Built in several stages over 20 years, the electric rail system in Portland, Oregon is varied in construction design and materials and presents a range of stray current conditions. The rail system is proximate to water supply mains, distribution mains, and services also constructed with varied materials and methods. This paper presents a parametric approach that characterizes stray current mechanisms, the magnitude of stray current effects, and the sensitivity of differing design and construction methods in mitigating stray current.
Streetcar trolleys and electric light rails are once again popular, and their resurgence has a large impact on the city street- above and below ground. City streets are both traffic rights-of-way and also buried utility corridors. On downtown streets, electric rail transit shares the road with water, sewer, natural gas and fuel lines, electric power, communication, and traffic utilities.
In the past, electric transit was constructed with poorly bonded rail, non-insulated embedded track, and directly grounded substations. As a consequence, large magnitude stray currents occurred along the length of the transit system. Today, stray current is mitigated by construction with continuously welded and cross-bonded rail, and high rail to earth resistance track. Substations are closely spaced and are diode grounded or are ungrounded.
The Portland Water Bureau has taken a parametric approach to stray current evaluation that incorporates sensitivity analysis and graphic methods that illustrate engineering fundamentals. Consultant recommendations have ranged from assumptions that would leave a myriad of thin-walled copper services within inches of the track, to poorly defined concerns that all existing pipes should be relocated out of the right-of-way. Portland's approach provides a basis to support a position between these extremes.
