Currents entering or leaving a structure as a result of any influences other than the cathodic protection system itself, or long cell action are termed “stray currents”.

Stray currents usually emanate from DC Traction Systems, where a portion of the traction current returns to the substation via earth, rather than through the return rail. Thus, the stray currents vary with time, depending on the location of the load and the power being drawn. Static stray currents, which are usually caused by other CP Systems, are generally referred to as “interference currents”.

Stray currents are detected by monitoring the structure/soil potential over a period of time. Current entering or leaving the structure causes variations in the structure/soil potential, which indicates the direction and intensity of the current.
Pipeline corrosion is characterised by a flow of current in the pipe wall between cathodic and anodic sites, which are often some distance apart. As long as the current flows within the metallic structure, no corrosion damage occurs. Corrosion of the structure only occurs at points where the current leaves the metal through defects in the coating or wrapping and returns to earth. This current loss is accompanied by a positive shift in the metal/electrolyte potential. Similarly, current entering the structure from the earth causes a negative shift.

In practice, data loggers or recorders are used to monitor the structure/electrolyte potential. These recordings are made over several hours in order to ensure representative results, as traction movement varies considerably over a 24hr period.

By synchronising the recordings, it is possible to identify stray current paths and determine the most effective location for future CP installations.

Close Menu