Electrical insulators, materials and equipment

An elongate high voltage insulator is formed of a rod or tube of insulating material, with a pair of electrodes spaced apart longitudinally thereof. At least part, and preferably the whole of the outer surface of the insulating material is covered by a layer of material including a particulate filler of varistor powder in a matrix having a switching electrical stress-controlling characteristic that is in electrical contact with each of the electrodes. The insulator core may be made of porcelain, and the stress-controlling material may be zinc oxide.