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BELL SYSTEM TECHNICAL JOURNAL 



together, the arrangement shown in Fig. 13 has been used. The percentage 

 of the current carried by the wires depends to a greater extent on their 

 inductance relative to that of the sheath, than on their resistance. Two 

 wires are employed, rather than a single wire of smaller resistance, in 

 order to obtain a lower inductance than with a single wire. 



On the route between Stevens Point and Minneapolis, where the shield 

 wires were installed after the cable was in place, two 165-mil wires about 

 twelve inches apart were plowed in some ten inches above the cable for a 

 distance of eighty miles. Surge measurements made after these wires were 

 installed indicated that the wires reduced the voltage between sheath and 

 core conductors about 60 per cent, in substantial agreement with theoreti- 



//////////////////////A'//////////// 



8" 



2" 



1 



S •- 



C 



O' 



o 



9" 



1 



T 



Figure 13 — Position of shield wires, S, when they are plowed in with cable, C. 



cal expectations. The cable would then withstand 75,000 amperes rather 

 than 30,000 without shield wires. If a cable of normal construction were 

 used on the above route, the shield wires would thus reduce the number 

 of direct lightning strokes that would be expected to cause insulation failure 

 about 3.3 times, from 50 per cent of the total without shield wires to about 

 15 per cent with shield wires (see Fig. 1, curve 2). If the insulation 

 strength is only 1000 volts, however, about 80 per cent of all strokes would 

 be expected to cause failure without shield wires and 40 per cent with 

 shield wires, so that the reduction would be substantially smaller, as ac- 

 tually appears to be the case on the above route. 



Aside from reducing core insulation failures, shield wires also minimize 



