592 



BELL SYSTEM TECHNICAL JOURNAL 



trates this effect. The actual circumstances concerned in this phe- 

 nomenon are, of course, that the wires which become grounded at 

 their terminals through the operation of the protectors exert a shielding 

 effect in exactly the same way as any other grounded conductor. 



VOLTAGE INDUCED BY 

 CURRENTS IN OTHER 

 TELEPHONE WIRES 



z LINE V"* " y 



VOLTAGE INDUCED BY 

 POWER CURRENT 



AAAr 



AAAr 

 -\A/V 



__/v A A 



■<2>WVG> 



<S>A/W<S> 



<5>VW<]I> 



Z EXPOSURE 



Z LINE 



-AW- 



AW 



AAA- 



AW 



TOTAL CURRENT 



CURRENT PER WIRE 



NUMBER OF WIRES 



Fig. 12 — Current in telephone wires. 



The resistance of an individual wire is relatively high but if numerous 

 protectors operate, the shielding may become fairly great, due to the 

 closeness of the wires to each other and to the fact that a substantial 

 amount of copper may be involved. Of course, this shielding is ob- 

 tained at the expense of at least momentary interruption of the circuits 

 on which protectors operate. 



The shielding effect of current in grounded telephone wires is exerted 

 on all telephone wires on the line regardless of whether the protectors 

 on them have or have not operated. Consequently, what may happen 

 on a large telephone line with a moderate induced voltage on it is 

 that enough telephone protectors on different circuits operate to give 



