Stray Current from Electric Railways

The first electrified transit system was developed in 1835 by Thomas Davenport, a blacksmith from Brandon, Vermont. The system was simply an electric railcar that ran on a circular steel track. The electric-powered vehicle contained a small electric motor powered by a battery. However, the battery-powered system was impractical for providing commercial railway service, since the battery needed to be recharged after short periods of time. It was not until the development of the dynamo in the late 1800s, which provided a continuous source of DC power for the railcar, that electrified rail transit systems became commercially viable. (reference)

The first commercially successful electric railway line in the United States began operating in 1888 in Richmond, Virginia. Within ten years, thousands of miles of electric railway were operating throughout the United States. Almost immediately, corrosion problems became noticeable by the telephone, water, and gas companies on their underground piping or cable that ran in close proximity to the railway. In addition, the transit agencies began to see corrosion damage on their rails and rail spikes. The corrosion damage, however, was first thought to be caused by the chemical makeup of the soil. It was soon concluded that soil chemistry could not have caused the severe corrosion problems encountered, and after some investigation it was discovered that current leakage off the railcar running rails was the primary cause of the corrosion problem.

Many early studies addressed the practical aspects of the problems and engineering solutions were implemented that mitigated the problem as best as the technology allowed. Most solutions had disastrous effects on the neighboring utilities. A common solution was frequent bonding of the rail return current path to a parallel water main or pipeline, the thinking being that the bond gave a metallic path for the current to follow rather than discharging off of the rail or other part of the transit structure. This exacerbated the problem since the utility line was now a part of the return circuit. Although bonding probably reduced stray-current corrosion on the transit system itself, corrosion would increase on many sections of the bonded utility line because the current was forced onto the utility structure, and when it left that structure to return to the transit system, corrosion could occur.

It was not until 1910, when the United States National Bureau of Standards (NBS) began their 11-year study of stray-current corrosion, that the problem was addressed systematically. In 1921, the National Bureau of Standards recommended the following measures to reduce the occurrence of stray-current leakage on the transit-system side:

Provide for adequate track-to-track bonding
Minimize the distance between the traction power substations, consistent with system economy
Insulate the negative feeders (rails)
Utilize a three-wire traction power system

The first three measures were implemented on many of the transit systems, resulting in decreased amounts of stray-current leakage. The fourth measure, a three-wire system design where the two running rails are neutral and a third and fourth rail are the positive feed and negative return, respectively, was not implemented by the transit companies. This was probably because of the added expense needed to construct a fourth rail to carry the return current back to the traction power station.

It was soon recognized that further mitigative measures were still needed to control stray-current leakage and the subsequent corrosion problems that were still occurring, especially on underground utility structures. Several recommendations were made in the National Bureau of Standards report that were applicable to the underground structures. They were the following:

Be selective in locating new construction near tracks
Avoid contacting cable with pipes and other structures
Use conduits in cable construction
Use insulating joints in pipes and cable sheaths
Shield structures with an insulating coating
Interconnect affected structures and railway return circuits

These measures, used in conjunction with the recommendations for the railway transit system, represented the best approach to reducing stray-current and corrosion in 1921. The general principles behind these measures remain valid today and form the basis for modern stray-current control design. Special note is given to the sixth measure, however. The installation of interconnections, or drainage bonds, between the underground structures and the return circuit, was recognized as acceptable only as a supplemental or temporary measure, since drainage bonds increase the overall amount and magnitude of stray current because of the lower resistance in the parallel resistance paths of the utility and the return rail. Drainage bonds should not be considered as a substitute to the design of return circuits with high, rail-to-earth resistances.

After the 1920s, construction of electrified-transit systems decreased dramatically, and the problem of stray-current corrosion was relegated to a low level. It was not until the 1950s and 1960s, when construction of new, electrified mass transportation systems increased that stray-current corrosion and its control once again became an important issue.

On the object of stray current corrosion see also: DC traction, Cathodic protection, Coating, Contour plots, Definition, Detection, Examples, External currents, Historical perspective, Impressed current, Interference, Mechanisms, Modeling. Pipeline, Potential distribution, Prevention, Stray fields and leakage, Transit systems