COOPERATIVE WORK ON JOINT USE OF POLES 239 



these factors and the results of the extensive studies of these matters 

 by the Joint Subcommittee on Development and Research are described 

 in the paper by Messrs. Wills and Blackwell. 



The various operating problems which have arisen almost since the 

 birth of the power and telephone industries and the investigations con- 

 ducted by the Joint Subcommittee on Development and Research 

 indicate the importance of giving careful consideration to the inductive 

 coordination features of joint use and of including this factor in studies 

 of the relative advantages and disadvantages of joint use as compared 

 with separate lines. This factor should, of course, be considered from 

 both its technical and economic aspects. 



Much can be accomplished in the inductive coordination of the two 

 distributing systems by cooperative advance planning. In urban 

 areas where the telephone circuits are largely in cable, there is about 

 a two to one ratio in the inductive effects between a joint line and sepa- 

 rate lines across the street. In rural areas where the telephone circuits 

 are largely open wire, the ratio of the inductive effects on joint lines 

 as compared with separate lines across the highway, is much greater, 

 other things being equal. 



In urban areas the power and telephone companies can through 

 cooperative planning frequently arrange to establish important power 

 feeders and telephone circuits on separate streets and thereby avoid 

 large inductive effects and permit more extensive joint use of branch 

 lines. A careful review of the equipment used on the powder and tele- 

 phone circuits and the introduction of operating practises designed to 

 limit the inductive susceptiveness of the telephone circuits and the 

 inductive influence of power circuits, form an important part of ad- 

 vance planning and cooperation. 



As described in the paper by Messrs. Wills and Blackwell, these 

 latter factors include such items as limitation of the odd triple fre- 

 quency series arising in Y-connected generators feeding directly on the 

 line and in single-phase service transformers. Suitable limitations 

 of the unbalances existing among the loads connected between the 

 three-phase conductors and the neutral, limit the ground return com- 

 ponents. 



Grounding of aerial telephone cable sheaths to provide for increased 

 shielding and the use of central office and station equipment providing 

 a higher degree of balance with respect to ground are helpful. 



The matter of joint use may involve both rural and urban commun- 

 ities. It is more generally associated with the latter because of the 

 severe limitations in physical space available for utility use. In the 

 case of rural lines where the telephone circuits are largely in open wire 



