IEEE Std 2720-2021 pdf download – IEEE Guide for Rail Potental Management for DC Electrifcaton Systems.
3. Defnitions, acronyms, and abbreviations 3.1 Defnitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 4 body voltage: The product of the current through the body and the body impedance. earth: The conductive mass of the earth, whose electric potential at any point is conventionally taken as equal to zero (also referred to as “remote earth”). earth electrode: A conductor or a group of conductors in intimate contact with, and providing an electrical connection to, earth (also referred to as a “grounding electrode”). NOTE—“Earth“ will be used in place of “ground” in this guide to avoid potential confusion with other IEEE defnitions of “ground.” 5 negative grounding device (NGD): a device that connects the substation negative bus to the substation grounding grid when the voltage between them exceeds a setpoint. The voltage setpoint can be a voltage magnitude with or without a time delay, or a voltage versus time curve. Also referred to as an automatic grounding switch, foating negative automatic grounding switch, or grounding contactor. negative return system: the equipment that connects the wheels of electrifed rail vehicles to the substation negative bus. This equipment can include running rails (track), impedance bonds, power cables, and fltering reactors. Fourth rail systems use a separate (4th) rail for return current instead of the running rails.
4. General The negative return portion of modern dc light and heavy rail power systems is intentionally isolated from earth under normal operations to the maximum extent practical. The purpose of this electrical isolation is the prevention of stray dc current flow into the earth and nearby infrastructure. The high levels of stray current flow associated with alternate methods of dc traction power system grounding, such as direct grounding and diode grounding, have rendered them essentially obsolete for new system construction Moody [B6]. 6 The steel running rails (tracks), which are the largest component of a negative return system, are typically isolated from the track bed by the use of components such as elastomeric pads and insulated fasteners. The isolation between tracks and ground is not perfect, with typical initial installation resistance-to-ground values of 250 and 500 Ω for 1000 ft of single track being specifed for ballasted and direct fxation track, respectively. Although the initial levels of isolation present at system commissioning tend to degrade with time, the resulting isolation of the tracks from earth remains suffcient to permit the buildup of voltage.