A major electric distribution company in a large metropolitan area had been maintaining and monitoring an effective cathodic protection system on its high-pressure, fluid-filled pipe-type cables, bringing high-voltage AC into downtown substations. The city has direct current (DC) transit systems for both internal public transportation and commuter rail. Although stray currents had been a persistent problem in this congested scenario, much progress had been made in identifying and mitigating the effects. A major supply component consisting of two 10-inch pipe-type cables with 5-inch oil circulation lines was being protected with regularly spaced rectifiers. Unexpectedly, routine cathodic protection monitoring uncovered a severe increase in detrimental stray current interference, with structure-to-electrolyte (S/E) potentials shifting several volts positive with swings of 1,800 millivolts observed over short periods of time.
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