Stray Current from a Nearby DC Transit Line on a Buried Pipeline
A coated steel pipeline runs parallel to a DC light-rail line for 2 km, about 30 m away. Pipe-to-soil potentials logged over 24 hours swing between -0.70 V and -1.35 V (CSE), with the most negative values during morning and evening service peaks. A close-interval survey shows the most positive reading at a point directly opposite a traction substation. The pipeline has a rectifier-based CP system of its own, set to hold -0.95 V at its drain point. The owner asks whether the existing CP system is adequate.
1. Is the fluctuation stray current or a CP system problem? How do you tell?
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The pattern is stray current. The swings correlate with transit service peaks, and the most positive reading sits opposite the traction substation, which is where return current enters the earth and where the pipeline is most likely to pick up current. A CP system fault would not produce a 24-hour cycle synchronized with train service.
The mechanism: the pipeline is a parallel conductor in the earth. Near the substation, current enters the pipe (making potentials more negative); farther along, it leaves the pipe to return to the rails (making potentials more positive and driving corrosion). The -0.70 V reading is the corrosion-risk location; the -1.35 V reading is the pickup location and is not itself a corrosion concern, though it can cause overprotection.
2. What does the existing CP system do to the stray-current picture, and what is the risk of simply raising its output?
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The existing rectifier holds -0.95 V at its drain point, which is fine for its own design but does nothing about the stray-current swing. Raising output to keep the -0.70 V location above -0.85 V would push the pickup location further negative (already -1.35 V), worsening overprotection and potentially increasing current discharge elsewhere. It treats the symptom at one point and worsens another.
The right response is to address the stray current at its source or with drainage: coordinate with the transit operator on return-current control, install a drainage bond or forced drainage system at the discharge location, and monitor potentials continuously at both the pickup and discharge points.
3. How would you verify that any mitigation actually works?
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Continuous potential logging at the discharge location (opposite the substation) and at the pickup location, over at least a full week including peak service, before and after mitigation. Confirm that the most positive potential stays at or above the criterion during peaks. If a drainage bond is installed, verify that it drains stray current without over-draining the CP system's own current, and check that the bond does not create a new discharge path. Include a 100 mV decay test at a representative location to confirm the pipe is polarized, not just shifted by IR drop from stray current.