Two cases are reported in which stray current potentials were mistaken for galvanic potentials from underground lead-sheathed telephone cables to ground. Lack of suitable meters prevented assessment of the true potentials of the cables which suffered severe corrosion damage.
Case 1 pertains to a relatively high stray current area adjacent to both the New York City subway system and the electrified New York Central Railroad. First tests showed steady positive potentials similar to those which might result from a galvanic source. When the fixed potential was blocked out by an opposite fixed potential, a marked fluctuation of potentials to earth was observed. Bonding reduced the cable-to-earth potential to a range of —0.1 to —0.4 volt.
Case 2 pertains to a section of cable where an average of 8.9 failures per 100 sheath miles per year occurred over an extended period of time. The damage began to occur shortly after discontinuance of a trolley system. A rectifier-powered cathodic protection system, installed in 1940, improved conditions on all but about 4,000 feet of the cable. Later, tests indicated alternating currents from the 11,000-volt, 25-cycle return of the New York, New Haven and Hartford main line were somehow related to direct currents on the sheaths; surges being correlated to railroad operation. The author postulates that the direct current may have been the result of the rectification of several AC potentials of different magnitudes over a large area. Corrections attempted by bonding a half-wave selenium rectifier to the adjacent railroad structure were successful, but a series of rectifier breakdowns, due to surges, was experienced. The use of a rectifier embodying a protective circuit, eliminated breakdowns, but additional testing and corrections are being made to bring the whole section to a potential which will reduce corrosion. 3.6.9
