The Inductive Coordination of Common- Neutral Power 

 Distribution Systems and Telephone Circuits * 



By J. O'R. COLEMANt and R. F. DAVIS 



Early installations of three-phase, four-wire power distribution 

 systems of the multi-grounded or common-neutral type in some 

 cases created noise problems involving neighboring telephone cir- 

 cuits. Operating experience, studies of specific situations and 

 comprehensive cooperative research over a period of years have 

 developed means of largely avoiding difficulties of this character. 

 The relative importance of various features of the power and 

 telephone systems which have been found to affect the noise induc- 

 tion problems involved is discussed and the general cooperative 

 procedures, most helpful in conversions to or extensions of these 

 types of power distribution systems, are outlined. 



Introduction 



PRIOR to about 1915, delta-connected 2300-volt, three-phase, pri- 

 mary circuits were used extensively for the distribution of electric 

 current. While some distribution networks throughout the country 

 still operate in this manner, the marked increase in load densities, 

 starting about 1915, often made the retention of the 2300-volt delta 

 system impracticable. In a few instances the development of the 

 particular network was at a point where it was feasible to change from 

 the 2300-volt delta to a 4600-volt delta arrangement but in other 

 cases the existing equipment represented too great an investment for 

 a complete change of this character. 



From studies of various methods of caring for the augmented load 

 densities it was found that the existing equipment could largely be 

 saved and the capacity of the distributing networks substantially 

 increased by converting them to a 2300/4000-volt, star-connected, 

 four-wire primary system. By about 1925 this system had extended 

 to most of the larger cities and most power companies had found it 

 economical for use in at least some parts of their territories. 



In using the 2300-volt equipment on the 4000-volt, four-wire system 

 it was necessary to stabilize the neutral conductor in some way. Most 

 of the four-wire systems had the neutral conductor grounded at the 



* Published in Electrical Engineering, January, 1937. 



t Mr. J. O'R. Coleman, joint author of this paper, is an engineer on the staff of 

 the Edison Electric Institute, New York City. 



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