Track Design Handbook for Light Rail Transit, Second Edition (2012) / Chapter Skim
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Pages 448-465
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From page 448... ...
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
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From page 449... ...
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)
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From page 450... ...
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.
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From page 451... ...
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.
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From page 452... ...
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.
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From page 453... ...
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.
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From page 454... ...
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.
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From page 455... ...
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)
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From page 456... ...
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.
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From page 457... ...
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.
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From page 458... ...
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.
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From page 459... ...
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.
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From page 460... ...
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.
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From page 461... ...
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.
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From page 462... ...
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.
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From page 463... ...
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.
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From page 464... ...
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.
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From page 465... ...
Corrosion Control 71-8 [5] American Railway Engineering and Maintenance-of-Way Association, Manual for Railway Engineering, AREMA – Part 13 Safety and Security.
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