684 ELECTRICAL MEASUREMENTS 



To locate the section in which the fault exists, compare the 

 resistance x as found by one of the previous methods, with l\K\, 

 then with l^Ki + 1 2 K 2 and so on. Suppose that the comparison 

 shows the fault to be in the third section; then 



x = hK, + hK 2 + I'JCt 



and the distance of the fault beyond the junction of the second 

 and third sections is 



, 



A 8 



Uncertainty as to the values of K introduces difficulties. The 

 average values of K for the different sections depend on the 

 diameters of the wires, their conductivities, the temperatures of 

 the sections, and in underground conductors of large cross- 

 section, where the lengths of the sections are short, on the 

 number of joints. 



In cables for large currents laid in ducts where the temperatures 

 may be high, there may be great uncertainty as to the tem- 

 perature and a consequent difficulty in correcting K\, K^, etc., 

 to obtain their values at the time of test. 



Locating Faults in Underground High-tension Cables. 2 In 

 locating faults in underground high-tension cables, special 

 difficulties are experienced because of the low resistance of the 

 conductor, which may be from 0.10 to 0.04 ohm per 1,000 feet. 

 On the other hand, such cables are readily accessible at the man- 

 holes, which may be about 300 feet apart. This makes it possible, 

 before cutting the cable, to verify the location of the fault as 

 given by a loop test, and for this purpose special apparatus has 

 been devised so that the particular length of cable in which the 

 fault is located may be identified with certainty. This verifica- 

 tion is necessary, for in a 10-mile length of cable an uncertainty 

 of 0.3 per cent in the resistance measurements corresponds to 

 an uncertainty of 158 feet or half a length of the cable between 

 manholes. 



In the long run, time will be saved by adopting an orderly 

 procedure and the following has been found satisfactory in dealing 

 with this class of faults: 2 



(A) Tests to diagnose the trouble and show the tester what 

 he has to deal with. 



