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BELL SYSTEM TECHNICAL JOURNAL 



(c) When resistance is added in one of the sheath-to-ground con- 

 nections, the shielding effect of the sheath and conductors is 

 reduced. 



{d) If the shield wire is grounded its effect is cumulative ^ with that 

 of the cable sheath and conductors. 



{e) The voltage along the open wires on the line is reduced when the 

 cable is grounded at both ends. Here again, of course, the 

 effect of the shield wire is cumulative ^ with that of the cable 

 sheath and conductors. 



The same shielding effects could be shown if the power instead of the 

 telephone circuit were in cable. Also, of course, if there is more than 

 one power or telephone cable, the shielding is increased. Iron armoring 

 also tends to increase the shielding. 



Power Circuit Conditions 

 Having illustrated some of the factors affecting coupling let us now 

 review briefly the factors affecting the current in the power line at 



POWER LINE 



Fig. 7 — Comparison of induced voltage with different amounts of residual current. 



times of ground fault. Not only is the current flowing to ground under 

 fault conditions on a power line usually greater than the current under 

 normal operating conditions, but it is also a fact that a given current 

 in a ground return circuit induces a larger voltage along a paralleling 

 telephone circuit than the same amount of current if confined to the 

 phase conductors. That the induced voltage depends on the amount 

 of residual current can be demonstrated as shown in Fig. 7. Here a 



^ While the benefits are "cumulative," the individual effects are not directly 

 additive due to mutual reactions between the different shielding conductors, the net 

 effect being less than the total of the effects of each shielding conductor acting alone. 



