388 BELL SYSTEM TECHNICAL JOURNAL 



With the battery switch in "Varley" position, a Varley measure- 

 ment is made by balancing the bridge to a rheostat value, V, at which 

 there is no galvanometer current. Then: 



A r -\r X 



A r - X + V 



x = |- (1) 



It will be noted that the fault resistance, F, is in series with the 

 battery and has no effect on the measurement except to limit the 

 sensitivity of the bridge. 



With the battery switch in "loop" position, a loop resistance 

 measurement is made by balancing the bridge to a rheostat value, L. 

 Then: 



L 

 2 



From these Varley and loop measurements the percentage location 

 of the fault, on a resistance basis, can be calculated as follows: 



V 

 From the distant end: — (100 per cent). 



L — V 

 From the measuring end: — j (100 per cent). 



Corrections for resistances of bridge leads, loading coils, etc., are then 

 made, the corrected percentage location is converted into feet, and the 

 location of the fault is determined by reference to cable records. 



These Varley circuits and formulas are well adapted to the toll 

 cable plant where wires are usually well balanced in conductor re- 

 sistance, and the resistance of the leads between the bridge and the 

 cable is small compared to the conductor resistance of the cable wires. 

 In exchange cable work, modified forms of the Varley loop, which do 

 not require that the "good" and faulty wires be of equal conductor 

 resistance and which correct automatically for the resistance of bridge 

 leads, are frequently used. 



Total Cable Failures 



In the case of total cable failure, due, for instance, to a wet spot, 

 there are no wires in the cable which are unaffected by the fault, and 

 the fault resistances of a large number of the wires are low, i.e., of the 

 same order of magnitude as the conductor resistances of the wires. 



