Status of Joint Development and Research on 

 Low-Frequency Induction * 



By R. N. CONWELL and H. S. WARREN 



This paper deals with coordination of power and telephone systems with 

 respect to induction at power system frequency, usually 60 cycles. The 

 principal problem in this held relates to effects produced under abnormal 

 conditions on power systems. The factors controlling the magnitude, 

 frequency of occurrence, duration, and effects, of induced voltages, are 

 discussed. Different types of protective measures, some applicable to 

 power systems and others to communication systems, are outlined, including 

 their respective advantages, limitations, and fields of application. The 

 reaction on this problem of lightning and of situations involving liabilit}' of 

 contacts between telephone wires and power wires is touched upon. The 

 whole matter is treated from the standpoint of the comprehensive joint in- 

 vestigation of the interference problem which is being conducted by the 

 N.E.L.A. and the Bell System. 



INDUCTION at power system fundamental frequency, commonly 

 called "low-frequency" induction, has different characteristics and 

 produces quite different effects from induction at the noise frequencies 

 discussed in the paper by Messrs. Blackwell and Wills. Smce very 

 little has been published on low-frequency induction, it seems desirable, 

 in order to make clear what the Joint Subcommittee on Development 

 and Research is doing on this subject, to explain the problem in some 

 detail. 



The disturbances in communication circuits due to low-frequency 

 induction are in general discrete occurrences, coincident with acci- 

 dental grounds or other faults on neighboring power lines, rather than 

 being continuous and due to normal power line operation. 



Three-phase power circuits, when operating normally, are so nearly 



balanced with respect to earth at their fundamental frequency, and 



telephone circuits of the ordinary type are relatively so insensitive at 



frequencies of 60 or 25 cycles, that induction at these low frequencies 



under normal power line conditions is rarely a practical problem. But 



when abnormal conditions, particularly faults to ground, occur on 



power lines, large unbalanced voltages and currents at fundamental 



frequency exist temporarily and at such times there may be induced in 



neighboring telephone circuits voltages which are hundreds of times as 



great as under normal operating conditions. The induced voltages 



under abnormal conditions may reach values sufficient to cause hazard 



* Part III of the Symposium on Coordination of Power and Telephone Plant. 

 Presented at the Winter Convention of the A. I. E. E., New York, X. Y., January 

 26-30, 1931. Published in abridged form in Eleclrlcal Engineering, April, 1931. 



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