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4 Literature Review 2.1 Introduction Rail networks have been critical components of transportation systems and the economy for more than 150 years everywhere in the world, particularly in urban areas (Hill 1997). Transportation systems carry approximately one-third of U.S. exports and deliver 5 million tons of freight every day (Weston et al. 2008). Electrical railways are categorized as light or heavy rail systems, according to their power-providing system (Zerbst and Beretta 2011). In heavy rail systems the third rail commonly supplies the positive polarity of the direct current (DC) power supply in the DC voltage range of 600 to 750, and the running rails provide the negative return. The third rail is an American invention, and its use dates to the dawn of the first subway systems in the 1860s. Some railway power-supply systems are limited to using overhead contact lines because their voltage levels are above 1 kilovolt (kV); however, third rails provide electric power to the railway vehicle by a rigid conductor that is placed between or along the rails (Montesinos et al. 2018). The train runs from the power drawn from a collector shoe in the third rail, which is usually found near the tracks and carries a high voltage that is extremely dangerous if touched (Montesinos et al. 2018). The third rail system is increasingly being used for new construction of subways; however, it faces clear challenges, including an aging infrastructure. It uses one contact variant among the top contact, side contact, and bottom contact. The top contact is not as safe as the other two and is susceptible to interruptions from ice and snow. Thus far, most of the research on third rail systems has been based on the low speed level [about 80 kilometers per hour (km/h)] (Montesinos et al. 2018); very little has concentrated on the high-speed level (120 km/h) and the improvement of the current collector (Verma and Reddy 2018). Vohra (2004) conducted research that analyzed the data of the contact force and the displacement of the collector shoe. The vibration rule of the shoe was obtained by combining the circular third rail test rig with the vehicle test device, and the results indicated that if the collector shoe lags functioning, it can cause the shoe to veer offline from the third rail. Kumosa et al. (2004) analyzed the motion features of the third rail slider by establishing the matching characteristic motion model of the contact surface between the third rail and the collector and identifying the factors that affect the running speed of the collector. Several other researchers illustrated the characteristics of normal contact stiffness on the contact surface of the third rails and analyzed the impact on the vibration response of the collector system (Vohra 2004; Montesinos et al. 2018). The insulators for third rail systems are of various sizes and materials, including wood, porce lain, and fiberglass, which lead to difficulties in designing a standard cleaning device C H A P T E R 2
Literature Review 5 (Vohra 2004). Mobasher et al. (2002) examined glass-reinforced plastic (GRP) rods to detect microscopic defects and found that chemical reactions promote cracking in the rods and play a major role in the failure process. Montesinos et al. (2018) studied micro-cracks in the GRP rods and found that environmental conditions can affect the insulator performance. Third rail systems and their corresponding components frequently deal with issues and challenges such as material corrosion caused by electrical erosion and dirt accumulation. The common approaches to reducing the amount of corrosion are cleaning the insulators at preset intervals, regular inspections, and preventive maintenance activities (Ibrahem 2017). Cherney et al. (2015) reviewed the works performed on the DC planes and recommended conducting erosion tests to identify the cause of erosion in insulating materials. Stewart et al. (2011) con- ducted a dynamic analysis of the corrosion caused by leakages in nongrounded DC railway systems and learned that the amount of corrosion was proportional to the leakage current integrated over time. In any electric railway system, including the third rail systems, a good current-collecting condition is a prerequisite for ensuring safe, reliable, and stable running of the vehicle and can reduce the maintenance costs and the damage rate of the current-collector components (Ibrahem 2017). The expansion of the metro line networks and the increase of passenger traffic have resulted in an increase in the running speed of the vehicles and higher requirements for the current-collecting performance of vehicles in high-speed and complex environments. 2.2 Third Rail Characteristics Third railâpowered city railways were implemented in the United States in 1895, when the Metropolitan West Side Elevated began operating in Chicago. Paris began using this system in 1900, in part of the mainline tunnel that connected the Gare dâOrsay to the rest of the Chemin de Fer de ParisâOrléans network. Technically, a third rail is a means for supplying traction current to a traction unit such as a train. The power is collected by a shoe that is mounted on a beam and located on its side. While many of the metro systems use overhead wires, third rails can be an appropriate choice for trains that are typically supplied by DCs where the voltage is less than 1,000 V (Hill 1997). Figure 1 depicts a third rail system schematically. Third rails can be several shapes, but they are all used to provide current to the moving train. The current is transmitted to the train by a sliding connection that is composed of springs, pistons, and a shoe gear. The material used for the shoe gears varies according to the material of the third rail. The third rail is often placed outside the running rails but is sometimes placed between the rails. To minimize resistance in the