In most cities underground telephone systems consist of bare lead cables that start from a central office and are routed in various directions therefrom through buried vitrified clay or creosoted wood conduits. Manholes are constructed at intervals along the conduit routes, primarily as cable splicing points. Corrosion surveys made periodically along these routes, include the measurements of potential between cable and earth at selected manholes. Where such potentials are governed by the effects of stray currents from ground return DC railways, these measurements usually provide a fair indication of the corrosion condition of the cables, not only in the manholes but also in duct sections adjacent to manholes.
Experience has shown, however, that sometimes exceptions exist when localized corrosion, not revealed by the tests at the manholes, occurs in the duct sections. While such conditions usually can be detected by a duct survey, described below, the expense of conducting such...
