BRIDGE MRTIIOns FOR LOCATING RESISTANCE FAULTS 387 



of several thousand megohms, while during summer, especially in 

 cables which have been in service for a number of years, the insulation 

 resistances between wires of some pairs in a 50-mile repeater section 

 may be as low as 25 megohms (1250 megohm-miles). 



Varley Loop Method 



The Varley loop circuits which are used ordinarily for locating 

 grounds and crosses on wires of toll cable are illustrated in Figs. 5 and 

 6. The Wheatstone bridge has equal ratio arms, A. The "good" 



jVarley 



Fig. 5 — Varley loop for grounds. 



and faulty cable wires have equal conductor resistances, r, and are 

 connected together at the distant end of the cable. F is the resistance 

 of the fault, and x is the conductor resistance of the faulty wire between 

 the fault and the distant end of the cable. 



iVarley faulty wire 



Fig. 6 — ^ Varley loop for crosses. 