electric circuit, the running rails are usually connected by wire bonds or other tools. The contact position between the train and the rail varies. Top contact was used in the earliest systems, but those developed later use side contact or bottom contact. Both contact types cover the conductor rail and protect it from dirt, snow, ice, and other sediments, and protect the track workers. Contact shoes can be positioned anywhere around the third rail, depending on the type of rail. In the United States, the conductor rails are usually made of steel. In some cases, compos- ite rails such as extruded aluminum with a steel surface are used to boost the conductivity. In other parts of the world, conductors are often made of extruded aluminum with a stainless steel contact surface because of its longer life, low electrical resistance, and light weight. Several countries, such as Japan, South Korea, and Spain, are adopting the use of overhead wires across
6 Third Rail Insulator Failures: Current State of the Practice urban railroads, but many new third rail systems have been established all around the world, including in many advanced countries such as China, Denmark, and Taiwan. 2.3 Third Rail Arrangement Third rails consist of running rails, insulators, protection boards, protection board brackets, and shoe contacts. Figure 2 shows the placement of these sections. 2.3.1 Running Rails The third rail is usually located outside two running rails, but on some systems it is mounted between them. In other words, on most of the third rail systems, the conductor rail is Figure 1. Schematic sections of third rails. Insulator Running rails Protection Board Brackets Third Rail Protection Board Contact shoe Figure 2. Position of third rail and other relative equipment in the railway system.
Literature Review 7 placed on the sleeper ends outside the running rails, but in some systems a central conductor rail is used. 2.3.2 Insulators Insulators are made of three components: fiber-reinforced plastic, metal end fittings, and silicone rubber (Gorur et al. 1999). They are usually made of fast-drying, nonconduct- ing materials such as porcelain, fiberglass, or composite materials, and are installed at each supporting bracket location. Rubber insulators are being used increasingly for transmission lines and have several advantages over conventional insulators: they are lightweight, have a slim design that is resistant to vandalism, and cause less pollution in the environment than conventional insulators. However, the main concern about rubber insulators is their dete- rioration of material properties caused by aging. 2.3.3 Protection Board Protection boards cover the top of the railwayâs electric third rail and are settled on the contact rail to protect personnel from coming in contact with the rail. These boards are typically made of materials such as fiberglass, plastic, and timber. 2.3.4 Protection Board Brackets The protection board brackets are used to support the protection board on the contact rails. Each of the brackets slides gently on the rail, is fitted over a board, and is mounted to it. 2.3.5 Shoe Contact Although third rails are usually placed outside the running rails, in some cases, they are located between two running rails. To transmit electricity to the train, a sliding shoe is used that is in contact with the side and bottom of the third rail and provides a protective cover for the top surface. When the shoe slides on the top surface, it is technically referred to as âtop running,â and when it slides on the bottom surface, it is referred to as âbottom running.â The bottom running shoe is less affected by snow, ice, or leaves (Gorur et al. 1999; Verma and Reddy 2018) and negates the possibility of electrocution. 2.4 Third Rail Mechanism Electrical current in third rails can be collected by a variety of methods and designs. The top contact method is the most modest and oldest; however, practitioners and experts in third rails have identified several of its drawbacks. The safety risk of the exposed electric conductor is very high; it collects leaves and ice, rendering the trains inoperable, and the remedies that are effective for resolving these problems are costly. The bottom contact method is widely considered the best way to collect third rail current, as most parts of the rail are effectively covered and protected from any environmental issues. The third rail shoe acts as a conductor power feed. Typically, the wheel to the connection of the rail works as a return. 2.5 Third Rail Material Types Three conventional types of rails are used in third rail systems. The first type, steel, dates to the 1890s and is the traditional technology used for this purpose. A majority of the operating third rails in the United States are made of steel because they have the longest operating
8 Third Rail Insulator Failures: Current State of the Practice life. The second type, aluminum, is bonded by bolts to a steel rail conductor, and is also widely used in the United States. The use of aluminum allows matching the size with the steel rail, and it has lower electrical resistance than steel. Aluminum/stainless steel (ALSS) has been used for third rails since the 1960s. It consists of a cap made of stainless steel that is affixed to an aluminum extrusion using different methods. Currently, there are more than 4,000 km of ALSS conductors in operation worldwide. Several types of materials and composites, such as bulk molding compound (BMC), GRP, fiberglass, wood, porcelain, and others are used to make third rail insulators. Rapid rail transit systems in many parts of the world use composite insulators, which are typically placed internally at a distance of 3 meters. Their performance largely depends on local environmental conditions. 