LIGHTXIXG PROTECTIOX OF BURIED TOLL CABLE 



269 



In Fig. 6 is shown the N'arialion in llie voUage calculated from (38), 

 together with that ol:)served in the tests referred to before. That the meas- 

 ured decrease in the voltage is smaller than calculated is due to the fact 

 that the earth resistivity at the test location increases with depth. Earth 

 resistivity measurements made by the four-electrode method show that the 

 resistivity is about 400 meter-ohms for electrode spacings up to about 20 

 feet and then gradually increases, reaching about 700 meter-ohms at 300 

 feet, 1200 meter-ohms at 1000 feet and approaching 1500 meter-ohms for 



& 



o 



■p 

 a 



o 



u 



20 



40 



60 



80 



100 



Distance from Cable - Feet 



Figure 6 — Reduction in voltage between sheath and core with increasing distance from 

 cable to point where current enters the ground. 



1: Measured when remote ground representing cloud is at a distance of 1000 ft. 



2 : Calculated for uniformh' conducting earth with remote ground at distance of 1000 ft. 



3: Calculated for uniformly conducting earth with remote ground at infinity. 



large electrode spacings. The measured variation in voltage with separa- 

 tion is in substantial agreement with that calculated for an earth structure 

 of this type in the manner outlined in Section 1.9. 



1.8 Discharges Between Clouds 

 In considering voltages due to discharges between clouds, the lightning 

 channel is assumed to parallel the cable. Due to magnetic induction, the 

 lightning current will give rise to an impressed electric force along the 

 cable sheath. Without much error it may be assumed that there is no 

 impressed force outside the exposed section of the sheath and that the 

 electric force in the e.xposed section due to a sinusoidal current / is E (x) = 



