Understanding Stray Current Mitigation, Testing and Maintenance on DC Powered Rail Transit Systems

All direct current traction power systems using rails for return of traction current have a level of current leakage. This leakage of current is dependent on both design and operating factors affecting the efficiency of the rail return path and is referred to as stray current. Stray currents have been detected since the first electric railways were placed into operation during the latter half of the nineteenth century and have serious effects on utility structures and the neighboring infrastructure at large. Stray currents can create safety hazards thereby rendering the design of stray current mitigation an important element of the overall design of a rail transit system. Like any other design/construction project, a baseline survey is an important and significant step in the data collection and fact finding process for a light rail system. Such a survey would aid in finding the soil resistivity data and the results of the stray current levels on existing buried metal utilities. Similarly defining the design criteria for stray current mitigation, monitoring, and testing for a new light rail design project is also important. Most of the design criteria for the older rail transit systems have been developed as an aftermath of the corrosion problem and/or after the design of new extension to the system. Some older transit systems still do not have a specified design or mitigation criteria for stray current, and corrosion issues are handled as they surface and are prioritized based on severity. In the absence of guidelines, it is hard to understand the reasoning behind the limiting criteria suggested in the transit agency manuals particularly when there is no record of testing or soil resistivity investigation. For these older transit systems the limiting criterion was developed based on the information from other transit services. Having applicable design criteria for stray current control and mitigation will help standardize the process for the transit and will lower the cost of mitigation. This paper has been written by a Civil Engineer with an effort to understand the source and the scientific reasoning behind the limiting values suggested by the transit agencies associated with stray current testing procedures and its control. In order to understand the limited stray current corrosion criteria and the respective testing, various transit agencies were interviewed. These interviews were supplemented by a thorough review of the respective transit agency criteria manual/guidelines (where such information was available and accessible). Critical evaluations of the testing procedures were conducted to analyze if these tests and mitigation methods were effective.