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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 agenciesâ and corrosion consultantsâ interview process, and stray current testing observations. Using these findings, pro- active sequential steps are presented in the guidebook for stray current isolation and quality control. These steps include measures that need to be taken at the inception of design, at pre- construction, at construction, and at postconstruction, leading into the maintenance and testing program phase during the revenue service of the transit system. These recommendations, if followed, will assure achievement of uniform stray current isolation and quality control for a DC-powered transit system. ⢠Design essentials â Baseline survey, â Traction power model, and â Track design. ⢠SCC â Control at source and isolation techniques, â Mitigation, and â Stray current collection. ⢠Maintenance and testing program â Coordination and communication, â Maintenance, and â Testing. ⢠Design criteria document for the transit agency 4.1 Design Essentials 4.1.1 BaseLine Survey A baseline survey from the inception of the transit system plays an essential role in the design of the rail transit system and helps develop a proper model for the operations stage. The baseline survey is an integral part of the initial design for corrosion control and consists of the following important parts: ⢠Soil corrosion characteristics: these include resistivity tests, pH tests, sulfate content, and chloride content tests. Soil corrosion characteristics like resistivity, pH, sulfate content, and chloride content determine CP needs, cement types for concrete, coatings for structures, and ground bed and grounding grid design. C H A P T E R 4 Stray Current Control Provisions for DC Transit Systems
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. ⢠Utility location survey and coordination: this includes voltage potential collection on existing utility structures, initiating the line of communication with utility owners, and actual physical survey of the nearby utilities. Existing stray current activity that may already be present from various other sources of DC is indicated by measuring voltage potentials on utility structures. ⢠Surrounding infrastructure: this includes checking for grounded connections for any metallic infrastructure near the rail line. This also includes conducting initial surveys to find out the utility and other metallic infrastructure near the rail line. ⢠Education and participation: this includes educating the relevant transit agency staff and other key stakeholders on stray current corrosion and reaching out to local corrosion societies. ⢠Risk matrix: this includes developing a risk matrix for existing and potential corrosion issues Most of the data gathering of the baseline survey elements involves surveying the surrounding infrastructure, educating the transit agency staff, and coordinating with utility owners. Soil resistivity and its testing are important components of the baseline survey that require special attention and assist in identifying the track-to-earth resistance for the transit system. The impor- tance of identifying and maintaining the right track-to-earth resistance for the control of stray current is emphasized in detail throughout this guidebook. It is good transit industry practice to perform track-to-earth resistance testing as part of the baseline survey. The testing can be performed during and after construction is complete and before the revenue service starts. This process not only helps in setting up the pre-operation baseline characteristics of the system but also aids in setting up the conformance criteria. Due to the fast-track nature and budgetary constraints of DC rail transit projects, this step is mostly skipped prior to pre-revenue operation. This is unfortunate in that a solid baseline can only be established when trains are idle and the trackway is pristine. If this step is skipped before revenue service, the only way to test and establish some modicum of a baseline is during revenue service on a thoroughly cleaned and dry track. Revenue service creates dynamic condi- tions that render confirmation of compliance challenging and thus testing in such conditions has its drawbacks. Additionally, stray current leakage control has been found to be difficult to achieve after a system has been in service for a few years. As with any other design or construction project, irrespective of size, a baseline survey (focused on stray current in this context) is the foremost imperative step in the data collection and fact-finding process for a transit system. Defining the design criteria for stray current mitiga- tion and monitoring and testing for an LRT/HRT design project is equally important. However, without the baseline survey data, there are no source data or findings with which to compare the testing results. Soil Resistivity The measurement of soil resistivity along the ROW of any transit system is essential for the corrosion control study. The soil resistivity measurements are integral in many aspects in many areas of the transit system design, including grounding, corrosivity to the underground infra- structure, and the design of the required track-to-earth resistance. Therefore, the study should include closely spaced locations along the entire ROW as well as locations of intended TPSs. The measurement procedures should follow these industry standards: ⢠ASTM G51-77âfor pH of soil for use in corrosion testing. ⢠ASTM G57âStandard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method.
