NDUCTION at power system fundamental fre-conditions, create a problem of low-frequency induction quency, commonly called "low-frequency" induc-in paralleling telephone circuits in addition to setting tion, has different characteristics and produces quite up high-frequency disturbances as explained in the different effects from induction at the noise frequencies Blackwell-Wills paper. This is particularly true where discussed in the paper by Messrs. Blackwell and Wills. the exposed telephone circuits are used for special serSince very little has been published on low-frequency vices such as the transmission of radio broadcasting proinduction, it seems desirable, in order to make clear grams. Grounded types of telegraph and other signal what the Joint Subcommittee on Development and circuits also are sensitive to low-frequency induction. Research is doing on this subject, to explain the prob-CLASSIFICATION OFFACTORS RESPONSIBLE FOR lem in some detail.The disturbances in communication circuits due toINDDCT TVEEFFECTS low-frequency induction are in general discrete occur-The same three classes of factors which combine to rences, coincident with accidental grounds or other underlie the noise-frequency problem appear also in the faults on neighboring power lines, rather than being low-frequency problem. As they appear in the latter, continuous and due to normal power line operation. these are: Three-phase power circuits, when operating normally, 1. "Influence factors" in the power system, which are so nearly balanced with respect to earth at their are concerned with the magnitude, duration, and frefundamental frequency, and telephone circuits of the quency of occurrence, of unbalanced voltages and ordinary type are relatively so insensitive at frequencies currents. of 60 or 25 cycles, that induction at these low frequencies 2. "Susceptiveness factors" in the communication under normal power line conditions is rarely a practical system, which are concerned with the nature and problem. But when abnormal conditions, particularly seriousness of the effects produced by the induced faults to ground, occur on power lines, large unbalanced voltages.voltages and currents at fundamental frequency exist 3. "Coupling factors" which determine the magnitemporarily and at such times there may be induced in tude of the voltages induced in the communication neighboring telephone circuits voltages which are hun-system, per unit unbalanced voltage or current of the dreds of times as great as under normal operating power system. conditions. The induced voltages under abnormal conIn the low-frequency induction problem, the coupling ditions may reach values sufficient to cause hazard to factors are largely dependent upon the characteristics of telephone employees or interruption to service. Al-the earth and the relations of power and telephone though such abnormal conditions occur infrequently systems to the earth. If the earth were an insulator and usually last for only the very short period required instead of a conductor there could, of ...
