How Effective Are Automatic Grounding Devices at the Floating Negative Rail System of a DC Powered Rail Transit System?

Currently most modern dc rapid transit systems around the world use running rails or a negative rail as the negative return circuit for the dc traction current. These rails are intentionally kept ungrounded from ground (earth) by using insulating material between the rail and earth to minimize dc stray current. This configuration of return current rail develops varying voltage with respect to ground along the entire length of the rail when trains are in operation. Probability of dc or ac power system faults exist in the vicinity of the rail tracks which leads to coupling of fault voltage to these rails. This varying voltage of the return rail at the train location gets transferred to the train vehicle shell due to design configuration of the vehicle propulsion system injecting negative current to these rails. To minimize the unpredictable threat of electric shock hazard, automatic negative rail grounding (NRG) devices that continuously monitor return rail voltage with respect to ground are implemented at appropriate locations such as substations, passenger stations and at other accessible locations. This paper provides recommendations to establish a uniform application approach for NRG devices in the transit industry with a clear understanding that it is the dc current through the human body and not the dc rail voltage that poses electric shock hazard.