LIGHTNING PROTECTION OF BURIED TOLL CABLE 299 



thick, with diametrically opposite seams having an overlap of about \" . 

 The outside lO-mil copper shield is corrugated to facilitate bending and has 

 a \" overlap at the seam. The thermoplastic coating is flooded with 

 thermoplastic cement to reduce moisture absorption. A photograph of this 

 cable is shown in Fig. 14. 



SUMMARY 



Current in the sheath of buried cables, due to direct lightning strokes, 

 strokes to ground near the cables or discharges between clouds, gives rise to 

 voltages between the cable conductors and the sheath. The voltages are 

 practically proportional to the square root of the earth resistivity and to 

 the direct-current sheath resistance. For the latter reason they are sub- 

 stantially larger for carrier cables of the size now used than for the much 

 larger voice-frequency cables. While direct strokes are usually most im- 

 portant, strokes to ground must be considered when the cables are of small 

 size, even when the surface resistivity is low, provided the resistivity at 

 greater depths is high. Under the latter conditions it is possible that for 

 small cables, discharges between clouds over the cable may also cause failure. 



For cables with thermoplastic or rubber coating the voltages between 

 the sheath and the core conductors are much the same as for jute-covered 

 cables. The coating of such cable is likely to be damaged by direct strokes 

 and strokes to ground near the cable, in which case corrosion of the sheath 

 may occur at such points. 



Based on theoretical lightning expectancy curves, the incidence of light- 

 ning troubles increases faster than the sheath resistance or the earth re- 

 sistivity. When the breakdown voltage of the core insulation is doubled by 

 use of extra core wrap, or when shield wires are installed in situations where 

 lightning damage is anticipated or has been experienced, a substantial re- 

 duction in lightning failures is to be expected. Shield wires will, however, 

 not prevent damage to the sheath and the sheath coating. 



Where the earth resistivity is very high and lightning storms occur fre- 

 quently, doubled core insulation together with shield wires may not provide 

 sufficient protection, even for cable of substantial size. Protection against 

 various forms of lightning damage may then be secured by use of thermo- 

 plastic sheath coating of adequate dielectric strength together with an out- 

 side concentric copper shield. 



References 



1. E. D. Sunde: "Lightning Protection of Buried Cable," Bell Telephone Laboratories 



Record, Vol. 21, No. 9, May 1943. 



2. B. F. J. Schonland: "The Lightning Discharge," Clarendon Press, Oxford, England, 



1938 or "Thunderstorms and Their Electrical Effects," Proc. of the P/ivs. Soc., Vol. 

 55, Part 6, No. 312, Nov. 1, 1943. 



