Stray Current and Interference
The mechanism
Stray current is current flowing through the earth or a structure on a path not intended by the CP designer. The rule to memorize: current discharging from metal into electrolyte causes corrosion; current entering metal is protective. Interference is therefore strongest where a foreign structure sits between a current source and its intended return path - near DC transit systems, welding operations, other companies' rectifiers, and HVDC electrodes. AC interference from power lines is a separate issue, causing shock hazard and AC corrosion.
Static versus dynamic
Static interference comes from steady sources and can be corrected with a fixed bond. Dynamic interference varies with train position or load and needs automatic control. Detection relies on structure-to-soil surveys, current mapping (a span or clamp-on ammeter along the line), close-interval surveys with synchronized interruption, and comparing potentials with the foreign source on and off.
Mitigation
Options include resistance bonds or direct bonds to the source structure, diodes or electronic (controlled) bonds that drain only in one direction, galvanic anodes to intercept the discharge, coating improvements, relocating or deep-well siting of the offending groundbed, and drainage to a railway negative bus. Bond sizing must be checked so the bond does not over-drain the protected structure.
Traps
Assuming any bond is safe - an oversized bond can cause interference elsewhere or over-drain the CP system; ignoring polarity in diode selection; treating a dynamic reading as static; forgetting AC mitigation (grounding mats, decoupling) and step/touch potential.
Worked example
A pipeline crossing a transit line shows −0.95 V CSE with trains absent and −0.72 V CSE with trains present at the same location, with current leaving at that point. The pipeline is picking up current on one side and discharging at the crossing. A controlled bond or an intercepting galvanic anode at the discharge point is the typical remedy.