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34 Stray Current Control Design, Mitigation, and Testing of DC Rail Transit Systems Stray current leakage and the corrosion caused by this leaked current in DC traction power systems have combined to be an ongoing issue especially in slab or embedded tracks. These tracks typically run through urban traffic areas, city centers, tunnels, and between utility lines. Their location requires the rail to be continuously isolated to provide adequate track-to-earth resistance. Ballasted and direct fixation tracks provide much better track-to-earth resistance when equipped with isolation pads under the rail and exhibit higher stray current protection. The transit agencies surveyed during the research for this guidebook are cognizant of this issue and of the benefit of defining the limiting values for track-to-earth resistance versus current leakage. However, in the absence of defined guidelines and initial soil investigations, it is hard to maintain the tracks and define the limiting values relevant to the local conditions. 3.1 Transit Agency Surveys An initial desk study of numerous DC-powered transit systems was carried out to understand the standard practices adopted for the control of stray current by these transit agencies. The data collected included information on design criteria, performance specifications, constructability issues, and physical environment. Based on the findings of the literature review and the list of transit agencies provided by TCRP (and through personal contacts of the lead investigators), a mix of 30 transit agencies (21 national and 9 international) were contacted. Data and infor- mation on existing stray current mitigation and collection procedures, methods of stray current testing and measurement, criteria for acceptable levels including rail-to-earth resistance, and agency specific criteria was collected to understand existing or previous issues. Table 7 provides the list of national and international agencies that participated by responding either to one of the questionnaires or by agreeing to a telephone interview. Appendix A presents the introductory questionnaire (short questionnaire) that was e-mailed to the national and international transit agencies listed in Table 7. The questionnaire was formatted to get to the premise of the limited stray corrosion criteria and guidelines used by these transit agencies. Though most of the transit agencies requested anonymity, a few agencies completed the questionnaire under conditions of complete anonymity. Therefore, their names are not listed in Table 7. Appendix B provides a matrix listing the responses received from some of these transit agencies, with responses to the questionnaire based on the survey results between 2012 and 2014. The answers provided by the transit agencies have not been vetted with the actual data, design, or configuration of their transit systems. Following is a synopsis of the key findings and responses gathered from short questionnaires and from face-to-face interviews with the transit agencies or their consultants. ⢠Fifty percent (50%) of the transit agencies or their consultants reported being aware of a stray current corrosion issue at their system. Sixteen percent (16%) of the remaining responded C H A P T E R 3
Stray Current Control Design, Mitigation, and Testing of DC Rail Transit Systems 35 that they were not aware of any stray current corrosion issues at their system. The rest did not respond to this query. ⢠One hundred percent (100%) of the transit agencies responded to the question on sub- station spacing. Eighty percent (80%) indicated that the spacing between most of the TPSs was <1 mile. However, there are few sections where the distance ranged from >1 mile to <2 miles, which is a preferred SCC measure. ⢠Eighty percent (80%) of the transit agencies responded Yes to the question on having conducted the baseline survey, with roughly 30% acknowledging the fact that the survey was only conducted on some of the newer lines. ⢠All U.S. transit agencies, with the exception of one transit agency, responded to the question on limiting values for rail-to-earth resistance maintained on their tracks. Following is the breakdown of their responses: â Twenty-three percent (23%) maintain 250 Ω/1,000 track feet; â Fifty-four (54%) maintain 500 Ω/1,000 track feet; and â Fifteen percent (15%) maintain 1,000 Ω/1,000 track feet. The remaining U.S. transit agencies either did not respond to this question or stated that it was maintained lower than 100 Ω/1,000 ft. Compared with this, 50% of the international transit agencies responded that they follow the BS EN 50122-2:2010 and the remaining international agencies either did not respond or presented numbers ranging from 5 Ω/1,000 track feet to 250 Ω/1,000 track feet. The rail- to-earth resistance is a critical parameter as it is one of the first lines of defense against stray current leakage. This short questionnaire served as a first blush analysis on the pulse of the industry, as the industry views itself, related to the current practice on stray current corrosion, mitigation, and testing. In general, the data sample collected as part of the short questionnaire served the purpose of an initial analysis and warranted the development of a more detailed questionnaire (long questionnaire). Based on the willingness of transit agencies to participate in a more detailed questionnaire and to advance the survey of the transit agency stray current corrosion data, a long question- naire with 51 questions was e-mailed to the transit agencies. Eighty-five (85%) of agencies had initially agreed to contribute to this long questionnaire; 35% of them actually responded. Transit Agency Name and Geographic Location MTA Maryland, Baltimore New Jersey Transit, Newark BART, Oakland, CA RTA, New Orleans, LA Charlotte Area Transit System, NC New York City Transit CTA, Chicago, IL SEPTA, Philadelphia, PA City of Calgary, Canada Valley Metro, Phoenix, AZ Copenhagen Metro, Denmark TriMet, Portland, OR RTD, Denver, CO Public Transport Victoria, Australia Edmonton Transit System, Canada Rio Metro Concession, Brazil GCRTA, Greater Cleveland, OH Sacramento Regional Transit, CA METRO, Houston, TX UTA, Salt Lake City LA METRO, Los Angeles, CA Sound Transit, Seattle, WA Manchester MetroLink, United Kingdom Toronto Transit Commission, Canada MTA Maryland, Baltimore Via Quatro Line 4, Sao Paulo, Brazil MBTA, Boston, MA WMATA METRO, Washington, D.C. Metro Transit, Minneapolis, MN Yarra Trams, Victoria, Australia Table 7. Transit agencies that responded to the questionnaire.
36 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Out of the 35%, some of the agencies completed the questionnaire with a request for anonymity. Appendix C presents the long questionnaire. Appendix D presents a matrix listing the summary of the agency response findings. Following is a synopsis of the key findings and responses gathered from long questionnaires and face-to-face interviews with the transit agencies or their consultants. ⢠All but one of the transit agencies answered the question on the type, size, and cross section of the rail. The rail cross section is an important design parameter for the control of stray current as it defines the rail resistivity, the concept and relevance of which are elaborated in the literature research chapter. ⢠One hundred percent (100%) of the transit agencies responded to the question on TPS spacing. This longer questionnaire included a query about the largest spacing between two TPSs. Most of the transit agencies stated that the largest spacing between two TPSs was < 2 miles, which is a preferred SCC measure. ⢠In response to the question on guidelines followed for the control of stray current leakage and mitigation design, most of the U.S. transit agencies mentioned that they have their own design criteria. They elaborated that these criteria define the limiting values for track-to- earth resistance and stray current testing and maintenance procedures, which helps them in maintaining the stray current leakage. Conversely, 100% of the international transit agencies referred to the BS-EN standards developed for the stray current leakage. ⢠Thirty-three percent (33%) of the U.S. transit agencies stated that they measure the track- to-earth resistance as part of their regular testing. These agencies have local design criteria manuals that define the limiting values and warrant testing and maintenance plans. However, the absence of regular testing to ascertain if recommended values in design criteria manuals are maintained indicates the need for better understanding and implementation of the guidelines proposed in those manuals. ⢠Ninety-nine percent (99%) of the transit agencies acknowledged that they either have stray current issues or have encountered them in the past. Most of the agencies also mentioned that in the absence of regular testing they usually find out about the stray current problems when a utility or third party complains about their pipes getting affected by the stray cur- rent leakage. ⢠With the exception of one international transit agency, none of the transit agencies shared their historical data on stray current leakage and corresponding mitigation measures. The agency that provided data requested to keep it confidential. ⢠Most of the U.S. transit agencies agreed that they would like to see a national guidebook for stray current design and mitigation measures for DC-powered rail. They emphasized the fact that a step-by-step guide for mitigation and then maintenance and testing will help the transit agency in keeping stray current leakage in check. ⢠A few of the agencies shared information on the cost of mitigating or repairing track due to stray current issues. 3.2 Transit Agency and Corrosion Consultant Interviews During the coordination process for the questionnaires, it was concluded that some of the transit agency employees either are not properly trained or knowledgeable on the specifics of the corrosion mitigation for their system or were reluctant to release the information. In some cases, the questions were left either unanswered or delegated to their corrosion consultants. A supple- mental interview of a cross section of corrosion consultants was warranted to get a viewpoint based on their experiences with some of these agencies. Table 8 highlights the list of corrosion consultant experts that participated in the interview process. Corrosion Consultants Corrpro (International) UTRS (now with STV) V&A Engineering LTK Parsons (International) Intertek (International) Table 8. Corrosion consultants interviewed.