Stray Current Control Provisions for DC Transit Systems 49 ⢠ASTM D1557âfor moisture density relations of soils and soil aggregate, mixtures using 10-lb rammer and 18-in. drop. ⢠ASTM G165âfor rail-to-earth resistance measurement. ⢠NACE (53). ⢠IEEE Standard 81-1983âIEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System. ⢠BS 7430:1998âCode of Practice for Earthing. The resistivity of soil varies widely throughout the United States and changes significantly within small areas. Resistivity is also affected by the moisture content of the soil and by the chemical composition and concentration of salts dissolved in the contained water. The influence of seasonal moisture depends on the background characteristic of the top soil layer, the resistivity of the deeper layers, and the grounding topography. A typical soil resistivity study would include measurement locations at 500-foot spacings and at depths of 2.5 feet, 5.0 feet, 7.5 feet, 10 feet, and 15 feet. A Barnes layer analysis (53) of the soils will provide key information on the stratification of the soils, which aids in the evaluation of soil classification along the ROW. The calculated Barnes layer resistivities should be statistically analyzed using a probability distribution to determine the overall soil characteristics along the ROW or sections of the ROW. A value of soil resistivity at a given probability level should be selected to provide the design level for determining allowable earth potential gradient and stray current leakage from the rails. A design level between 80% and 90% probability is selected to cover a range of soils along the ROW. The selected soil resistivity level can determine the allowable earth potential gradient develop- ment over a given length from the rails. This theoretical determination is made to simulate the perpendicular impact on utilities along the ROW. The allowable stray current level is deter- mined by selecting an allowable earth potential gradient and performing the calculations for the current. The typical level of allowable earth potential gradient is about 75 millivolts over a 1,000-foot earth span perpendicular to the rails. Variations in this parameter simulate various conditions such as close proximity utility structures or long crossing or paralleling pipelines. The allowable stray current level determined in this phase of the design will be compared with the traction power load flow model to calculate the required track-to-earth resistance levels. Atmospheric Corrosion Characteristics Climatological conditions comprise gathering local weather data including, but not limited to, temperature, relative humidity, and precipitation. Air quality data of a local area constitute determination of local area pollutants and their concentration in comparison with the appro- priate local air quality standards. A matrix format and analysis in an interpretive report collate these data to validate the influence on corrosion of the rail infrastructure. Additionally, it is important to identify the location and source of existing areas with cor- rosion issues within the project boundary to document existing concerns and mitigation methods implemented to control corrosion. The contractor should identify the locations of the existing corrosion issues and prepare a matrix of the locations within the track ROW. Surrounding Infrastructure and Utility Location Survey and Coordination Maintaining effective communication with the local utility companies is also an important aspect of monitoring the performance of the SCC and mitigation system. In the case that a utility company has their own existing stray current monitoring systems or where such systems have been installed as a result of stray current mitigation activities, notifications by the utility companies that a stray current activity has occurred should be investigated and recorded.
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) measurements for all utility structures within the proposed project boundary, including the vicinity of maintenance and yard facilities, using a copperâcopper sulfate (Cu/CuSO4) half-cell reference electrode that serves as a reference point to ground. ⢠Record structure-to-earth potentials for at least 24 hours at 2-second intervals at each record- ing location. ⢠Record at least 2 minutes at 2-millisecond intervals to confirm if there is significant mains- frequency voltage or other higher frequency components present at each recording location. ⢠Record data over approximately equally spaced locations (300 feetâ500 feet) and at critical utility crossing locations within the proposed project boundary. ⢠Identify utilities that have cathodic protection and further designate as either impressed current or sacrificial anode cathodic protection systems. ⢠Coordinate with utility and pipeline owners to identify and agree on the exact location and connection point for each recording of potentials. ⢠Document all communications between the contractor and utility companies during the survey process on a regular basis. Education and Participation It is important to have trained corrosion control staff on the transit agency payroll. Transit agencies are aware that stray current is a serious issue, and it would benefit them greatly if they train their staff on the fundamentals of stray current control. This would not only help address any potential stray current issues early on but would also aid the transit agency in conducting early testing of rail track. Participation in the current local corrosion committees that exist or in any stakeholder engagements is recommended. This participation would help build awareness of stray current interference and their limits, should any exist, so that all transit agency testing activities and measurements can be compared against them and mitigations applied appropriately. Additionally, keeping a log of the corrosion issues caused by stray current and the money spent to mitigate those corrosion problems would be extremely beneficial to the rail industry in assessing the economic and logistic burden borne by rail transit agencies as a direct impact of stray current corrosion. Risk Matrix The following items should be further developed as part of the risk matrix during the design phases once baseline surveys have been carried out and the design is progressing, as this is the only point when the items can be ascertained: ⢠Detail all project-specific components both within and outside the project boundary that are at risk from stray currents from the DC transit system. Base this on survey results and means of protection for each asset and maintain a risk register that details all assets at risk from stray current. ⢠Identify any residual stray current. 4.1.2 Traction Power Model Designing the transit system requires understanding the transit system demand. Commer- cially available professional simulation models are used for the design of traction power.
