JOINT DFA'ELOrMENT AND LOW-FREQUENCY INDUCTION 209 



Residual voltages and currents are equivalent to single-phase vol- 

 tages and currents applied to a circuit consisting of the three line con- 

 ductors in parallel as one side, and the earth as the other side. Their 

 large inductive effects are due to the great dimension of the loop formed 

 by this earth return circuit, much of the return current being effectively 

 so deep in the earth that its neutralizing action is small. In the case 

 of the balanced components, the inductive efTect due to the voltage or 

 current of one conductor is largely neutralized by the voltages or cur- 

 rents of the other two conductors. 



The chief characteristics which determine the magnitude of the resid- 

 ual voltages and currents are (1) the power circuit voltage, (2) the 

 impedances of the neutral ground connections, (3) the line and appara- 

 tus impedances, (4) the fault and earth impedances, (5) the sources of 

 power supply, (6) the character of ground wires if used, and (7) the 

 circuit configuration including ground wires. 



When a fault occurs between a phase conductor and earth on a power 

 system having neutral ground connections, these neutral connections, 

 together with the fault, line conductors and earth, form a closed circuit 

 for the residual current. Unless the neutral impedance is very high, 

 e.g., approaching that of an isolated system, the shunting effect of the 

 capacitance to ground of the line conductors may for most purposes be 

 neglected and practically the same value of residual current exists at 

 all points along the line between the fault and the neutral connection 

 to ground. For simplicity, a system with a single line and single 

 neutral ground connection may be assumed. With this picture in 

 mind, it is clear that the value of the neutral impedance may be an 

 important factor in determining the magnitude of the residual current. 

 If the fault occurs near the point where the neutral is grounded, the 

 line and apparatus impedances being low, a small impedance in the 

 neutral may control the current. On the other hand, for faults occurr- 

 ing at points remote from where the neutral is grounded, the impedance 

 in the neutral connection may have to be relatively large to materially 

 reduce the residual current. 



As one limit there is the solidly grounded neutral, i.e., no impedance 

 is inserted and as good a ground as practicable obtained. This 

 obviously permits ma.ximum residual current when ground faults 

 occur. Unless the grounding impedance is very high the residual cur- 

 rent, and not the residual voltage, is the controlling factor in grounded 

 neutral systems. 



As the other limit there is the isolated neutral, i.e., the impedance 

 from neutral to earth is infinite. In this case no residual current passes 

 through the neutral. At the ends of the line the residual current is zero. 



