A plasma torch (11) is supplied with its d.c. operating voltage by a drooping transformer (33) and a rectifier (31), for striking an arc between an electrode (19) and a workpiece (13). If the arc strikes between the electrode and the torch casing (17), the operating voltage drops. A control circuit (51) senses when the operating voltage is below a threshold value and turns off the power supply by opening a solenoid switch (39, 41, 43). This action is briefly disabled each time the power supply is turned on to allow the operating voltage to build up to its normal level, above the threshold.