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Track Design Handbook for Light Rail Transit, Second Edition (2012)

Chapter: Chapter 8 - Corrosion Control

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Suggested Citation:"Chapter 8 - Corrosion Control." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 8 - Corrosion Control." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 8 - Corrosion Control." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 8 - Corrosion Control." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 8 - Corrosion Control." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 8 - Corrosion Control." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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8-i Chapter 8—Corrosion Control Table of Contents 8.1 INTRODUCTION 8-1  8.1.1 A Primer for LRT Stray Current 8-1  8.1.2 The Interdisciplinary Corrosion Control Team 8-3  8.2 TRANSIT STRAY CURRENT 8-4  8.2.1 Stray Current Circuitry 8-4  8.2.2 Stray Current Effects 8-4  8.2.3 Historical Background 8-5 8.2.4 Design Protection Components 8-6  8.2.4.1 Traction Power 8-6  8.2.4.2 Track and Structure Bonding 8-6  8.2.4.3 Drain Cables 8-6  8.2.4.4 Trackwork 8-7  8.2.4.5 AC versus DC Considerations 8-7  8.2.4.6 Effect of Grounding and Bonding on Corrosion Control 8-8  8.3 TRACK ALIGNMENT AND TPSS LOCATION FACTORS 8-9  8.4 TRACKWORK DESIGN 8-10  8.4.1 Rail Continuity 8-11  8.4.1.1 Rail Joint Bonding 8-11  8.4.1.2 Cross Bonding 8-11  8.4.2 Ballasted Track Materials 8-12  8.4.2.1 Concrete Cross Ties 8-12  8.4.2.2 Timber Cross Ties 8-12  8.4.2.3 Ballast 8-13  8.4.3 Embedded Track Issues 8-13  8.4.4 Direct Fixation Track Issues 8-13  8.4.5 Yard and Shop Track Issues 8-14  8.4.6 Track Appliances 8-14  8.4.6.1 Impedance Bonds 8-14  8.4.6.2 Switch Machines 8-15  8.4.6.3 End-of-Track Bumping Posts and Buffers 8-15  8.4.7 Stray Current Tests and Procedures 8-15  8.5 SUMMARY 8-16  8.6 REFERENCES 8-16 

8-1 CHAPTER 8—CORROSION CONTROL 8.1 INTRODUCTION Electrified rail transit systems, both light and heavy rail, typically utilize the track system as the negative side of the electrical circuit in the system’s traction power network. In light rail transit systems, the positive side, which carries direct current (DC) electrical current from the substation to the transit vehicle, is typically an overhead contact wire system or catenary. Because perfect electrical insulators do not exist, electrical currents will leak out of this circuit and escape into the soil to find the path of least resistance back to the substation. The amount of such stray currents will be inversely proportional to the efficacy of the electrical insulation provided and directly related to the conductivity of the soil and any alternative current paths back to the substation such as pipes, cables, reinforcing steel, etc. Where this current leaves a conductor to jump to another, corrosion occurs by electrolysis. In addition to stray current that originates at an electrified rail transit line, other sources of stray current can be found in virtually any metropolitan area. It is important to identify these so that the transit line isn’t blamed for all corrosion. For the same reason, regular monitoring of the structures for stray current and corrective maintenance of any leaks are both extremely important. While corrosion can occur for many reasons, stray current is responsible for a large proportion of the corrosion damage that occurs on electrified rail transit systems. Controlling that damage requires controlling the stray current at its source—the track structure. The track engineer, working together with other disciplines, obviously plays a key role in that mission. 8.1.1 A Primer for LRT Stray Current In 1967, Mars G. Fontana, a professor at The Ohio State University, published Corrosion Engineering, one of the seminal textbooks on corrosion. In it, he stated: The term stray current refers to extraneous direct currents in the earth. If a metallic object is placed in a strong current field, a potential difference develops across it and accelerated corrosion occurs at points where current leaves the object and enters the soil. Stray current problems were quite common in previous years due to current leakage from trolley tracks. Pipelines and tanks under tracks were rapidly corroded. However, since this type of transportation is now obsolete, stray currents from this source are no longer a problem. [1] That text was representative of its times since, by the mid-1960s, streetcars had been eliminated from all but a handful of North American cities. However, since then, “trolley tracks” have evolved into light rail transit (LRT) lines, reintroducing the possibility of stray currents from LRT operations and the attendant corrosion problems. Typically, unless a fault has occurred in an insulator, stray currents from the positive side of the light rail transit traction power circuit (e.g., the catenary) are minuscule. Stray currents from the track, on the other hand, are common and can get quite large due in no small part to the proximity of the track to the ground. Once in the soil, stray currents will follow any available conductor to