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), signal performance, and schedule performance (54). The corrosion engineer uses this traction modeling data to calculate the stray current leakages, including track-to-earth resistance. This calculation requires early coordination between the traction power designer and the corrosion engineer. Using a static simulation package is also popular, incorporating worst-case conditions of vehicle acceleration and load current to evaluate stray current leakage. The package uses the typical worst-case locations (passenger stations) to evaluate the highest possible load currents based on train size and full acceleration. As the name suggests, this method pro- vides the worst-case analysis and will typically yield higher values of track-to-earth resistance requirements. The following protective measures should be studied during traction power design to ensure that stray currents are maintained within the acceptable range. TPS Spacing TPSs should be adequately spaced (preferably <1 mile apart) and provided with SCC devices to allow the connection of the negative bus to the station ground mat. The substation is arranged so that direct current does not flow into the substation structure earth. Risks from stray current relating to the earthing of equipment due to maintenance work is taken into account. The return bus bars in substations and similar installations are operated so that they are insulated from earth. When and if needed for safety reasons, a voltage-limiting device to connect between the return bus bar and earth is provided. It is preferable to implement a test facility to allow for periodic monitoring of the stray current return to identify changing conditions associated with the track-to-earth resistance. It is ideal to provide remote monitoring systems to record the negative bus-to-earth potential, negative return shunt, track-to-earth potentials, and the stray current return circuit. The remote monitor- ing system should consist of either a stand-alone data acquisition module and communications package or a supervisory control and data acquisition interface. Positive Distribution System The positive distribution system should be operated as an electrically continuous bus, with no breaks, except during emergency or fault conditions. Intentional electrical segregation of mainline, yard, and maintenance positive distribution systems should be the only type of segre- gation permitted. Overhead contact systems that consist primarily of support poles, the contact wire, and, where applicable, the messenger wire must be designed to minimize the generation of stray current. Third rail (conductor rail) should be on the side of the track away from the platform wherever practical. This applies to all cases except where a single track lies between two platforms. Mainline Negative Return System The mainline running rails should be the main medium of the negative return system and thus the longitudinal resistance of the running rails should be low. The mainline running railsâincluding special track work, grade crossings, and all ancillary system connectionsâ should be designed to have a minimum, uniformly distributed, in-service track-to-earth resistance. Appropriately designed insulating track fastening devicesâsuch as insulated tie plates, insulated rail clips, direct fixation fasteners, or other approved methodsâare to be in place.
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. In the case of conductor rail systems with third and fourth rails, each conductor rail is to be insulated from earth depending on the nominal voltage of the system. System Grounding It is recommended to insulate the traction supply and return circuit from earth and to prefer- ably design the traction supply and return circuit from earth as a non-earthed, floating system. Safe earthing and bonding measures in accordance with the relevant standards and requirements should be designed to minimize potential hazards to persons or damage to systems equipment arising from the operation of electrical systems. Touch Potential Step-and-touch potentials between the running rails and the adjacent ground or structures should be limited to 120 volts under normal circumstances (depot workshops should be limited to 60 volts). The system should be designed and installed such that this limit will not be exceeded during service operation as long as the degree of wear of the running rails remains within agreed limits. Performance of the equipment regarding stray current will, in the absence of direct evidence to the contrary, be acceptable so long as the touch potential, at all places on the system and at all times (including failure conditions), remains within 60 volts with respect to adjacent ground and structures. The agreed levels of rail-to-earth resistance are also to be maintained throughout the system. The design, however, should ensure that in normal operating conditions touch potentials lower than 60 volts are achieved. 4.1.3 Track Design Though both ballasted track and embedded track designs require equal consideration, the embedded track design necessitates a more complex level of electrical isolation as compared with ballasted track and thus demands careful design and an early contribution from the corrosion engineer. Following are some of the key elements of the track design, along with some recom- mended standards, that must be cross checked with the corrosion engineer at an early design level (during the traction power modeling design) to avoid potential short- and long-term stray current leakage issues. Rail Resistivity and Cross Bonding The longitudinal resistance of the running rails should be low. The longitudinal resistance can be reduced by the use of rails with greater cross section or cross bonding of the running rails or the tracks where signaling considerations allow. Rails should be continuously welded where practicable and sufficiently well bonded across any discontinuities such as expansion joints and fish-plated joints. Bonding should also be provided to ensure continuity across switches and crossings, at which insulated cable should be used where appropriate. There are exceptions. An exception is where block joints are needed, for which impedance bonds should be used at the ends of test lengths, where insulated rail joints shunted by removable bonds should be used; the interface between the storage and maintenance yards and the main line, where insulated rail joints should be used; and test track-isolating points, where suitable continuity bonds should be installed in a safe and reliable manner. The two rails of each track and, where practicable, the two tracks of each route should be cross bonded at regular intervals not exceeding 500 feet using insulated cable. It is preferred to pro- vide shunts at each substation to facilitate measurement of the current in the traction feed and return circuits from each route section served by the substation. It is also preferred to provide
