BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 383 



by the fault or because the fault resistance is high. The methods for 

 locating insulation faults discussed in this paper are especially applic- 

 able to such cases. 



Resistance unbalances on cable wires are of relatively infrequent 

 occurrence and are usually difficult to locate. A method frequently 

 employed for locating such faults is to measure the impedance un- 

 balance at various frequencies of a circuit containing the faulty wire 

 and to analyze periodic impedance-frequency curves plotted from the 

 measurements. 1 The methods for locating series resistance unbalances 

 discussed in this paper involve the use of ordinary Wheatstone 

 bridges, are simple to apply, and give results which are believed to be 

 comparable to those obtained by the impedance-frequency method. 



NoR:iiAL Insulation Resistance of Cable Wires 



The values of insulation resistance obtained by measurements on 

 cable wires which are not faulty are dependent on the circumstances 

 in which the measurements are made. In the case of paper-insulated 

 telephone cable the most important factors affecting insulation re- 

 sistance are electrification period and temperature. 



The following discussion of normal insulation resistance refers 

 particularly to measurements between wires of pairs in a typical 

 repeater section of aerial toll cable approximately 50 miles long, the 

 wires being at ground potential at the time of application of the 

 testing potential. Insulation resistance to ground is also of interest, 

 but is difficult to measure accurately in long lengths of cable because 

 of interfering potentials. As a rough approximation, normal insulation 

 resistance between a wire and ground can be considered to be about 

 two thirds as great as normal insulation resistance between wires. 



A curve illustrating the variation of insulation resistance between 

 wires of a typical cable pair over a 30-minute electrification period is 

 shown in Fig. 1. In general, the electrification periods necessary for 

 obtaining reasonably constant values of insulation resistance differ 

 appreciably for different pairs, and for the same pair at different times. 

 The usual period ranges from 15 minutes to an hour for a pair 50 miles 

 long. Routine measurements are generally made, however, using 

 electrification periods of one minute. 



The paper used for insulating the wires of telephone cable has an 

 appreciable negative temperature coefficient of insulation resistance. 

 This is indicated by the curve of Fig. 2 which shows variations of 

 average insulation resistance with temperature. The points for the 



1 "Telephone Circuit Unbalances," by L. P. Ferris and R. G. McCurdy, A. I. E. E. 

 Transactions, 1924, Volume XLIII, page 133L 



