BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 403 

 are eciual : 



and therefore: 



Re- F 



D = t/^ . (11) 



Applicntion: It will be clear from the above theory that Formula 

 (11) will give accurate results only if the following requirements are 

 met: 



1. The resistance, F, must be the only unbalance on the wires. 



2. The resistance of the unbalance must remain constant throughout 



a set of measurements to determine i?o and Re. 



3. The conductor resistances of wires 1 and 2 must be equal. 



4. The capacitive reactances of wires 1 and 2 to 3-4 must be large 



as compared to the conductor resistances of the wires and the 

 fault resistance. 



5. Capacitance unbalances of wires 1 and 2 to 3-4 must be negligible. 



In general, the short cable method is suitable for locating, with a 

 fair degree of accuracy, series resistance unbalances ranging from a 

 few ohms to several hundred ohms on non-loaded cable not exceeding 

 three or four miles in length. In cases of unbalances of only a few 

 ohms resistance, however, it is essential that the wires of the faulty 

 quad be very well balanced in conductor resistance; and the bridge 

 rheostat should be variable in steps of 0.1 ohm. Usually, best results 

 are secured when measurements are made from the cable end nearer 

 the fault. 



The bridge voltage used should be as small as practicable in order 

 to minimize changes in fault resistance. A sufficient number of separ- 

 ate determinations of the location should be made to insure that con- 

 sistent results are being secured. 



The measurement with the distant ends of wires 1, 2, 3 and 4 con- 

 nected together is made merely to obtain the actual value of fault 

 resistance. The value of fault resistance can be obtained instead by 

 a d.-c. Varley measurement, if desired. If this is done, however, 

 arrangements should be made so that the bridge connections can be 

 changed rapidly, as it is desirable to make measurements of R^ and 

 Re in quick succession to avoid errors due to changing fault resistance. 



The short cable method is applicable to paired cable as well as to 

 quadded cable. In the case of paired cable, ground may be substi- 

 tuted for wires 3-4, and measurements made of impedance to ground 

 rather than of impedance between wires. Usually in these circum- 

 stances, however, the bridge cannot be balanced very sharply. 



