BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 389 



Two methods by which such faults can be located are discussed below: 

 A "Corrected Varley" method which may be used provided two wires 

 having fault resistances to ground differing by at least 25 per cent are 

 available for measurements; and a "Straight Resistance" method 

 which does not require that the two wires have faults of unequal 

 resistances. 



Corrected Varley Method 



Consider a cable in which all wires have low insulation resistance to 

 ground because of a wet spot, and assume that from among the faulty 

 wires two wires are selected for a Varley measurement. Assuming a 

 bridge having equal ratio arms, A, the Varley network can be repre- 

 sented as shown in Fig. 7, where M and F are the effective resistances 

 of the faults on the two wires, r is the conductor resistance of either 

 wire, and x is the resistance of that portion of either wire which is 

 between the distant end of the cable and the faults.^ 



I 



Fig. 7 — Schematic circuit — corrected Vailey method. 



The Varley circuit of Fig. 7 is equivalent to the Varley circuit of 

 Fig. 8, where the "tt" type network formed by the three resistances, 

 M, F and 2x, has been replaced by a "T" type network having resist- 

 ance values as indicated. When the bridge is balanced by adjustment 

 of the rheostat to a resistance, V, at which there is no galvanometer 



2 The actual faults form a "tt" type network consisting of a ^resistance between 

 wires and a resistance between each wire and ground. The "tt" type network has 

 been replaced by a "T" type network having resistances, M and F, between the two 

 wires and the branch point of the network, and a third resistance connectmg the 

 branch point to ground. This third resistance is in series with the bridge battery 

 and its only effect is to limit the sensitivity of the bridge. To simplify discussion 

 the resistances, M and F, are shown connected directly to ground, and the third 

 resistance is considered to form a part of the resistance shown connected between the 

 battery and the junction point of the ratio arms of the bridge. 



