The Location of Opens in Toll Telephone Cables 



By p. G. EDWARDS and H. W. HERRINGTON 



Synopsis: Improved methods have recently been developed for the 

 location of opens in toll cable conductors. The discussion of these methods 

 is prefaced by a review of older practices. 



This improved open location method and equipment are sufficiently 

 accurate that in practically all cases a fault in a 60-niile length of cable may 

 be located within a maximum variation of plus or minus one half the length 

 of a cable section (a vSection is the length of cable between splices — about 

 750 feet), and therefore enables one to select, prior to the opening of the 

 cable, one or the other of the two splices between which the fault lies. 

 This degree of accuracy is very desirable for practical reasons. 



In this development, the line characteristics are considered. The 

 accuracies of calculated locations, assuming no errors in measurements, 

 are compared for different lengths of lines. The impedance bridge circuit 

 is treated to bring out the method of obtaining a balance. The effects of 

 several frequencies of testing potential are analyzed. The probable errors 

 and inaccuracies of measurement which would interfere with the correct 

 location of faults are classified and methods for their correction are de- 

 veloped. The accuracy of the method and the sensitivity of the apparatus 

 are given. 



THE location of "opens," or breaks in the continuity of telephone 

 conductors, has always been an important problem in the testing 

 and maintenance of the toll cables of the telephone plant. Although 

 the number of opens encountered in the cable circuits is relatively 

 small as compared to aerial wire lines, their location is more difficult 

 because of certain electrical characteristics of the cable circuits. This 

 condition, coupled with the fact that any work on a toll cable may 

 interfere with a large number of facilities, renders the quick and 

 accurate location of such faults imperative. 



A high resistance voltmeter was used in the first feasible attempt 

 to locate opens by means of electrical measurements. The original 

 method of locating opens by the use of a voltmeter consisted essentially 

 of a series of continuity tests. The faulty conductor was connected 

 to ground or to the other wire of the pair at succeeding test stations 

 until the fault was isolated between two adjacent test stations. The 

 trouble was then found, either by inspection of the entire line between 

 test stations, or by further continuity tests with a lineman, first near 

 the middle of the section and later at points gradually approaching 

 the location of the fault. 



The inconvenience of such a proceduie, however, led to an improved 

 use of the voltmeter. The method employed afforded a rough com- 

 parison of the capacitance to ground of the portion of the faulty wire 

 adjacent to the measuring station with that of its good mate or 

 another conductor (of like gauge) following the same route. This 

 was done by allowing the wire to become charged through a volt- 



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