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Stray Current Corrosion Mitigation on Light Rail Transit Systems

By Glenn H. Willson, P.E.

Rapid growth has prompted some cities to invest in light rail transit (LRT) systems to relieve traffic congestion and revitalize the city. Light rail systems being installed are powered by DC rectifiers. If left uncontrolled, stray currents originating from the LRT rails and substations can cause corrosion of buried structures and piping. Because V&A is experienced in the testing and mitigation of stray current for LRT systems, we can share some insights into this matter.

Substations with rectifiers supply power to the light rail train through an overhead cable. The overhead cable is connected to the positive side of the rectifier. The rails on which the train travels are used as the negative return conductors connected to the negative side of the rectifier.  Figure 1 shows a general schematic of a DC Powered system.

Figure 1. Schematic of Stray Current Emanating from the LRT System.

Light rail systems are sometimes located in city streets and in close proximity to many underground utilities. Utilities such as water, gas, or sewer pipelines may be affected by stray current corrosion which can be caused by low rail-to-earth resistance, unbonded connections across mechanical joints or a high resistance path of the return current along the rails. Other factors that effect stray current are the spacing of passenger stations, high-voltage substations that are far from passenger stations, and new tracks.[1] The end result is corrosion at locations where current is discharged from reinforcing steel on bridges, tunnels, and passenger stations or in buried utilities that cross or run parallel to the LRT tracks. Once the LRT system is fully operational, a repair at one of the utilitiy-dense intersections may be difficult because the light rail system has to be shut down prior to excavation. Therefore, many transit operators have taken several steps to ensure that stray current has been addressed in the design and construction of the LRT systems.

Combating Stray Current

There have been several improvements in the materials being used over the years to mitigate stray current in LRT systems.  Some equipment that may help decrease stray current includes higher resistance rail insulating fasteners and boots similar to that which is shown in Photo 1 and Photo 2, respectively.

Photo 1. Insulating fasteners	Photo 2. Embedded trackwork

Other improvements include the use of computer simulations to yield data through which stray current can be reduced and optimal locations for electrical substations and passenger stations can be found.[2]

The following equipment can be used to help monitor the amount of stray current from LRT systems:

1. Install permanent reference electrodes under the tracks where utility pipelines cross underneath LRT system tracks.
2. Install insulating flanges  with four-wire test stations on either end of a metallic utility pipeline crossing the LRT system alignment.
3. Install permanent ammeter hoop around piping to detect current flow.
4. Install permanent reference electrodes along tunnels, in passenger and power station areas, by major piles, and adjacent to bridge structures.
5. Install continuous wire fabric mesh underneath embedded or direct fixation trackwork.

V&A recommends that the following  tests be conducted on an LRT system to determine the magnitude of stray current present:

1. Rail-to-Earth Resistance Tests - Measures the resistance between the rail sections and earth. A high resistance reduces stray current.
2. Stray Current Testing – Measures the amount of stray current that will be discharged from the rails into the earth.
3. Electrical Substation Voltage Tests – Measures the resistance between substation negative connection and the substation bus. Requires access to the interior of each traction power substation.

V&A recommends that the following tests be conducted on buried utilities and structures to determine the magnitude of stray current present:

1. Structure Potential Tests – Measures the voltage between a structure and a copper copper sulfate reference electrode. Data is collected on bonded reinforcing steel structures, buried metallic utilities and other utility structures in the trackwork area. The test requires that data be collected before and after the train is operational in order to compare the results.

V&A’s Role in Light Rail Transit Systems

V&A’s Corrosion Division provides stray current testing services and has extensive experience in designing stray current corrosion control and monitoring systems.  Readers should contact V&A at the number below if any further information or guidance is needed with respect to these services.

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V&A Consulting Engineers
1999 Harrison Street, Suite 975, Oakland, CA 94612
Tel. (510) 903-6600, Fax (510) 903-6601
www.vaengr.com
Copyright 2005