Some Aspects of Low-Frequency Induction Between Power and Telephone Circuits

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 operation of power and telephone systems although abnormal conditions 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 t h a t 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.