Power limited fire alarm circuits are critical to the safety and well being of building occupants who find themselves in a burning building. During such an event it's critical that fire safety equipment such as fire pumps, fire alarm signaling equipment, elevators, and alarms remain operational for a specified amount of time. These circuits are considered essential communications paths during an evacuation or relocation of building occupants during a fire or emergency situation.
A change to 760.61, Application of Listed PLFA Cables, in the 2005 NEC now permits the use of Type FPLP-CI cable on installations where fire-alarm circuits must continue to operate during a fire. Circuit integrity (CI) cables are designed to provide a minimum 2-hour operational rating when exposed to extreme heat and fire. They're also designed to limit the fire load in a space and produce a low smoke output when burning. Many in the industry agree this is a positive change in the Code.
But what might be even more interesting with respect to this section of the Code is what didn't get changed.
Turning up the heat. Additional changes to 760.61 were meant to correlate with NFPA 90A, “Standard for the Installation of Air-Conditioning and Ventilating Systems — 2002,” by removing the option from the NEC to place cable in ducts and plenums [300.22(B)], and providing a listing, marking, and unique application requirement for limited combustible cable in other spaces used for environmental air, such as ceiling cavity plenums and raised floor plenums. However, controversy arose from the proposals submitted, which would have:
Deleted permission to install unlimited quantities of combustible plenum cable in air ducts and in plenums, other than ceiling cavity plenums and raised floor plenums.
Correlated NEC terminology with that of NFPA 90A.
The proposed effect of these changes would have been to modify applications of plenum cable from ducts, plenums, and other spaces used for environmental air to suitable for use in ceiling cavity plenums and raised floor plenums. But as you might imagine, there is much concern surrounding cable being installed in air ducts.
The purpose of a plenum is to transmit environmental air throughout a building. Therefore, the flame and smoke characteristics of all materials located within these spaces are very important. The proposed changes to 760.61 would have prohibited the placement of cables in ducts and plenums [300.22(B)], even though unlimited quantities of cables have been placed in air distribution systems since this requirement was added in the 1975 NEC.
In the 2002 NEC cycle, concerns over the increasing amounts of combustible cables in plenums were addressed when proposals were accepted to remove accessible portions of abandoned cables. But not all abandoned cables are accessible. Due to changes in construction of a building (i.e., where the area above a ceiling plenum has been sheet-rocked over, or where interlocking ceiling tiles are installed with no provision for access to the area above) some abandoned cables are inaccessible. This may be the case in commercial buildings.
Working on a solution. Anticipating that cables will be installed in ducts, plenums, and other spaces used for environmental air, what can be done to increase safety and reduce potential fire hazards, including toxicity of burning cables? A simple solution would be to ban the installation of all cables in these spaces. But that would still leave all of the existing cables that have already been installed in these spaces. The alternative is to specify cables with a very low smoke-producing characteristic that won't propagate flame when burning, which is included in Fine Print Notes. This is where NFPA 90A comes back into the picture.
During the 2005 NEC Code-change cycle, the NFPA Standards Council assigned primary jurisdiction for combustibles in plenums to the Technical Correlating Committee on Air Conditioning, and directed it to seek the cooperation of the committees on Fire Tests, National Electrical Code, and Safety to Life. The Technical Committee submitted proposals to correlate their efforts in revising NFPA 90A with those of the NEC.
NFPA 90A, which allows cables to be installed in plenums, requires cables installed in ceiling cavity plenums and raised floor plenums to be one of three types:
Listed noncombustible cable
Listed limited combustible cable
Listed cable that meets the requirements of NFPA 262, “Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces”
Each type of cable has different fire resistance characteristics. These cables are listed in descending order of fire resistance, similar to Table 760.82(I). The Fine Print Note (FPN) of 760.82(I) states, “Cable types are listed in descending order of fire resistance rating.”
For a cable to be classified as limited combustible it must have a maximum potential heat of 8,141 J/g and a maximum flame spread index of 25. One method of defining a low potential heat cable is establishing an acceptable value of potential heat when tested in accordance with NFPA 259, “Standard Test Method for Potential Heat of Building Materials.” One method of defining a low flame spread cable is establishing an acceptable value of flame spread when tested in accordance with NFPA 255, “Standard Method of Test of Surface Burning Characteristics of Building Materials.” Listed combustible cable is defined by these three requirements:
- Maximum potential heat
- Maximum flame spread index
- Maximum smoke developed index
NFPA 90A requires that supplementary materials for air distribution systems have a maximum flame spread index of 25 and a maximum smoke developed index of 50.
This leaves you with two types of cable that are currently suitable for this type of installation. Cables that meet the requirements of supplementary material could be used in limited applications, but they couldn't be used in ceiling cavity plenums and raised floor plenums. Limited combustible cables could be used because they meet all the requirements for supplementary materials while also meeting the requirements for use in ceiling cavity and raised floor plenums.
But this also potentially opens the door for the use of new fire alarm power-limited duct cable (FPLD), which is a limited fire hazard cable that offers a significantly lower fuel load and lower smoke production features than conventional Type FPL plenum cables. However, there are concerns related to the use of “duct cable” or “limited combustible cable,” including toxicity of the newly specified product and relative incapacitation factor presented by the chemical constituents of the polymer in the new cable design.
The jacketing and insulating materials used in duct cable and limited combustible cable is subject to heat decomposition and the emission of sub-lethal toxic fumes. Some of these fumes can incapacitate the building occupants. A recent study by the NFPA Fire Protection Research Foundation has advanced an international effort to make certain that people can escape a burning building before being overcome by smoke or gases generated by thermal decomposition. This work is part of a revolution in fire safety in which codes and standards are beginning to address how much smoke or gases generated by thermal decomposition will incapacitate people, rather than how much will kill them. Some people will also argue that the requirements for using duct cable have failed to recognize toxicity or emissions that are essentially colorless, such as hydrogen fluoride, which converts to hydrofluoric acid upon contact with any mixture, and other toxic gases may be generated.
Nowhere in the NEC will you find the term “limited combustible” cable. That's because the NFPA Standards Council has concluded that the term “limited combustible” isn't appropriate to cable. Although there was talk of adding this term to the 2005 NEC, the Standards Council agreed the best course of action for them to take with regard to this subject was to refrain from making revisions that would interrelate with NFPA 90A, “Standard for the Installation of Air-Conditioning and Ventilating Systems.”
The Standards Council is now waiting for completion of the NFPA 90A revision cycle before it proceeds with making further changes to the NEC. Many issues must be addressed and agreed upon throughout the 2008 NEC Code change cycle. One thing is a sure bet: Code-Making Panels No. 3 and 16 have a lot of work ahead of them related to these delicate cabling issues.