INDUCTIVE COORDINATION 79 



was largely in aerial cable. The investigation was continued in Elmira, 

 New York in 1926 to 1929 to embrace the factors introduced when the 

 exchange telephone plant was of open-wire construction.^ Sup- 

 plementing these detailed technical studies, an investigation of certain 

 economic features of various arrangements of power and telephone 

 distributing methods and of their practical application under varying 

 conditions was carried out in California in 1928 and 1929. As a 

 result of these investigations the various factors involved in the co- 

 ordination of multi-grounded or common-neutral power systems and 

 telephone distribution systems were determined and certain practices 

 developed for the coordination of these systems under various condi- 

 tions in urban areas. ^ 



The purpose of this paper is that of briefly outlining a few of the more 

 important features of power and telephone circuits affecting noise 

 coordination. Following such a review there is presented a list of 

 measures which extended experience has shown will, where given proper 

 consideration by both parties, enable multi-grounded or common- 

 neutral power circuits and telephone circuits to live harmoniously. 

 No attempt is made, however, to reiterate the extensive technical 

 information obtained from the investigations outlined above as these 

 are adequately covered in the references cited. 



Recent Trends 



During the past few years there has been extensive conversion from 

 other types of urban power distribution to the multi-grounded or 

 common-neutral system of primary distribution. Where there exists 

 a three-phase, three-wire delta circuit the system is converted by 

 making the secondary neutral network continuous, reinforcing it 

 where necessary, and making the required changes in transformer 

 connections. Where there is a three-phase, four-wire uni-grounded 

 primary system the conversion is, as previously mentioned, made by 

 interconnecting the primary and the secondary neutral at each load 

 transformer generally removing the primary neutral only at the time 

 of major rebuilding. In either case extensions are usually made using 

 a single neutral in the secondary position. 



In the urban areas most of the multi-grounded or common-neutral 

 systems are of the 2 300/4000- volt class, although there are a few 

 instances where 4600-volt systems have been converted. At the 

 present time there is being constructed a 6900/1 2, 000-volt common- 

 neutral distribution system at Wichita, Kansas. 



The distinct trend in power distribution practice has been, in no 

 small measure, influenced by the improved overall protection features 



