Lightning Current Observations in Buried Cable 



By H. M. TRUEBLOOD and E. D. SUNDE 



Results are given of observations of lightning currents, voltages, and charges 

 in a buried cable over most of three lightning seasons. These are compared with 

 theoretical expectations. Data regarding the incidence of lightning strokes to 

 ground, as observed with automatic recording equipment, are also reported, 

 together with comparisons with similar data published previously. 



Introduction • 



LIGHTNING currents in buried telephone cable are of considerable 

 ' importance in that they may cause excessive voltages between the 

 cable sheath and the conductors with resultant insulation failure, and may 

 also cause severe damage by crushing the cable or fusing holes in the sheath. 

 The incidence of lightning strokes to buried cable, the resulting voltages, 

 and lightning trouble expectancy, have therefore been subjects of theoreti- 

 cal, experimental, and field studies, which, together with operating 

 experience, have pointed the way to improvements in the design of com- 

 munication cable to minimize its liability to lightning damage, and in the 

 application of remedial measures where excessive lightning trouble has 

 been experienced.^ 



The territory around Atlanta, Georgia, has appeared to be particularly 

 severe with respect to these lightning hazards, because of high earth re- 

 sistivity and high thunderstorm rate. Buried cables initially installed in 

 this territory were accordingly provided with protective measures in the 

 form of increased core-sheath insulation and shield wires buried with the 

 cable. In spite of these measures, however, a substantially higher rate 

 of lightning damage than anticipated was experienced, as a result of which 

 a new design was adopted for the transcontinental coaxial cable westward 

 from Atlanta. In this cable, the lead sheath was insulated from an out- 

 side corrugated 10-mil copper shield by a 100-mil layer of thermoplastic 

 insulation intended to prevent the entrance of lightning currents into the 

 sheath and thereby to minimize voltages between the sheath and the cable 

 conductors. 



Simulative tests with surge currents, believed to have a wave shape 

 representative of lightning stroke current, had indicated satisfactory agree- 

 ment between measured and calculated voltages between sheath and cable 

 conductors. It appeared, therefore, that the departure from predicted 



* References are listed at end of paper. 



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