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) that are mounted on or mechanically connected to the rails should be either electrically insulated from the rails, insulated from earth, or, where appropriate, provided by suitable insulating joints. This includes signaling equipment. Rail-to-Earth Resistance The insulation arrangements adopted for ballasted and embedded track should achieve a rail-to-earth resistance designated in the design criteria manual (based on the baseline survey) of the transit system after all construction works are complete, including surface finishing. The contractor should provide method statements for verifying the levels of rail-to-earth resistance achieved both on the test lengths and throughout the route, during the following stages. They are construction, postconstruction, testing and commissioning, and operation (revenue service). Rail and Track Insulation A high level of insulation from earth of the running rails and of the complete return circuit is required when the running rails are used as part of the return circuit. To reduce stray currents, no part of the return circuit should have a direct conductive connection to metallic installations, components, or metallic structures that are not insulated from earth. This insu- lation of the rails can also be achieved by using the following methods: ⢠Rail boot construction, ⢠Insulation of the entire trough that carries the rail, ⢠Insulation of fasteners, and ⢠Plastic, concrete, composite, or wooden ties. Isolation of Storage and Maintenance Yards Since the tracks in storage and maintenance yards are concentrated on a small area, no major voltage drop arises in these areas. It is recommended to electrically separate storage and mainte- nance yards from the running lines on both the supply and return circuits. On the supply side, each yard should be provided with a dedicated rectifier, preferably a dedicated TPS, to power movements within the yard and to meet the standing loads imposed by rolling stock. On the return side, insulated rail joints should achieve segregation. The positions of these joints should be coordinated with the positions of section breaks or overlaps in the overhead contact system, and both should be located such that the LRT system should not need to come to a stand bridging two electrical sections during foreseeable maneuvers. Exceptions are allowed provided a stray current study proves no negative effects can occur. Drainage at and Around the Tracks The track designer and then later the contractor should make every effort to secure and main- tain effective drainage of the track throughout the route. Specifically, rainwater, leakage water from water mains, and other sources should drain quickly away from the vicinity of the rails leaving no standing water in contact with the rails or metalwork connected to them. Return Cables Return cables connect the running rails with the TPS. The connections for the return current at substations and any other connections to the rails should be made using an insulated cable. It is preferred that the design should provide at least one representative length for insulation test purposes in each type of track form. Such test lengths should form part of the normal running
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. The test lengths should be located to afford ready access to the system earth at the nearest TPS. Stray Current Leakage Path Control The design should be such that no stray current leakage paths form between structural units, between the structure and piped services, handrails, and other metallic components located along the track. Electrical insulation from the transit or depot structures should be required for the following installations located along the tracks: ⢠Signaling equipment or their supports, ⢠Metal pipes, ⢠Lightning protection system to bridges, ⢠Earthing cables, and ⢠Sectionalizing switches, high-speed circuit breakers, and their supports. Bonding of External or Nonrailway Adjacent Structures Critical metal structures that are not part of the transit system but that have railway safety implications should be suitably insulated or should be bonded to the traction return system via spillover devices. The number of such devices should be kept to a minimum and consistent with the required electrical performance. Aerial track structure (bridges) or elevated structures should be designed to provide stray current isolation, rail insulation, effective drainage, and proper grounding, and should achieve the suggested rail-to-earth resistance. The implementation of SCC on bridges should follow the requirements described below. ⢠Provisions should be made in the reinforcing or steelwork to provide electrical conductivity in the bridge deck, parallel to and under the track. ⢠All longitudinal bars in the top layer of reinforcement should be tack welded at all overlaps to ensure electrical continuity and to achieve low resistance joints. ⢠Structural deck members should be electrically insulated from support piers and abutments. ⢠Any metallic handrail, anchor bolts for bridge bearing, fascia units, walkway, and the like along the bridge should be electrically insulated from the steel reinforcement in bridge beams and cross heads. The stray current design should incorporate a suitable cable route between the connection points on the bridge deck and the points of connection to the running rails. All such connections should be made by means of insulated bolted terminals. Underground track structures (tunnels) requirements are the following: ⢠Reinforcing steel in underground track structure inverts should be made electrically continuous. ⢠Provisions should be made in the steelwork to provide electrical conductivity, parallel to and under the track. ⢠Steel liner tunnel construction should be reviewed to determine the need for special measures, such as increased liner thickness, external coating systems, or CP. ⢠Evaluation of increased corrosion control measures should be based on the corrosivity of the local soils.
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). Utilities ⢠Utility pipes and cables on the underground section of the railway, together with their fixings, should be electrically isolated from any structural reinforcing or metalwork that may carry stray current or be raised in potential by the flow of stray current, or where the pipe or cable may conduct stray current into the structural metalwork. ⢠All nonrailway metallic utility pipes passing through or embedded under the track should be insulated from the structure by a plastic (nonconductive) sleeve. ⢠All connections from external utilities to the underground section of the track, including pipes, sheaths for power cables, communications cables, and earth systems should be electrically insulated from the transit system structures and systems. ⢠Corrosion control requirements for buried utilities installed by the utility owner as part of transit construction should be the responsibility of the individual utility owner. ⢠The electrical continuity of utility structures (e.g., duct banks or steel casings) is essential. The requirements for determining the proper electrical characteristics of these structures should be incorporated into the design of the structure. 4.2 Stray Current Control Based on the literature review and verification through actual survey and testing of selected transit agencies in earlier sections of this guidebook, it is evident that SCC starts with the notion of âcontrol at source.â Mitigation of the stray current, the collection of stray current leakage, and then finally the ongoing planned maintenance and testing of the tracks should subsequently follow. The following sections further emphasize the measures adopted and discussed previ- ously for SCC followed by mitigation and collection techniques. 4.2.1 Control at Source and Isolation Techniques ⢠Floating or ungrounded system, ⢠Designing traction supply circuits with low resistance, ⢠Designing traction return circuits with high resistance, ⢠Increasing the cross-sectional area/size of the rail (90â120 pounds, 40â80 mΩ/km), ⢠Maintaining a continuous electrical path for the negative current by using continuously welded rails, ⢠Using frequent cross bonding (250â500 feet), ⢠Using substation spacing (<1 mile or between >1 mile and <2 miles), ⢠Applying coatings to the affected structure to reduce the overall level of stray currents in the structure due to an increase of the structure-to-soil resistance. Coatings must have established performance records for the intended service and be compatible with the base metal to which they are applied: â Coating the rail trough of the embedded rail with a dielectric insulating material to act as a barricade to other connecting materials. â Coating of the rail surface with a dielectric insulating material (epoxies such as coal tarâ special cases only). A number of precautions are typically followed if epoxy coated rail (ECR) is used for SCC. Care is taken during placement of the rails to minimize coating damage, because ECR will corrode at an accelerated rate at epoxy coating flaws (holidays).
