The negative return portion of a modern direct current (dc) traction power system, which includes the tracks (the running rails), 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 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. For modern dc traction power systems in particular, however, this resistance is high enough for the rails to be considered essentially ungrounded with respect to local electrical ground (earth). 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 substations. Since the shells of rail vehicles are typically at the same voltage as the wheels and rails, this voltage difference could be impressed on a passenger entering or exiting a train from a grounded platform. Or they could be impressed on a person walking along the tracks. In the USA, these voltage differences have generally been limited through system design; North American standards for substation grounding are referenced for design purposes, in particular IEEE Standard 80, Guide for Safety in Substation Grounding. In Europe, a standard has been developed specifically to address control of voltages between rails and structures, BS EN 50122-1 (IEC 62128-1), Railway Applications - Fixed Installations - Part 1: Protective Provisions Relating to Electrical Safety and Earthing. Voltage-limiting equipment that can be installed in passenger stations and other accessible locations has been developed in response to the requirements of EN 50122-1. These devices quickly connect the running rails to the station structure to eliminate unsafe voltage differences. If an earth fault occurs (broken catenary conductor falling on the ground, for example), there may not be a low-resistance circuit back to the substation due to the electrical isolation between running rails and earth ground. Without a low-resistance path back to the substation, there may be a resulting low-level short circuit current flow insufficient to operate the substation protective systems. As a result, the area in the vicinity of the fault may potentially be elevated to unsafe voltage levels. Equipment intended to detect this condition and connect the substation negative dc bus to the substation grounding grid is gradually being incorporated into modern North American dc traction power substation design. These devices are known by several names such as "substation grounding contactors", "automatic grounding switches", and "negative grounding devices". Devices built to comply with EN 50122-1 are termed "Voltage Limiting Devices". EN 50122-1 includes voltage-time curves that dictate the maximum permissible magnitudes and durations for ac and dc voltages; equipment built to EN 50122-1 must clamp the highest voltages in no more than 20 milliseconds. This paper will review current American and European standards and codes for maximum permissible rail voltage on direct current traction power systems. The maximum permissible voltage levels will be explained and compared. The principles of negative grounding device operation and corresponding voltage settings will also be discussed.
