Lightning Protection of Buried Toll Cable

P R A C T I C A L L Y all of the toll cable installed since 1939 has been of the carrier type and most of it has been buried in order to secure greater immunity from mechanical damage. It was realized, however, that burying the cable would not prevent damage due to lightning and that, on account of their smaller size, more damage was to be expected on the new carrier cables than on the much larger voice-frequency underground cables then in use. Moreover, when damage by lightning does occur, such as fusing of cable pairs or holes in the sheath, it is not so easy to locate and repair as on aerial cables, since excavations may have to be made at a number of points. Studies were therefore made of the factors affecting damage of buried cables by lightning and remedial measures were devised and put into effect in cases where a high rate of lightning failures was anticipated on new installations, or was experienced with cable already installed. Most of the cable installed was thus provided with extra core insulation, and shield wires were plowed in on many of the new routes. It was recognized early in these studies that more effective lightning protection might be secured by providing the lead sheath with a thermoplastic coating of adequate dielectric strength and an outside copper shield, and that such cable might be required in territory where the earth resistivity is very high. This type of cable has recently been installed on a route in highresistivity territory where experience has indicated t h a t other types of construction would probably be inadequate and, since it has advantages also from the standpoint of corrosion and mechanical protection, it may be used also where lightning is not of such decisive importance.