Sensitivity Characteristics of a Low-Frequency Bridge Network for Locating Opens in Telephone Circuits

The problem of locating opens in telephone cable conductors involves the determination of impedances. A study has been made of the degree of accuracy and sensitivity obtainable in impedance measurements with different frequencies of supply voltage. For long cables the input impedance is a hyperbolic rather than linear function of the characteristic impedance. The error in impedance measurement arising from this functional departure proves to be least for the lower frequencies. On the other hand, the bridge sensitivity is improved by somewhat higher frequencies. A mathematical and experimental analysis of the sensitivity of impedance measurements in cable fault location by means of a de Sauty bridge indicates the desirability of using frequencies of the order of four cycles. The sensitivity is further increased by controlling the phase of the field excitation of the bridge galvanometer. This improved open location method and equipment are sufficiently accurate that in practically all cases a fault in a 60-mile length of cable may be located within a maximum variation of plus or minus one-half the length of a cable section (a section is the length of cable between splices??about 750 ft.), 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.