80 BELL SYSTEM TECHNICAL JOURNAL 



readily obtained by the multi-grounded neutral arrangement as well 

 as by certain equipment savings. The recent emphasis placed on the 

 electrification of rural areas and the distinct need for maximum 

 service continuity on rural power circuits has increased the interest 

 in the use of the multi-grounded or common-neutral method of 

 distribution in rural areas. The rural systems are generally of the 

 7600/13, 200-volt class, although 4600/8000-volt circuits have been 

 used to some extent. 



The factors affecting inductive coordination involved in the use of 

 the common-neutral method of distribution in rural areas, are some- 

 what different from those encountered in urban areas. This is largely 

 due to the lower load densities, the greater lengths of circuits, higher 

 operating voltages and to the somewhat different types of equipment 

 employed in rural telephone distribution. These factors were in- 

 vestigated by the Joint Subcommittee on Development and Research 

 during the summers of 1935 and 1936 and the more important con- 

 siderations determined.^" 



Factors Influencing Inductive Coordination 



In any problem of inductive coordination it is convenient to sub- 

 divide the factors influencing coordination into those relating to the 

 inductive influence of the power circuit, the inductive susceptiveness 

 of the telephone circuit and the inductive coupling between the two 

 types of circuits. As far as urban distribution circuits are concerned 

 the load current unbalance of the power circuit is usually the controlling 

 influence factor. For rural distribution circuits the unbalanced 

 charging currents are generally more important than the unbalanced 

 load currents. Likewise, in an exchange telephone circuit the admit- 

 tance and impedance unbalances of the two sides of the circuit are 

 usually the controlling factors in its inductive susceptiveness. As 

 far as coupling between the power and telephone circuits is concerned 

 this is largely controlled by their relative positions and the lengths 

 of the exposure. For urban areas their relative positions are largely 

 fixed by the normal arrangement of conductors and cables on jointly- 

 used poles. In rural areas power and telephone circuits are generally 

 at roadwa\' separation although some joint use exists. In urban areas 

 considerable control can often be exercised over the coupling by 

 planning the routes of the main feeds of the two services so as to avoid 

 long sections of close exposure. In rural areas where there are no 

 paralleling routes close together it is generally necessary for both 

 services to use the same roads and therefore the opportunity to control 

 the coupling by the cooperative planning of routes is much reduced. 



