674 ELECTRICAL MEASUREMENTS 



The total line resistance is supposed to be known from previous 

 measurements made while the line was perfect; denote it by L. 

 Two measurements of the resistance to ground are made by the 

 observer at the sending end, one, RI, with the far end insulated 

 and a second, # 2 , with the far end grounded. Then, referring 

 to Fig. 416, 



L = x + y 

 fli = x + g 



gy 



-L Ohm- 



Bendlni j I Far 



* nd j X Obnn >| - y Ohou *\ End 



> - BeiliUnce 



to 

 < Ground 



Q round 



Fio. 416. Blavier and earth overlap tests for fault location. 

 Eliminating g and j/, 



x = #, - V(L - R*) (R, - R t ) (1) 



This gives the resistance from the sending station to the fault. 

 The corresponding distance is calculated by aid of the known 

 resistance per unit length of the cable. 



The resistance measurements may be made in any convenient 

 manner, as by the volt and ammeter method. A practical 

 difficulty is that the resistance to ground, g, is variable, being 

 influenced by the amount of moisture present and the action 

 of the current at the fault. Also, the resistance, g, may be so high 

 that it exerts very little shunting action when y is placed in 

 parallel with it by grounding the far end of the line. 



The Earth Overlap Test. In applying this test it is neces- 

 sary to make resistance measurements from both ends of the 

 line. With the far end grounded, the resistance, R\, to ground 

 is measured from the near end. The line is then grounded at 

 the near end and the resistance to ground, Rz, is measured from 

 the far end. Then 

 L = x -f y. 



gx 

 g + x 



