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BELL SYSTEM TECHNICAL JOURNAL 



magnitude of the induced voltage. Figure 13 also shows the set-up 

 by which this fact can be demonstrated. It will be noted that, while 

 there is a fairly high longitudinal voltage, the voltage between the wires 

 and ground with both ends of the circuit grounded is negligible at all 

 points along the line. 



If the telephone line extends beyond the exposure, the effect of this 

 portion of the line is to add impedance between the exposure terminal 

 and the protector without adding a corresponding induced voltage. 

 If a power line fault occurs at the end of the exposure, a voltage-to- 

 ground will exist at this point equal to the current in the telephone 



POWER LINE 



I^ 



TELEPHONE LINE 



TO AMPLIFIER --- 

 AND METER ~- 



1 



A — DEMONSTRATION 



z z zezezez 



■A/V \N VV<5>AV<2>V\K]5>A/\n 



~ B — THEORY ~ 



Fig. 14 — Voltage to ground for telephone line grounded at both ends where fault is 



inside of exposure. 



line times the impedance outside the exposure. This situation can 

 be illustrated in the set-up of Fig. 13 by leaving the power fault at 

 the end of the exposure and adding a small impedance in the ground 

 connection to the telephone line at one end. When this is done it 

 will be noted that a voltage-to-ground exists at the end where the im- 

 pedance is added and that the voltage-to-ground decreases as the 

 measuring point is moved toward the other end. 



To illustrate that voltage-to-ground may occur under other con- 

 ditions even though the telephone line is solidly grounded at the 

 exposure terminals, consider the situation shown in Fig. 14. This 