Stray Current Control Design, Mitigation, and Testing of DC Rail Transit Systems 37 The feedback from general consultants indicated the need for experienced consultants for stray current testing and control or for focusing on getting the transit agency staff more trained and comfortable with carrying out stray current measurements. The consultants also unanimously agreed that there is a dire need for a guideline to address the design, testing, and maintenance of the stray current control process. Many of the agencies did not want to share transit agency specific data and requested anonymity. But the previous data collection exercise, through questionnaires and interviews, not only helped in understanding the physical and environmental settings of the transit systems but also provided an insight into the many means and methods used by the transit agencies to mitigate and collect stray current leakage. Judging from studying the responses to both of the questionnaires and interviews with transit agency personnel and corrosion consultants, the critical needs of the industry follow: ⢠Implementation of improved rail insulation (track-to-earth) techniques; ⢠Guidelines for acceptable SCC; ⢠Ongoing track maintenance program (keeping rail track-bed areas clean and drained); ⢠Proper placement of a TPS along the track using traction power and stray current corrosion modeling; ⢠Standardization of a regular testing program for transit agencies; and ⢠Standard testing methods for stray current and their limiting measurements (based on baseline survey). 3.3 Transit Agency Essentials and Corrosion Issues (Case Studies) A decision tree, presented in Appendix E, was developed to narrow a select list of transit agencies for further detailed analysis and presentation. Four representative DC-powered rail transit systems were shortlisted for case studies to evaluate effective practices for SCC and control of track-to-earth/rail-to-earth voltages. To ascertain that the data collected from these agencies was a true cross-sectional representative of the industry, this sample set included (a) an agency with relatively newly constructed tracks, (b) an old agency, (c) an agency with tracks under construction [both LRT and heavy rail transit (HRT)], and (d) an international transit agency with overhead contact systems. Subsequent sections describe a summary of the case studies on these four agencies. Face-to- face interviews and site visits were conducted between 2012 and 2014. The transit agencies may have expanded their system, changed their system configuration, or updated their stray current collection and mitigation techniques. Names of transit agencies are anonymous at the request of transit agency personnel. This information has been gathered not only from in-person interviews and site visits but also by collecting live testing and maintenance track data. 3.3.1 Transit Agency 1 Transit Agency 1 comprises approximately 13 miles of LRT and is powered by 750 volts DC via an overhead catenary system (OCS). This includes the recently opened extension along one of the routes. The LRT runs through the urban shared and dedicated right-of-way (ROW) with embedded, direct fixation track, and ballasted track sections. The TPSs are spaced at not more than 1 mile apart (with an exception of one location where the distance is close to 1½ miles). The negative return system is through the running rails. A shop and yard facility is electrically isolated from the mainline system. At the time of the interview, the extension line was not in revenue service so the interview focused on the issues on the existing line.
