BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 391 



this opposite end, using a bridge having equal ratio arms, 7i'itli the 

 distant ends of the wires open, and the resistance in the bridge rheostat 

 at balance is designated Lq: 



il/ + F = Lo - 2.r. 



If a Varley measurement is made from the same end, using a bridge 

 having equal ratio arms, it'ith the distant ends of the ivires open, and the 

 resistance in the bridge rheostat at balance is designated Fq: 



M - F = Fo. 



Substituting these values of {M + F) and {M - F) in (2) : 



x=^^. (3) 



"" 2 Fo ^ ^ 



Application: To apply the Corrected X'arley method, an ordinary 

 Varley measurement is made from one end of the cable, and additional 

 loop resistance and Varley measurements, as described above, are 

 made from the opposite end. The values of balancing resistance thus 

 obtained are substituted in Formula (3). The location of the trouble 

 on a resistance basis, x/r, can then be calculated, and the location can 

 be converted into feet in the usual manner. 



Usually it will be necessary to determine the loop conductor re- 

 sistance, 2r, of the faulty wires from cable records rather than by 

 measurement at the time the location is being made. A measurement 

 of loop conductor resistance would be in error because of the low 

 resistance shunt {M -f F) on the portion of the loop between the faults 

 and the short-circuited ends of the wires. The accuracy of location 

 is dependent, therefore, on the accuracy to which conductor resistance 

 can be estimated. 



In cases where it is desirable to use the Corrected Varley method 

 the fault resistances will be low, so that usually the balancing resist- 

 ance, Lo, will not exceed 10,000 ohms. If Lo is too high to measure 

 using a bridge with equal ratio arms, unequal ratio arms, A and B, 



A 

 may be used and the quantity -^ Lo substituted for Lq in Formula 



(3). Measurement of Fo, however, should be made using a bridge 

 with equal ratio arms. 



The Corrected Varley method will give accurate results only under 

 the following conditions: 



(1) Both faults must be at the same point along the cable. 



(2) The fault resistances must remain constant throughout the test. 



