Enforce the 'no smoking' policy: limited-combustible fiber-optic cables resist the spread of fire and smoke: limited-combustible cables such as the one above do not ignite as easily or burn independently under the same fire conditions that a plenum cable will ignite and propagate flames

Communications News, July, 2005 by Todd Harpel

The advent of client-server computing in the early 1980s and the development of the LAN to support the rapid increase in the use of personal computers since then has drastically increased the amount of installed communication cabling. As a result, building code officials are concerned about the buildup of cables in air-return plenum spaces, where much of this cabling is installed, because of its relatively easy access and almost universal proximity to work areas.

In addition, as new generations of LAN cabling systems are installed, the unacceptable practice of leaving the old cables abandoned in place has forced code officials and the National Fire Protection Association (NFPA) to look at the fire safety level of LAN cables and other low-voltage cables.

Until 1975, low-voltage cables were not required to be installed in conduit, including communications, signal and control, fiber-optic, fire alarm and CATV/ broadband cables. This changed when the National Electrical Code (NEC) mandated that all low-voltage circuits be installed in metal conduit.

This change was reinforced by a number of devastating fires, the most influential of which was the MGM Grand Hotel fire in Las Vegas. In this fire, combustible materials, including cables in concealed spaces, burned with such speed and intensity that a "flashover" situation developed in the casino and 84 people lost their lives.

In 1978, AT&T and DuPont developed cables that exhibited flammability properties that were similar to the fire safety properties of cable in conduit. Underwriters Laboratories (UL) then developed a standardized test that allowed this cable option to be approved and adopted by the NEC--known today as the "plenum cable test." Although plenum cables have always been more expensive than standard riser-rated PVC insulated cables, they cost significantly less than having to install cables in metal conduit.

As the amount and density of plenum-rated cables installed in plenum spaces has increased over the past 25-plus years, there has been a growing concern regarding abandoned cables, as well as the fuel load represented by these cables. By the early 1990s, work was initiated by code officials to mandate the removal of abandoned cables and to develop a cable that was more flame retardant than plenum cable. Ideally, code officials wanted a cable that was as flame retardant as the building materials used to construct air return plenums.

LIMITED-COMBUSTIBLE CABLE INTRODUCED

In 2001, the first "limited-combustible" LAN cables were introduced. These cables exhibit similar fire-performance characteristics as those required for other building materials. The first limited-combustible LAN cables to be listed by UL were copper twisted-pair cables, which produce up to 20 times less smoke and represent three to five times less potential fuel load than similar plenum-rated cables. Today, there are several manufacturers who offer limited-combustible copper LAN cables.

As the focus turned to developing optical-fiber cables that would meet the limited-combustible requirements, of particular concern were the identification or development of the basic cable materials necessary to obtain the fire performance desired and the cable's ability to comply with existing mechanical, environmental and physical performance specifications. These challenges were overcome with the listing of the first limited-combustible optical-fiber cable by UL in January.

Limited-combustible cables resist the spread of a fire more effectively than today's plenum cables. Demonstrations and testing done on both limited-combustible and plenum cables have shown that limited-combustible cables do not ignite as easily or burn independently under the same fire conditions that a plenum cable will ignite and propagate flames along the cable itself.

More importantly, however, is that limited combustible cables generate up to 20 times less smoke than plenum cables. Smoke inhalation is the cause of most injuries and even deaths in a fire event. Smoke also has the potential to damage electronic equipment even when a fire has not.

Compounds that are present in the smoke can deposit on sensitive electronic circuits and cause chemical reactions that can eventually destroy them. In addition, many of the particles that are part of the smoke itself can become electrically charged, causing short circuits when they come into contact with circuit boards and other sensitive electronics.

STANDARDS BEING HARMONIZED

Because limited-combustible optical-fiber cables generate such a small amount of smoke during a fire, they can help protect the life safety of the building occupants, as well as provide improved asset protection for mission-critical electronic equipment.

Although there has been recognition within the standards bodies of the existence of the new limited-combustible cable, it has not yet been referenced or required by the codes that govern fire safety in buildings today. NFPA 70, also known as the National Electrical Code, is the reference document used to certify cable installations. The NFPA 90A Heating and Ventilating Equipment Code, however, is the "landlord" for the air-return plenum space. This code was the first to allow plenum cable in the plenum space.