included in a different embodiment of the invention is a system whereby, in a multiple gap output module power supply, responsive to failure of an output transistor in any one of the output modules, the entire power supply is cut off.