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. ⢠Isolating other structures so there is no unintentional direct metal contact with stray current sources or other metal structures: â Insulated fastener clips, â Insulating direct fixation fasteners, â Maintaining the ballast at a minimum of 1 in. below the bottom of the rails and preferably at 2 in., â Using elastomeric rail boot, â Filling the entire trough of the embedded rail with dielectric polyurethane or a combina- tion of other suitable material (like cork or polyurethane), and â Insulating the anchor bolts that penetrate beyond the insulated rail trough ⢠Providing metal or fiber reinforced U-shaped boxes for the rail trough with cork spacers to align the rail and fill the gaps in the trough. ⢠Using high resistivity concrete mixes with mineral admixtures (chloride freeâthis can also be part of a mitigation measure). ⢠Using corrosion-resistance steel or reinforcing steel with improved corrosion resistance, such as stainless steel and galvanized steel. However, they are not typically considered adequate protection against stray current-induced corrosion. Galvanized steel can only provide corrosion protection to the base steel for a limited time. Precautions should be taken so that no conductor rail will connect to earth so as to cause impermissible touch voltages on the running rails or cause a risk of fire or thermal damage to equipment. 4.2.2 Mitigation CP System or Sacrificial Anode Although insulation of the metal (utility pipe) if perfectly applied and maintained gives decent protection against stray current corrosion, defects in insulation are unavoidable. Because of the defects, the current tends to be concentrated in the small defect area where corrosion can take place at a relatively high rate. Therefore, it is preferable to supplement the insulation with the CP of the system. For a steel structure or pipe, it is preferable to keep the potential between 0.85 volts and 2 volts negative to soil; damage such as hydrogen embrittlement or disbonding of insulation may take place at higher negative voltages. Drainage Bonds In the case of anodic interference, a drainage bond between the structures may be considered to limit the positive potential shift to within the limits. If necessary, a resistor may be included to restrict the current flow. This option is implemented in rare scenarios when other alternatives cannot be used, and extra care should be used during the design. Impressed Current CP System Impressed current CP has been applied to most types of reinforced concrete structures in all types of conditions. CP applied to an RC structure can become the source of stray current to another part of the structure or to another structure in the vicinity, and thus needs to be carefully analyzed before implementing.
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). For example, in high-traffic urban areas and areas where utilities and other metal structures are more concentrated, it is recommended to increase the rail-to-earth resistance by providing secondary measures to overcome the rail boot defects. These collection mats are used to intercept and retain stray current for embedded track sections that are laid on concrete slabs with steel reinforcement such as tunnels and viaduct. In such instances, a stray current collection mat in the concrete below the tracks provides a low resistance path to intercept and retain the stray current leaving the rails. These collection mats must be continuously bonded together along their length to provide the stray current with a low resistance path. Insulated cables are provided between the mesh and the respective traction substation to offer a controlled path for the return of the stray current from the mesh to the negative bus of the traction substation instead of the alternate paths through earth. These insulated cables, usually copper, should directly connect to the mat at a regular interval of 300 to 1,000 feet (100 to 300 m) and carry the current to the point where it re-enters the substation or the running rail. This alleviates corrosion damage to supporting and third party infrastructure. The stray current collection system should fundamentally provide electrical continuity in the reinforcement, with drain-off cables to a stray current collection cable. These cables should be electrically continuous and looped into the substations for monitoring purposes. This provision should apply to all track forms within the areas of stray current leakage. In the areas other than the ones previously mentioned, the designer may choose to provide a reinforced concrete track slab, in which case a stray current collection system should also be provided. A recommended method for providing the required electrical performance is to weld together sufficient lengths of the reinforcing rods of the concrete slab, principally parallel to but also at right angles to the track, to form a mesh with a conductivity parallel to the rails. Alternatively, a recommended method is to incorporate a proprietary preformed steel mesh into the track slab, provided it can be demonstrated to have equivalent electrical characteristics. An insulated stray current collector cable per route, having a conductivity at least equal to that of a 70 mm2 copper conductor, should be run over all parts of the route containing a stray current collection system. A dedicated duct should be provided for the stray current collection cable wherever a stray current collection mat is to be provided. Connections to the stray current collection cable should be made at intervals not greater than every 1,000 feet (300 m) in a roadside watertight recess. The design of the recesses should allow easy access for monitoring and should where practical be located outside the swept path of the LRT. During the design of the collection mat, it must be assumed that all the current will transfer from the mat, which is located directly under the rails, to the collector cables and then to the substation. The stray collection mats are generally recommended where stray current leak- age is considered to be high. Extremely high efficiencies can be achieved when the material surrounding the stray current collection system is highly resistive. At low soil resistivity, a stray current collection system with a high efficiency is more challenging to achieve. In such cases, it may be more economical to consider other ways to reduce the stray current level at the source, such as insulating the entire trough that carries the rails or in combination with high resistivity concrete. Alternatively, for low resistivity soils it may be efficient to place the rail in the rubber boot, fill the entire trough, and insulate the anchor bolts that penetrate beyond the rail trough. A detailed analysis and design are required to control the impact of stray currents on these structures.
