210 BELL SYSTEM TECHNICAL JOURNAL 



52;raclually increasing to a maximum at the point of fault. The circuit 

 for residuals is through the capacitance of the line to ground, the mag- 

 nitude of this capacitance controlling the magnitude of the residual 

 current, which is much less than with grounded neutral systems except 

 in cases of double faults when it may be very large. With a single 

 fault the residual voltage may be a more important factor in respect 

 to induction than residual current. 



The impedance of the fault itself depends upon a number of things, 

 including the type of line construction and the earth conditions. The 

 subcommittee has under way investigations to gather data on the 

 range of fault impedances under different conditions. To determine 

 the maximum residual current, the fault impedance may be taken as 

 zero. In many instances, this approximation gives sufficiently close 

 results, particularly if the fault is remote from the grounded neutral 

 so that line, neutral and apparatus impedances are controlling. In 

 case of conductors falling upon the ground, local earth conditions 

 largely determine the fault impedance. On a steel tower line an insula- 

 tor breakdown results in a relatively short arcing path to grounded 

 metal, whereas, in wood pole construction, the pole itself introduces 

 considerable impedance unless nullified by guys or other metal. 



The foregoing discussion of residual current has been confined prac- 

 tically to the situation brought about by single faults to ground. 

 Double faults at separate locations sometimes occur and these are 

 equivalent to a phase-to-phase short circuit through the earth, giving a 

 large residual current in the intervening section of line. If the two 

 faults in such a case are on opposite sides of an exposure, very severe 

 induction may result. Experience shows that double faults at separate 

 locations constitute only a few per cent of the total faults occurring on 

 grounded-neutral power systems but are a much larger percentage of 

 the total faults on systems normally isolated from ground. 



The presence of ground wires on a line may have considerable in- 

 fluence on fault impedance. Being connected to ground at frequent 

 intervals, such wires decrease the impedance to ground where a break- 

 down occurs between a phase conductor and a ground wire or any 

 metal in contact with a ground wire. A ground wire tends to increase 

 the total residual current but on the other hand its controlling function, 

 from the induction standpoint, is that of a shielding conductor tending 

 to decrease the induced voltage. 



Circuit configuration does not have a large influence on unbalances 

 due to abnormal conditions, but it has an important eftect upon any 

 unbalance of a power circuit under normal operating conditions. To 

 be balanced, the phases of the power circuit must be symmetrical with 



