Power Distribution and Telephone Circuits Inductive and Physical Relations

Consideration of the relations between power distribution and telephone systems is naturally involved in any comprehensive review of the problems of the rapidly expanding power-distribution networks in this country. Avoidance of contact and provision of suitable working conditions for employees in situations of close proximity are dealt with in the National Electrical Safety Code and in State Regulations which provide arrangements for safety where complete separation is not feasible. Induction from distribution, circuits has heretofore had less general attention than induction from power-transmission lines. Recently the Joint General Committee of the National Electric Light Association and the Bell Telephone System has undertaken comprehensive investigations of these problems. Of particular interest is the study of induction under joint use conditions now progressing actively at Minneapolis. Pending completion of this and other studies, a preliminary and qualitative discussion is here given. Situations of exposure fall into three groups determined by the character of the area served: (1) ``downtown´´ districts; (2) residential urban districts. (3)rural districts. The major problems arise in the second group. A wide variety of arrangements characterize both systems, and require consideration. A mong technical features, coefficients of induction for close exposures, shielding action of metallic cable sheaths for both power and telephone circuits, and ``ground potential´´ effects are distinctive problems. Where both classes of circuits are in cable with suitable precautions as to grounding, interference is rarely to be anticipated.