200 BELL SYSTEM TECHNICAL JOl'RNAL 



pedance relay connected in series with one side of the central office 

 interconnecting circuit. The selective ringing scheme involves con- 

 necting the ringer windings from one side of the line to ground at the 

 station set. For the coin box service, a coin-collect relay winding is 

 connected between one side of the station set and ground. These 

 unbalances have been investigated in detail by the committee and the 

 results have been published ^ as described later. 



The unbalance of party lines due to the ringer ground is usually 

 much more important than that of the central office interconnecting 

 circuit due to the supervisory relay. Both are, in general, more impor- 

 tant than the cable unbalances. Coordination difficulties between 

 telephone exchange systems and power distribution systems thus 

 usually involve the party-line circuits before the individual-line circuits 

 are affected. 



The controlling unbalance in the exchange plant when in cable 

 being in the nature of an inductance between one side of the line and 

 ground, its importance decreases with increasing frequency of the in- 

 duced longitudinal voltage. This effect largely counter-balances the 

 increase in coupling with frequency. Thus, in most situations involv- 

 ing joint use of poles by distribution circuits and exchange cable tele- 

 phone circuits, induced currents of the third and fifth harmonics of the 

 power circuit fundamental frequency assume chief importance. 



Exceptions are cases where outstanding harmonics in the range 

 between 800 and 1500 cycles are present on the power circuits. In 

 these cases, particularly where the exposures are long, the central office 

 apparatus unbalances may be more important than those of the party- 

 line station apparatus. 



The method which has been found most generally applicable for 

 reducing the susceptiveness of exchange cable circuits is the grounding 

 of the cable sheaths. This reduces through shielding the magnitudes 

 of the longitudinal voltages and currents. Special station sets having 

 lower susceptiveness have been used in specific cases where their use 

 appeared to be the best method. 



Power Level and Sensitivity. The magnitudes of the induced currents 

 in the telephone system having been determined by the influence 

 factors, the coupling, and the unbalances of the telephone circuits, 

 the degree to which they impair telephone service depends upon their 

 intensity as compared to the intensity of the telephone currents. 



Consideration has been given by the subcommittee to the possibility 

 of increases in power levels (a) on local exchange circuits and (b) on toll 

 circuits. Little promise has been found in the proposal to raise voice 

 power levels in the local exchange plant as a means of reducing the 



