Fighting Fire with Wire

Limited Combustible — The New Standard for Fire Safety

By Daniel Kennefick,
Limited Combustible Product Business Manager, Berk-Tek, A Nexans Company

Since the advent of client server computingin the early 1980s, building codeofficials have concerned themselves withthe buildup of cables in air return plenumspaces. The development of local area network(LAN) cables to support the rapid increase inpersonal computer use, plus the unacceptablepractice of leaving these cables “abandoned inplace,” has forced code officials and theNational Fire Protection Association (NFPA) totake a hard look at the fire safety level of LANcables and other low voltage cables.

Fortunately, new low voltage cables were developed to dramatically improve fire safety properties. The new fire safety level these cables offer is called limited combustible (LC) — “the new standard for fire safety.”

A History of Cable Fire Safety

Until 1975, low voltage cable included communications, signal and control, fiber optic, fire alarm, and CATV/broadband. Installation in conduit was required for these cables. Building power cables, however, has required installation in conduit for some time to both protect people from electrical shock and deliver a cable fault to ground. In addition, the NFPA required the use of conduit for building power circuits to prevent the spread of fire in the event of a catastrophic cable failure.

With respect to low voltage circuits, the NFPA and national codes officials say the energy developed in a low voltage circuit was not enough to create a spark or a fire. The volume of low voltage cables in a typical building was not substantial enough to warrant any unique fire safety precautions, such as the use of conduit.

The first major change to cable fire safety came in 1975 when the National Electrical Code (NEC) mandated that all low voltage circuits be installed in metal conduit. This change was brought about by a number of devastating fires, the most influential of which was the MGM Grand Hotel and Casino 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.

Around the time this change was implemented, DuPont®, AT&T, and UL developed and introduced a cable that had similar fire safety properties as cable in conduit. The test, developed at UL, was a variant of the Steiner Tunnel test used to list building materials such as ceiling tile and wallboard. The pass/fail criteria for the test were calibrated to the cable in conduit performance, and this new cable and test were introduced to the NEC during the 1978 code cycle. This cable option was approved and adopted by the NEC in 1978.

This “exception” test is what we know today as the “plenum cable test.” Despite the high cost of this new cable versus the nonplenum cable used at the time, the use of this “exception cable” quickly became the norm across the country. Users quickly found that because they no longer had to use conduit, the installed cost of low voltage cabling dropped by at least 50 percent when they used plenum cable. The

Development of LC Cable

As noted, client server computing quickly took off in the 1980s, and the volume of low voltage cables installed in buildings began to rise exponentially. Many of these cables were left abandoned in place as network technologies and their supporting cables evolved.

By the early 1990s, this increase of cable density in building air return plenums caught the attention of code officials, and work was initiated to both eliminate the abandoned cables and develop a more flameretardant cable than plenum cable. Ideally, code officials wanted a cable that was as flame retardant as the building materials used to construct air return plenums.

This development was completed in 2001 with the announcement of the first LC cable design. This cable was burned in the same test as ceiling tile and wallboard and passed the same smoke emission and flame spread criteria. This new fire safe cable design offered two particular attributes for which code officials were looking:

• Up to 20 times lower smoke emissions than plenum rated cable

• Three to five times lower fuel load than plenum rated cable

Lower smoke emissions and less fuel load are two attributes that will help reduce the effect cable will have on building fire safety now and in the future.

Comparing LC Cable with Plenum Cable

To significantly improve the fire safety of LAN copper cable, material changes were required. As an example, the individual conductors in Category 5e and Category 6 plenum rated cables are insulated with FEP (fluorinated ethylene propylene). This material is more familiar to some by the DuPont trade name Teflon®. The FEP cable core is then jacketed with a flame-retarded, smoke-suppressed grade of PVC.

To create an LC cable, the PVC jacket is replaced with a specially formulated FEP jacket (see Figure 1). The FEP compound used by Berk-Tek in copper and fiber optic LC cables is an exciting new development from Alpha Gary and DuPont. It offers the low smoke emission and flame spread of FEP but is flexible and can be pigmented like PVC.

