|NFPA Guide on Methods for Evaluating Fire Hazard to Occupants of Passenger Road Vehicles|
Issue 44: NFPA Guide on Methods for Evaluating Fire Hazard to Occupants of Passenger Road Vehicles
By Marcelo M. Hirschler, Ph.D.
NFPA 556, Guide on Methods for Evaluating Fire Hazard to Occupants of Passenger Road Vehicles1
is the first document issued by a standards organization to highlight
the problems of fires in road vehicles, especially automobiles. The
guide focuses on the high levels of fire losses (particularly fire
fatalities) not associated with fuel tank fires.
NFPA 556 identifies various scenarios for a fire to start: inside the
passenger compartment, inside the engine compartment, inside the trunk
or load carrying area, from pool fires resulting from fuel tank failure
and burning under the vehicle or from other external heat sources. In
every case, the key issue is fire penetration into the passenger
Insufficient road vehicle fire safety results, at least partially, from regulatory reliance on an inadequate fire test2
(Figure 1) designed to protect against cigarette ignition of interior
materials. There is no requirement for assessment of heat release of
individual materials, components or vehicle sections. NFPA 556 points
out that, as long as FMVSS 302 continues being the only fire safety
tool, vehicular fire losses are likely to remain high. This has been
discussed extensively elsewhere.3,4
The NFPA 556 structure is as follows:
Chapter 1: Scope, purpose, application, symbols
Chapters 4-5 provide background information. Figure 2 shows how very small fires starting in the engine compartment of vans quickly result in high duct temperatures, illustrating the speed of fire growth. Figure 3 shows how the van/car duct material has much higher heat release, when tested in the cone calorimeter (ASTM E 1354), than a fire retarded material of the same polymeric composition (polypropylene).
Engineering design approaches can be used to mitigate the effects of
fires on vehicle occupants (such as using proper barriers to separate
the engine compartment or to prevent penetration from pool fires).
However, the key means to decrease fire hazard is the use of materials
and/or products with appropriately improved fire properties, especially
lower heat release. Abundant work has shown the key importance of heat
release rate in fire hazard.5,6,7,8
Chapter 10 describes evaluation methods and tools suitable for assessing the fire behavior of components, for any of the key fire scenarios. This includes both fire test methods suitable for fire safety engineering calculations (and does not include FMVSS 302) and guidance documents. Reaction-to-fire testing (typically heat release and ignitability) is essential but is not always enough, and fire resistance testing is also needed in some areas (such as fuel spills, windshields and fire stops). Specific tests for individual products (such as cables or carpets) will also probably play a role. The cone calorimeter is the premier test that can be used to obtain needed fire test data (Figure 5). However, the guide notes that test methods alone may not be sufficient to complete a fire hazard analysis and NFPA 556 references standard documents which offer additional ways of assessing fire hazard, including vehicle redesign.
Chapter 12 summarizes the guidance to minimize fire hazard for each
scenario. Full scale tests conducted to assess the fire performance of
road vehicles have often analyzed one or more of the scenarios outlined
in the guide as most likely to cause harm. Such tests are most useful if
they assess heat release properties, and, additionally, are used to
obtain as many other relevant fire properties as possible (smoke
release, combustion gas release, heat fluxes, temperatures, and mass
loss). This will provide information on potential drawbacks of
alternative designs, with respect to properties other than heat release.
The testing of sections (individual compartments or individual fuel
packages), e.g. in a furniture calorimeter, will lead to an
understanding of local interactions. NFPA 556 recommends the cone
calorimeter as a suitable tool for choosing materials with desired fire
performance properties, especially because the instrument is capable of
assessing all the properties deemed to be most critical in one test.
NFPA 556 also describes the mass loss cone fire test, ASTM E 2102, as a
way to obtain ignitibility data under the same fire exposure conditions
as in the cone calorimeter. Mass loss data from the mass loss cone test
parallels heat release data from the cone calorimeter, at a much lower
instrument cost. This guidance chapter reinforces the concept that
testing needs to include also an overall analysis that indicates that
there will be an actual improvement in road vehicle fire safety.
It is hoped that the issuance of NFPA 556 by a consensus standards
development organization will lead to improvements in such fire safety,
perhaps including regulatory changes.
Marcelo Hirschler is with GBH International
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