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From page 47... ... 47 Key decision matrices associated with implementing, maintaining, and testing of SCC and safety control of rail-to-earth potentials were developed by using the information collected from the literature review, questionnaires, data gathered during the transit agencies' and corrosion consultants' interview process, and stray current testing observations. Using these findings, proactive sequential steps are presented in the guidebook for stray current isolation and quality control. Read the entire page →
From page 48... ... 48 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook • Atmospheric corrosion characteristics: these include weather variations, determination of pollutants, and anticipated life of galvanizing. Sources of hostile pollutants and anticipated life of galvanizing are determined using the atmospheric corrosion characteristics. Read the entire page →
From page 49... ... Stray Current Control Provisions for DC Transit Systems 49 • ASTM D1557 -- for moisture density relations of soils and soil aggregate, mixtures using 10-lb rammer and 18-in. drop. Read the entire page →
From page 50... ... 50 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Surrounding infrastructure and utility location survey and coordination, referred to as a stray current survey, should include the following activities: • Perform field surveys to locate existing underground utilities and identify all structures that may be subject to corrosion due to the project within the project ROW and vicinity. • Record stray current potential (voltage) Read the entire page →
From page 51... ... Stray Current Control Provisions for DC Transit Systems 51 These simulation models evaluate how well the design of the transit system complies with the environmental analysis for allowable stray current. The load flow program is run for various parameters associated with the transit system, including vehicle performance, traction power performance (e.g., positive distribution, negative distribution, traction power substation, or AC/DC feeders) Read the entire page →
From page 52... ... 52 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook A conductor rail insulated from earth, also referred to as a fourth rail, can be used for the traction return current. If this is a live part and not connected to the running rails, usually no stray currents occur. Read the entire page →
From page 53... ... Stray Current Control Provisions for DC Transit Systems 53 shunts in the current collection cable at specific substations, which will be routed into these substations for monitoring purposes. All fittings (clips on concrete ties) Read the entire page →
From page 54... ... 54 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook lines and be electrically isolated from the adjoining track at each end by means of insulated rail joints, to be shunted during normal operation by removable continuity bonds. The arrangements at these joints should facilitate the connection of recording equipment to each end of each test length. Read the entire page →
From page 55... ... Stray Current Control Provisions for DC Transit Systems 55 At grade crossings, where the running rails are embedded in ground, care should be taken that the value of the track-to-earth resistance does not exceed the value of the connecting tracks (neighboring tracks) Read the entire page →
From page 56... ... 56 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook • Using sealants to seal all gaps with a polysulfide, polyurethane, or silicone sealant that provides a nonconductive path between the rail and surrounding earth. • Isolating yards and storage areas. Read the entire page →
From page 57... ... Stray Current Control Provisions for DC Transit Systems 57 4.2.3 Stray Current Collection There are situations in which mitigation measures must be augmented by the use of collection systems (like steel collection mats) Read the entire page →
From page 58... ... 58 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Conclusion Use of the previously mentioned SCC techniques varies on a case-by-case basis and their individual or combined use largely depends on the environment and geographical location of the tracks within the transit system. Following are various other methods or techniques that are used as stand-alone or in combination with each other to achieve SCC: • Diode earthed and solidly earthed schemes, • Grounded systems and substations, • Insulating switch machines at the switch rods, • Properly insulating switch blower machines and their ducting, • Insulating the impedance bond tap connections from the housing case, • Placing substations near points of maximum train acceleration, • Maintaining electrical continuity in tunnel liners and reinforcing steel, • Epoxy coated reinforcement (not common) Read the entire page →
From page 59... ... Stray Current Control Provisions for DC Transit Systems 59 The key fundamental maintenance essentials for ballasted and embedded track system must include the following: • Maintain the ballast at a minimum of 1 in. below the bottom of the rails (preferably 2 in.) Read the entire page →
From page 60... ... 60 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook the corrosion issues caused by stray current early on and then helping to mitigate those corrosion problems based on the data gathered from such testing. • All structures that are to be electrically continuous should be tested for electrical continuity, compared with theoretically based criteria, to validate that they meet or exceed the accepted criteria. Read the entire page →
From page 61... ... Stray Current Control Provisions for DC Transit Systems 61 Track Slab Current Measurement or Ground Current Survey Current flow in the track slab provides an insight into the magnitude and direction of the possible current leaking from the rails into the earth. Many transit agencies consider track slab current measurement as the most effective test to evaluate the current leakage, where the top layer of reinforcing steel is welded to make it electrically continuous. Read the entire page →
From page 62... ... 62 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook 500 ohms/1,000 track feet for single track has been made known to be an achievable and reasonable value based on the literature review. This value varies for track laid on timber tie-stone ballast, concrete tie-stone ballast, embedded rail boot, and direct fixation tracks. Read the entire page →
From page 63... ... Stray Current Control Provisions for DC Transit Systems 63 4.4 Criteria Document A thorough criteria document with corrosion control design should be prepared and used as a guide for implementing the design and maintenance requirements for stray current corrosion control and stray current mitigation systems. The document should include the procedures and evaluation criteria for rail-to-earth resistance, stray current corrosion control design, and stray current testing and maintenance guidelines. Read the entire page →
From page 64... ... 64 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook affected systems are street railways or trolleys, the areas in which the railways were built were also most likely to have underground metallic structures like utility piping, thus making it necessary to have stray current leakage control. Although stray current corrosion is more of an issue in embedded tracks and tracks with low soil resistivity, it is a concern for all kinds of track and needs to be addressed during the design, construction, and maintenance of DC-powered rail transit systems. Read the entire page →

From page 47...

... 47 Key decision matrices associated with implementing, maintaining, and testing of SCC and safety control of rail-to-earth potentials were developed by using the information collected from the literature review, questionnaires, data gathered during the transit agencies' and corrosion consultants' interview process, and stray current testing observations. Using these findings, proactive sequential steps are presented in the guidebook for stray current isolation and quality control.