492 BELL SYSTEM TECHNICAL JOURNAL 



the admittance unbalances between the various wires and 

 ground may be fairly large. 



2. In considering the effects of the admittance unbalances, there are 



two conditions which must be considered; where the trans- 

 formers supplying the three-phase portion are "Y grounded" 

 on the line side, and where they are "delta" on the line side. 

 When the supply transformers are connected delta on the line 

 side, there is no path for residual current into the transformers 

 and the voltages of the conductors to earth adjust themselves 

 so that the net charging current to earth is zero (although there 

 will be some interchange of charging current between various 

 portions of the network). This condition requires unequal 

 voltages to earth, the voltages of the wires having the higher 

 capacitances being lower than those of the lower capacitance 

 wires. This generally gives a residual voltage. 



3. When the supply transformers are connected Y-grounded on the 



line side, the voltages of the wires to ground are controlled by 

 the transformer voltages and the principal effect of a single- 

 phase extension is a tendency to cause residual current. 



The discussions above apply particularly to power systems which 

 are electrically short at all of the important harmonic frequencies 

 present. If the systems are long enough so that propagation effects 

 (particularly "quarter wave-length" effects) must be considered at 

 any of the important harmonic frequencies present in the voltage or 

 current waves, these simple analyses must be modified. These 

 propagation effects cannot be demonstrated with the apparatus 

 available and will not be discussed further except to point out that they 

 are not infrequently encountered in field problems. 



Inductive Susceptiveness of Telephone Circuits 

 As pointed out previously, theoretically a telephone circuit could 

 be constructed so that it would not be affected by any fields which 

 would be set up by nearby electrical systems and hence would have 

 zero susceptiveness. However, as in the case of the power line, it is 

 not practicable to build such ideal telephone lines and consequently, 

 the consideration of telephone lines in inductive exposures has to do 

 with the deviations from perfection in this respect. 



As was indicated earlier in this article, the metallic type of telephone 

 circuit is now usually used. The grounded system which uses one 

 wire with earth return, was employed exclusively in the very early 

 days and is still used in some cases, particularly in sparsely settled 

 areas. 



