LIGHTNING CURRENTS IN BURIED CABLE 279 



lightning trouble expectancy in the earlier cables was due to one or more 

 of the following conditions: a higher rate of occurrence of lightning strokes 

 during thunderstorms, higher stroke currents than in other parts of the 

 country, a longer duration of the lightning currents than assumed, or a 

 higher incidence of strokes to buried cable than predicted theoretically. 

 The observations described here, the larger part of which have extended 

 over a period of three lightnmg seasons, were intended to secure informa- 

 tion on these points. The data forming the principal subject of this paper 

 were obtained from a section of the coaxial cable mentioned above, which 

 for a number of reasons was particularly suitable for the purpose. 



I. Theoretical Expectations 



1 .0 General 



As mentioned above, the observations were made on a cable route 

 through territory of high thunderstorm rate and high earth resistivity, 

 both of which facihtate measurements of currents along the cable. As a 

 result of the high thunderstorm rate, the incidence of strokes to ground is 

 high, and because of the high earth resistivity, the number of strokes to 

 ground near the cable which flash to latter is also high. Another result of 

 the high earth resistivity is that the attenuation of current along the cable 

 is relatively low, so that currents and voltages may be observed at appre- 

 ciable distances along the cable from the points of the lightning strokes. 

 The rate of attenuation is, furthermore, smaller the longer the duration of 

 the lightnmg current, that is, the longer the time to half-value. Since 

 lightning trouble experience in this territory indicated the possibility of 

 currents of rather long durations, this was an additional factor favorable to 

 the purposes of the tests, although, like the others, it increases the liability 

 of cables to Hghtning damage. 



Though the relationships of the various factors mentioned above to earth 

 resistivity and to lightning current wave shape have been dealt with in 

 detail in the study^ referred to above, they are briefly reviewed here to 

 facilitate comparisons with and discussions of the observations. 



1.1 Incidence of Strokes to Buried Cable 



The current in a lightning stroke to ground spreads in all directions from 

 the point where it enters the earth. If a cable is in the vicinity, it will 

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

 cable and in both directions along the sheath to remote points. The cur- 

 rent in the ground between the lightning channel and the cable may give 

 rise to a voltage drop along the surface of the earth sufficient to exceed the 

 breakdown gradient of the soil, particularly when the earth resistivity is 



