BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 399 



As indicated by the above discussion, the hniitations of the Double 

 Varley method are as follows: 



1. There must be only one actual fault on any one cable wire. 



2. The fault resistances must remain constant throughout a set of 



measurements to determine Fi and V^- 



3. If b(-th of the wires used for the Varley measurements are faulty, 



tlie faults must be at the same point on each wire, the re- 

 sistances of the faults must be unequal, and the resistance 

 of the fault on at least one of the wires must be high compared 

 to the conductor resistance of the wire. 



4. If the fault resistances are high enough to be comparable in 



magnitude to the normal insulation resistances of the faulty 

 wires, the normal insulation resistances must be equal, and 

 correspondingly distributed along the wires. 



5. The conductor resistances of the wires must be equal. 



It will be understood that since the Double Varley method is ap- 

 plicable only when the resistance of the fault, M, is high compared to 

 the conductor resistances of the wires, the Corrected Varley method 

 or the Straight Resistance method should be used in cases where M 

 is comparable in magnitude to the conductor resistances. 



Series Resistance Unbalances 



The methods for locating series resistance unbalances discussed in 

 this paper involve essentially the balancing of the faulty wire against 

 a "good" wire of equal capacitance by adding resistance to the "good" 

 wire at the testing end until the effective impedances of the two wires 

 are equal. A simple relationship then exists between the balancing 

 resistance required, the resistance of the fault, the length of the faulty 

 wire between the distant end of the cable and the fault, and the total 

 length of the faulty wire. The circuit arrangement used depends on 

 whether the cable under test is long or short. 



The circuit arrangement for applying the test to short cables is 

 shown in Fig. 13. 



T 



Audible 

 frequency 



Shielded-ir 

 Transformer 



Fig. 13 — Schematic circuit — short cable method for locating a series resistance 



unbalance. 



