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3 Stray current and stray currentâinduced corrosion have been causes of concern among transit agencies, electrolysis committees, utility owners, providers, and electric railway carriers not only in the United States but also throughout the globe. With initial principles and mitigation methods dating back to the early 1900s, the first preliminary report on stray current corrosion and mitigation in the United States was prepared in 1916. The document was published as a progress report after the end of the World War I in 1921 (1). To date, most of the principles identified and the mitigation measures recommended in the 1921 electrolysis report are still adopted by transit service providers, the engineering community, and corrosion consultants. Stray current leakage and the corrosion caused by these currents is more of an issue in low resistivity soils and embedded tracks, which typically run through major traffic areas, city centers, and threads between utility lines that require the rail to be continuously isolated to provide superior track-to-earth resistance (2). Conversely, ballasted tracks have relatively minimal stray current leakage because the entire rail does not require continuous isolation from earth, and separation is needed at the contact points, which are mostly insulated. Maintenance, periodic testing, and quality control (QC) of stray current control (SCC) still remains an issue, as does the introduction of standardized guidelines to mitigate and collect stray currents. A general understanding in the engineering and the transit communities is that due to the lack of technical guidelines, proper maintenance, and testing procedures the approach to address stray current issues in the rail transit industry is more reactionary than preemptive. A uniform system for rail track isolation supplemented by QC testing and maintenance guidelines would not only help reduce stray current corrosion faced by transit agencies but also would lessen the ever-rising cost of corrosion repairs to DC-rail transit systems. Much of this repair cost is associated with the restorations performed either to the DC-running rail itself or to the neighboring infrastructure elements such as utilities (3, 4). The absence of specific national SCC or mitigation standards or guidelines in the United States necessitates the need to produce contemporary standards and guiding principles for transit providers and corrosion consultants. Real case studies on issues related to stray current effects from transit agencies were used in preparing these consolidated guidelines and recommendations. Some of the case studies involved the following: ⢠Stray current analysis of national and international transit agencies, which includes European transit agencies, an Australian transit agency, and a Southeast Asian transit agency. Introduction C H A P T E R 1
4 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook ⢠Analysis of stray current corrosion and damage to neighboring utilities from the operation of multiple national and international agencies. ⢠Review and participation in national and international transit agency maintenance and testing programs. This TCRP guidebook is a resource to be used immediately by stray current corrosion consultants, transit agency owners, and corrosion testing and maintenance providers. 1.1 Research Methodology The literature research conducted as part of the guidebook development process helped in achieving an understanding of current industry practices, both national and international, and criterions for the mitigation of stray currents. In the absence of any national standards and guidelines, international standards were studied, especially with respect to their relevance to U.S. transit systems. Figure 1 provides a graphic representation of the approach taken to complete various tasks for the research carried out under TCRP Project D-16. Figure 1. Process layout for SCC research.
