INDUCTIVE COORDINATION 81 



Certain quantitative indications of the extent to which this measure 

 of coordination is applicable in the two types of areas are shown in the 

 illustrative examples in the Appendix. 



Power Circuits 



Power systems operate, for the most part, at frequencies of 60 cycles 

 and below. Telephone circuits, on the other hand depend mainly 

 upon frequencies above about 200 cycles for the transmission of speech. 

 Ordinarily, therefore, the effects of induction from the fundamental 

 frequency currents and voltages in neighboring power lines are negli- 

 gible as far as telephone circuit noise is concerned. It is quite generally 

 recognized, however, that it is impracticable commercially to build 

 rotating machinery and transformers which are entirely free from 

 harmonics. There are, therefore, harmonics present on all operating 

 power systems and it is the harmonic-frequency components induced 

 into telephone circuits from these power system harmonics that are of 

 major importance from the noise standpoint. ^^ 



In any distribution circuit the harmonic currents present will fall 

 within the following classes: — load currents, transformer-exciting 

 currents and line-charging currents. With a uni-grounded neutral 

 the load currents and the transformer-exciting currents are practically 

 entirely confined to the wires of the circuit. Where the neutral is 

 multi-grounded, the vector sum of the currents in the phase conductors 

 (residual current) will divide between the neutral conductor and the 

 paralleling earth path as determined by the relative impedances of 

 these two paths. W^hile there is some variation in the division of the 

 return current between the neutral and ground paths, for most practical 

 purposes this division may be assumed to be about half in each path 

 at all the frequencies of interest. 



As pointed out above, in the case of a line operating with uni- 

 grounded neutral, the earth-return components of the load and 

 transformer-exciting currents are ordinarily negligibly small. How- 

 ever, this is not true of the line-charging current which is chiefly a 

 function of the magnitude and frequency of the impressed voltage, 

 the circuit length, and, at non-triple harmonic frequencies, of the 

 balance of the admittances to ground of the various phase conductors. 

 While multi-grounding the neutral ordinarily increases the earth- 

 return components of the load and transformer-exciting currents, it 

 has been found, due to the parallel path provided by the neutral wire, 

 on an average to decrease slightly the amount of charging current in 

 the earth. 



In an urban distribution system where the load density is relatively 