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 combi- nation 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), and ⢠Providing isolation transformers to supply traffic signals, roadway lighting, and light rail stops near light rail tracks. 4.3 Maintenance and Testing Program It is important to conduct regular inspections and testing of the tracks, including any mitigation techniques installed by the transit system, to ascertain that the stray current leakage is within limits and the mitigation measures are operational as designed. Coordination and communication with utilities, other infrastructure owners, and potential stakeholders in the vicinity is an important component to the success of a robust maintenance and testing program. 4.3.1 Coordination and Communication The development of a coordinated effort to sustain effective SCC requires education, communication, and cooperation of all stakeholders and concerned parties. Communication is the foundation of the effort to achieve and maintain effective SCC. Thus, it is essential to maintain regular exchange of information between the interested parties to develop an overall sustainability of effective and efficient SCC. As presented in earlier sections of the report, some of the agencies carry out pipe-to-soil potential measurement on the utility lines that cross their tracks and so do the utility owners. This redundant testing is a costly and inefficient duplication of efforts that can be avoided if such entities would coordinate and collaborate beforehand to ascertain that one consultant is hired to conduct the testing and implement subsequent mitigation measures, if needed. 4.3.2 Maintenance and Monitoring For ballasted tracks it is important to maintain the ballast at a minimum of 1 in. below the bottom of the rails (preferably at 2 in.), which is difficult to maintain at all times. Considering the operation frequency of the trains, a scheduled maintenance plan is vital to maintain the ballast levels. For embedded tracks, the rubber boot is the most widely used, cost-effective, and efficient mitigation method. The rail rubber boot, if installed and maintained properly, typically does not require any costly modifications. However, rail rubber boots inevitably end up getting damaged due to heavy wear and tear in urban surroundings coupled with the periodic upkeep required for track systems. Such unpreventable wear and tear warrants regular maintenance to be an essential element of the SCC regime.
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.). ⢠Maintain rail isolation from all metal objects. ⢠Maintain clean and dry tracks (control vegetation and sweep away the dirt and debris). ⢠Perform regularly scheduled visual inspection of the tracks. ⢠Maintain continuous welded rail and avoid rails cracks and gaps at rail joints. ⢠Check for voids or loose connections at the boot sleeves (where boot overlaps). ⢠Maintain proper drainage around the rail boot and the tracks. ⢠Perform regularly scheduled testing of the tracks. Figure 23 shows an example of maintenance work on a local LRT track. Here the rubber rail boot has been removed from a section of the track to allow room for rail lubrication equip- ment to help reduce wear of the rail on curves. In this particular example, since the isolation of the track was compromised due to the removal of the rubber boot, thus a polyurethane compound (Iso-flex) was used to provide the required nonconductive membrane between the rail and the ground (55). Because of the higher cost of this polyurethane compound compared with the rail boot, it is only used to repair smaller sections and where it is difficult to reinstall the rail boot. Maintenance can install a permanent monitoring system in a concrete structure for monitoring stray current interference. This system performs the following tasks: ⢠Verifies the extent of polarization effects on reinforcing steel bars caused by stray current interference. ⢠Follows the performance and effectiveness of preventive measures used, such as electrical isolation systems, and monitors possible modifications with time, during the lifetime of the structure. ⢠Allows performance of measurements at positions that are not easily accessible during normal service. 4.3.3 Testing A robust testing plan needs to be charted and then implemented to carry out the necessary upkeep of the tracks and the traction power system. Such a plan would benefit from first identifying Figure 23. Iso-flex replacing rail boot.
