JANUARY 31 2003
completely insulated from the earth. Some part of the load current may travel through the
earth. In the event of a track fault, these currents could be extremely high. Buried or
submerged metallic structures in the vicinity (several miles) of these tracks could be
subject to stray current effects. Pipelines that run parallel, cross under the tracks, or are
located near the DC substation, are especially prone to these stray currents. If there are
high resistance joints in the pipeline, the current may bypass the joint, leaving the pipeline
on one side of the joint, and returning on the other side. Since the source of the stray
current is moving, it may be necessary to monitor the metallic structure over a 24-hour
period to see if these currents affect it.
Figure 2-13. Stray Current Corrosion Cell Caused by a DC Transit System
Current Flow in the Earth
High Voltage Direct Current (HVDC) Electric Transmission Lines. Power
distribution systems are another source of stray currents. Most power systems are AC,
although sometimes DC systems with grounded neutral may be used. These
transmission lines, under fault conditions, may use the earth as the return path for the DC
current. Because DC requires only two-wire instead of three-wire transmission, it is
sometimes used when large amounts of power needs to be transported large distances.
Conversion units are located at each end of the transmission lines. Each of these
conversion units are connected to a large ground grid. Any unbalanced load would result
in a current in the earth between these two ground grids. These unbalanced currents are
naturally not constant--they vary in direction and magnitude. HVDC line voltages may be
750,000 volts or higher.