Abstract: The spark rate across electro-discharge machine tool electrodes and workpiece, is counted and compared with a given pulse output from a reference source. A constant ratio of spark pulses to reference pulses indicates a stable machining operation. Variation in the ratio indicates breakthrough of the electrode, into e.g. a passage in the workpiece. A signal is derived from the ratio variation and is utilized to operate a switch to cut off power to the electrode. The invention is particularly useful where a bank of electrodes machine a row of holes into a passage, for it may be that some electrodes break through before others, making it possible for them to machine the back wall of the passage if power is not cut off.