LOW- FREQUENCY INDUCTION 



593 



a shielding effect on the remaining wires sufficient to reduce the voltages 

 on them to values lower than will operate the protectors. 



Another important factor is the voltage-to-ground at various places 

 along the telephone circuit after protectors operate. With the pro- 

 tectors operated the voltage-distribution-to-ground can be evaluated 

 from the longitudinal induced voltages, the longitudinal currents, and 

 the series impedances in the circuit. As the simplest and perhaps 

 most striking case, consider a telephone circuit which is solidly 

 grounded due to operated protectors at the ends of a uniform exposure 

 with a fault on the power line at the end of or beyond the exposure. 

 This situation is illustrated in Fig. 13. The distribution of induced 



POWER LINE 



IMPEDANCE 

 REPRESENTING 

 ADDITIONAL 

 LINE ^ 



f^ 



TELEPHONE LINE 



TO AMPLIFIER 



AND METER 



n. 



A -DEMONSTRATION 



e z e Z e 



r^-V\K5>W<2>A/\r®-^W-^AV<I>Mr 



B-THEORY 



Fig. 13 — Voltage to ground for telephone line grounded at ends where fault is 



outside of exposure. 



voltage along the line is uniform and the longitudinal current is equal 

 to the total longitudinal voltage divided by the total series impedance. 

 If the net drop in voltage is taken from either end to any point along 

 the circuit, it will be found that the induced voltage accumulated 

 over this distance is equal and opposite to the voltage drop over this 

 same distance due to the current flow through the impedances in this 

 section. Consequently, under these conditions the voltage-to-ground 

 is zero at all points along the circuit. This is true regardless of the 