Track Design Handbook for Light Rail Transit, Second Edition 8-2 get back to the traction power substation. These paths can include the soil itself; buried utility pipelines and cables; and other metallic structures, such as bridges, along the way. If an alternative path offers less electrical resistance than another route, then the better conductor will carry proportionally more of the stray current. In extreme examples, particularly when the electrical continuity of the track structure is poor, more electricity will return as stray current than through the running rails. Some older elevated railway systems were actually designed for this occurrence. The problem with stray currents evolves from the fact that whenever electric current leaves a metallic conductor (i.e., a water pipe) and returns to the soil (perhaps because it is attracted to a nearby gas line), it causes corrosion on the surface of the conductor it is leaving. This is the same phenomenon that occurs when a metallic object is electroplated, such as when construction materials are zinc plated. In the case of transit system stray currents, the typical current path can involve several different conductors as the electricity wends its way back to the substation; therefore, corrosion can occur at multiple locations. This can create conditions that range from leaking water lines to gas line explosions. The rail itself will also corrode wherever the current jumps from it to reach the first alternative conductor. Structures along the transit line, particularly steel bridges and embedded reinforcing steel, are also at risk. Hence, multiple parties have an interest in controlling or eliminating the leakage of stray currents and minimizing the damage they inflict. Stray currents are common on a light rail transit system because its track structures are typically close to the ground. Grade crossings, embedded track, and fouled or muddy ballast are common locations for propagation of stray currents. Because of the maze of underground utility lines typically found in urban and suburban areas where light rail transit systems are built, abundant alternative electrical paths exist. Predicting the likely path of potential stray currents and defining methods to protect against them can be extremely complex. Because of this complexity, it is essential that the advice of a certified corrosion control specialist with stray current experience be sought from the beginning of design. One case study was conducted along a segment of an operating DC streetcar line where significant corrosion of a parallel water main was being experienced. The study evaluated the statistical relationship between rail volts and water main volts. The methodology involved the use of digital recording instruments and spreadsheet software to perform the required data acquisition and analysis. Under normal operating conditions, a correlation coefficient of 94% was calculated. A control measurement was obtained with the trolley line out of service that resulted in a correlation coefficient of only 3%, indicating that the operation of the railway had a significant impact on the corrosion of the water main. The old bolted joint rails, which were embedded in the street and over the centerline of the water main, were then replaced with continuously welded rail that was insulated with a rail boot. Post construction measurements showed that the correlation coefficient under normal operations had reduced approximately 10-fold—from 94% to 9%. Some of the principal measures that can be taken to minimize traction current leakage include the following: • If jointed track is used, install electrical bonding across the joints. One of the many advantages of continuous welded rail (CWR) is that it offers a superior traction power return.

Corrosion Control 8-3 • Insulate rails from their fastenings and encase rails in embedded track in an insulating material. • The steel reinforcement in the underlying concrete slab can be continuously welded / cross bonded. This will eliminate the electrical potentials that can develop between individual reinforcing steel bars and lead to corrosion. • In ballasted track areas, the ballast should be clean, non-conductive, well-drained, and not in contact with the rail. (This condition is often difficult to achieve and maintain in practice, particularly in the vicinity of highway grade crossings.) • Conduct corrosion surveys on susceptible metal structures both before commencing design of the system and again prior to the commencement of revenue service train operations. These surveys provide baseline information on any stray currents that pre-exist in the light rail system and benchmark the efficacy of the measures taken during design and construction. Subsequently, regularly scheduled monitoring for stray currents as part of a preventative maintenance program is also very important so that leakage problems can be identified and corrected before significant damage occurs. • Provide auxiliary conductors to improve and supplement the electrical capacity of the rail return system. This can be accomplished by cross bonding connections between all rails of the track(s) and/or by adding supplemental negative return cables. • If the bonded reinforcing steel mentioned above is bonded back to the negative side of the traction power substations, it can collect stray currents that get past the primary insulating system. (This is an extreme measure that should not be undertaken without extensive investigation of the alternatives.) Existing pipes and cables in the vicinity of the tracks must be investigated and protective action taken as necessary to protect them from stray current corrosion. Whether the light rail operator or the local utility takes responsibility, it is imperative that strategic action is undertaken to mitigate the effects of stray current corrosion in the design phase and during construction. This will avoid corrosion from becoming a costly and dangerous maintenance issue later. 8.1.2 The Interdisciplinary Corrosion Control Team An interdisciplinary approach is necessary to determine the required design for stray current mitigation. Recommendations are developed in coordination with all other rail system disciplines including track, traction power, signals, and communications systems, as well as facility infrastructure and systems such as civil, structural, electrical, mechanical, and plumbing. Corrosion control measures are needed for all those project elements, even though, at first glance, they seem entirely unrelated to traction power and stray traction power current. A few examples of the stray current problems that can arise due to the actions of other disciplines include the following: • Signals—“sneak paths” for stray current can occur on circuitry provided for broken rail detection.

Track Design Handbook for Light Rail Transit, Second Edition 8-4 • Track—poor drainage and the resultant fouling of ballast can bypass insulation measures and result in localized corrosion of rails and rail fastenings. • Communications—stray currents can sometimes occur over cable shields. 8.2 TRANSIT STRAY CURRENT 8.2.1 Stray Current Circuitry Traction power is normally supplied to light rail vehicles (LRVs) by a positive overhead contact wire system. The direct current is picked up by a vehicle pantograph to power the motor and then returns to the substation via the running rails, which become the negative part of the circuit. Unfortunately, a portion of the current strays from the running rails and flows onto parallel metallic structures such as reinforcing steel; utility pipes and cables; and other structures such as pilings, ground grids, and foundation reinforcing bars. 8.2.2 Stray Current Effects Corrosion of metallic structures is an electrochemical process that usually involves small amounts of direct current. It is an “electro” process because of the flow of electrical current. It is a “chemical” process because of the chemical reaction that occurs on the surface and corrodes the metal. Faraday’s Law relates the amount of metal lost in an electrolytic process to the electric current generated from that metal. For this reason, a unidirectional DC current along a buried utility line, such as a water line, will drive metal from the pipe when subjected to stray currents over a period of time. One ampere of direct current flowing for 1 year will corrode 20 pounds [9 kg] of iron, 46 pounds [21 kg] of copper, or 74 pounds [36 kg] of lead. Natural galvanic corrosion involves only milliamperes of current, so many buried structures can last several years or decades without structural distress or failure. Unlike the very small currents associated with galvanic corrosion, stray current corrosion from a transit system can involve the continuous flow of several hundred amperes. The same physical laws apply for corrosion of the metal, electron flow, chemical reactions, etc., but metal loss is much faster because of the larger amounts of current involved. For example, with 200 amperes of current discharging from an underground steel structure, 2 tons [1.8 tonnes] of metal will be corroded in 1 year: 20 pounds per ampere per year x 200 amperes = 4,000 pounds of steel corroded [9 kg per ampere per year x 200 amperes = 1,800 kg of steel corroded] If this current flow is concentrated at one location, structural failure can occur in a very short time period. Even if the leakage is spread out over a length of track, stray current from a light rail system will still aggressively corrode transit rails, rebar, and steel structural members and all adjacent underground metallic structures. While various methods are available for mitigating this issue, the most effective method is to control the current at its source, i.e., the surface of the rails. Faraday’s law cannot be applied to AC stray currents since they are not unidirectional and alternate between positive and negative polarities.

