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Normal fault-locating procedures become impractical in certain conditions, either for purely technical reasons or on account of the loss of service time involved. This paper commences by summarizing a theoretical investigation of the possibilities of applying pulse and frequency-modulation methods to this problem on wide-band coaxial telephone cables. A practical fault-locator employing d.c. pulses is then described. The problem is first contrasted with that of radar, and the factors controlling the choice of the transmitted waveform and those limiting the accuracy of location attainable are then discussed. The fundamental requirements of a frequency-modulation system are examined in some detail, and it is concluded that, although an f.m. instrument would be attractive in certain circumstances, the practical advantage lies with the pulse type of fault-locator owing to the clarity and reliability of its indications when more than one fault is present. The d.c.-pulse instrument described has been in use for some time, and faults on coaxial cables have been located within 1% of their true distances at ranges up to 10 miles.