Stray current measurement at the tramway infrastructure in Ostrava, Czech Republic

Abstract

In most transit systems, rails are used as return conductors for the current from the vehicle to the electrical substations. If the rails are not fully insulated from the ground, some of this current would leak and become stray current, causing stray current corrosion on the rails and metal objects (such as pipelines) in the immediate area. It is very difficult to measure stray current directly, but stray currents can be calculated by measuring other parameters. Stray currents were measured on a 1.3 km section of tramway infrastructure in Ostrava. The potential between rail and earth was measured on the basis of the standard EN 50122-2, where two reference electrodes were placed at an appropriate distance from the tram track at three measuring points in the ground - the first point was located at the beginning of the section, the second in the half of the section and the last at the end of the section. Rail currents were measured at two measurement points - the first point at the beginning of the section and the second point at the end of the section. Using the rail-to-earth potential and the equation from the standard EN 50122-2:2011, the rail-to-earth conductance per length was calculated. The conductance per length was also calculated using Ohm's law, where the current difference is a difference between two measurement points. Since the results obtained using the standard and Ohm's law did not agree, a detailed analysis of the tram section was performed and electrical drainage was found. The drainage represents an electrical connection of the protected metal structure in the area of the tram track by a cable with stray current source. Through the drainage, the stray currents are directly returned to the rail. In this measurement section, the drainage has influenced the current difference between the measurement points - without drainage, this difference would be much smaller.