Abstract: An insulator having a central portion or stem and one or more sheds is provided with a barrier over at least one of the sheds whereby a substantial improvement in withstanding flashover is obtained yet significant damage to the surface of the insulator is avoided. The barrier comprises a sheet of dielectric material positioned over the upper surface of a shed but spaced away therefrom, the sheet extending radially outwardly to overhang the periphery of the shed. The barrier is furthermore spaced radially away from the central portion or stem of the insulator over at least a substantial part of the inner periphery of the barrier. The barrier may be made of rigid material, for example, constructed from a plurality of sectors but is preferably made from a flexible material so that it can be fitted around an existing conventional insulator. The underside of the barrier is preferably provided with a number of projections for spacing it away from the upper surface of the shed.