The paper describes the design and operation of an apparatus for control of quality of extruded insulation of high-voltage polythene power cables by non-destructive detection and location of harmful electrical discharges.The apparatus scans polythene-insulated cable core at a speed comparable with that of the extrusion process of manufacture. With test voltages of up to 100 kV a.c.(r.m.s.), the sensitivity of detection is more than adequate for discrimination between harmful and harmless discharges. Discharge sites are readily located to within 3 in in cable cores of continuous lengths of more than 5000 yd.The apparatus does not require continual supervision by technical personnel.