38 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook The track is mostly embedded (95%) with an approximately 1-mile-long direct fixation track and a small section of ballasted track. The transit system uses 115 RE rail (i.e., a certain rail section, 25 rail) on most tracks, with rail boot system and flangeway filler for the embedded section and concrete ties with insulated rail clips for the ballasted section to control the stray current leakage. Additionally, it uses continuously welded rails, direct fixation fasteners, cross bonding, tie and ballast at-grade track, and a continuous reinforcing steel mat with bonding cables in the concrete section to provide electrical continuity. This steel mat collects the stray current escaping the rails and conducts it along the track to the point where it reenters the running rails. This mat not only assures structural continuity that brings back the current to the return path but also controls the stray current from taking any unwanted routes, eventually sparing the surrounding utilities and infrastructure from corrosion. Test stations are provided at approximately 300-foot (on newer track) to 500-foot intervals to provide measurements of track-to-earth resistance. At gaps around bathtubs, at concrete paver locations, and around special track work, FX-120 polymer fills in the gaps. The injection of silicon material along rubber boot interfaces increases the track-to-earth resistance in those areas. All items that connect to the running rails (e.g., switch heaters, signaling system, and rail lubricators) and to the negative buses within the sub- station are suitably insulated, ensuring that there is no link between the grounding structures and the negative return. An important element of the SCC for the transit agency is the liaison with the utility owners that have utilities near the tracks. Numerous utilities exist along the ROW, including some major lines like gas lines and municipal water pipelines. In addition to the stray current design pro- vided by transit agency, the local utility owners near the tracks conduct testing on their pipelines and maintain their own sophisticated CP systems to protect their infrastructure. The preferred rail-to-earth resistance of the embedded track is 100 Ω/1,000 feet of track and the conservative contract requirement is 250 Ω/1,000 feet of track. The corrosion consultant established these resistances after conducting the rail-to-earth testing of the tracks using ASTM standards. Figure 15 shows the rail boot installation on the rail on a section of the tracks under construction, and Figure 16 shows the section of the rail that sits on reinforced concrete slab, which also has a steel mat for corrosion control. The transit agency has established a track maintenance and inspection plan based on the design criteria manual, which includes Figure 15. Section of flangeway filler rail boot during construction.
Stray Current Control Design, Mitigation, and Testing of DC Rail Transit Systems 39 both LRT and bus rapid transit systems and a dedicated chapter on corrosion control comprising stray current corrosion. The transit agency carries out systemwide stray current testing such as track-to-earth resistance, track slab stray current flow, and audio frequency tracing measure- ments, if needed. Testing Methods The following tests are conducted as part of the overall track stray current testing and maintenance: ⢠Visual inspections, ⢠Structure and/or pipe-to-soil potential measurements, ⢠Utility testing, ⢠Track slab electrical continuity and current flow, ⢠Bridge stray current, and ⢠Cell-to-cell potential gradient measurements. The testing is conducted every 3 to 5 years based on track performance. Figures 17 and 18 illustrate the sampling stations/process. Findings from Testing and Corresponding Corrosion Issues Results of the testing conducted soon after the revenue service revealed that most of the track sections complied with the 100 Ω/1,000 feet of track resistance except in the following areas: rail anchors at the bridge expansion joints, track switch bathtubs, and concrete and brick pavers bridging effects at bathtubs. Most of the public utility pipes along the ROW were also tested where minor-to-negligible stray current effects were noticed. All these areas of potential concern were enhanced and from that point the transit agency now conducts testing of the track once every 3 years for the entire section and once a year along a select section of the track. To understand the testing procedure, the need, and the methodâs effectiveness, ground testing was observed and performed sometimes as part of the work for this guidebook. Details of the testing and respective results were incorporated into the development of guidelines and recommendations that are presented in Chapter 4. Figure 16. Steel reinforcement for the embedded slab track.
40 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Conclusion When asked what the key issues are, the Transit Agency 1 staff indicated that they would rather not construct an embedded track within the urban area due to high traffic volume and regular maintenance issues. In a situation where there is no other alternate route, they would like to ensure that the rail is completely isolated (use of modified rail boot) in combination with a possible stray current collection system. This would ensure minimal-to-no leakage of current to the earth or the neighboring utilities. Moreover, as recognized in the literature review, Transit Agency 1 would like to see some guidelines and principles supporting the limiting values for stray current mitigation. 3.3.2 Transit Agency 2 Since the earlier days of its service in the 1900s, Transit Agency 2 has grown to be one of the top 10 busiest subway systems in the world. Of its 840-mile-length of track, approximately Figure 17. Example 1: utility testing station. Figure 18. Example 2: utility testing station.
