IEEE Std 2720 pdf free download

IEEE Std 2720 pdf free download.Rail Potential Management for DC Electrification Systems.
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 [B6j.
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 50012 for 1000 ft of single track being specified for ballasted and direct fixation track, respectively. Although the initial levels of isolation present at system commissioning tend to degrade with lime, the resulting isolation of the tracks from earth remains sufficient to permit the buildup of voltage.
For dc traction systems in particular, the lack of an intentional connection between the tracks and earth allows voltage differences to occur along the rails, and between the rails and nearby structures. These voltage differences are caused by the flow of current through the running rails back to the source substations. The voltage difference along the rails is the result of voltage drop across the rail impedance, which is often termed “longitudinal voltage drop.” bbRail potential” is the difference in voltage between the tracks and ground at a specific location. “Earth” or “ground” in this instance means “remote earth” and “earth” in the terminologies ofArnerican (IEEE) and European (IEC, EN) standards and codes, respectively (a zero-potential reference).
Rail potential at any location along the tracks varies significantly due to the passage of trains, with the higher values typically corresponding to periods of peak train acceleration and therefore lasting on the order of seconds. The resulting peak rail potentials may or may not be significant, being dependent on the magnitude of the train load or braking currents, the resistance of the rail return circuit, and the degree of electrical isolation of the tracks from earth. The distance between substations and the outage of substation rectification equipment also affect rail potentials since these impact the resistance of the rail return circuit.
5. Review of Applicable Codes and Standards for Rail Potential
5.1 General
Rail potential can cause voltages to occur on metallic railway infrastructure that may be accessible to railway workers and the public. For example, since the metallic shells of rail vehicles are typically at the same voltage as the wheels and rails, a voltage difference could be impressed on a passenger entering or exiting a train from a grounded platform. Alternatively, persons may contact metallic infrastructure near the tracks that has elevated voltages due to rail potential, or they may directly contact the tracks or metallic equipment connected to the tracks. If the contact does not create a difference in voltage across the body (hand-to-hand. hand-tofeet. foot-to-foot). then the voltage does not present a hazard. However, if a significant voltage difference is impressed across the body, potentially dangerous electric currents could flow through the heart as a result.
IEEE Std 2720  pdf download.