LOW-FREQUENCY INDUCTION 585 



comparison is made between the induced voltage for two residual 

 currents, one greater than the other. With larger current, the voltage 

 indicated on the meter is larger. The resistance inserted in the fault 

 to reduce the current in this demonstration might be thought of as 

 simulating added impedance anywhere in the ground return circuit 

 through the fault. For example, it might be thought of as simulating 

 the effect of the line impedance which would be added if the fault 

 occurred at some distance beyond the end of the exposure. Also, its 

 effect is the same as would be produced by an increase in the reactance 

 of the supply transformers or in the neutral-to-ground connection; 

 or by an increase in the local resistance at the fault itself. 



In analyzing the impedances further, there are two general types of 

 power systems which must be considered: the "grounded neutral" 

 and "isolated neutral" systems. These are illustrated in Fig. 8. 

 In the grounded neutral system, the neutrals of one or more trans- 

 former banks are grounded directly or through impedance so that in 

 the event of a fault, a path for current is established from the fault 

 through the earth and back to the system through the neutral-to- 

 ground connections. In the isolated neutral system there are normally 

 no grounds on the system so that in the event of a fault the only path 

 for fault currents is through the capacitances of the unfaulted phases 

 to earth or through a second fault if one exists. Hence for a singfe 

 fault, the fault current is limited to the charging and leakage current. 



Figure 8-D shows for a grounded neutral system, the equivalent 

 single-phase circuit for residual currents. In an actual line, the 

 circuit conditions are, of course, usually much more complex than 

 those shown. In even the simplest situations, there are usually other 

 lines, generator points, or grounding points which supply some fault 

 current. However, for the purpose of examining the fundamental 

 phenomena, the simplified diagram can be used. As can be seen, the 

 impedances which control the residual current are those associated 

 with the fault, the line impedance, those in the transformer and 

 generating equipment and the impedance, if any, in the neutral-to- 

 ground connection. Impedances in any of these places tend to limit 

 the fault current. 



Figure 8-C shows a simplified diagram of the equivalent single- 

 phase circuit of an isolated neutral system with a single-phase fault- 

 to-ground. For this condition, it is evident that the fault-current 

 path includes the capacitances to ground of the unfaulted phase 

 conductors. In a small system these capacitances will be small and 

 the fault current will, therefore, also be small, particularly if the 

 voltage is not high. In extensive systems or systems having much 