Stray Current Control Design, Mitigation, and Testing of DC Rail Transit Systems 41 60% of the track miles are underground, with more than 470 station locations. The systems design represents three distinct styles, with the primary difference being the platform lengths. Since most of this system was built in the early to mid-1900s the system has seen a lot of upgrades. The upgrades include the one ongoing at the time of the site visit in November 2011 and then again in March 2012 involving additional tracks on the existing and new transit routes. Transit Agency 2 is powered by 650 volts DC via the third rail with substations receiving as much as 27,000 volts from the power plants whereas the signals, station lighting, tunnel light- ing, ventilation, and other miscellaneous line equipment is powered by AC. The substation spacing varies from 0.5 miles in the newer lines to not more than 1.5 miles in some of the older lines (this is after construction of some interim TPSs to reduce the spacing). The track is mostly ballasted or concrete with recently upgraded insulated clip-type fasteners for rail-to-tie connection and continuous welded sections of rail. There are still some sections with wooden ties and spikes as well; however, they are gradually being replaced (see Figure 19). The traction power system is isolated with floating negative return running rails (no grounded system by design). The rail-to-earth resistance of the track is approximately between 1 to 10 Ω/1,000 feet at its worst. Diode drains along with track bonding are used to provide SCC with cross bonding every 500 to 1,000 feet. Transit Agency 2 does not have specific criteria or principles for the operation and main- tenance of stray current corrosion and instead maintains/retains a corrosion control task force that handles the corrosion-related complaints. Thus, as stated in the literature review, the approach to address stray current issues is more reactive than proactive. A corrosion control guide was structured in 1984 that is used by the agency staff for reference and corrosion mitigation control. Testing Methods There is no periodic inspection or testing schedule in place for Transit Agency 2. The cor- rosion staff is responsible for the corrosion surveys, and testing is conducted mostly to address a prevailing complaint rather than as a proactive approach. The corrosion testing crew is on duty 24 hours a day with the goal to keep the corrosion issues at a minimum. The crew has been diligently maintaining the systemâs stray corrosion under control (see Figure 20). Figure 19. Section of ongoing construction with concrete and wooden ties.
42 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Findings from Testing and Corresponding Corrosion Issues Because this system is older, issues are inevitable and they have been fixed in the past. However, the recent upgrades to the tracks and the overall system have rendered the previous corrosion records obsolete. Since Transit Agency 2 does not keep a database of existing or past corrosion issues, there are no reports generated to depict stray corrosion best management practices or documented mitigation measures that have been successfully adopted. Following are some of the issues that the agency staff mentioned that they have to address on a recur- ring basis: ⢠Water main failures or corrosions. ⢠Corrosion of rail spikes. ⢠Loss of expansion joint bonds of the elevated structures leading to corrosion of steel components. ⢠Failure of old lead cables. Conclusion When asked what the key issues are, the agency staff indicated that they would like to upgrade all the tracks that are old or have not been upgraded yet, change the fastener clips, and replace the old lead cables. During the interview, they also implied that not having a source document (guideline or recommendations) for reference and standards makes their job difficult and that they would like to see a typical guideline to be followed across the industry. 3.3.3 Transit Agency 3 Since its opening in the late 1990s, Transit Agency 3 has grown to become an integral part of the countyâs transit system. The transit agency has a combination of light rail and heavy rail and operates approximately 100 miles of LRT and HRT, with more than 100 station locations. The heavy rail lines share the ROW for a short length of the route whereas the light rail lines run on their own ROW except at grade crossings, with expected utilization of the shared ROW. The routes run in a mix of at-grade, elevated, and underground environments. Seven hundred fifty (750) volts power the HRT system via the third rail. The LRT system is powered by 750 volts DC via the OCS with running rails providing the negative return for both Figure 20. Water pipe utility test on bridge.
