Maximum Permissible Rail Potential

This article reviews the current American and European standards and codes for maximum permissible rail voltage on DC traction power systems. The principles of negative grounding device (NGD) operation and its corresponding voltage settings are also briefly discussed. The negative return portion of a modern DC railway power system, which includes the running rails (tracks), is normally isolated from earth to the maximum extent practical. The purpose of this isolation is to prevent stray DC currents from flowing through the earth and potentially causing corrosion of nearby metallic infrastructure. The isolation of the tracks from the earth is not perfect. Each track tie and insulated running rail fastener assembly can be electrically represented as a resistor of high-ohmic value connected between the rails and the earth. With many of these resistors in parallel over miles of track, a distributed leakage resistance is established between the rails and earth. However, for modern DC traction power systems, in particular, this resistance is high enough for the rails to be considered essentially ungrounded with respect to local electrical ground (earth).