Corrosion Control 8-5 8.2.3 Historical Background The phenomenon of stray currents from electrified street railways was observed almost immediately when the first electric trolley lines were constructed in the 1880s. The importance of maintaining good electrical continuity of the rails was quickly recognized, and many trolley systems welded rail joints 60 years before the process was widely accepted on “steam” railroads. Where rails could not be welded, they were electrically bonded to each other with copper cables. These measures reduce stray currents, but cannot eliminate them. No matter how good a conductor the track system is, some portion of the traction power current will always seek an alternative path back to the substation. Utility companies fought this problem, both in the courts and in the field. Once the legal issues were resolved, the most effective means of minimizing stray current damage that was available in the late19th century was to make the buried utility network as electrically continuous as possible. Copper bonds were placed around joints in buried pipes and crossing utility lines were electrically bonded to each other. Finally, the entire utility network was directly connected to the negative bus of the traction power substation by “drain cables” so that any stray currents could return without causing significant corrosion along the way. All big city utility companies, plus the local streetcar company, participated in a “corrosion control committee” with the objective of ensuring that all new facilities were properly integrated into the system, thereby preserving the delicate balance of the network. (In many cities, a single holding company might own most of the utility companies and the trolley company as well; thus, such committees were not necessarily combative congregations.) Such methods were generally effective; however, a side effect of the improved underground electrical continuity was that the utility grid typically became better bonded than the track structure. As such, a significant portion of the traction power current would perversely elect to stray from the rails and use the buried utilities to get back to the substation. Where such currents left the track to seek the alternative paths, significant corrosion would occur at the base of the rails. It was not uncommon that track maintenance work would reveal that the base of rail had corroded away completely where it crossed above a particularly attractive utility line. When trolley systems were abandoned in most cities, the corrosion committees were disbanded and the utility companies became less zealous about bonding their networks. In many cases, the introduction of non-metallic piping created significant electrical discontinuities in utility systems. Such gaps were of no consequence in a city without a local originator of significant levels of stray currents and their associated corrosion issues. With no trolley network in the neighborhood, incidental corrosion potential could typically be neutralized using sacrificial anodes. However, if a light rail system is introduced or reintroduced into such a city, sacrificial anodes are insufficient as they could corrode away in an extremely short time. The result can be corrosion problems not unlike those that occurred 100 years ago, with stray currents leaping off metal pipes when they reach an electrical dead-end at a non-metallic conduit or cross another potentially attractive conductor. Reverting to the continuous utility bonding and drain cable methods of the past is typically neither a practical nor completely effective methodology of achieving stray current control. Because of the widespread use of non-metallic buried pipe and the subsequent high expense of recreating an

Track Design Handbook for Light Rail Transit, Second Edition 8-6 electrically continuous path through the utilities, it is typically much cheaper—and arguably easier—to attempt to effectively insulate the track structure from the ground so that stray currents are minimized from the beginning. Such insulation, coupled with other protective measures, including very selective bonding of utilities and drain cabling, is the foundation of stray current corrosion control measures on modern light rail transit systems. This controlled approach also protects rails and other transit structures that would be subjected to these stray currents. 8.2.4 Design Protection Components 8.2.4.1 Traction Power Since the 1960s, increased efforts to reduce stray currents have been made through modifications to traction power substations. Typical modern substations are either ungrounded, “floating” above ground potential, or are grounded only through diodes that prevent stray currents from passing from the ground to the negative bus. This frequently reduces stray currents from hundreds of amperes to near zero. Completely ungrounded systems exhibit the greatest improvement in stray current control. Nevertheless, stray currents are still possible in an ungrounded system as it is entirely possible for current to leak out of the track at one spot, travel along alternative paths in the ground, and then return to the track at another location. Since the track itself must eventually be directly connected to the negative substation bus, stray currents can still occur and cause damage en route back to the traction power substation (TPSS). 8.2.4.2 Track and Structure Bonding Achieving electrical continuity of the track structure is of paramount importance in keeping negative return current in the rails. The use of continuously welded rail, together with the installation of bonding cables around unavoidable bolted joints, provides most rail transit systems with an excellent current path through the rails. Stray current corrosion of transit structures can typically be controlled through electrical bonding. Since the 1960s, it has been common practice to also bond reinforcing steel in concrete structures so as to provide a continuous electrical path. The bonding is typically concentrated in reinforcing bars in the lowest portions of the structure and those surfaces in contact with the earth such as retaining walls. Bonded reinforcing steel networks can provide a shielding effect for outside utility structures. Many light rail systems have been built with heavily reinforced slabs beneath the track to provide both structural support and a barrier against migration of stray currents into the ground. 8.2.4.3 Drain Cables Stray current drain cables are common on legacy rail transit systems that predate the development of effective methods of electrically isolating the tracks. Drain cables are sometimes provided for future use on modern light rail systems, but are not necessarily connected to the utility system. Utility companies monitor their pipelines for any stray currents and, if problems are detected, the companies have the option of connecting to the drain cable as a last resort. Coupled with other protective systems, such cabling provides a secondary approach to corrosion protection in the event that the primary measures are ineffective at locations where excessive leakage from the rails occurs. However, since drain cables effectively decrease the resistance along a negative return path through the utility grid, their indiscriminate use can actually increase the amount of traction power current that leaves the rails. Therefore, drain cables should only be

