Some Aspects of Low-Frequency Induction Between Power 

 and Telephone Circuits * 



By H. R. HUNTLEY and E. J. O'CONNELL 



This article discusses the phenomena involved in low-frequency induction 

 between power and telephone circuits and describes a demonstration which 

 has been developed to illustrate certain of them. 



Introduction 



TN the practical problem of inductive coordination between power 

 -*■ and telephone circuits, there are two aspects to be considered : 



1. Induction within the frequency range used in transmitting speech 



which may result in disturbing noise in telephone receivers. 

 This phenomenon is usually associated with the normal opera- 

 tion of power and telephone systems although abnormal con- 

 ditions in either system may result in a large increase in the 

 noise. 



2. Induction at the fundamental frequencies used in the transmission 



of power. This is commonly referred to as "low-frequency 



induction" and in some cases may reach such magnitudes as 



to interrupt telephone service, constitute a hazard to telephone 



employees and produce other detrimental effects. Induced 



voltages of magnitudes sufficient to cause operating difficulties 



in telephone circuits occur usually only under abnormal power 



circuit conditions which produce large currents in the earth. 



Under normal circuit conditions, three-phase power circuits are 



so nearly balanced with respect to ground at their fundamental 



frequency that induction at this frequency is rarely sufficient 



to seriously affect well balanced telephone circuits. 



Both types of induction have been and are being intensively studied 



cooperatively by the power and telephone industries. Much of this 



work has been handled through the Joint General Committee of the 



National Electric Light Association and Bell Telephone System which 



was formed in 1921 and it is now being carried forward by the Joint 



General Committee of the Edison Electric Institute and Bell Telephone 



System. 



* This paper appeared in somewhat different form in Amer. Railway Assoc. Proc, 

 June, 1934, under the title "Demonstration of Low-Frequency Induction Between 

 Power and Telephone Circuits" by H. R. Huntley. 



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