Abstract: The suspension insulator comprises an insulating skirt of quenched glass or of porcelain having an underside in which a pin is fixed by a cement-based binder, a sacrificial ring made of zinc in contact with the binder and surrounding the pin, and an annular plug of electrically conductive material disposed between the insulating skirt and the sacrificial ring in such a manner as to provide sealing that prevents leakage currents from passing through the binder. The plug serves to avoid corrosion phenomena of the pin and the risk of the insulator exploding.