Track-to-Earth Potentials and Stray Current Monitoring on Portland TriMet MAX Light Rail System

The track is insulated in a DC railway system. Even when the track is new, the insulation between track and earth is never ideal. The insulating capacity of the new track diminishes in the course of time due to ageing and dirt. The insulation can also be affected due to improper track construction. It is, therefore, necessary to monitor stray current activity to detect early insulation deficiencies to prevent any possible damage due to stray current corrosion of metal objects such as underground utility pipelines or building reinforcements. One approach to monitoring stray current activities is through the measurement of track-to-earth potentials. Dynamic stray current effects can normally be identified by fluctuations in the track-to-earth potentials with time. This paper describes the Siemens SITRAS SMS stray current monitoring system used on the Portland IMAX Light Rail extension. The SITRAS SMS system consists of a central evaluation PC located at TriMet Systems Engineering Office with track-to-earth voltage sensors installed at each of the six traction power substations on the IMAX line rail extension. The sensors and the PC are connected to each other through an ETHERNET network. The measured data are stored and archived in three different stages: instantaneous, short-term, and long term average values. This paper will present and analyze the data colleted on the IMAX line rail extension. Basic stray current modeling and analysis will also be discussed.