LIGnTNING CURRENTS IN BURIED CABLE 



285 



value in 20 microseconds, with half-value in 260 microseconds, the Same 

 crest current as that found before at .v is obtained at twice the distance .v. 

 This follows because all component frecjuencies of the hrst surge are related 

 to corresponding components of the second surge by the same factor, 

 viz. 4. 



0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 



DISTANCE ALONG CABLE (FROM STROKE POINT) IN KILOFEET 



Fig. 5 — Variation in crest current in cable and in voltage between sheath and copper 

 shield for stroke currents having various times to half-value as indicated on curves, for 

 an earth resistivity of 1000 meter-ohms. 



In Fig. 4 are shown curves^ from which the attenuation may be obtained 

 for a surge-current reaching its crest value in 5 microseconds and its half- 

 value in 65 microseconds, as shown by the curve for .r^/p = 0. The crest 

 current has attenuated by 50 per cent when x^'p = 25 or .v = 5p''"-^. Thus, 

 for an earth resistivity of 1000 meter-ohms, .v = 160 meters =» 525 feet 

 when the time to half-value of the current at the stroke point is 65 micro- 

 seconds, while X = 1050 feet when the current at the stroke point reaches 

 its half-value in 260 microseconds. In Fig. 5 are shown crest current 



