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BELL SYSTEM TECHNICAL JOURNAL 



Double Varley Method ^ 

 The distributed normal insulation resistances of cable wires can be 

 considered, in so far as fault locating measurements are concerned, as 

 though they were single resistances concentrated at some point along 

 the wires (Rule of Resultant Faults). Consider two wires having 

 equal and correspondingly distributed normal insulation resistances, 

 N, which appear to be concentrated at some point b ohms from End 2 

 of the wires, and assume faults of effective resistances, M and F, on 

 the wires at a point x ohms from End 2. Let r be the conductor 

 resistance of either wire, and Vi and F2 the balancing resistances for 

 Varley measurements from Ends 1 and 2 of the wires, respectively, 

 using bridges with equal ratio arms as indicated in Fig. 11. 



Fig. 11 — -Schematic circuit — -double Varley method. 



Applying the Rule of Resultant Faults, let Ci be the apparent loca- 

 tion, in ohms from End 2, of the resultant of M and TV, and let C2 

 be the corresponding location of the resultant of F and A''. Then: 



M 



N 



and correspondingly: 



Cl 



c<> 



F+ N 



The equivalent resistance of the resultant of .1/ and N is MN/M + N. 

 and of the resultant of F and N is FN/ F -\- N. Let cs be the apparent 



'The Double Varley method has been described in "Cable Testing," a paper 

 read by E. S. Ritter before the Nottingham Centre of the Institute of Post Office 

 Electrical Engineers (British), May 25, 1922. In that paper it is stated that the 

 method is due to Mr. H. T. Werren. 