Corrosion Control 8-7 used as a last resort and only after extensive investigation of the problems and alternative solutions. Even if utilities are not connected to them, drainage cables are often used to provide an electrically continuous path between metallic components of the transit infrastructure (such as reinforcing steel in bridges and tunnels) and the negative side of the traction power substation. These drain cables protect those items from stray current corrosion. As a secondary benefit, the drain cable can prevent build-up of an electrical charge on the protected item, which could otherwise be hazardous to employee safety. 8.2.4.4 Trackwork Ultimately, electrical insulation of the track structure offers the first line of defense against stray currents. Keeping the rails clean and dry is important, as are good insulators between the rail and the ties. Good storm water drainage is also critical since slowly moving runoff can deposit conductive sediments that can bypass insulators. Rails laid in street pavements need to have insulating coatings to maintain electrical isolation. Since track design is the focus of this Handbook, track insulation will be discussed in detail in Article 8.4. It must be emphasized, however, that providing track insulation alone is not a panacea, particularly if the track insulation systems are not regularly maintained and cleaned. If track insulation systems are compromised, for example by fouled ballast or dirty insulators, stray current leakage is inevitable. Thus, the required level of maintenance should be considered during design. 8.2.4.5 AC versus DC Considerations It is important to note the distinction between the effects of stray currents from alternating current (AC) versus direct current (DC) sources. Stray currents that are generated by the DC electric traction system may cause severe corrosion to metallic components of the infrastructure. It is for that reason that the return components of DC electric traction systems are properly isolated from the ground. Alternating currents, on the other hand, cause cyclic anodic and cathodic conditions to occur at the same point as the current alternates from positive polarity (which is characterized by the discharge of current via the loss of metal ions into the surrounding media and which constitutes an anodic site) to negative polarity (which is characterized by the pickup of current via the plating of metal ions from the soil and which constitutes a cathodic site). It can be stated that the metal is corroded on the anodic or positive half-cycle, when current is discharged as metal ions, and protected on the negative or cathodic half-cycle, when current is received as metal ions are plated back onto the metal substrate. It should be noted that the cathodic or negative half-cycle provides protection of the metal against corrosion since current is flowing into the metal rather than discharging from the metal as metal ions. This principle is referred to as cathodic protection and is a major component of corrosion control for metallic structures. While corrosion caused by AC current can occur due to the alternating positive and negative half-cycles, it is normally not significant in comparison with corrosion caused by DC current. It is therefore necessary for an LRT design team to know whether the measured values of stray currents detected during a baseline survey are in fact corrosive DC stray currents or AC ground currents from electric utility operations. This can have a significant impact on the prescribed mitigation measures and hence the project cost.

Track Design Handbook for Light Rail Transit, Second Edition 8-8 The identification of AC versus DC components of stray current should be considered in the determination of stray current mitigation requirements. It is not unusual for both DC traction power return currents and 60-Hz AC electric utility ground currents to be present in a given measurement of stray current activity. Sampling rates that are adequate to identify the various sources may be applied to identify the fundamental and harmonic frequencies of the AC components although the DC component, which is represented by the average value of the waveform, is of primary concern for purposes of stray current mitigation. Measurement techniques that do not permit the determination of the relative contributions of AC versus DC components can result in an incorrect quantification of DC stray current levels, falsely attributing a corrosive condition to AC stray current. This, in turn, could result in unwarranted recommendations for stray current mitigation and unnecessary cost to the project. It is, therefore, necessary that the LRT designer understand the relative contributions of AC versus DC components in order to specify corrosion control measures that are commensurate with the level of exposure. 8.2.4.6 Effect of Grounding and Bonding on Corrosion Control The grounding and bonding of the infrastructure is intended to minimize touch and step potentials by providing a low resistance for current flow under both normal and fault conditions and hence reduce the voltages to which personnel can be exposed to within tolerable limits. The unintentional flow of electric traction power return currents over non-current-carrying metallic components of the infrastructure is in conflict with the intent of the grounding and bonding requirements of publication NFPA 70: National Electrical Code (NEC). The NEC is published by the National Fire Protection Association (NFPA). Article 250.6(A) of the NEC states: The grounding of … normally non-current-carrying metal parts of equipment shall be installed and arranged in a manner that will prevent objectionable current.[6] The word “normally” in the quote above was added in the 2008 edition to acknowledge that fact that non-current-carrying metallic components that do not carry current under normal conditions may carry current under fault conditions until the fault is cleared. It should be noted that railroad systems are not within the scope of the NEC, as indicated in the following excerpt from Article 90.2(B) Not Covered(3): Installations [on] railways for generation, transformation, transmission, or distribution of power used exclusively for the operation of rolling stock or installations used exclusively for signaling and communications purposes.[6] Railroad traction power, signals, and communications systems are therefore not within the scope of the NEC. However, railroads typically invoke the NEC for various aspects of railroad installations and electrical installations at passenger stations, operations and maintenance facilities, control centers, and other facilities that represent places of assembly and occupancy classes where the safety of employees and the public is an issue. Since those facilities are normally grounded and bonded in accordance with NEC requirements, the flow of objectionable currents from the electric traction return system may occur and must be considered in the grounding and bonding plan.

