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15 Case Examples 4.1 Case 1 Case 1 provides bus, heavy rail, and demand-responsive transportation services. The nominal traction power voltage of the third rail is 600-750 V, which is common in third rail systems all around the world; however, this system is equipped to employ a maximum voltage of 750-1,000 V for voltage regeneration during operation. This system is responsible for more than 100 million passenger trips annually. Table 3 summarizes the basic information pertaining to Case 1. Approximately 176,000 insulators of different types are implemented in this transit system, in and out of tunnels. Out of tunnels, 59,000 porcelain, 60,000 fiberglass, and 40,000 wood insulators are used, while all the 17,000 insulators used in tunnels are made of porcelain. Case 1 encounters an average of 11 to 15 insulator failures per year. Thus, the rate of insula- tor failure is approximately 1 in 15,000, which causes a loss of 1,000 to 5,000 passenger hours annually and a total delay of less than 10 hours per year. The main impacts of insulator failures on the operation of this transit system are staffing reductions from normal maintenance and increased labor costs for emergency callouts. Little effect on retention of customers is probable in this system, as it improves its infrastruc- ture by renewing and replacing components and improves its design methods as needed. The rate of failures was observed to have significantly decreased to fewer than five incidents per year in the segments of the infrastructure that were improved. Hot weather, cold weather, humidity, and weathering are all factors for consideration in the Case 1 transit system. In the tunnels, water infiltration is a common problem that leads to corrosion of the mounting hardware and buildup of salt and dirt on insulators. Saltwater from deicing roads adjacent to the tracks over bridges is another issue, as it drips or splashes onto the insulators and causes them to fail. Of the physical and chemical causes of insulator failure, dirt buildup and erosion are the most commonly experienced. Rust contaminants, carbon dust, dirt, and grime also affect Case 1 transit-system composite insulators. Case 1 transit system insulators are exposed to rain and pollution, both of which can create a low-resistance electrical path that leads to fire. Strain insulators experience the lowest number of fires. In addition to the delays and passenger losses, costs of more than US$400 are expected as a result of the operational and maintenance costs of each insulator. The Case 1 agency performs semiannual preventive maintenance inspections, event-driven inspections (e.g., weather special events such as snow, icing, tornadoes, and the like), and post incident examinations, and uses historical maintenance data to identify failure trends. It also conducts research and improves designs to reduce the maintenance requirements of the C H A P T E R 4
16 Third Rail Insulator Failures: Current State of the Practice insulators and employs maintenance-management tracking software and standard operating procedures for inspections and maintenance. The third rail safety standards and regulations are updated by the agency every 4 or 5 years to provide continual improvement to the systemâs reliability. Every 6 months, it conducts safety compliance inspections and investigations. Multiple vendors provide different makes and models of insulators to Case 1, which enables the agency to implement a variety of mounting configurations. Gravity or clipped third rail on cast-iron top and insulated post with a reinforced base or a cast-iron base bolted to a tie are the most commonly used. The Case 1 agency provides 6 to 10 inches of clearance distance around and above insulators during installation. The insulator installation process for Case 1 is to ⢠Install cable in duct banks, ⢠Protect cables with pothead covers where they exit from ducts, ⢠Provide slack for vibration and movement of third rail, and ⢠Cover surfaces with insulating hose. All the contact rails in the Case 1 transit system are top contact. Of the 242 miles of contact rail in this system, 25 miles are steel, 5 miles are aluminum, and 212 miles are composite. To prevent insulator failures or at least minimize the rate of their occurrence, the Case 1 transit agency established a dedicated maintenance crew to clean the insulators and power components in the tunnels and walkways. Their cleaning requirements include ⢠Restrictions on apparel that can be worn during the cleaning of insulator surfaces, ⢠Rules pertaining to electrical safety, and ⢠Rules pertaining to health and safety. Truck-exhaust stack brushes are used to scrub the insulators, using water and an electrical cleaner, and they are dried with towels. The main drawbacks of Case 1âs insulator cleaning are that it is labor- and time-intensive, but the insulators are positioned in the tunnels so that the cleaners do not encounter any issues. The Case 1 transit system implements safety considerations; however, safety failure events (e.g., smoke events resulting from an arcing phenomenon in insulators, high-intensity fires resulting from short circuits, and the like) have been reported. In addition, the system has experienced damage to electrical propulsion equipment and poor track conditions as a result of electrolysis and corrosion from stray currents. Agency Case 1 Third Rail System Voltage (V) 750-1,000 Number of Third Rail Insulators 176,000 Insulator Material Porcelain, fiberglass, andwood Number of Passengers (Million Passenger Trips/Year) 100 Insulator Failure Rate 1/15,000 Transit Modes Bus, heavy rail, anddemand-responsive Loss of Passenger Hours (Passenger Trips/Year) 5,000 Average Annual Delays (Hours) 10 Operational Cost per Insulator Failure ($) 400 Types of Contact Rail Top contact Insulator Cleaning Status Implemented Average Cost of an Insulator ($) 300 Table 3. Summary information for Case 1.
