Low-Level DC Leakage and Fault Currents in Transit Systems: Developing a Prototype for Detection and Mitigation (2025)
Chapter: 1 Background
CHAPTER 1
Background
Low-level electrical fault currents are a phenomenon found in direct current (DC) traction systems used in public transit systems worldwide. These low-level currents are typically caused by small and sporadic failures of insulation within the electrification system, which makes them difficult to locate, measure, and control (Cingoz et al. 2015). The apparent effects of these faults go unnoticed for long periods of time because of their slow and progressive nature; however, if these faults are left undetected, existing evidence shows extensive damage to infrastructure of transit systems and that of adjacent private/public utilities could result.
Leakage currents due to insulator failure or contact with nonconductive surfaces fall below the detectable level of conventional relays. Since these faults are often caused by energized conductors within public reach, they pose a great threat to personal safety and property security. The main concern for private/public utilities is the significant corrosion of subsurface utilities caused by the compounding effects of low-level faults. They may also create a safety hazard to transit patrons and the general public, as contact with any metallic structures (e.g., fences) is potentially lethal when they become energized to dangerous voltages. Furthermore, smoke generated by burning cable insulation in tunnels and confined areas creates additional safety hazards for transit patrons.
More often than is desirable, the service that electric railway systems provide to commuters is interrupted due to electric power disruptions (Sozer 2020). Theoretical aspects of short-circuit faults on traction and transit systems have been analyzed in Cotton et al. (2005) and Boler et al. (2020). Ibrahem et al. (2017) note that “the amplitude of short-circuit currents in . . . DC traction systems can be in the same order of magnitude as that of the maximum start-up current, due to the relatively low voltage level of the supply line and its high impedance.”
In order to detect low-level fault conditions, it is necessary to conduct extensive testing, which is extremely costly and difficult to accomplish, particularly in areas remote from traction power substations. With current operating budget restrictions in the industry, consideration of this type of testing is not feasible. To minimize service disruptions, there is a need to design and develop a cost-effective smart sensor system to do predictive electric system monitoring and to keep faults from occurring.
The objective of this project is to integrate the proposed technology into systems that can be applied to third-rail DC transit systems in a variety of monitoring and reporting applications. The use of continuous system condition information will enable predictive maintenance, outage avoidance, and real-time evaluation of the electric network condition.
The concept of the final sensor system developed could be turned over to the private sector for engineering and offered for sale to the transit industry. The models produced should not require the shutdown of an electrified system except during connection of the module.