Corrosion Control 8-9 It should be noted that the NEC is purely advisory from the perspective of the NFPA but may be invoked by an “Authority Having Jurisdiction” (AHJ), such as a local building code enforcement officer, as a legally required standard. The provisions for safeguarding of persons from hazards arising from the installation, operation, or maintenance of conductors and equipment in electric supply stations and overhead and underground electric supply and communications lines are defined in publication IEEE C2— National Electrical Safety Code (NESC), which is published by the Institute of Electrical and Electronic Engineers, Inc. (IEEE).[7] Grounding rules make up a significant portion of that standard. In 1970, the section of the NESC entitled “Rules for the Installation and Maintenance of Electric Utilization Equipment” was deleted since those rules are now largely covered by the National Electrical Code. The following points are noted: • The NESC covers utility facilities and functions up to the service point. • The NEC covers utilization wiring requirements beyond the service point. The NESC specifically includes railways, except for rolling stock, in its definition of utilities. The grounding requirements of the NESC are given in Section 9, “Grounding Methods for Electric Supply and Communications Facilities.” However, the scope of the grounding requirements specifically excludes both the grounded return of electric railway traction power currents and lightning protection wires that are normally independent of supply or communications wires or equipment. The grounding of static wires that serve the dual role of lightning protection for the OCS contact wire assembly and electric traction return are therefore not covered by the NESC. The NEC and NESC are voluntary standards but have been adopted in whole or in part by various states and local jurisdictional authorities. The determination regarding the legal status of the NEC and NESC in any particular state or locality is made by the AHJ. The LRT design team needs to identify the AHJ for their project and evaluate the impact of federal, state, and local laws. 8.3 TRACK ALIGNMENT AND TPSS LOCATION FACTORS LRT routes that are circuitous instead of generally straight can induce stray currents as well. This is because the current may leave the track at one bend in the route, cut across several city blocks using the utility grid, and then jump back onto the rails at another point. LRT route decisions are typically made during early planning and environmental studies, and there often will be no corrosion or traction power engineers on the project staff at that time. The track alignment engineer may be one of the few engineering professionals involved during planning studies. By notifying the project planners that certain routings might have a greater or lesser propensity to instigate stray current problems, the track alignment engineer can possibly save the project much grief and cost at a later date. Traction power engineers also play a key role in minimizing the propensity of return current to stray. Possibly the most important of these is the location and frequency of the traction power substations (TPSSs). It is highly desirable that each TPSS be located immediately adjacent to the route and generally no more than a mile apart along the route. TPSSs that are located at a

Track Design Handbook for Light Rail Transit, Second Edition 8-10 distance from the route of even a few city blocks can increase the propensity for stray current to take a “shortcut” back to the substation. This is one reason that TPSSs are preferably floating above ground potential—thereby forcing any stray current to return to the rails, as they will be the only route back to the substation. 8.4 TRACKWORK DESIGN LRT systems utilize direct current electrical power to propel the vehicles. This current arrives at the vehicle via the OCS and is normally returned from the LRV to the substations through the rails. While many modern light rail vehicles utilize AC motors, DC is still used for power distribution due to certain efficiencies. The DC power is inverted to create AC current on board the vehicle. However, DC power has a tendency to produce stray current over unintended paths, particularly on the track side of the system. Therefore, stray current control is a necessary element in the design of the track system. Modern designs for DC transit systems include the concept of “source control” at the rail surface to minimize the generation of stray currents. Electrical isolation of the rail using insulation is necessary for the protection of utility pipelines and steel structures along and near the LRT route.[2] In addition, if the track is shared by railroad freight traffic during non-revenue hours, insulated rail joints are required at all rail sidings and connections to adjacent rail facilities. The route of an LRT system is not generally within a totally dedicated right-of-way; therefore, the various types of track construction each require individual attention. The essence of state-of-the-art technology in the design of modern transit systems is the concept of controlling stray current at the rails. Operation of the traction power system with the substation negatives isolated from ground (floating) will result in a higher overall system-to-earth resistance. The goal is to maximize the conductivity of the rail return system and the electrical isolation between the rails and their support systems. The following are generally accepted design measures that the trackwork designer can use with various trackforms so as to create an electrically isolated rail system that controls stray currents at the source: • Continuous welded rail. • Rail bond jumpers at mechanical rail connections (especially special trackwork). • Insulating pads and clips on concrete cross ties. • Insulated rail fastening system for timber cross ties and switch timber. • A minimum separation of 1 inch [25 millimeters] between the bottom of the rail and the ballast on ballasted track. • Insulated direct fixation fasteners on concrete structures. • Electrical insulation measures, such as enclosing the rail in a rubber boot or using a dielectric coating material at all roadway and pedestrian crossings. • Special insulation materials for embedded rails, such as a rubber rail boot, an elastomeric grout, or surrounding the track slab in an insulating membrane.

