New developments in fire-reducing technologies help keep
plenum spaces out of sight and out of mind
Concerns are rising about the growing amount of voice, video, data, and security cables in commercial buildings required to service the ever-increasing demands of IT networks. More workstations, each with expanding capabilities and increasing bandwidth requirements, are taxing many buildings' communications infrastructure. And communications cabling, which carries important data packets to and from computer rooms, is now as common in building plenum spaces as ductwork. While most cable selections are based on electrical performance requirements, certain fire-rating factors that go beyond the minimum requirements are often overlooked.
This fact places each building at a greater fire risk with each new installation of communications cabling. As a result, much discussion in the industry has been focused on the requirements in the 2002 NEC, which call for the removal of the accessible portion of abandoned communications cabling. These concerns are also the thrust behind the genesis of a new cable technology that significantly reduces these fire safety risks.
National codes and standards. The communications cable most often used in commercial buildings is called “plenum” cabling. The construction of a typical 4-pair UTP (unshielded twisted-pair) plenum cable is shown in the Figure on page 20. It's designed for use specifically in hidden spaces within dropped ceilings that handle return airflows, also known as the plenum space. The National Fire Protection Association (NFPA) sets the plenum cable requirements based upon three mandatory attributes: smoke generation, flame spread, and fuel load.
In the early to mid '70s, plenum cables greatly increased the severity of the damage caused by several significant fires, among them the One World Trade Center in 1975. As a result, a consortium of industry partners convened to address communications cabling types used in this application. In the early '80s, the NFPA Committee 90A, which was primarily responsible for developing standards for the HVAC plenum space, issued primary requirements for all materials “exposed to the air flow” to be noncombustible or “limited combustible.”
Technically speaking, a limited combustible material is categorized as one with a smoke generation index of less than or equal to 50 (as compared to red oak which has a smoke generation index of 100), a flame spread index of less than 25, and a potential heat (fuel load) of less than 8.1 mega Joules/kilogram. The fuel load is the heat energy contained in a cable that could be released in the event of a fire.
At that time many communications cables were tested, but none passed such stringent flame, smoke, and fuel load requirements while also meeting the electrical performance characteristics outlined by the NEC and the Telecommunications Industry Association (TIA). The result of this testing and development was a compromised selection — allowed only as an exception to the original NFPA ruling — based on the best cable construction available at the time, which was combustible in nature and hazardous with respect to smoke generation levels.
More than 25 years later, this selection of combustible communications cable still represents the plenum cable we know today. But times have changed and the cabling industry is now able to meet NFPA's original challenge with a limited combustible cable.
Differences in materials equal differences in fire safety. One of the core challenges the cabling industry has faced has been finding materials that excel simultaneously at three often divergent goals:
Excellent electrical insulation properties on both copper and fiber.
Reasonable processing parameters for cable manufacturing.
High ratings on flammability, smoke generation, and fuel load.
The three most commonly used materials in cabling today are polyethylene (PE), polyvinyl chloride (PVC), and fluorinated ethylene propylene (FEP). Several different formulations of PVC jacketing materials and PE insulations are used in this industry.
When the performance of each is compared, PE offers excellent electrical properties for insulating copper. However, in a fire, it's highly combustible, extremely high in fuel load, and readily generates dense smoke, which creates significant life and equipment fire safety hazards and risks. PVC has poor electrical properties but offers better fire performance than PE. However, it's difficult to melt and has poor flexibility in cable applications. To resolve these issues, other materials, such as lead, are added to achieve greater processability, flexibility, and aging stability. The resultant PVC compound represents an inexpensive material that creates a relatively safe jacket for most plenum cables, but remains combustible in nature. Lastly, FEP has electrical insulation characteristics on copper equal to polyethylene and meets the performance criteria for flame spread, fuel load, and smoke generation.
In contrast, limited combustible cable encompasses several improvements that include optimum fire-rated materials of construction and a reduced cable size; it has a 10% smaller jacket.
Limited combustible cable. Through collaboration among industry customers, competitors, suppliers, and research firms like UL and NFPA's Fire Research Foundation (NFPRF), limited combustible cables are now readily available. In fact, the 2002 edition of NFPA 90A, Installation of Air Conditioning and Ventilating Systems, recognizes this cable requirement and listing as approved for use in plenum spaces. And the NEC is currently in the process of reviewing the limited combustible cable technology and may establish specific requirements as early as in the 2005 edition of the Code.
Limited combustible cables significantly exceed the flame, fuel load, and smoke ratings of today's minimal code-approved exception cables, enough to earn the limited combustible rating. In addition, they must pass a full array of other tests, including temperature aging, humidity, and jacket slitting, all of which may compromise the plenum cable safety and the integrity of data signals over time. These cables are also made with less plastic, which makes them easier to work with.
Many leading cable manufacturers currently offer 4-pair UTP, coaxial, and fire alarm limited combustible cables. The enhanced 4-pair UTP constructions come in everything from Cat. 3 to 6e (TIA Standards). The average increase in total installation costs of a system that includes limited combustible cables is less than 10% compared to conventional plenum cable. Limited combustible cable might even reduce overall installation costs if local codes require plenum sprinklers in sprinklered buildings. Check with your local inspector and reference the 2002 edition of NFPA 13, Installation of Sprinkler Systems.
Guerin is market development manager for DuPont Communications Cabling Solutions in Wilmington, Del.