Case Examples 17 4.2 Case 2 This agency operates both heavy and light rail and handles an average of 50 million pas- senger trips per year. The nominal traction power system voltage is 750 to 1,000 V, and the system can regenerate its voltage while it is operating. Of the approximately 19,100 insulators, 100 are porcelain, 1,500 are epoxy, and 17,500 are fiberglass. All the porcelain insulators are installed outside the tunnels, and 66% of the epoxy insulators (1,000) and 43% of the fiber- glass insulators (7,500) are installed in the tunnels. Metal frames or pipes are used to mount the insulators. The 28 miles of contact rails are steel and of the top contact type. A summary of the information pertaining to Case 2 is shown in Table 4. An average of 25 insulator failures per year in this transit system equates to a rate of 1 out of 1,000 insulator failures, which cause a loss of less than 1,000 passenger hours per year. Strain insulators are favored by the agency because they are less prone to fire, which is a main concern in third rail power systems. A clearance distance of 16 to 20 inches is provided around the insulators for safety purposes and more reliable operation. In addition, a safety cover, as well as brackets and anchors, are used to protect them from environmental events and to prevent the public from touching the insulators. Annually, an average delay of 30 to 40 hours is attributed to the failure of insulators. Dirt buildup and damages from impacts are the most frequent causes of insulator failures, while vandalism and lightning are not problems at all. Flashover/arcing is a rare cause; the rest of the causes only occur occasionally. Voltage surges are also reported by the Case 2 agency. Five types of contaminants affect this system: iron, rust, carbon dust, dirt, and grime. Maintenance operations cost more than US$400 per insulator failure in the Case 2 tran- sit system, but conducting maintenance and inspections helps reduce the number of fail- ures. Various preventive actions are taken, including cleaning the insulators and surfaces. A specialized pressure washer is used for the surface cleaning because there is insufficient clearance around the insulators for the cleaners to reach the bottom of the insulator. The insulator cleaning device is adjustable and helps the cleaner to clean more thoroughly. An annual inspection of all the insulators is performed by the agency to identify the insulators that are more likely to fail. Cleaning and maintenance are performed every month, both inside and outside the tunnels. Hipot insulation testing and infrared inspections are performed to avoid insulator fires. Inspections are performed frequently to increase the reliability of the Case 2 system and reduce insulator failures, and the substationâs maintenance and third rail maintenance procedures are documented as a preventive measure. Table 4. Summary information for Case 2. Agency Case 2 Third Rail System Voltage (V) 750-1,000 Number of Third Rail Insulators 19,100 Insulator Material Porcelain, fiberglass, andepoxy Number of Passengers (Million Passenger Trips/Year) 50 Insulator Failure Rate 1/1,000 Transit Modes Heavy rail and light rail Loss of Passenger Hours (Passenger Trips/Year) 1,000 Average Annual Delay (Hours) 35 Operational Cost per Insulator Failure ($) 400 Types of Contact Rail Top contact Insulator Cleaning Status Implemented Average Cost of an Insulator ($) 350
18 Third Rail Insulator Failures: Current State of the Practice Several safety-failure events have occurred in the Case 2 systems, including smoke events caused by arcing, damage to electrical propulsion equipment, and poor track conditions caused by corrosion and electrolysis. Safety compliance inspections and investigations are conducted once a year, which increases the safety of the workers. The maintenance workers also document any unsafe conditions, and the managers correct them. The porcelain insulators used by this agency cost more than US$500, while the fiberglass insulators cost between US$250 and $500. Electrical current leakages cost the system between US$20,000 and $30,000 annually. 4.3 Case 3 The agency in Case 3 provides commuter rail, subways, buses, and paratransit services. Even though it operates four of the major terrestrial mass-transit vehicles, approximately 40% of the 50 million passengers who use this transit system in a year are transited by a third rail system. Voltage of 600-750 V is used for this system where third rail is implemented, and regeneration of less than 600 V is possible when the system is operation. The rated insulator voltage is 1,000 V/min for a 72-hour water soak and 2,200 V/min for a 3-minute water soak with dielectric strength of 465 V/mile. The common mounting configuration of insulators is performed by inserting lagging screws into wood ties. All the contact rails are made of composite and are of the top contact type. All the insulators in the Case 3 third rail system