Solving Stray Current Mitigation In Portland’s Rail System

By Aaron Eder, P.E. | August 2014, Vol. 69, No. 8
Rendering of the Portland-Milwaukie light rail system. (photo courtesy of TriMet)

Due to safety constraints, construction crews are restricted from working too closely to moving trains or the high-voltage catenary power lines installed overhead. Further, excavation beneath or bordering the track is restricted due to the potential of undermining the track as well as reducing the load on the existing utility. Construction parallel with rail systems can be more challenging than crossing under the track, because these excavations warrant trench safety systems such as shoring to support the trench and protect crews. In streets that are congested with other utilities, such as downtown Portland, shoring can prove to be a challenging task in itself (see Figure 2). An additional challenge with excavations parallel to rail systems is the physical limitations of the heavy machinery used to install and maintain utilities. Construction machinery generally spans the excavation, which may infringe upon the rail system right-of-way and impede train movement.
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Bonding wires installed across each pipe joint creates a continuous electric circuit between corrosion test stations. (photo courtesy of Portland Water Bureau)

The second, and less obvious, impact of a new light rail system is stray current corrosion. Light rail trains are powered by electricity. Electrical current travels in a circuit from the power station to the train via the overhead wires above the tracks through the train’s electric engine and back to the power station via the rail and ground. Because the soil serves as a parallel conductor to the track, a portion of the current will return to the power station through the ground. The current returning through the ground is known as a “stray current,” due to the fact that it follows a path other than that intended. Electric current in the ground will use the most conductive medium to return to the power station. Iron and steel water pipes are often good conductors. However, when the current leaves the pipe, it takes electrons with it and corrodes the pipe.