82 BELL SYSTEM TECHNICAL JOURNAL 



high, the load currents and transformer-exciting currents are relatively 

 large and the line-charging currents are usually negligible. In such 

 a system multi-grounding the neutral results in an increase in the 

 current returning through the earth and a consequent increase in the 

 inductive influence of the power distribution system. 



In rural areas, however, where the load density is low and the load 

 currents and transformer-exciting currents are relatively small, the 

 line-charging currents become significant. In general, under such 

 conditions the multi-grounding of the neutral does not increase the 

 magnitude of the ground-return current at frequencies of interest from 

 the noise induction viewpoint. Under certain conditions the mag- 

 nitude of this ground-return current may actually be substantially 

 decreased by the multi-grounding of the neutral. This effect is more 

 marked for the higher voltage circuits. 



The harmonics present in a distribution circuit may be divided 

 into (1) triple harmonics, that is, the third harmonic and odd multiples 

 of it, and (2) non-triple harmonics, that is, the odd harmonics, starting 

 with and including the fifth, which are not multiples of three. The 

 triple harmonics in a three-phase system are in phase in the three line 

 conductors so that their residual value (vector sum) is the arithmetic 

 sum of their magnitudes in the three-phase wires. The non-triple 

 harmonics are spaced, in time phase, the same as the 60-cycle funda- 

 mental and the magnitude of the residual current (vector sum) for 

 these harmonics is usually much less than their arithmetic sum. If 

 these harmonics were perfectly balanced the residual current for these 

 frequencies would be zero. In exposures involving three-phase sec- 

 tions of line the balance of the non-triple harmonics between phases 

 is influenced by the degree of balance of the loads and single-phase 

 branches and therefore has an important effect in reducing the overall 

 influence of the power system. In exposures involving single-phase 

 extensions, or extensions consisting only of two-phase wires and a 

 neutral wire this advantage of the balancing of the non-triple har- 

 monics is, of course, not obtainable. 



The extent to which induction from the non-triple harmonic voltages 

 and currents in power distribution circuits can be controlled by power 

 circuit transpositions is ordinarily very limited. Usually, due to the 

 large number of exposure discontinuities arising from changes in the 

 power or telephone circuits, the power circuit transpositions are quite 

 ineffective. This is particularly true in cases where the induction from 

 the ground-return current is controlling. In specific cases where 

 considerable wave shape distortion exists and the induction from the 

 balanced voltages and currents may therefore be relatively important, 

 transpositions in power distribution circuits may be found helpful. 



