LO W-FREQ UENC Y IND UCTION 



583 



While the demonstration shows only the effect of a grounded wire 

 on or near the power or telephone lines, similar effects in varying 

 degrees may be caused by such grounded metallic structures as under- 

 ground pipe lines, railroads where the rails are bonded in long lengths, 

 trolley lines, etc. In many situations the shielding effects of such 

 structures may be substantial. 



Another type of grounded conductor which may give substantial 

 shielding is the metallic sheath of a telephone or power cable. A cable 

 sheath will effect some shielding on conductors which are not enclosed 

 by it in the same way as any other grounded metallic conductor, but 

 the major shielding effect is experienced on conductors within the 

 sheath. The shielding effect of a cable is, as in the case of a shield 

 wire just demonstrated, determined to a considerable extent by its 

 impedance with ground return. 



Shielding due to a telephone cable can be demonstrated using the 

 set-up shown in Fig. 6 and it is noted that: 



POWER LINE 



rO AMPLIFIER 

 AND METER 



OPEN WIRE TELEPHONE LINE 



MEASURING 

 TELEPHONE CABLE / PAIR 



-1_-^ 



OTHER PAIRS 

 IN CABLE 



;^ 



""^SHE 



Fig. 6 — Effect of cable shielding. 



(a) The voltage along a conductor inside the cable is reduced when 



the sheath is directly grounded at both ends. 

 (6) If the effective resistance of the sheath is reduced by paralleling 



with it some of the conductors inside, the shielding is increased 



(i.e., the reduction in voltage is greater). 