are made of epoxy, and this transit agency has had recent insulator material issues. In particular, cracking of the insulator between the base and stem has happened frequently, and the fracture lines accelerate insulator failure with carbon and dirt buildup. For a period, the agency had suspended purchasing insulators from a longtime supplier until quality assurance/quality control issues were addressed. After several in-depth studies of the failed insulators, it was determined the resin-fiberglass ratio was not being met. This agency concluded that not enough resin was being used in the insulator production. The agency has since changed their specifications to address future purchases, and they are evaluating other insulator epoxy/fiberglass designs. This agency has reported an average of 15 insulator failures every year, inside and outside the tunnels, and they cause loss of less than 1,000 passenger hours per year, which means a maximum passenger loss rate of 2/100,000. The number of failures in renewed and replaced infrastructures is five per year, which is one-third of the average of all insulators. A maximum of 20 hours of delay is expected every year in the current Case 3 system resulting from insulator failures, and the maintenance and operational cost per insulator failure is around US$250 to $300. The average cost of an epoxy insulator used in Case 3 is US$50 to $100. Clearance of 16 to 20 inches around and above the installed insulators is mandatory. The summary of information relative to Case 3 is shown in Table 5. The insulators in this transit system are exposed to both wet and dry weather conditions, which can increase the rate of failure. Water infiltration is an environmental issue for the insu- lators in tunnels, as is the saltwater resulting from deicing roads adjacent to the tracks over bridges that drips and splashes onto the insulators and causes them to corrode. Carbon dust is the most common contaminant that causes insulator issues in this system, and water infiltration and dirt buildup are the most frequent causes of insulator failures. Snow and ice accumulation, sunlight/ ultraviolet (UV), mechanical stress, salt fog/air, erosion, lightning, and vandalism are not recog- nized as main causes of insulator failure. The following safety failures have occurred in Case 3: ⢠Smoke events as a result of arcing of insulators, ⢠Explosions due to flashovers,
Case Examples 19 ⢠Fires caused by electrical short circuits, and ⢠Poor track conditions due to electrolysis and corrosion. Safety compliance inspections and investigations are conducted by the Case 3 agency every 2 years. They rely on the lessons learned from previous failures to minimize their insulator failures. Visual inspections and heat testing also serve as predictive indicators of future failures, and the identified insulators are replaced. 4.4 Case 4 The Case 4 transit system is heavy rail with nominal traction power voltage of 600-750 V, and its dielectric withstand for wet and dry over-voltage is 15,000 V and 60 hertz (Hz), respec- tively. It conducts approximately 50 million passenger trips per year. Approximately 134,000 insu- lators are used in this system, of which 106,000 are made of porcelain and 28,000 are made of fiberglass. The maximum rate of insulator failure in this system is 1/10,000. Fire instances some- times occur in the insulators of Case 4, which all are of the post type. For safety purposes, a clearance space of 11 to 15 inches is provided. A summary of information pertaining to Case 4 is shown in Table 6. An average of 1 hour of delay is estimated for each of the insulator failures in the system, which equates to 20 hours of delays annually. The estimated operational and maintenance cost per insulator failure in Case 4 is around US$350 to $400, while new fiberglass insulators cost less than US$50 and porcelain insulators cost US$50 to $100. Agency Case 3 Third Rail System Voltage (V) 600-750 Number of Third Rail Insulators 140,000 Insulator Material Epoxy Number of Passengers (Million Passenger Trips/Year) 50 Insulator Failure Rate 1/50,000 Transit Modes Subway, bus, commuter rail, and paratransit Loss of Passenger Hours (Passenger Trips/Year) 1,000 Average Annual Delay (Hours) 20 Operational Cost per Insulator Failure ($) 275 Types of Contact Rail Top contact Insulator Cleaning Status Not implemented Average Cost of an Insulator ($) 75 Table 5. Summary information for Case 3. Top contact Not implemented 75 Case 4 600-750 134,000 Fiberglass and porcelain 50 1/10,000 Heavy rail 1,000 20 375 Types of Contact Rail Insulator Cleaning Status Average Cost of an Insulator ($) Agency Third Rail System Voltage (V) Number of Third Rail Insulators Insulator Material Number of Passengers (Million Passenger Trips/Year) Insulator Failure Rate Transit Modes Loss of Passenger Hours (Passenger Trips/Year) Average Annual Delay (Hours) Operational Cost per Insulator Failure ($) Table 6. Summary information for Case 4.
