Lightning Protection of Buried Toll Cable 



By E. D. SUNDE 



A theoretical study of lightning voltages in buried telephone cable, of the 

 liability of such cal^le to damage by lightning and of remedial measures, together 

 with the results of simulative surge tests, oscillographic observations of light- 

 ning voltages and lightning trouble experience. 



Introduction 



PRACTICALLY 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 bury- 

 ing the cable would not prevent damage due to lightning and that, on ac- 

 count of their smaller size, more damage was to be expected on the new car- 

 rier 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 num- 

 ber 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 pro- 

 tection 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 high- 

 resistivity territory where experience has indicated that other types of con- 

 struction 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. 



When lightning strikes, the current spreads in all directions from the 

 point where it enters the ground. If there are cables in the vicinity they 

 will provide low resistance paths, so that much of the current will flow to the 

 cables near the lightning stroke and in both directions along the sheath to 

 remote points. The flow of current in the ground between the lightning 

 channel and the cables may give rise to such a large voltage drop that the 

 breakdown voltage of the soil is exceeded, particularly when the earth 



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