Introduction 5 1.2 Clarification of Terms The following list presents and defines some terms used throughout the guidebook to avoid confusion in later discussions throughout the chapters. Anode. The electrode of an electrochemical cell at which oxidation occurs (loss of electrons). Electrons flow away from the anode in the external circuit. Corrosion usually occurs and metal ions enter the solution at the anode. Ballasted rail. Area where the running rails are laid above the ballasted track surface. Bond. The bond is a metallic or reinforced concrete structure made electrically continuous by welding the appropriate components and/or by installation of cables between compo- nents to provide a path for the stray current to return to its source without damage to the structure. Cathode. The electrode of an electrochemical cell at which reduction is the principal reaction. Electrons flow toward the cathode in the external circuit. Cathodic protection. Electrochemical immunity produced by an appropriate cathodic polarization. Conductive part. Part that can carry electric current. Contact line system. Support network for supplying electrical energy from substations to electrically powered traction units that covers overhead contact line systems and conductor rail systems. The electrical limits of the system are the feeding point and the contact point to the current collector. Corrosion. Progressive degradation or deterioration of a material commonly referred to as rusting, due to its interaction with the environment, that is, air, water, or soil. Cross bond. Electrical connection intended to connect in parallel the conductors of the return circuit. Direct drainage bond. Device that provides electrical drainage by means of a direct bond between an affected structure and the stray current source. The bond may include a series resistor. Drainage. Conductance of electric current from an underground or submerged metallic structure by means of a metallic conductor. Drainage bond. Transfer of stray current from an affected structure to the current source by means of a deliberate bond. (Forced) drainage bond. Device that provides electrical drainage by means of a bond between an affected structure and the stray current source. The bond includes a separate source of DC power to augment the transfer of current. (Unidirectional) drainage bond. This type of drainage bond will include a diode to ensure that the current flows in one direction only. Dynamic stray currents. Stray currents that continually vary in amplitude and/or continually change their electrolytic paths, such as DC-powered mining operations, DC welding equip- ment, railway systems, and lightning. Earthing. Connection of conductive parts to an appropriate earth electrode Electric traction system. Electrical distribution network used to provide energy to rolling stock. Electrolysis. Production of chemical changes of the electrolyte by the current flowing through an electrochemical cell. Embedded rail. Area where the top of the running rails is at the same level as the surrounding surface. Equipotential bonding. Provision of electric connections between conductive parts, intended to achieve equipotentiality.
6 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Fault condition. Non-intended condition caused by short-circuit. Fourth rail. Conductor rail used instead of the running rail for the return currents. Galvanic corrosion. Accelerated corrosion of a metal because of an electrical contact with a more noble metal or nonmetallic conductor in a corrosive electrolyte. High voltage. Nominal voltage exceeding alternating current (AC) of 1,000 volts or DC of 1,500 volts. Indirect contact. Electric contact of persons or animals with exposed conductive parts that have become live under fault conditions. Isolating joint. A joint that breaks electrical continuity in a structure but does not affect the mechanical integrity of the structure. Insulating rail joint. Mechanical rail joint that longitudinally separates the rail electrically. Low voltage. Nominal voltage up to and including AC of 1,000 volts or DC of 1,500 volts. Overhead contact system. Electric contact system using an overhead contact line to supply current for use by traction units or rolling stock. Polarization. The change from the open-circuit potential resulting from current across the electrode/electrolyte interface. Potential shift. A change in measured potential of a metallic structure caused by the applica- tion of current from an external source. Rail potential. Voltage occurring between running rails and earth. Rail-to-earth resistance. Electrical resistance between the running rails and the earth or structure earth. Rail-to-rail cross bonding. Electrical bond that interconnects the running rails of the same track. Return circuit. All conductors that form the intended path for the traction return current to the substation. Return conductor. Conductor that parallels the track return system and connects to the running rails at periodic intervals. Short circuit. Accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these conductive parts to be equal to or close to zero. Static stray currents. Stray currents that maintain constant amplitude and constant paths, such as high voltage DC power transmission lines. Stray current. Part of the current caused by a DC-traction system that follows paths other than the return circuit. Stray current corrosion. Corrosion resulting from stray current leakage. Telluric current. An electric current that moves underground or through the sea. Telluric currents are phenomena observed in the earthâs crust and mantle. Third rail/conductor rail. Rail mounted on insulators located parallel to the running rails to provide the positive feed to the transit vehicle. Touch potential. Voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away. Touch voltage. Voltage between conductive parts when touched simultaneously by a living object (person or an animal). Track circuit. Electrical circuit, of which the rails of a track section form a part, with usu- ally a source of current connected at one end and a detection device at the other end for detecting whether this track section is clear or occupied by a vehicle. Track return system. System in which the running rails of the track form a part of the return circuit for the traction current. Track-to-track cross bonding. Electrical bond that interconnects tracks.
Introduction 7 Traction power. Propulsion power for rail transit vehicles. Traction power substation. Installation to supply a contact line system with power. The substations may convert energy from a utility to the form that matches the transit vehiclesâ needs. Traction return current. Sum of the currents returning to the supply source, the substation, or regenerative braking vehicles. Voltage limiting device. Protective device whose function is to prevent existence of an impermissible high touch voltage.