Stray Current Control Design, Mitigation, and Testing of DC Rail Transit Systems 43 systems. The substation spacing varies from 0.5 miles to 2 miles for both the LRT and HRT systems. The track is mostly ballast with concrete ties and insulated rail fasteners. The traction power system is ungrounded with continuously welded rails, cross bonding, clip fasteners with insulation padding, and sacrificial anodes to mitigate the stray current. The design rail-to-earth resistance of the track is 500 Ω/1,000 feet of rail whereas the embedded section at-grade crossings/city streets is 300 Ω/1,000 feet of rail. Wooden ties and spikes are used for the tracks in shops and yards and are reasonably well isolated from the main line. The agency does not have any track maintenance inspection plan or design criteria manual that includes guidance on stray current corrosion. The corrosion issues are managed as they arise (see Figure 21). The transit agency is currently working with a consultant to conduct the survey and testing of sections of the line and to recommend suggestions to mitigate the stray current corrosion. Testing Methods There were no inspections or surveys conducted on Transit Agency 3 until recently. Typically, utility companies or local residents would report potential stray current issues to the transit agency staff who, in turn, would take any mitigation action as needed. However, due to the increasing corrosion issues and the expansion of the LRT system, the agency now uses the services of a corrosion consultant. The consultant conducts track-to-earth resistance and pipe-to-soil corrosion testing and provides mitigation recommendations. The same consultant is also working on preparing a detailed operations and maintenance manual, including main- tenance procedures, and is providing stray current training to the transit agency staff. Findings from Testing and Corresponding Corrosion Issues Results of the testing conducted by the Transit Agency 3 consultant reveal that the stray current corrosion activity has generally increased. Following are some of the ongoing stray current corrosion issues: ⢠Corrosion of the fire protection pipe system due to failure of the CP system. ⢠Number of other CP locations not functioning as designed. ⢠Corrosion of rail spikes. Figure 21. Corrosion of pipes at station facility.
44 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook ⢠Areas of low track-to-earth resistance along the lines. ⢠Possibility of a substation being grounded, which results in stray current leakage. Conclusion Periodic testing and monitoring of the testing locations were identified as the key issues for the tracks. The staff at the agency indicated that they would like a consultant to keep a restricted online database of the system, including the results along with the GPS locations of the testing areas. This will provide Transit Agency 3, the consultant, and the utility owner a log of the test results along with locations. Additionally, Transit Agency 3 would like to see some more guidelines and principles to support the ongoing testing and maintenance of the tracks to avoid stray current corrosion issues. 3.3.4 Transit Agency 4 The system of Transit Agency 4 comprises approximately 19 miles (32 km) of LRT/tram line powered by 750 volts DC via an OCS and a suburban rail line (rapid transit). The LRT includes the recently opened extension line (approximately 7.5 km) along one of the routes (also powered by 750 volts DC). The LRT runs through the urban shared and dedicated ROW with embedded, dual-block slab, and ballasted track sections with a total of 52 stops (stations). The TPSs are spaced at not more than 1 mile apart, with an exception of one span where the distance between the TPSs is slightly longer than 1 mile. The negative return system is through the running rails, cross bonding, and the collector cable, which is bonded to the stray current mat (wire mesh). The existing depot is divided into two areas: a storage area (yard) and a maintenance area (shop). The yard area uses the floating earth system employed on the main line, while the maintenance area is directly grounded to prevent touch potentials. The maintenance area is energized when the LRT is driven into and out of the building. At the time of the interview, another extension to the existing LRT line was under construction; however, it was not ready for revenue service. Transit Agency 4 uses T-rail (also referred to as flat bottom) 113-pound rail with concrete ties with insulated rail clips for the ballasted section, 80-pound rail for the slab-in track section, and Corus 59R2 coated rail or Corus 35GP rail with rail boot system with flangeway filler for embedded track. Additionally, Transit Agency 4 uses continuously welded rails, direct fixation fasteners, cross bonding, tie-and-ballast at-grade track, and a steel mat stray current collector system with bonding cables in the concrete section to provide electrical continuity. This steel mat collects the stray current escaping the rails and conducts it along the track to a copper cable bonded at every 300 meters. Test stations are provided at approximately 300- to 500-foot intervals to provide measurements of track-to-earth resistance (see Figure 22). An important element of the SCC for Transit Agency 4 is the stray current collection system and the liaison with the utility owners that have utilities near the tracks. In addition to the stray current design provided by Transit Agency 4, the local utility owners near the tracks conduct testing on their pipelines and maintain their own sophisticated CP systems to protect their infrastructure. Testing Methods The following tests are conducted as part of the overall track stray current testing and maintenance: ⢠Visual inspections, ⢠Structure or pipe-to-soil potential measurements, ⢠Utility testing,
