LOW-FREQUENCY INDUCTION 587 



due to the fact that for the isolated system, full phase-to-phase or 

 possibly higher voltage is impressed between the unfaulted phases 

 and ground, thus increasing the voltage stress on the insulation of the 

 entire system during the time of fault. 



Figure 9 shows a demonstration set-up to illustrate the effects of 

 faults on an isolated neutral system which is small enough so that the 

 capacitances are negligible. It will be noted that when a single fault 



POWER LINE 



Fig. 9 — -Demonstration of efifect of faults on isolated neutral system. 



is put on the line at the far end, no appreciable rise in voltage on the 

 telephone line occurs. However, when a second fault is put on another 

 phase at the near end, the induced voltage immediately rises. In 

 order to illustrate that the current is residual only between the faults, 

 the faults can be moved closer together. When this is done, the in- 

 duced voltage decreases until, with the faults at the same point, it 

 practically disappears. The small remaining voltage is largely due to 

 balanced current induction due to the heavy load on the power line 

 when two of the phases are shorted. 



A system grounded through a neutral impedance, such as when 

 resistance or reactance is included in the neutral-to-ground connections 

 or when a high reactance grounding bank is used, partakes of some of 

 the characteristics of an isolated neutral system. Generally speaking, 

 the addition of neutral impedance tends to reduce the fault current, 

 this effect being proportionately larger for faults near the neutral 

 grounding points. This reduction in fault current tends to reduce 

 the voltage induced on nearby telephone lines and in some cases may 

 reduce the "shock" to the power system and the damage at the point 

 of fault. On the other hand, increasing the neutral impedance tends 