By using these materials in combination, the cables become “DuPont certified” and are marketed by Berk-Tek as DuPont-certified LC cable. As an all FEP design, Berk-Tek’s DuPont-certified LC cable offers many attributes that are superior to plenum cable:

• Less smoke emissions and flame spread

• Better low temperature and high temperature UL ratings

• Improved electrical performance at elevated temperatures

• Contains no lead

• Easier to pull — low coefficient of friction

• More flexible than a natural FEP jacketed LC cable

• Smaller than plenum cable — greater tray and pathway fill

• 100% recyclable

Who’s Interested in LC Cable?

Since its development, two primary groups of end users have taken an interest in LC cable — those who need to create more “time to exit” large numbers of potentially slow-moving people and those protecting mission-critical facilities (MCFs).

Numerous end users in both of these categories are considering specifying LC cable or have already installed it to improve building fire safety.

Expected Code Evolution of LC Cable

Listing programs for LC cables exist at both major independent test labs in the United States — UL and ITS/ETL Semko. Berk-Tek has DuPont-certified limited combustible LAN copper cable listings for Category 3 through LANmark-2000 enhanced Category 6. Berk-Tek is also the first to launch an LC fiber optic cable — Adventum LC.

These cables can be installed today in any fire safety classification environment, including a plenum environment. LC cable is considered “best practice” and has fire safety performance above and beyond plenum cable. For LC cable to achieve “mainstream” use, it must appear in the industry’s primary fire safety documents. Figure 3 shows the fire safety codes and standards under the NFPA, which influence the wire and cable industry.

Electrical inspectors have historically used NFPA 70 (the NEC) as a reference document to certify cable installations. Over the years, the NFPA has done a fairly good job of ensuring codes and standards with common areas of interest are harmonized. With the addition of LC cable to the code development process, we are at the initial stage of a major change that will take a number of years to harmonize throughout the affected codes and standards.

The NFPA 90A Heating and Ventilating Equipment Code is a very important code for the wire and cable industry, as this group acts as the “landlord” for the air return plenum space. This group was the first group to allow plenum cable in its space. The NEC followed NFPA 90A in 1978 to harmonize the two groups.

The 90A group is currently in the middle of its 2006 code cycle and is considering the addition of LC cable to its code because of a special clause in the NFPA 13 Sprinkler Code. The Sprinkler Code states:

8.14.1.5: Localized Protection of Exposed Combustible Construction or Exposed Combustibles … concealed spaces having exposed combustible construction, or containing exposed combustibles …

The combustibles shall be protected as follows:

If the exposed combustibles are in the horizontal plane, the area of the combustibles shall be permitted to be protected with sprinklers … If LC cable is used in these sprinkler buildings in a concealed space, such as an air return plenum, then the following applies from NFPA 13:

8.14.1.2: Concealed Spaces Not Requiring Sprinkler Protection

8.14.1.2.1: Noncombustible and limited combustible concealed spaces with no combustible loading … The space shall be considered a concealed space even with small openings such as those used as return air for a plenum.

Thus, in sprinkler buildings, if combustible materials (plenum cables are considered combustible because they are not LC) are installed in concealed spaces such as air return plenums, then the space must be sprinkler. If LC cable is used, then the space does not need to be sprinkler.

NFPA 90A is attempting to harmonize with the Sprinkler Code on this issue. Thus far, this recommendation has passed through the committee and will undergo a vote in the NFPA national meeting. If it passes the national vote, it will become part of the 2006 90A Code.

To complete the harmonization process, the inclusion of this Sprinkler Code clause will be recommended for the 2008 NEC in fall 2005. This recommended addition will go through the balloting and comment process for the next two and a half years.

If the cycle is completed, then the sprinkler clause will be added to the 2008 NEC. If the sprinkler clause for LC cable is included in the 2008 NEC, intense lobbying will occur with code officials, such as electrical and fire inspectors, to adopt this version of the code at their respective local levels. The NFPA 90A and NEC Fire Safety Hierarchy would then appear as shown in Figure 4.

Recommendations

While plenum cable has served the industry well, it is a 30-year-old technology that is no longer the best available. LC cable provides users with an improved level of fire safety that will allow them to more effectively protect people-dense buildings and MCFs such as data centers.

Berk-Tek’s DuPont-certified LC cables also offer additional benefits, such as improved flexibility and strip ability as well as an easy-to-read print legend. As the codes evolve to this more fire safe technology, users will utilize this cable in concert with other building fire safety improvements to create safer structures and more time to exit their occupants.

 

For more information about Berk-Tek’s DuPontcertified LC cables, visit www.berktek.com or call 1-800-BERK-TEK.