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BELL SYSTEM TECHNICAL JOURNAL 



lems arise in designing switching devices to apply the test potential 

 between all conductors. 



Defects found by continuity, cross or dielectric strength tests must 

 be located within the cable in order that repairs may be made. The 

 point of break in open conductors is located by comparing the capaci- 

 tance between the defective conductor and the adjacent conductors 

 with the capacitance between a conductor known to be good and its 

 adjacent conductors. Preliminary locations of crosses between con- 

 ductors and between conductors and the sheath are made by means of 

 the modified Murray Loop test. Final locations are made by means of 

 a search coil and telephone receiver which responds to currents of 

 audible frequency circulated through the crossed conductors. 



Fig. 30 — Typical test set installation at Baltimore plant. 



Fig. 30 shows a closer view of a section of the test mezzanine at the 

 left of the cooling area. The test desk in the foreground is designed 

 for making insulation resistance, D.C. capacitance, A.C. capacitance, 

 and conductor resistance tests. The test desk in the center is a 

 shielded precision bridge for making capacitance and conductance 

 measurements at audio frequencies. Two test desks in the back- 

 ground are capacitance unbalance bridges. All desks on the mezzanine 

 floor are provided with test leads which terminate in outlet boxes on 

 the test floor below. 



Figs. 31 and 32 show a front and rear view respectively of the 



