680 



ELECTRICAL MEASUREMENTS 



With the apparatus arranged as in Fig. 422 and the switch in 

 the position shown, the bridge will balance when 



M r 



~N = JTP7 



Pi being the resistance unplugged at P, 



(x + r)N - P,M 



or 



M + N 



(8) 



FIG. 422. Connections for Varley loop test for fault location. 



To measure (r + x), the total resistance of the loop, the key 

 K is thrown to the dotted position and a second balance obtained, 

 using the apparatus as an ordinary Wheatstone bridge. 



Determination of the Total Resistance of the Defective Con- 

 ductor. If there is only one perfect wire between the stations, 

 the total resistance of the faulted line cannot be determined by 

 measurements made from one end of the line. 



FIG. 423. Pertaining to determination of resistance of faulty conductor. 



The determination may be made by tests from both ends. 

 The line is first looped with Z at B and x determined by one of 

 the previous methods. Then the loop is made at A and the 

 observer at B measures y. The total resistance is obviously 

 (x + y). (See Fig. 423.) 



When there are two perfect wires between the stations, 

 measurements from one end of the line suffice. When dealing 

 with a multiple-conductor cable, two unfaulted wires in the same 

 cable may be used as the auxiliary wires (see Fig. 424). 