Corrosion Control 8-11 • Cross bonding cables installed between the rails to maintain equal potentials of all rails and reduce resistance back to the substation. • Insulation of the impedance bond tap connections from the housing case. • Insulation of switch machines at the switch rods. • Installation of rail-insulated joints to isolate rail-mounted bumping posts. • Installation of insulated rail joints to isolate the main line from the yard and the yard from the usually grounded maintenance shop area. • Separate traction power substations to supply operating currents for the main line, yard, and shop. • Rail-insulated joints to isolate the main line rails from freight sidings or connections to other rail systems. 8.4.1 Rail Continuity Continuous welded rail is the generally accepted standard for main line light rail construction. CWR creates an electrically continuous negative return path to the substation, in addition to other well-known benefits. There are, however, additional measures that can be taken to maximize the conductivity of the track system, thereby providing traction power less reason to stray. 8.4.1.1 Rail Joint Bonding The rail configuration at special trackwork, turnouts, sharp curves, or crossovers may require jointed rails. Jumper cables, generally called “rail bonds,” permanently connected to the rail on either side of the bolted rail joint connections, ensure a continuous electrical path across the mechanical connections. Bond cables may be used to bypass complex special trackwork to provide continuity and also protect track maintenance workers from electrical shock when they are replacing special trackwork components. The use of jumpers must be carefully coordinated with the design of the signal system. See Chapter 10, Article 10.2.11 for additional discussion concerning rail bonding. 8.4.1.2 Cross Bonding Periodic cross bonding of the rails and parallel tracks provides equivalent rail-to-earth potentials for all rails along the system. Using all parallel rails to return current provides a lower negative return resistance to the substation, since the return circuit consists of multiple paths rather than individual rails. In “open” trackforms (e.g., ballasted or direct fixation track), where train control systems based on track circuits are employed, cross bonds are generally installed at impedance bond locations on rails to avoid interference with rail signal circuitry. Cross bonding is accomplished by attaching insulated cables to the rails. Both rails are connected in single-track locations, with all four rails cross bonded in double-track areas. The recommended method for cable connections to the rails will vary depending on the size of the cable. See Chapter 10, Article 10.2.11, for additional discussion on electrical bonding of rails for both traction power and signaling.

Track Design Handbook for Light Rail Transit, Second Edition 8-12 Cross bonding in embedded track sections requires an alternative design approach since the signaling system is not carried through the embedded track area. This is typically the case as most embedded track light rail systems run on “line-of-sight” operating rules coordinated with street traffic signal patterns. To provide cross bonding of embedded tracks, insulated conduits are generally installed between rails prior to installation of the concrete for the initial track slab. Insulated cables are attached to each rail to obtain electrical continuity. These rail connections are typically enclosed in a covered box attached to the side of the rails so that the connections can be inspected and repaired if necessary. These boxes must be accounted for in the track slab design, including both storm water drainage and electrical isolation. Smaller cables may be used to provide an easier turning radius to the rails in the rail trough zone and facilitate attachment of the cables to the rails in constrained spaces. It is common design practice to install the cables at 1,000-foot [305- meter] intervals throughout the embedded track zone, with one such location being directly opposite each substation. 8.4.2 Ballasted Track Materials The choice and details of materials used in track construction can have a significant effect on preventing stray current. 8.4.2.1 Concrete Cross Ties Concrete cross ties with an insulated rail seat (generally consisting of a rail pad and clip insulators) provide good rail isolation. The rail seat pad is generally fabricated of thermo-plastic rubber, ethyl vinyl acetate, or natural rubber. It is approximately ¼ to 5/8 inches [6 to 16 millimeters] thick and is formed to fit around the iron shoulders embedded in the concrete cross tie. The clip insulator is typically a glass-reinforced nylon material formed to sit between the rail base, the shoulder, and the elastic rail clip, electrically isolating each from the others. This assembly, plus a pad beneath the base of rail, electrically isolates the rail from the concrete tie. Insulating at the rail base is important because concrete cross ties, with their reinforcing steel, are not good insulators. 8.4.2.2 Timber Cross Ties While wood is generally a good insulating material, timber cross ties are only marginal insulators because they are treated with preservative chemicals and they absorb moisture as they age. While timber cross ties provide sufficient insulation against low-voltage, low-amperage signal system currents, they offer little resistance to high-voltage, high-amperage traction power current. Timber cross ties with insulating components at the fastening plate, as shown in Chapter 5 (Figure 5.4.2), can be used on main line track and at special trackwork turnouts and crossovers to reduce leakage. Electrical insulation can be achieved by inserting a polyethylene pad between the metal rail plate and the timber tie, installing an insulating collar thimble to electrically isolate the steel plate from the anchoring lag screw, and applying coal tar epoxy to the hole for the lag screw. The insulating pad and collar thimble afford insulation directly between the two materials. Coal tar epoxy applied to the drilled tie hole fills any void between the end of the collar thimble and timber tie and affords some insulation between the lag screw and wood tie. The insulated tie plate pad should extend a minimum of ½ inch [12 millimeters] beyond the tie plate edges to afford

