UGIirNING PROTECriON OF BCRIJW TOLL CABLE 273 



dx (51) 



r" 1 



/ \ / ^ —ar —Vx 



m(>') = / - ^ « 



^1 1 + 3'(to + r) 



= ^"g 3.(^„ + r) ~ "^^'"^ ^"^"^^^ ^^^^ 



^e'^'^My) when 703/ » 1 (53) 



In applying the above expressions, a rough value of pe is first assumed in 

 calculating \(o) and ^^((7) and a more accurate value next obtained from 



100 



50 



10 



2 

 3 



.1 



100 



Distance from Cable - Feet 



1000 



Figure 7— Reduction in voltage between sheath and core with increasing distance from 

 cable to point where current enters ground. 



1 : Upper layer of 400 meter-ohms and 30 ft. depth. Lower layer of 4000 meter-ohms 

 and infinite depth. 



2: Uniformly conducting earth. 



3: Upper layer of 1500 meter-ohms and 30 ft. depth. 

 Lower layer of 150 meter-ohms and infinite depth. 



(49). If the value of pe thus obtained differs materially from the assumed 

 value, a second calculation may be required. In the expressions for X 

 and p. the resistivity pe is to be used in calculating T, the latter being taken 

 as {v/2pjy where / is the time to crest value of the voltage as before. 



In Fig. 7 is shown the manner in which the voltage decreases with in- 

 creasing separation for three assumed earth structures. The resistivities 

 and the depth of the upper layer were selected such that the equivalent 

 earth-resistivity, and thus the voltage in the case of a direct stroke, is the 

 same in all cases and equal to 1000 meter-ohms. It will be noticed that 

 in the case where the resistivity of the lower layer is high, the voltage due 

 to a stroke at a distance of 200 ft. is 50% and at a distance of 1000 ft., 25% 

 of the voltage due to a direct stroke. When the cable is small, insulation 

 failures may thus be occasioned by strokes to ground at considerable 



