NFPA 921 Sections 12-2.1 through 12-6
[interFIRE VR Note: Tables and Figures have not been reproduced.]
12-2.1 Accidental Fire Cause. Accidental fires involve all those
for which the proven cause does not involve a deliberate human act to ignite
or spread fire into an area where the fire should not be. In most cases
this classification will be clear, but some deliberately ignited fires can
still be accidental. For example, in a legal setting, a trash fire might
be spread by a sudden gust of wind. The spread of fire was accidental even
though the initial fire was deliberate.
12-2.2. Natural Fire Cause. Natural fire causes involve fires
caused without direct human intervention, such as lightning, earthquake,
wind, and the like.
12-2.3. Incendiary Fire Cause. The incendiary fire is one deliberately
ignited under circumstances in which the person knows that the fire should
not be ignited.
12-2.4. Undetermined Fire Cause. Whenever the cause cannot be
proven, the proper classification is undetermined. The fire might still
be under investigation, and the cause may be determined later. In the instance
in which the investigator fails to identify all of the components of the
cause of the fire, it need not always be classified as undetermined. If
the physical evidence establishes one factor, such as the presence of an
accelerant, that may be sufficient to establish the cause even where other
factors such as ignition source cannot be determined. Those situations are
also encountered to a lesser degree in accidentally caused fires. Determinations
under such situations are more subjective. Therefore, investigators should
strive to keep an open unbiased thought process during an investigation.
12-3. Source and Form of Heat of Ignition. The source of ignition
energy will be at or near the point of origin, although in some circumstances
the two may appear not to coincide. Some sources of ignition will remain
at the point of origin in recognizable form, whereas others may be greatly
altered or even completely destroyed. Nevertheless, the source should be
identified in order for the cause to be proven. Sometimes the source can
only be inferred, and the cause as found will be the most probable one.
A competent ignition source will have sufficient temperature and energy
and be in contact with the fuel long enough to raise it to its ignition
The ignition process involves generation, transmission, and heating.
(a) The competent ignition source will generate a level of energy sufficient
to raise the fuel to its ignition temperature and will be capable of transmitting
that level of energy to the fuel.
(b) Transmission of sufficient energy raises the fuel to its ignition
temperature. Where the energy source is in direct contact with the fuel,
such as the contact of an overheated wire with its insulation, the transfer
is a direct conduction from the source to the fuel. Where there is a separation,
however, there should be a form of energy transport. This can be by contact
with the flaming gases from a burning item, by radiation from the flame
or surfaces or gases heated by that flame, or a combination of heating by
the flow of hot gases and radiation.
(c) Heating of the potential fuel will occur by the energy that reaches
it. Each fuel reacts differently to the energy that impacts on it. Some
may be reflected, and some may be transmitted through the material. Some
is dispersed through the material, and some heats the material, causing
its temperature to rise. The term thermal inertia is used to describe the
response of a material to the energy impacting on it. Thermal inertia is
defined as the product of thermal conductivity, density, and specific heat.
These three properties determine the manner in which a material will transmit
heat from the exposed surface to its core or an unexposed surface and distribute
and absorb heat within the element itself. The surface temperature of a
material with a low thermal inertia (such as foam plastic) will rise much
more quickly when exposed to energy from a high-temperature source than
a material with higher thermal inertia (such as wood paneling). Thin materials
will also heat more quickly from a given source of energy.
Once the area and possibly the point of origin is identified, the investigator
should identify the heat-producing device, substance, or circumstance that
could have caused the ignition. Heat-producing devices can include fixed
and portable heaters, gas-fired or electric appliances, furnaces, water
heaters, wood stoves, lamps, internal combustion engines, clothes dryers,
and incendiary devices.
The investigator should also look for devices that may have malfunctioned.
Such devices include many of the foregoing plus electrical service equipment,
receptacles, kitchen and laundry appliances, motors, transformers, and heavy
Sources of ignition for gases or vapors include arcs from motors with
brushes, arcs from switches that are not explosionproof, gas or electric
pilots, or flames in gas appliances.
Flammable gases or liquid vapors, such as those from gasoline, may travel
a considerable distance before reaching an ignition source. Only under specific
conditions will ignition take place, the most important condition being
concentration within the flammable limits and an ignition source of sufficient
energy located in the flammable mixture. This separation of the fuel source
and the origin of the fire can cause confusion.