Corrosion Control 8-13 a higher resistance path for surface tracking of stray currents. Chemical compatibility between the pad and epoxy material must be verified during design. Timber cross ties can appear attractive compared to concrete ties on a first cost basis; however, the cost of materials and labor associated with adding insulated rail fastenings to timber cross ties can tip the scale toward the use of concrete cross ties. This is particularly true if high-quality timber ties are specified. Because of this factor, the longer life expectancy of high-quality concrete cross ties, and the fact that fewer concrete cross ties are usually required, most rail transit projects have concluded that concrete cross ties are more economical on a life cycle cost basis. 8.4.2.3 Ballast To eliminate the path for stray current leakage from rail to ballast, the ballast section should be a minimum of 1 inch [25 millimeters] below the bottom of the rails. The clearance requirement pertains to rail on both concrete and timber cross ties for both main line and yard trackage. This is essential to increase the rail-to-earth resistance and assist in minimizing the stray current leakage to earth. Ballast should be clean and well-drained not only when initially constructed, but also during service. Regular maintenance is required, as described in Chapter 14 of this Handbook. The use of metallic slags as ballast is not recommended. Rail grinding should be done with vacuum systems to minimize contaminating ballast with metallic grindings. 8.4.3 Embedded Track Issues Embedded track is often used on portions of a light rail system where the tracks are located within an urban street. Electrical isolation of embedded rails can be provided by insulating all the surfaces of the rail with the exception of those in contact with the wheels, insulating the trough that the rail sits in, or a combination of both. Track may also be isolated by insulating the perimeter of the entire concrete base slab, a concept often called the “bathtub” stray current isolation concept. The materials used to provide this insulation generally consist of polyethylene sheeting, epoxy coal tar coating, elastomeric grout, or natural rubber sheeting. Rail boot, first used on Toronto’s streetcar system in the 1990s, has very nearly become the default method of electrically isolating plain embedded track, but several insulating systems have been used successfully. The specific design for stray current control is selected by the track designer with recommendations from the corrosion control specialists. See Chapter 5 for additional information concerning details and materials for insulation of embedded track. 8.4.4 Direct Fixation Track Issues Direct fixation track is generally located on aerial sections or in tunnels in light rail transit systems. The direct fixation fasteners, as described in Chapter 5, provide electrical insulation between the rails and the concrete structure. The typical direct fixation fastener consists of a sandwich of steel plates and rubber pads, with the latter providing both electrical isolation and acoustic attenuation. An elastomer of the proper resistivity provides excellent insulation and deters current leakage. Fastener inserts are often epoxy coated to further isolate the rails from the concrete slab.

Track Design Handbook for Light Rail Transit, Second Edition 8-14 Despite the use of insulated rail fasteners, stray current leakage often occurs in damp tunnels that are subject to ground water seepage. The fasteners can become coated with a wet conductive film. The problem can be particularly acute in snowbelt climates, as the entire tunnel track system can become coated with a brine composed of de-icing chemicals mixed with snow that is carried into the tunnel from surface portions of the system. At stations, even those outside of tunnels, the area below the rail and between direct fixation fasteners will often become filled with a mixture of water, brine, grease and the sand that is dispensed by the LRVs to improve traction when braking—creating a direct short to ground. A regular maintenance program is essential to keep the track structure clean in tunnels, since natural precipitation is not available to wash the track in these areas. The importance of routine housekeeping to flush these contaminants off the track cannot be overstated. See Chapter 14 on LRT track maintenance for additional discussion on this topic. 8.4.5 Yard and Shop Track Issues Maintenance shop tracks are grounded to protect workers by reducing the touch potential between the rail car and ground to zero. Maintenance yard tracks are generally floating or non- grounded, but insulation is rarely included between the rails and the timber cross ties. This design decision is based on economic considerations, as well as the fact that yard tracks are generally used only at low speed and consequent low current draw, and a separate traction power substation is used to supply operating current for train movement in the yard. The only time the yard rails become electrically connected to the main line or shop rails is when a train enters or leaves the yard or shop. This is a short period and does not result in any harmful sustained current leaking into the earth. Note that transit system structures within a yard complex may have to be protected against locally originated stray currents between yard trackage and the yard substation. Consequently, underground utilities in yards are often constructed with non-metallic materials such as PVC, FRE, and polyethylene. 8.4.6 Track Appliances Several items that are attached to the track can circumvent barriers to stray current if not detailed correctly. Many of these are related to the LRT train control system. The paragraphs below discuss some of the devices that can require special attention so as to not become stray current leakage locations. 8.4.6.1 Impedance Bonds Leakage of stray currents into the earth can be a significant problem if the cables from the rails are electrically connected to an impedance bond housing case that is in contact with the earth. This type of grounded installation can result in a continuous maintenance problem if an effectively high rail-to-earth resistance is to be achieved. Instead, the housing case should be mounted clear of any concrete slab conduits, reinforcing bar, and contact with the earth. Impedance bond housing cases for a light rail transit line are generally located at-grade along the right-of-way. The cases are mounted on timber tie supports in the ballasted area either between or directly adjacent to the rails. In order to eliminate possible points of contact with the earth, the

