Bridge Methods for Locating Resistance 

 Faults on Cable Wires 



By T. C. HENNEBERGER and P. G. EDWARDS 



In this paper are discussed bridge methods for locating resistance faults 

 on cable wires, with special reference to the theory of methods for (1) locat- 

 ing insulation faults which cause complete cable failure, (2) locating insula- 

 tion faults of high resistance, and (3) locating series resistance unbalances. 



The methods described are better adapted to the toll than to the exchange 

 telephone cable plant, since they require that the conductor resistances of 

 the wires used for measurements be equal and, in general, that measurements 

 be made from each end of the faulty cable. 



IN the toll telephone plant, insulation faults such as "grounds" and 

 "crosses" are usually located by the "Varley loop" method, which 

 involves essentially the measurement of the d.-c. resistance of the 

 faulty wire between the point of fault and one end of the cable, and 

 the comparison of this resistance with the total conductor resistance 

 of the wire to obtain the "percentage location" of the fault on a re- 

 sistance basis. Corrections are then applied to account for such 

 factors as the resistance of the leads between the cable and the bridge, 

 the resistances of loading coils, and non-uniformity of conductor 

 resistance caused by temperature differences between underground and 

 aerial sections of the cable. After all corrections are applied the 

 corrected percentage location is converted into distance from one 

 cable end to the fault. 



In general, the most troublesome insulation fault to locate is a 

 "wet spot" due to absorption of moisture by the insulation through a 

 defect in the lead covering of the cable, which results in low insulation 

 resistance between wires and to ground. Standard apparatus now 

 available for locating grounds and crosses is sufficiently sensitive to 

 permit accurate locations of wet spots up to about five megohms in 

 resistance. The Varley loop methods ordinarily employed in con- 

 nection with the apparatus will give accurate results provided a wire 

 of very much higher insulation resistance than the faulty wire is used 

 as the "good" wire for measurements. These are the conditions 

 which usually are found when wet spots occur. Cases occur occa- 

 sionally, however, in which a "good" wire having sufficiently high 

 insulation resistance as compared to the faulty wire cannot be obtained, 

 either because all of the wires available for measurements are affected 



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