Safety in public places is one of the most important considerations for civil engineers and public service providers. High-traffic and frequent-use areas are especially crucial for ensuring that all functional assets are operational and secure, as well as having specific measures in place to prevent accidents from escalating when they occur. Public transport is one of the main facilities that many rely on daily. It is essential that the entire network is in good working condition, as any downtime or incidents can be critical not only to its users, but also its providers and local authorities.
The United States has a mass transit tunnel system spanning millions of passenger miles, where trains, trams, buses, and other vehicles operate, carrying millions of commuters on board. While tunnel accidents are rare, the consequences can be quite dramatic when they do happen. Examples of well-known fire incidents include Caldecott Tunnel in California (1982), Mont Blanc Tunnel in France (1999), Howard Street Tunnel in Maryland (2001), and the Channel Tunnel between the UK and France (2008) that together claimed numerous victims.
Fire incident data obtained from eight of the world’s top 12 transit agencies in North America and Europe over the period 1998-2009 revealed that 88 percent of fires occur in the trainway or the station, and 12 percent of the reported fires involve the passenger vehicle (Aon, 2011). These enclosed spaces pose significant danger due to blocked exits by fire or vehicles, limited visibility due to smoke, as well as potential damage by incoming transportation.
Reviewing the standards
There are codes in place to help stop the tunnel infrastructure, such as key electrical life safety circuits, from catching fire and failing to maintain emergency lighting, alarms, ventilation fans, and exit signs. However, not all are able to withstand extreme temperatures, and subsequently can cause further harm. Code NFPA 130: Standard for Fixed Guideway Transit and Passenger Rail Systems is an international safety regulation used for the design of mass transit systems. It includes guidelines for ensuring life safety and fire protection to public transport, emergency lighting and alarms, ventilation, and control systems.
NFPA 130 determines the design and selection of materials that are fire safe by limiting flammability, requiring circuit integrity, and lowering smoke emission. When cables catch fire, the entire network of emergency assets can short circuit and fail. This would mean no lighting and no ventilation or smoke extraction. In addition, if wiring is unable to withstand high temperature, it creates smoke, which limits visibility and also increases the risk of toxicity, asphyxiation, and damage to electronics.
Underwriters Laboratories (UL) conducted a series of tests in 2012 and discovered a high rate of failure in two-hour fire-rated cables, with subsequent assessment also revealing inconsistent results in using certain cables (UL, 2015). As a result, UL delisted all two-hour fire-rated cables in September 2012.
UL’s research found that the main reason for wiring failing circuit integrity testing was that the zinc melting in rigid metal conduits was combining with the copper conductors of the wire. When zinc reacts with copper conductors, forming brass, it lowers the melting temperature and causes a circuit integrity failure. Consequently, the support for emergency lighting or ventilation fan operation in the tunnel is stopped.
Another challenge UL identified is that cables were not reaching the two-hour fire-resistance level and failed within 30 to 40 minutes, which did not meet the industry safety standard. As a result, UL voided all UL 2196 two-hour fire-rated cables that required conduit, and prohibited their use if they included zinc. This required a resubmission for all cables in order to gain the new certification that would meet the updated standards.
Low-smoke zero halogen solution
While UL has yet to formalize a new UL 2196 test standard, it has implemented an interim certification program for all manufacturers and providers of fire-rated cables. The interim test process has added a number of key changes to help ensure the testing is adequate. Among the changes are increasing sample size, testing all electrical conductor sizes, toughening the requirements for vertical assessment, and only allowing listings for material actually tested, with no exceptions. This directory requires all fire-rated cable products to be retested in order to ensure consistency and a high level of safety across all applications.
One available solution to ensure maximum safety in transit tunnels and other applications is to use low-smoke zero halogen (LSZH) mineral-insulated (MI) fire-rated cables. These MI cables can support emergency lighting circuits and exhaust fans, especially when one- or two-hour circuit integrity is needed. Critical circuits, such as emergency lighting and ventilation, are required to use cables that are LSZH, or be completely encased in concrete.
Smoke density in a tunnel is a significant safety concern where visibility is minimal and smoke inhalation a potential risk. Halogens are also toxic and can corrode sensitive electronics. The key benefits of the LSZH MI cables are that they operate fully in fire well in excess of two hours and meet all the requirements of low-smoke cables.
LSZH MI cables are also smaller in diameter and do not need other protection, such as conduit, concrete encasement, or redundant routing. They are easier to install and maintain, resulting in reduced costs. These cables are suitable for applications where an outer jacket helps to stem oxidation that can occur in direct burial or concrete encasement.
Ensuring safety with new safety tests
LSZH MI cables have been tested according to the UL 2196 and ASTM E119 time and temperature curve parameters. Fully loaded circuits are attached to lights outside of a test furnace for a minimum of two hours at a peak temperature of 1,850°F (1,000°C) while the lights are required to stay lit during that time. Following this, the cables are sprayed with a fire hose and then reconnected to check whether the lights switch on again. If they do, the cables pass the test successfully and can be certified under the UL 2196 and ULC S139 standards.
Don Tremaglio is sales manager for Pentair Thermal Building Solutions (thermal.pentair.com). Pentair’s Thermal Building Solutions’ LSZH MI cables, Pyrotenax, fully comply with the UL requirements and can be used in a number of applications, including transit tunnel infrastructure. With a basic inorganic construction of a copper sheath and conductors, together with a mineral insulant (magnesium oxide; MgO), the cables have a long history of meeting industry standards and providing reliability in electrical circuit protection.