Corrosion Control 8-15 center taps of the impedance bonds are insulated from the mounting case by installing a clear adhesive silicone sealant between the center taps and the case. While the track engineer is generally not involved in the placement of impedance bonds, he or she should examine the details proposed by the signal and traction power design staff so as to be certain the installation provides proper isolation. On more than one occasion, the track design has been blamed for a stray current leak that turned out to be the fault of a deficient impedance bond installation detail. See Chapter 10, Article 10.2.2, for additional discussion concerning impedance bonds. 8.4.6.2 Switch Machines For several reasons, including the safety of the signal maintainers, electrically powered switch machines need to be grounded. This requires that they be electrically insulated from the track structure. See Chapter 10 for discussions concerning switch machines. 8.4.6.3 End-of-Track Bumping Posts and Buffers It has been common practice to electrically isolate rail transit end-of-track devices from the track by installing insulated rail joints in the track ahead of the striking face of the unit. This is not actually necessary for all such installations. There are several issues involved: • Making certain the end-of-track device does not create a shunt between the rails, thereby giving the signal system (if present) a false indication of track occupancy. • Making certain that the end-of-track device does not become a stray current leak. • Making certain the working parts of a hydraulic buffer end-of-track device are protected from corrosion. Static fixed bumpers with no moving parts and friction buffers that control train deceleration solely with rail skates and/or spring-loaded heads require only a single insulated joint in one rail to interrupt signal shunt current. So long as the track they are sitting upon is isolated from the earth with the ordinary details for that trackform, full isolation of such devices from the running rails is not necessary. The practice of two insulated joints is likely a holdover from the days when ballast piles over the rails were used as an end-of-track control device. Since the ballast pile grounded the rails, the insulated joints were necessary. End-of-track buffers with hydraulic heads may require grounding to protect the hydraulic working parts from corrosion. Since the grounding system would create a stray current path, those types of end-of-track devices should have an insulated joint in both rails. 8.4.7 Stray Current Tests and Procedures The track isolation methods discussed in the articles above, when properly constructed, will provide good initial values of rail-to-earth resistance. However, as these components deteriorate, they become dirty and require maintenance to maintain their original resistivity. Periodic tests are also required to locate and remove direct shorts that occasionally occur as discussed in the

Track Design Handbook for Light Rail Transit, Second Edition 8-16 following section. Stray currents can rise to harmful levels if short circuits to ground are not detected and removed. Once the LRT system is in operation, regularly scheduled tests are recommended to monitor and maintain the integrity of stray current control systems. The most common tests are rail-to-earth resistance tests, substation-to-earth voltage tests, and structure-to-earth tests. Transit agencies use a broad spectrum of approaches for these test programs ranging from infrequent use of consultants to permanent in-house corrosion control personnel. Typically, the greatest efforts are made only after stray current problems have already damaged piping, utility structures, trackwork components, or signal circuits. Such troubleshooting can resolve problems, but it is much more effective to conduct regularly scheduled, routine monitoring for stray currents so that problems can be detected and corrected before they manifest themselves in the form of measurable corrosion or degraded signal system performance. Track resistance to ground is a major indicator of the isolation of the electric traction return system from the infrastructure. The generally recognized methodology for the determination of track resistance is given in ASTM G165 – 99 Standard Practice for Determining Rail-to-Earth Resistance. Sections of track should be tested as they are constructed rather than only upon the completion of construction in order to immediately identify and repair a failed section. See Chapter 13 for additional discussion on this topic. 8.5 SUMMARY Corrosion from stray electrical currents is an important interdisciplinary issue that requires the close attention of the design team. There are several effective methods that have been used by light rail system operators and builders to either avoid or mitigate the effects of stray current corrosion. The designer must seek the advice of experts in this complex field, as well as coordinate with utility companies and the signal system designer. It is also important to recognize that track component specifications should include appropriate electrical resistance features to accomplish the goals of the corrosion control plan. However, the designer needs to be certain that specified products will actually meet the specified performance requirements for track-to- ground resistance of the entire track system. 8.6 REFERENCES [1] Fontana, Mars G. Corrosion Engineering, McGraw-Hill Book Company, Third Edition, Fontana Corrosion Center, Ohio State University, 1988. [2] Sidoriak, William, & McCaffrey, Kevin, Source Control for Stray Current Mitigation, APTA Rapid Transit Conference 1992, Los Angeles, California, June 1992. [3] Moody, Kenneth J., A Cookbook for Transit System Stray Current Control, NACE Corrosion 93, paper No. 14, New Orleans, Louisiana, February 1993. [Deleted from 2nd Edition] [4] NACE International, “Stray Current Corrosion: The Past, Present, and Future of Rail Transit Systems,” NACE International Handbook, Houston, Texas, 1994. [Deleted from 2nd Edition]

Corrosion Control 71-8 [5] American Railway Engineering and Maintenance-of-Way Association, Manual for Railway Engineering, AREMA – Part 13 Safety and Security. [Deleted from 2nd Edition] [6] National Electrical Code, NFPA 70, National Fire Protection Association. [7] National Electrical Safety Code, Institute of Electrical and Electronic Engineers. [8] Code of Federal Regulations Title 49 Transportation Part 236 — Rules, Standards, and Instructions Governing the Installation, Inspection, Maintenance, and Repair of Signal and Train Control Systems, Devices, and Appliances. [Deleted from 2nd Edition] [9] IEEE Std 1313.1 IEEE Standard for Insulation Coordination – Definitions, Principles, and Rules. [Deleted from 2nd Edition] [10] Title 29 Code of Federal Register Labor Part 1910 - Occupational Safety And Health Standards (29CFR1910). [Deleted from 2nd Edition] [11] Title 29 Code of Federal Register Labor Part 1926 - Safety and Health Regulations for Construction (29CFR1926). [Deleted from 2nd Edition]

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TRB’s Transit Cooperative Research Program (TCRP) Report 155: Track Design Handbook for Light Rail Transit, Second Edition provides guidelines and descriptions for the design of various common types of light rail transit (LRT) track.

The track structure types include ballasted track, direct fixation (“ballastless”) track, and embedded track.

The report considers the characteristics and interfaces of vehicle wheels and rail, tracks and wheel gauges, rail sections, alignments, speeds, and track moduli.

The report includes chapters on vehicles, alignment, track structures, track components, special track work, aerial structures/bridges, corrosion control, noise and vibration, signals, traction power, and the integration of LRT track into urban streets.

A PowerPoint presentation describing the entire project is available online.

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