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Motor Vehicle Fires vs Structure Fires

October 23, 2017


Many fire investigators prefer to only do structure fire origin and cause investigations.  They tend to shy away from or sometimes flat out refuse to do investigations for motor vehicle fires, claiming that they are too complex. But are they really?

Fundamentally, all fires develop and spread in a similar manner with the main differences between fires being the compartment configurations, fuel loads, ventilation and ignition sources.  These are all required considerations of every fire investigation.  The principles of fire dynamics will be the same whether in a vehicle or structure, therefore, the investigative methodology is essentially the same.

NFPA 921: Guide for Fire and Explosion Investigations classifies automobiles, trucks, heavy equipment, farm implements and recreational vehicles (motor homes) all as motor vehicles. Considering all the variations between types of motor vehicles (cars, trucks, buses, loaders, combines, motor homes), manufacturers (Ford, Dodge, Volvo, Toyota, Caterpillar, Kenworth), systems (fuel, emission control, electrical, braking), equipment, materials and so on, motor vehicle origin and cause investigations present themselves as a daunting task for a fire investigator.

Beyond the fire origin and cause investigations, vehicle fire investigations also pose additional safety concerns compared to structure fire investigations because of the presence of fluid leaks, undeployed airbags and stored electrical energy (batteries and capacitors).

The main difference between the two types of fires is that vehicle fires often deal with a small compartment size which can translate into more rapid-fire ventilation driven growth than with similar situations in larger compartments such as those found in structure fires.

To conduct a vehicle fire investigation, the area of origin must first be identified. NFPA 921 separates motor vehicles into five (5) different regions: exterior, engine compartment, passenger compartment, cargo compartment, and underbody or under chassis. Fire patterns, degree of damage, witness information, even the way the windshield reacts can all be used to identify the region the fire originated, even potentially narrowing it down to a smaller area of origin within that region. As with structure fires, ventilation effects and fuel packages require consideration during the origin determination.

Once the area of origin is determined, then all of the potential ignition sources within that area must be considered. With all the variations in motor vehicles, this is where the investigator must turn to other sources for information. Knowing the Vehicle Identification Number (VIN) for the vehicle will provide essential information to the investigation, including pertinent recalls and Technical Service Bulletins (TSB – recommended repair precipitated by several occurrences of a problem). Another good source of information would be examination of an exemplar vehicle. While with structures, there is rarely, if ever, an exemplar structure to examine, exemplar vehicles are often easier to locate. An exemplar will give the investigator information as to locations of vehicle specific large fuel packages, potential ignition sources, potential problems with maintenance or manufacturing, and other crucial information. Other resources include information directly from the manufacturer, repair invoices, information from the owner/operator or online repair websites, such as AllData, Mitchell and Fleet Cross. Generally, there is abundant information available to help familiarize an investigator with the multitude of systems found in motor vehicles.

Once all the potential ignition sources are identified, the same process used for structure fire investigations is then followed. The investigator must develop an ignition scenario hypothesis once all the data is gathered and analyzed. Was it hot surface ignition of a leaking fluid? Or was the ignition of combustibles by electrical arcing, overloaded wiring or mechanical friction? Each plausible hypothesis must be tested to try to determine the one final hypothesis that matches all the gathered evidence.

So, while there are differences between a structure fire and a motor vehicle fire, there are a lot of similarities too.  Both structure and motor vehicle fire investigations require a systematic approach utilizing the information available to the investigator.   In the end, while vehicle fires can be more technically challenging, the methodology is the same.

Authored by Carly Cooke


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