CABLE TESTING f>77 



placed in series with x, the other in series with r, as indicated 

 in Fig. 419. 



The resistances of the two ammeters and the connections are 

 assumed to be negligible. The polarity of the battery should be 

 such that the fault resistance is a minimum. If the readings 

 of the ammeters are I x and I r , 



I r x I r x 



- or 



I, ~ r "' I, + I, ~ x + r 



(5) 



The total resistance of the loop, (x + r), may be determined by 

 the volt-ammeter method. 



Formula 5 gives the resistance in ohms from the sending 

 end to the fault. If the conductors are both of the same mate- 

 rial and size and at the same temperature, the resistance per 

 unit length will be the same for both and 



distance to fault = total length of loop X j / j-j (6) 



The resistance, y, from the far end of the line to the fault may 

 also be obtained, for 



I* - r 



I r X 



or 



r x 



I x -I r r-x 2 



I x + I r r + x r + x 



2 



For a loop of uniform resistance per unit length, 

 distance from far end of line to fault = 



length of one wire X j^rry ' 00 



The two-ammeter method, using alternating currents, has 

 been employed by Nicholson to locate broken or otherwise de- 

 fective insulators on long high-voltage transmission lines. 1 Few 

 of the methods of fault location are applicable to this case, for 

 high voltages must be employed in order that the defective 

 insulator may arc over to the metal supporting pin and thus 

 establish the fault. Practically full-line voltage may be required. 



