UGHTMXG CURRENTS IX BURIED CABLE 301 



As mentioned before, the theoretical expectancy of lightning damage and 

 of strokes to the cable discussed in this pa])er has been based on an incidence 

 of 2.4 strokes per square mile for 10 thunderstorm days, a value derived 

 from magnetic link obsers-ations of the rate of stroke occurrence to the 

 aerial supporting structures of transmission lines, on the assumptions that 

 they attract lightning strokes in accordance with laws established from 

 laborator\^ observations on small-scale models, and that, the average height 

 of the ground wires is 70 feet above the earth or adjacent trees. ^ If this 

 lieight had been taken as 60 feet instead the incidence would have been 

 2.8, in substantial agreement with that obtained from our observations for 

 northern territor}' — in the main the territor}^ traversed by the transmission 

 lines from which the data were obtained. 



The cur\'es shown in Fig. 13 include substantial areas and a rather large 

 amount of data and should, therefore, be fairly representative. Thus a 

 radius of four miles corresponds to an observation area of 50 square miles. 

 Within this area a total of 342 strokes was recorded during 1948 at the ob- 

 ser\-ation point in the test section near Atlanta. One of the storms during 

 this period, in which the antenna was struck, passed directly over the ob- 

 servation point and provided a considerable amount of the data. How- 

 ever, even if the observations during this storm were omitted, the total for 

 the season would have been reduced by less than 10 per cent, while the ob- 

 ser\'ations during July, August, and September would have been reduced 

 about 20 per cent and would have been slightly lower than in the same 1947 

 period. The data thus indicate that the yearly incidence per square mile 

 of strokes to ground is about 2.8 per 10 thunderstorm days in northern 

 parts of the country, but may be as high as 4.3 in those southern parts 

 where more severe types of thunderstorms occur. Considering, however, 

 both the 1947 and 1948 observations in Georgia, it appears that an incidence 

 of 3.7 would be a reasonable expectation for an entire season. With this 

 incidence, rather than 2.4 as assumed in Fig. 12, Curves 1 and 2 in that 

 figure would approximately correspond to earth resistivities of 500 and 1000 

 meter-ohms, respectively. 



Conclusions 



The observations indicate that the duration of lightning currents in the 

 southern territory under observation is substantially longer than the aver- 

 age ordinarily assumed. The time to half-value of intense currents, which 

 are of main importance as regards Uability to lightning damage, is of the 

 order of 150 microseconds, and for lower currents even larger. This, 

 together with the higher incidence of strokes to ground and the high earth 

 resistivity, would appear to account for the high rate of lightning damage 

 experienced in this territory- in cables of earlier design than the copper- 



