Abstract: A composite insulator (1) for overhead power lines comprises a rigid core extending axially between two attachment fittings (6) and having placed thereon a shell having sheds (5, 5?) made of electrically insulating synthetic material. It includes a detector device for detecting surface leakage currents, which device includes an electrically conductive metal annular ring (7) surrounding the shell between an end shed (5?) adjacent to one of the two fittings (6) and another shed (5) adjacent to the end shed (5?) that extends radially over the ring (7). The ring (7) is connected by a conductive wire (8) passing through the end shed (5?) to be connected to an electronic unit (9) of the detector device.