Abstract: Electrical discharge machining (EDM) apparatus includes an arc protection circuit which monitors the values of the ignition voltages applied across the gap between an electrode and a workpiece. A signal conditioning circuit includes a peak detector and averaging circuit conditions the voltage appearing across the gap and the arc protection circuit compares the value of the conditioned signal to the value of one of two reference voltage signals depending on whether the gap ignition voltages are increasing or decreasing. When the ignition voltages have decreased below the first reference level, the arc protection circuit provides a first control signal which causes a servo drive mechanism to increase the distance between the electrode and the workpiece. The first control signal also causes a pulse generator circuit to increase the off time between successive electrical discharges.