David B. Kiddoo
AlphaGary Corp.
Just as we do for all decisions in life, we must continue to look for the best balance of choices in making decisions for fire-safe cables. Flame spread, smoke generation and carbon monoxide are more likely to kill people during a fire than acid-gas generation from a halogen cable. While undesirable, acid gases generated by cable combustion in a fire are a minor factor in fire-fighting and saving lives, compared to these other hazards. As the cabling industry moves toward more-stringent cable standards for flame spread, smoke containment and corrosion, and away from the "halogen vs. non-halogen" approach, what is advocated is to find the best balance of cable materials--halogen and non-halogen--to minimize risk.
The National Electrical Code (NEC) does not specify materials to be used in any cable construction. Rather, the code is performance-based, with the specific objective of "prescribing minimum requirements for safety to life and property." The National Fire Protection Association (nfpa--Quincy, MA), in developing the NEC, recognized that to address the risk of fire properly requires a total risk assessment. This involves various factors, including ease of ignition, flame-spread characteristics, rate of heat release and smoke yield.
Balance of properties
The NEC also strongly promotes "application-based" cable-testing criteria that simulate the environment of a building; for example, plenum or riser areas. It is not a question of a single material--such as the halogen sheath on a cable--but a balance of properties that minimizes the risk of fire hazards in a particular environment.
In fact, a halogen jacket and insulation in a cable are extremely effective fire suppressants.
In a letter to the industry, J. Michael Davis, global business manager, and James R. Hoover, global regulatory manager, at DuPont (Wilmington, DE), the leading manufacturer of FEP (fluorinated ethylene propylene), have clarified some facts about halogen materials, including the following:
Halogen-containing cables can emit toxic materials upon combustion. However, non-halogen materials also emit toxic materials, and at significantly lower combustion temperatures.
Non-halogens, which burn at lower temperatures, will begin to emit carbon monoxide, a lethal gas, much earlier than halogens. Halogen-containing materials, in fact, do not burn at all at these same lower temperatures.
There is no statistically significant difference in the LC50 (life-count 50%) toxicity values for halogen-containing cables compared with the most commonly used non-halogen cables.
DuPont is unaware of any human fatalities associated with combustion of any DuPont fluoropolymer or halogen-containing cable in any industry or application spanning over 50 years of sale and use.
International standards for cabling address three independent testing criteria: flame spread (how far the cable burns), smoke density (how much smoke is produced) and toxicity, while U.S. codes address application-based, full-scale environmental testing criteria.
U.S. standards for fire resistance are more stringent than the international codes, and promote human safety by setting criteria that allow time for evacuation in a developing fire. Three levels of fire-resistance are defined. The first applies to general-purpose cabling. The second applies to cables in building risers. The toughest standard is for cabling installed in plenum spaces.
In the NEC, a plenum space is defined as "a compartment or chamber to which one or more air ducts are connected and that forms part of the air-distribution system." Effectively, a plenum space can be any concealed area in a building where air can transfer horizontally from office to office--for example, a drop ceiling or a raised floor.
Alternatives to using plenum-rated cable include using non-halogen cable in metal conduit and plenum-rated cable trays. Also, placing adequate firestopping materials according to code in a cable installation will provide additional fire-safety protection.
The nfpa and international engineering standards groups constantly assess fire-hazard risks in each unique environment. They recommend testing standards that are relevant to the particular scenario.
David B. Kiddo is business manager for wire and cable insulation adn jacketing products at AlphaGary Corp., Leominster, MA.