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 corro- sion 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. ⢠The transit agency and the utility operators should jointly determine the need for stray current monitoring facilities for utility structures. ⢠Test facilities may be installed at select locations to evaluate stray earth current effects during start-up and revenue operations. ⢠Testing facilities may be installed at all utility crossings with the system and on utilities that are within 300 feet (100 m) of the track and parallel to the system ROW. ⢠On completion of track work and before testing, commission, and revenue service of the traction supply system, the initial reference electrical condition on all facilities fitted with monitoring terminals, both on and off the railway, should be measured. ⢠Ongoing testing and monitoring of stray current and rail insulation, conductance, or potential should be conducted. ⢠Effective faultfinding methods should be incorporated and the technical specifications of instruments and equipment used to locate stray current leakage paths should be specified. The recommendation, based on the research from the literature review and from feedback from transit agency personnel, is to conduct the testing of the entire transit system at least once every 3 years on newer systems and once every 1 to 2 years on older systems. The following tests are recommended based on the type, size, and physical environment of the system: ⢠Visual inspections, ⢠Structure or utility pipe-to-soil potential measurement, ⢠Track slab current measurement (ground current survey), ⢠Track-to-earth resistance survey, and ⢠Audio frequency signal tracing (where required). Visual Inspection Visual inspections may be conducted to identify any uncharacteristic structure item or impact from other miscellaneous factors at each special track elements like bathtub, rail lubricators, switches, curves, rail and boot joints, traffic intersections, and along the entire track. These inspections help to identify concrete curb joining in the bathtub membranes, broken or miss- ing rail clip isolation, and rail boot or polymer separations along rail lubricators. If required, conduct visual inspections in conjunction with physical testing of the tracks. Structure or Utility Pipe-to-Soil Potential Measurement These tests are conducted to specify whether the structure or pipe is influenced by the stray current and if the current is leaving or entering the pipe. Negative potential denotes current pickup by the pipe whereas positive potential is indicative of current discharge. Table 9 provides the basis for further testing recommendations. Potential Shift (mV) Stray Current Influence Category and Remedy < 25 (N) Negligible 25â75 (L) Lowâno further evaluation recommended 75â150 (M) Moderateâfurther evaluation recommended based on structure and protection levels >150 (H) Highâfurther evaluation recommended Table 9. Recommended potential shift limits.
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. Track slab current testing or electrical continuity testing should be performed by impressing a test current across a structure span and measuring the voltage drop caused by the test current. The measured resistance of the structure span is compared with a theoretical calculation of the structure span. Typical acceptance criteria for the field-measured resistance range from a maximum of approximately 110% to 120% of the theoretical value. Electrical continuity testing may be performed before concrete placement, after concrete placement for reinforced concrete structures identified for electrical continuity bonding, and as part of the maintenance testing during revenue service. Electrical continuity testing should also be performed before and after backfilling for pipelines that require electrical continuity bonding. Track-to-Earth Resistance Survey The track-to-earth resistance of the running rails is the primary barrier for the control of stray current discharge from the negative system and should be the primary construction acceptance test addressed by the transit agency. Testing to meet compliance with the established acceptance criteria may be performed as construction progresses to identify and correct deficiencies in an efficient and cost-effective manner. The track-to-earth resistance test procedure depends on the track configuration. The testing procedures should generally follow the industry standard ASTM G165 but may be adjusted in some cases to suit field conditions. The basis of the test procedure is to apply a DC test current between the track section under test and a remote earth ground. The resistance values for each test point are averaged and normalized to a 1000-foot (300-m) length of track for evaluation of the criteria. The rail-to-earth resistance achieved by each test section should be tested in dry conditions on completion of construction (concrete pour) and before revenue service and compared with the initial results. Rail-to-earth resistance measurements require sections of track to be electrically isolated and cannot be performed during train traffic. Transit systems with small windows of time without train traffic may not have the resources to disconnect all bonds, track circuit leads, return cables, test, and then restore the system. For embedded track, assurance of the track-to-earth resistance should be tested before the track is embedded. Once the rails are embedded in concrete or insulating compound it will be more difficult and costly to find and rectify any issue with the track insulation resistance. This testing is conducted by impressing a test current with the help of 12 volt batteries at one location along the rails and measuring the track-to-earth voltage shift and then returning current spans along the rail at specified locations along the ROW. Measurements are recorded over 24 hours to document the transit agency peak and off-peak periods taken between the pipe or structure and earth, between the pipe or structure and rails, and between the negative bus and earth. The testing equipment requires multimeters, preferably two 12 volt DC batteries, an automatic timer, and cables to run between the testing stations. The main reason for this resistance test is to locate and remove any track work discontinuity and to document the long- term variations in the resistance values. Each transit system requires different track-to-earth resistances fluctuating from 1 ohm/1,000 track feet to 1,000 ohms/1,000 track feet. Track-to-earth resistance value of