2.6 Third Rail Advantages and Disadvantages The major advantages and disadvantages of third rails are presented in Table 1. The size and speed of the trains are constrained because of voltage limitations. As an illustration, trains that use third rail systems cannot provide a high amount of air conditioning because of electricity limitations. Experience shows that 100 mph is the highest speed that can safely be attained when using third rails. Above this speed, the shoes and rail contacts encounter problems that can lead to an unstable situation. (It should be noted that the speed record of third rails worldwide is 108 mph.) One significant advantage of third rails is that they cost less than diesel or steam locomotives to operate and are more environmentally friendly than diesel systems. The main advantage of third rails over overhead wires is that it is less expensive because there is no need for construction through the train path. Electric shock is a hazard presented by the third rails, but installing screen-door platforms or placing the conductor rail away from the platform is a solution. Another disadvantage is that the third rail systems with top contact are susceptible to the accumulation of snow, which can cause interruptions in system operation. 2.7 Third Rail Issues and Challenges Third rail system failures can be caused by environmental, mechanical, electrical, and opera- tional problems. Researchers have suggested methods to prevent and remediate the failure of the transmission insulators (Kumosa et al. 2004) that frequently occur inside tunnels when rust particles and carbon dust cause a short circuit in the insulator and initiate smoke, explode the insulator, and cause the wooden ties to catch fire. Vohra (2004) indicated that fiberglass insulators can also burn in such a situation, and porcelain insulators can melt. Table 2 lists the categories and relative causes of issues inherent in third rail systems. Weather conditions affect the operation of third rails. Those with top contact are prone to snow accumulation, which can cause service interruptions, and thunderstorms with strikes of Third Rail Advantages Third Rail Disadvantages Little visual intrusion on the environment Speed limitation Inexpensive installment procedure Involves safety hazard No vertical clearance required Limited capacity because of low voltage More robust than overhead line systems In high currents, high voltage drops Easy to reach and maintain Cannot be used for high-speed trains Table 1. Major advantages and disadvantages of third rails.
Literature Review 9 lightning can disable the power. It is, therefore, important to find ways to control the effects of the weather. Partial arcing is a result of degradation in third rail systems, and the degradation can generate combustion and ignite the flammable debris (Stewart et al. 2011). The smoke from the ignited debris can interrupt the third rail service and incur safety risks as well. Some insula- tors, such as BMC insulators, accumulate dust and cause contamination because of brushes of carbon, which lead to deterioration, short circuiting, and failure of the insulator and the third rail. Composite insulators are also subject to failure because of iron particulates, rust, and so forth from the maintenance tracks. Degradation alters the normal functioning of polymeric insulators (Verma and Reddy 2018). Reddy (2019) conducted an analysis to determine the cause of degradation and arcing in dry, wet, and contaminated conditions. The results interestingly showed that in wet conditions, the current leakage increased 6 times more than in dry conditions. Verma and Reddy (2018) suggested that regular maintenance and coating mechanisms might reduce the degradation and increase the performance life of an insulator. All third rail systems are susceptible to electrical erosion that results from the arcing pheno menon between the rail contact and the collector shoe. When there is repeated contact loss at a location on the face of the rail, the conductor material becomes eroded. The conditions that can lead to collector loss include engagement and disengagement under full power at ramps, insufficient collector dynamic response, discontinuities in rail contact surface, and poor align- ment between the third rail and the running rails. Corrosion is a problem that must be taken into consideration for all electrical supply systems, especially for DC third rail systems. The major factors that contribute to corrosion are mois- ture, competing fields of electricity, impedance bonds, and cab signal assets. The distortion extension is widely relative to the ratio of distorting power to the power of the short circuit, and even small distortions can impact the voltage when the short-circuit power is low enough (Cherney et al. 2015). Supply voltage distortion can be boosted by parallel resonances, which is very normal in electrical systems that use capacitors, and accelerates the degradation and reduces the reliability of the electrical components, including the capacitors, cables, and transformers (Gorur et al. 1999). 2.8 Regular Inspections Conducting inspections before failures occur helps reduce the number of third rail failures by detecting the components that are prone to fail (Vohra 2004). The contact rail should be inspected to ensure that insulators are present, clean, in good condition, located on brackets, and exposed with an insulator cap that is placed by a lug hole (Verma and Reddy 2018). Other third rail components also need to be inspected regularly. The contact rails need to be inspected to determine whether they are exposed uniformly and evenly on all the insulators and to detect any damage on the contact surface. The protection board should be inspected Table 2. Major issues of third rail systems. Category Causes Environmental Erosion, lightning, sunlight, and saltwater penetration Mechanical Cracks/fractures, mechanical stress, damage from impacts, and aging Electrical System voltage fluctuations, corrosion, distortion, andflashovers/arcing Operational Dirt buildup, defective product materials, water infiltration, ice and snow accumulation, and vandalism
