228 BELL SYSTEM TECHNICAL JOURNAL 



in cases of contact, wire or structure failures are generally involved while 

 faults may cause induction which do not involve falling wires. Con- 

 tacts impose on the telephone line the full voltage to ground of the 

 power conductor at that point, whereas induced voltage is usually only 

 a fraction of the power circuit voltage. This does not mean that the 

 imposed voltages due to contact are always higher than those due to 

 induction, because the majority of exposures to contact do not involve 

 the higher voltage circuits while the opposite is true regarding inductive 

 exposures. In cases of contact only part of the wires of the telephone 

 line are usually involved whereas in the majority of induction cases sub- 

 stantially the same voltage is induced on all the wires. The voltages 

 imposed on a telephone line by contact as well as those by induction 

 may extend over the full length of the conductors involved. 



In addition to the effects of contact between wires of the two systems, 

 there is a distinct class of hazard to linemen of both utilities introduced 

 by situations of insufficient clearance due to improper construction or 

 inadequate maintenance on the part of one or both utilities. 



Voltages on telephone lines by lightning produce effects somewhat 

 similar to the effects produced by power lines but lightning voltages 

 differ from the other voltages in that their duration is much shorter. 

 Lightning makes necessary protector discharge gaps of very high speed 

 of operation in order to prevent serious over-voltages on the telephone 

 system, whereas contacts with power circuits make necessary a pro- 

 tector of high current-carrying capacity. 



Committee's Program of Work 



The program of work on low-frequency induction undertaken by the 

 Joint Subcommittee on Development and Research through its project 

 committees is laid out to develop the essential facts bearing on the 

 problem of telephone protection in a broad sense, including causes, 

 effects, and remedial measures. The program covers not only the 

 technical but also the economic aspects of the problem. The problems 

 of lightning and physical contact under conditions of conflict or joint 

 use are also included, as the measures finally adopted must protect 

 against voltages from these sources as well as voltages induced by 

 power systems. 



Extensive field trials of all promising protective measures, are under 

 way in order to determine their practicability under operating condi- 

 tions. As the work progresses, it is expected to issue from time to 

 time reports covering the applicability, efficacy, limitations, and condi- 

 tions of use, of various measures. This should result in a better under- 

 standing of the problem and more effective and economical solutions of 

 specific situations as they arise. 



