CABLE TESTING 679 



stone bridge, the other two arms being made up of resistances 

 under control of the observer. Fig. 421 shows the scheme of 

 connections. 



The relative positions of the galvanometer and battery are 

 important; with the connections as shown, earth currents have 

 no effect on the readings. 



r E 

 FIG. 421. Connections for Murray loop test for fault location. 



When the galvanometer stands at zero, 



M _ r_ M + N r + x 



N ~ x N x 



N 



The total resistance of the loop (r + x) is obtained by a bridge 

 measurement. 



If uniform wires are being dealt with, 



/ N \ 

 distance to fault = total length of the loop X \jjl~+Jj) ' 



A potential divider or a slide-wire arrangement with extension 

 coils may be convenient for M + N, in which case M + N is 



constant. 



A cross between two conductors in a multiple-conductc 

 is located in a similar manner, the only difference being that the 

 battery, instead of being connected to ground, is attached 1 

 of two faulty conductors, the other being looped with an unfaulted 



wire. 



Varley Loop Test.-In this method a fixed bridge ratio, is used 

 and the balance obtained by adding resistance to the smalle 

 section of the loop as indicated in Fig. 422. 