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. In the case of new transit systems where insulation pads are used under the rails with concrete and wood ties, values of 500 ohms/1,000 track feet or greater are assumed. Track-to-earth resistance values of embedded tracks are generally less than the ballasted track construction and range from 100 ohms to 250 ohms/1,000 track feet. Track-to-earth resistance for direct fixation tracks is higher, ranging from 500 ohms to 1,000 ohms/1,000 track feet and may be higher. However, all these values are merely recommendations and must be calculated based on the results of the baseline survey. Monitoring of Rail Potential It is difficult to insulate completely the rail, and the rail-to-ground resistance will drop on the rail with time thus making it important to detect insulation deficiencies early. This early detection is a necessary measure to prevent rail potential drop and is accomplished by continu- ously monitoring the rail potential at dedicated locations like substations and passenger stations. A change in the average potential is compared with the value of the reference situation for that system. The change in the maximum and minimum potential along the rail line will indicate the changes in the rail insulation. Stray current monitoring data acquisition systems from different vendors are available in the market and can be installed at dedicated locations to measure the potential drop repeatedly. An average of 24-hour time interval readings are recorded to counter the vary- ing traffic loads throughout the day. This method works continuously and does not affect train traffic. Audio Frequency Signal Tracing Where Needed An audio frequency detector is used to locate discontinuities in the electrical circuit. The equipment includes an oscillator that converts low voltage (12 volts) DC from the battery to a stable audio frequency AC and a receiver that employs an integrated circuit amplifier. The oscillator is connected to the battery and to the rail whereas the tester walks with the receiver along the rail to locate the discontinuities. The voltage suddenly drops to a low or null level, where there is a short or discontinuity identified along the traverser distance. This is assumed to be a point directly above the contact. In areas where there may be a complicated network of continuous structures, it is difficult to pinpoint specific locations and other methods may need to be employed. Audio frequency signal tracing results are used in conjunction with the track-to-earth resis- tance data to pinpoint local low-resistance areas requiring further investigation. These tests are conducted by impressing a 750-hertz signal onto the rail in various configurations and measuring the signal strength along the rails. Figure 24 shows a typical audio frequency tester. In transit agencies that have a regular maintenance and testing plan along with correctly designed mitigation measures, it was observed that they have a better handle on stray current leakage. This makes the regular testing of the tracks an important aspect, and this cost should be included in the maintenance budgets for transit agencies. Only a handful of the transit agencies surveyed, questioned, and tested as part of this guidebook development perform such regular testing of the tracks. It was easily recognizable from the results of the survey questionnaire that those agencies that do perform such regular testing of the tracks have fewer stray current problems.
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. The design criteria document should summarize the baseline survey and be updated peri- odically to include stray current surveys. The document should highlight the implication of the baseline survey findings on the design of corrosion and stray current mitigation systems for the transit system and underground utilities that are installed or relocated as part of the LRT system construction. The design criteria document should discuss the site specific (geographical) conditions for the transit agency, identify necessary deviations from the design criteria require- ments due to site specific conditions, and provide the rationale for each deviation. The principles provided in the design criteria document are only for guidance. 4.5 Chapter Summary Assessment of the potential corrosion resulting from stray current should be an integral part of the planning and design process at the inception of all projects. Additionally, the testing of stray current corrosion must continue through the course of revenue service. Transit agencies are aware that stray current is a serious issue, and it would benefit them greatly if they train their staff on the fundamentals of stray current control. This would not only help address any potential stray current issues proactively but would also aid the transit agency in conducting early testing of rail track. Many transit agencies do not maintain a log for stray current corrosion issues and the money spent to mitigate those corrosion problems. This kind of tracking would prove beneficial to the rail industry in assessing the economic and logistic burdens borne by the transit agencies as a direct impact of stray current corrosion. Stray current issues have been around since the first electric railways were placed into operation and can create safety hazards and have serious effects on utility structures and the transit infrastructure. Since most of the heavily Figure 24. Audio frequency test setup.
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 neces- sary 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. For embedded tracks, the design must have an electrical barrier to insulate the rail from the conductive parts having the potential of carrying the current to the earth. Even though bituminous asphalt and different mixes of concrete embedment have been used in the track design in combination with other epoxies, the rail boot has proved to be the most effective and cost-efficient SCC measure. Rail boots not only provide vibration isolation but they also buffer the rail and its supports from the surrounding structure, thereby providing resistivity to fugitive stray current. Thus, the rail boot not only protects the rail but also protects the surrounding infrastructure from corrosion. Some key benefits of the rail boot are as follows: ⢠Quick and easy installation without the need for specialized technical crews. ⢠Rail is completely electrically isolated. ⢠Air-borne and ground-borne noise reduction. ⢠Galvanic corrosion of rail foot near embedded steel structures and utilities is avoided. ⢠Rubber boot track system is simple to construct. ⢠Minimal maintenance of paved track system as compared with other techniques. The inconsistent design of the extension of an existing system is a major contributing factor to stray current leakage. The exceptions are when the older track is electrically isolated from the new track or the older track was designed or improved to a similar or as stringent an SCC design as the new track. In conclusion, it would be easier to implement most of the preceding isolation, mitigation, and collection options on a newer transit system with proper foresight and planning. Not all the options and recommendations discussed in this section apply to older systems or systems that are building extensions to their existing systems. It is thus the responsibility of the design engineer or consultant, in conjunction with the transit agency, to design an SCC system tailored to each unique scenario. The key to achieve a leakage-free transit system is to follow the logical sequence of the design process and then maintain a stringent maintenance and testing regime.
