What the IEC Tells Us About Stray Currents: Guidance for a Practical Approach

It is well known that in dc-electrified rail lines, the running rails are used to return the traction current to the rectifier substations. However, we cannot avoid some part of this current leaving the rails and instead passing through the earth to rejoin the return circuit at a point that is closer to the rectifier substation, and so the current reaching the negative busbar of the rectifier substation. These leakages, called stray currents, seek the least-resistant paths and, in particular, piping and cable systems of any kind placed in the neighborhood of tracks. Thus, a stray current is the part of a current caused by a dc traction system that follows paths other than the return circuit. The major effects of stray currents can be the corrosion and subsequent damage of metallic structures at the points where stray currents exit. The metallic mass dissipated onto the soil can be estimated through the Faraday's laws of electrolysis. For example, considering 1 A of stray current flowing continuously through a steel structure, the metallic mass of corroded steel dissipated onto the soil is equivalent to 9.13 kg per year. There is also the risk of overheating, arcing, fire, and subsequent danger to people and equipment both inside and outside the dc traction system.