The transit industry uses insulated cable for traction power supply and negative return cables between traction power substations, tie switches, and points of delivery to the vehicles, whether third (contact) rail or overhead contact system (OCS). In addition, third rail systems use jumper cables to provide continuity of power around third rail gaps and expansion joints. The conductor insulation system used for traction power cables as well as for lower voltage power distribution and signal/communication systems typically has a specified life of 30 years. Many of the transit systems in the United States are that age or older with original cables still in service.
Many cables within transit tunnels have had failures because of the water penetration through and around the cable insulation, causing damage to the insulation and corrosion of the copper conductor. In some subway tunnels, the conduits themselves have degraded to the point where they cannot be cleaned out and reused for their original purpose. Transit systems have experienced cable fires and system-wide closures due to the degradation of traction power cable insulation. In recent years, major storms affecting the United States have inundated transit tunnels with fresh and contaminated water and exacerbated cable lifecycle concerns.
Research is needed to answer key questions and to determine end of life and replacement criteria for transit traction power cables.
The objective of this research is to develop guidelines for determining when to replace transit system cables. The research will include a study of current practices for (1) assessing insulation aging and (2) replacement of cables before failure, including (a) approaches used in transit systems for which there are no monitoring programs and/or testing systems, (b) factors affecting life of cable, and (c) effective practices from transit agencies.
The research plan shall be divided into phases and tasks and shall describe in detail the work proposed in each task. The research plan shall describe appropriate deliverables that include, but are not limited to, the following (which also represent key project milestones):
- A literature review and survey/interview process to document standards and current practices through case studies covering maintenance practices from a mix of systems (small, large, old, and new) by engaging industry subject matter experts experienced in the design, construction, maintenance, and operation of traction power systems. Cable monitoring systems, such as monitors that measure cable conductor to earth ground resistance trends, should also be reviewed.
- An interim report and in-person presentation at a panel meeting. The panel meeting will take place after the contractor has delivered an interim report presenting the results of the early tasks. The interim report and panel meeting should occur after the expenditure of no more than 50 percent of the project budget. The interim report should include draft guidelines that can be piloted in Phase II.
In Phase II, it is anticipated that at least two transit operators will host 1-day pilot training sessions applying the draft guidelines. Using feedback from the pilot training sessions, the research team will refine the guidelines prior to submitting draft final deliverables.
At a minimum, the guidelines should answer the following questions:
Recognizing that cable insulation degrades over time, how long is the useful life of insulated cables?
How do you determine the degree of degradation of insulated cables?
What are the diagnostic indicators for when insulated cables should be replaced?
What factors enable the cable to achieve its maximum useful life or degrade more rapidly?
What can you do to cost-effectively replace or extend the useful life of insulated cables before they fail?
Is there a “smart replacement strategy” for insulated cables?
Also, the guidelines should include factors affecting useful life and procedures for
- Cable inspection and maintenance (including intervals and types).
- Return on investment (ROI) for maintenance interventions (including trade-offs of improving conditions versus replacing cables to extend life).
- Cost of inaction/non-intervention.
- Remaining useful life determination
- by type of cable insulation.
- by environmental condition (e.g., temperature, humidity).
- by application condition (e.g., current loading, voltage).
- by installation location (e.g., tunnels, surface, water).
Remaining useful life determination and valuation of transit traction power assets will be required of transit operators by October 1, 2018, under FTA’s Transit Asset Management rule. See Special Note G. Timely deliverables addressing remaining life valuation are anticipated to be of high interest to transit operators responding to the FTA Transit Asset Management rule over the years to come.
Note: For this project, the term cables applies to traction power cables, including return cables and jumpers.
A useful resource for this project is the FTA Transit Asset Management Plan guidance related to valuation of traction power cables. See particularly page 27.
STATUS: Research in progress. Draft final deliverables are anticipated in March 2021.