10 Third Rail Insulator Failures: Current State of the Practice to check whether the brackets are located on the timber ties, accurately gaged, exposed on the tie, bolted by screws, and installed correctly on the running rail (Vohra 2004). Technically, visual inspections include using vision and all the human senses while inspect- ing equipment (Stewart et al. 2011). Daily and weekly visual inspections of tracks reveal defects, corrosion, erosion, and so forth, and for accurate inspections, appropriate tools can be applied and/or tests can be conducted. For example, the insulation resistance test, infrared test, and so forth can be used to gauge an insulatorâs performance (Stewart et al. 2011). 2.9 Maintenance Programs Some agencies only perform the minimum maintenance because of the high cost of a more extensive maintenance program. Some, however, regularly maintain the third rail compo- nents, including insulators, since renewal of the failed components reduces the number of delays resulting from a system shutdown, and other failure consequences are more costly than the maintenance (Verma and Reddy 2018). Maintenance tasks fall into two main categories: preventive and corrective. Transit agencies try to avoid corrective maintenance because of its high expense. The reliability of the system is significant relative to the presence of the appropriate voltage, which increases the importance of regular maintenance of all the components (Zerbst and Beretta 2011). To save labor and perform optimal maintenance, some agencies implement condition- based maintenance, which means conducting maintenance tasks for essential components and within specific parameters. This method is accurate but can be performed only by trained personnel. Cleaning can help to reduce the likelihood of dirt buildup, voltage fluctuations, and snow and ice accumulations (Vohra 2004). Some of the transit systems dedicate a maintenance crew to clean the insulators and power components, specifically in the tunnels and walkways (Gorur et al. 1999), since surface cleaning is important for avoiding third rail issues (Vohra 2004). During installation, an appropriate clearance space should be provided around the insulators, where possible, to ensure the safety of the cleaning crew, and the cleaning device should be adjustable to different-sized insulators (Vohra 2004). Automatic cleaning devices can accelerate the cleaning process and eliminate the occasional occurrence of labor injuries to those who perform the manual cleaning. Detecting which components need to be replaced or repaired because of aging, damage, failure, or vandals is a significant challenge. Predicting models that can be developed by a statistical method and historical data can be used to identify which insulators may fail in the near future (Gorur et al. 1999). Cathodic protection is a useful method for insulating the third rail components and allevi- ating the effects of weather such as erosion, corrosion, salt fog, sunlight, water infiltration, and saltwater penetration. Maintenance-management tracking software can assist with tracking maintenance actions and minimize the relevant costs. Implementing this type of software can also be a tool for documenting lessons learned. 2.10 Design, Installment, Safety, and Regulation Specifications The first step in improving safety regulations and standards is to frequently inspect the rail tracks and all the third rail components to detect unseen threats to public and worker health and safety. Records of previous incidents, specifically safety-event failures, need to be consulted
Literature Review 11 to determine the factors that affect safety, and the workers, inspectors, and the public need to be trained in and updated on safety regulations and standards. Documentation of the important practices after failures is important for future reference. Flaws in the installation, the use of inappropriate materials and insulator types, the lack of access to insulators in crowded areas, using insulators prone to fire, and so forth can be avoided if problems are documented and considered when renewing third rail infrastructures. Third rail design methods also need to be upgraded to reflect new technologies and improvements (Montesinos et al. 2018). Another implemented and suggested strategy to enhance worker safety is having the main- tenance workers document unsafe conditions so that the managers can take remedial actions. This leads to more efficient use of time and money (Kumosa et al. 2004). Specifications that define the requirements for accurate installation and functioning should be developed for third rail components such as the contact system and insulators. Specifications for safety considerations should also be developed to prevent safety-failure events such as smoke events caused by arcing insulators (Kumosa et al. 2004). The installation of a cover board mitigates many of the causes of failure, including ero- sion, cracking, mechanical stress, damages from impacts, lightning, water infiltration, and saltwater penetration. In fact, all the studied transit systems use cover boards to preserve their components from many environmental conditions and consequently from failure (Verma and Reddy 2018). Using heaters to prevent the third rails from icing is one of the strategies that helps mitigate the weather-related issues for third rails. Several insulator placement and configuration methods are available for third rails. Gravity or a clipped third rail on a cast-iron top and insulated post with a reinforced base, a cast- iron base bolted to a tie, and lagging screws in wood ties are examples of such configurations. The appropriate configuration needs to be selected by each agency, according to their environ- mental conditions and the access of the public to the insulators (Verma and Reddy 2018).