Stray Current Control Design, Mitigation, and Testing of DC Rail Transit Systems 45 ⢠Track slab electrical continuity and current flow, and ⢠Cell-to-cell potential gradient measurements. This testing occurs every 5 years based on the track performance and current leaks. Findings from Testing and Corresponding Corrosion Issues There are areas of railway structure where the basic transit agency criteria for SCC has not been adequately achieved to ensure control of stray current to an acceptable level. Additionally, there is a potential risk of corrosion to third party structures such as utility pipes. In these areas, it is recommended to add measures or changes to the basic requirements. Options for these measures include fixing the epoxy coating between the rail boot and the ground in damaged areas, keeping the track clean from trash and debris, and reducing the rail potential by adding traction return cables to reduce the return circuit resistance. To understand the testing procedure, the testing need, and the testing methodâs effectiveness, actual ground testing was observed and performed as part of the work for this guidebook. Conclusion When asked what the key issues are, Transit Agency 4 staff indicated their satisfaction with the adequacy and efficiency of the stray current corrosion levels and mitigation measures established by their consultant. With regular maintenance and testing, the stray current leakage is kept within limits described in the transit agency criteria. As for the utilities, a detailed assessment based on tests and monitoring is undertaken to assure that the stray current leakage is kept within the agency criteria. In response to the question on standards and guidelines, Transit Agency 4 mentioned that they follow the BSI standards. However, they indicated that additional step-by-step guidance on stray current leakage, mitigation, and testing will help the agency in streamlining their stray current corrosion control process. 3.4 Chapter Summary The literature review, transit agency surveys, and case studies highlight a need for uniform design guidelines for stray current isolation and track maintenance for the U.S. DC rail transit community. Guidelines, complemented by a track maintenance and testing plan, will not only Figure 22. Embedded and ballasted section.
46 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook help transit agencies in keeping the stray current leakage to a minimum but will also help in implementing QC measures. The implementation of recommendations and best management practices in these guidelines, coupled with a preplanned maintenance regime, comes with an initial cost; however, such proactive measures will help reduce the unpredictable and repetitive cost of repair and breakdown down the line. Assessment of potential corrosion resulting from stray current should be part of the planning and design process at the inception of any project. Furthermore, testing for stray current cor- rosion must continue throughout revenue service. Based on interviews conducted during the course of compiling data for this guidebook, it was apparent that most of the transit agencies had not conducted prerevenue testing and did not have a regular testing and maintenance plan. It was also observed that transit agencies are not keeping a log of the corrosion issues caused by stray current or tracking the money spent to mitigate these issues. This kind of tracking would be beneficial to the rail industry in assessing the economic and logistic burden borne by the rail transit agency as a direct impact of stray current corrosion. Most of the transit agencies interviewed had at least one corrosion staff or traction power engineer on their payroll. But because of limited knowledge and understanding of stray current corrosion issues coupled with the absence of guidelines, they are forced to rely on outside consulting resources, as verified when transit agency staff forwarded the survey questionnaires to their respective consultants to complete. The DC rail transit agency staff understands that stray current corrosion is a serious issue and wants to have an amicable solution to control stray current leakage. However, they do not necessarily have the means and methods to keep the stray current leakage within limits and would like to see some defined criteria to control stray current corrosion. Transit agency staff is also aware of the need, benefit, and importance of stray current corrosion testing, maintaining the track, and keeping it clean of debris and dirt. However, they report that due to the lack of available transit agency funds, they cannot carry out the desired testing and maintenance. A potential alternative to this lack of funding issue is that the transit agency staff could be trained on the fundamentals of stray current corrosion, track testing, and control and mitigation techniques to conduct these services in house. These measures could keep stray current levels under control and minimize the preemptive system repair costs.
