394 BRLJ. SYSTEM TECHNICAL JOURNAL 



In connection with botli the Corrected \'arley method and the 

 Straight Resistance method, it is possible to modify the measuring 

 schemes and obtain somewhat more compHcated formulas for the 

 location of the faults. The specific measuring schemes which have 

 been described are those which it is felt are most practicable for fault 

 locating work on toll cable. 



Insulation Faults of High Resistance 



In order to locate faults of high resistance, sensitive galvanometers 

 and highly insulated bridges must be employed, and the fault locating 

 methods must correct for factors peculiar to the locating of such faults. 

 If the resistance of the fault is high enough to be comparable in mag- 

 nitude to the normal insulation resistance of the faulty wire, the effect 

 of normal insulation resistance must be taken into account. In the 

 case of a high resistance wet spot, it may happen that all wires in the 

 cable are affected to some extent by the fault so that no wire of high 

 insulation resistance compared to the selected faulty wire is available 

 for measurements. 



The solutions of the \^arley networks for high resistance faults are 

 more readily obtained by approximate than by exact mathematical 

 reasoning, and will be worked out by the process of combining all of 

 the "effective faults" on the wires into a single resultant fault and then 

 solving the bridge network for this fault. The approximate solution 

 is based on a principle which for the purposes of the present discussion 

 can be stated as follows: 



Any two shunt faults of high resistance along a ware can be replaced 

 by a single resultant shunt resistance located between the two 

 faults at a point the distance of which from either fault is 

 directly proportional to the fault resistances. 



Thus, if M and F are the resistances of two faults at separated points 

 along a wire, and m and / are their respective distances from the re- 

 sultant, then: 



M _ m 



~F~7' 



The application of this "Rule of Resultant Faults" to Varley 

 measurements can be shown as follows : Let M and F be the effective 

 resistances of the faults on two cable wires at the same point along the 

 cable; let r be the conductor resistance of either wire between the 

 cable ends, and x the resistance of that portion of either wire which 

 is between End 2 of the cable and the faults. Let V be the value of 

 balancing resistance for a Varley measurement made from End 1, 

 using a bridge with ecjual ratio arms, as indicated in Fig. 10. 



