BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 393 



sistance, that the fault resistances are comparable in magnitude to 

 the conductor resistances, and that the fault resistances remain 

 constant while a set of measurements is being made. 



Referring to Fig. 9, assume that, from among the faulty wires, two 

 wires are selected having a fault of low effective resistance, {M + F), 

 between wires. Let r be the conductor resistance of either wire be- 

 tween cable Ends 1 and 2 ; and let {r - x) and x be the conductor 

 resistances of either wire from Ends 1 and 2, respectively, to the fault. 



With the wires open at End 2, the resistance between wires is 

 measured at End 1 by means of a bridge having equal ratio arms and 

 arranged for an ordinary loop resistance measurement. Calling the 

 rheostat resistance at balance, Loi: 



Loi = 2{r-x) + 0/+F). 



Similarly, with the wires open at End 1, the resistance between wires 

 is measured at End 2. Calling the rheostat resistance at balance, L02: 



L02 = 2x + (.1/ + F). 

 Combining the equations for Loi and L02: 



Z/02 — Loi ~ 4.A; — If. 



and therefore: 



^ = 2a- + (Los - Loi) ^ ^^^ 



4 



N 2/- - (L02 - Loi) (z^ 



{r - x) = ^ • l^J 



Application: The Straight Resistance method involves only simple 

 resistance measurements, Loi and L02, from the two ends of the cable. 

 The loop conductor resistance of the faulty wires is obtained from 

 cable records. The values thus secured are substituted in Formula 

 (4) or (5), and the location, x or {r - x), is converted into feet in the 

 usual manner. 



Since the conductor resistances of the faulty wires must be equal, 

 measurements should be made on the two wires comprising a pair 

 when practicable. The effective fault resistance between wires should 

 be low; otherwise slight errors in measurement might cause large 

 errors in calculated location. However, in cases where the fault re- 

 sistances are too high to be measured using bridges with equal ratio 

 arms, unequal arms, A and B, may be used and the quantity 



^ (L02 - Loi) substituted for (L02 - Loi) in the formulas. 



X5 



