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 compartment.

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

Figure 1: Photograph of FMVSS 302 Fire Test Apparatus

The NFPA 556 structure is as follows:

Chapter 1: Scope, purpose, application, symbols
Chapter 2: Referenced publications
Chapter 3: Definitions
Chapter 4: Types of vehicles
Chapter 5: Passenger road vehicle fires, statistics and background
Chapter 6: Approach to evaluating vehicle fire hazard
Chapter 7: Objectives and design criteria
Chapter 8: Selecting candidate design
Chapter 9: Typical fire scenarios to be investigated
Chapter 10: Evaluation methods and tools
Chapter 11: Individual fire scenarios
Chapter 12: Further guidance
Annex A: Explanations of issues from Chapters 1 through 12
Annex B: Fire retardants
Annexes C and D: Referenced publications and additional bibliography

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).

 

Figure 2 - Duct temperatures in van fire tests with a small
ignition source in engine compartment

Figure 3 – Cone calorimeter heat release rate for car duct
material (polypropylene) compared to a fire retarded
polypropylene, at an initial test heat flux of 40 kW/m2


Chapter 6 contains a flow chart describing how to use NFPA 556 (Figure 4). The chapter describes the performance-based approach that the guide recommends to design fire safe passenger road vehicles. The guide points out that fire safety is not the only concern and when evaluating fire properties; it is essential to ensure that all functional and other safety properties remain in place for the vehicle. However technology exists to ensure a successful combination of properties to achieve both.

 

Figure 4 - Flow chart contained in NFPA 556 (Chapter 6)


Chapters 7-9 discuss the methodology and point out that the extent and type of testing and/or analysis needed are a function of the extent of the necessary design change. While small- or intermediate-scale tests for ignitability and heat release would be adequate to replace a part of an existing vehicle, full-scale tests or mathematical modeling of fire growth and spread are probably needed for design of a complete new vehicle or of a major section. All analyses must consider the various scenarios of interest (as discussed above).

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.

 

Figure 5 – Cone calorimeter


Chapter 11 describes details of all important road vehicle fire scenarios. Fires starting in the engine compartment and spreading into the passenger compartment via the engine cover/bulkhead, the ductwork or the windshield are particularly critical. Collision damage often provides alternative ways in which fire penetrates into the passenger compartment. These fires typically start in one of three ways: via an electrical fault following a collision, via an electrical fault independent of collision or via non electrical ignition sources. Engine compartments contain abundant electrical wires and cables; fire safety requirements for electrical products are available for many environments, including the National Electrical Code (for buildings) and regulations associated with other vehicles.

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

  1. NFPA 556, "Guide on Methods for Evaluating Fire Hazard to Occupants of Passenger Road Vehicles," National Fire Protection Association, Quincy, MA, 2011.
  2. Federal Motor Vehicle Safety Standard 302, "Flammability of Interior Materials - Passenger Cars, Multipurpose Passenger Vehicles, Trucks, and Buses," National Highway Traffic Safety Administration, Washington, DC 1972.
  3. Janssens, M. and Huczek, J. "Comparison of Fire Properties of Automotive Materials", in Business Communications Company Fourteenth Ann. Conference on Recent Advances in Flame Retardancy of Polymeric Materials, Norwalk, CT, 2003.
  4. Digges, K. et al. "Human Survivability in Motor Vehicle Fires", Fire and Materials, 32, 249-258, 2008.
  5. Babrauskas, V. and Wickström, U. "The Rational Development of Bench-Scale Fire Tests for Full-Scale Fire Prediction", Fire Safety Science - Proc. of the Second International Symposium, Hemisphere Publishing, New York 1989.
  6. Babrauskas, V. and Peacock, R. "Heat Release Rate: The Single Most Important Variable in Fire Hazard". Fire Safety Journal, 18, 255-272, 1992.
  7. Hirschler, M. et al., "Rate of Heat Release of Plastic Materials from Car Interiors", Business Communications Company Eleventh Ann. Conference on Recent Advances in Flame Retardancy of Polymeric Materials, Norwalk, CT, 2002.
  8. Hirschler, M. "Improving the Fire Safety of Road Vehicles", Advances in Fire Retardant Materials, pp. 443-466, Woodhead Publishing Ltd., London, UK, 2008.
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