20 Third Rail Insulator Failures: Current State of the Practice The configuration of most of the insulators is a base with two holes, through which bolts are installed and anchored in concrete. All the contact rails are the top contact type; 80 miles are made of steel and 20 miles are made of composite. Cracks/fractures and flashovers/arcing are the most frequent causes of insulator failure in the Case 4 third rail system. Insulators also fail because of stray currents, water infiltration, and weathering. Once an arcing insulator is reported, the crew replaces it during the night. Thermal imaging is used to detect which insulators are hot; however, not all the hot insulators catch fire, and they may remain in the system for a long time without causing any issues. The specifications for the insulator materials that are currently used in their systems ensure that the smoke from burning materials is as minimally toxic to individuals as possible. The agency is studying thermoplastic and fiberglass insulators with smooth surfaces for future use in their system. They update their regulations frequently and annually update the risk-based model that guides their inspectors. 4.5 Case 5 The agency for Case 5 operates commuter rail, bus, rapid transit, and light rail. The nominal traction power voltage is 600 to 750 V, and the regeneration capability is between 750 and 1,000 V. The rated insulator voltage and its dielectric withstand for wet and dry over-voltage is 750 V. The transit system makes more than 100 million passenger trips annually. Over 72,000 insulators are used in this system: 55,000 out of tunnels and 17,000 inside tunnels. Approximately 10,000 insulators are made of porcelain and 62,000 are made of fiber- glass. The average cost of a porcelain insulator is US$50 to $100, which is far less than the US$250 operational cost of an insulator failure. Annually, six to 10 insulator failures occur in the Case 5 third rail system, which equates to a maximum failure rate of 2/10,000. The failure rate is the same for every type of insulator, irrespective of whether the insulators or infrastructure have been replaced with new ones. The failures cause a passenger loss of around 1,000 hours and 10 hours of delays every year. Hangars, pedestals, and stands are used to form the common insulator configuration. The impacts of failure on Case 5 operations are waste of labor, accelerated material costs, loss of ridership, and lost trust of passengers. The summary of information for Case 5 is shown in Table 7. All the 86.6 miles of contact rails in Case 5 are made of steel, with 23.6 miles of bottom contact type and 63 miles of top contact type. Brackets, safety covers, and anchors are used to restrict Agency Case 5 Third Rail System Voltage (V) 600-750 Number of Third Rail Insulators 72,000 Insulator Material Fiberglass and porcelain Number of Passengers (Million Passenger Trips/Year) 100 Insulator Failure Rate 1/5,000 Transit Modes Bus, rapid transit, commuter rail, and light rail Loss of Passenger Hours (Passenger Trips/Year) 1,000 Average Annual Delay (Hours) 10 Operational Cost per Insulator Failure ($) 250 Types of Contact Rail Top contact and bottomcontact Insulator Cleaning Status Not implemented Average Cost of an Insulator ($) 75 Table 7. Summary information for Case 5.
Case Examples 21 access to the insulators. Snap-on cover boards on under-running third rails and protection boards on over-running third rails limit access and require outages for removal and service. Dirt buildup, water infiltration, and mechanical stress are the most observed causes of insu- lator failure, and hot weather and dry weather are the basic environmental causes of insulator failures. Water infiltration in tunnels also causes insulator failures by straying currents that lead to flashovers. The biggest problem is saltwater resulting from deicing roads adjacent to the tracks or over bridges that drips or splashes onto the insulators. The transit agency in Case 5 has a dedicated maintenance crew to clean tunnel walkways of debris. Station areas are cleaned every week, and the entire tunnel is cleaned once a year. Various contaminants that can cause insulator failures in Case 5 third rail systems are iron, rust, carbon dust, dirt, and grime. Periodically, the voltage is measured on hook bolts to check whether current is leaking through worn insulators. To minimize insulator failure, Case 5 is using flex seal over covers/ joints in known problem areas to prevent saltwater from penetrating. They have moved many of the third rails from under platforms and outlawed salt on platforms where it was still being used. The regulations are updated frequently, and safety compliance inspections and investiga- tions are performed as well. The case example questionnaire and case example interview guide are shown in Appendices C and D.