Information should be obtained from owners or occupants when possible
about what potential ignition sources were in the area of origin, how and
when they were used, and recent activities in the area. That type of gathering
of information is especially important when the source of ignition does
not survive the fire. The information would also be helpful in alerting
an investigator to small or easily overlooked items when examining the area
of origin. When electrical energy sources are considered as potential producers
of the heat of ignition, the investigator should refer to Chapter 14 of
12-4. First Material Ignited. The first material ignited (initial
fuel) is that which first sustains combustion beyond the igniting source.
For example, the wood of the match would not be the initial fuel, but paper,
flammable liquid, or draperies would be if the match was used to ignite
The physical configuration of the fuel plays a significant role in its
ability to be ignited. A nongaseous fuel with a high surface-to-mass ratio
is much more readily ignitible than a fuel with a low surface-to-mass ratio.
Examples of high surface-to-mass fuels include dusts, fibers, and paper.
If the initial fuel has a high surface-to-mass ratio, then the intensity
and duration characteristics for a heat source become less stringent. The
higher the surface-to-mass ratio of the fuel, the less energy the heat source
should produce to ignite the fuel, although the ignition temperature is
the same. Gases and vapors are fully dispersed (in effect an extremely high
surface-to-mass ratio) and can be ignited by a low heat energy source instantly.
The initial fuel could be part of a device that malfunctions. Examples
include insulation on a wire that is heated red hot by excessive current
or the plastic case on an overheating coffee maker.
The initial fuel might be something too close to a heat-producing device.
Examples are clothing against an incandescent lamp or a radiant heater,
wood framing too close to a wood stove or fireplace, or combustibles too
close to an engine exhaust manifold or catalytic converter.
The initial fuel is important for understanding the events that caused
the fire. For example, if the remains of a match were found on the burned
surface of a wood end table in the area of origin, one should not jump to
the conclusion that the match ignited the wood tabletop. The match almost
certainly would go out without igniting the solid wood surface. Maybe the
match had been blown out and dropped there by an occupant. Was there any
paper or other light fuel that could have carried flame to a chair or other
fuels? Remember that the initial fuel must be capable of being ignited within
the limitations of the ignition source. The components in most buildings
are not susceptible to ready ignition. For example, flooring, drywall, structural
lumber, wood cabinets, and carpeting do not ignite unless they are exposed
to a substantial heat source.
Unusual residues might remain from the initial fuel. Those residues could
arise from thermite, magnesium, or other pyrotechnic materials.
Gases and vapors can be the initial fuel and can cause confusion because
the point of ignition can be some distance away from where sustained fire
starts in the structure or furnishings. When ignition causes a low order
explosion, it is obvious that a gas, vapor, or dust is involved. Layered
vapors of gasoline might not ignite violently so that, unless evidence of
the accelerant is found, the source of ignition many feet from where the
puddle burned might be difficult to associate with the fire.
12-5. Ignition Factor (Cause). A fuel by itself or an ignition
source by itself does not create a fire. Fire results from the combination
of fuel and an ignition source. Therefore, the investigator should be cautious
about deciding on a cause of a fire just because a readily ignitible fuel
and a potential ignition source are present. The sequence of events that
allow the source of ignition and the fuel to get together establishes the
To define the ignition sequence requires determining events and conditions
that might have occurred or have been created in the past. Furthermore,
the order in which those past events occurred might have to be determined.
Consider a fire in a restaurant kitchen that started when a deep-fat fryer
ignited and spread through the kitchen. The cause is more than simply the
deep-fat fryer overheated. Was the control turned up too high? Did the control
contacts stick? Why did the high temperature cut-off not prevent overheating?
Those factors could make a difference between a minor incident and a large
hostile fire. In each fire investigation the various contributing factors
should be investigated and included in the ultimate explanation of the ignition
The investigator is cautioned not to rule out a cause merely because
there is no obvious evidence for it. Do not rule out the electric heater
because there is no arcing in the wires or the contacts are not stuck. Obviously,
arson is not eliminated because the lab did not find accelerant in the evidence.
The same standard applies to accidental fire causes. Potential causes should
be ruled out only if there is definite evidence that they could not have
caused the fire. The electric heater could be ruled out if it was not plugged
in. A smoldering cigarette can be ruled out if the room was well involved
10 minutes after a reliable witness passed through and saw no smoke.
12-6. Opinions. When forming opinions from hypotheses about fires
or explosions, the investigator should set standards for the degree of confidence
in those opinions. Use of the scientific method dictates that any hypothesis
formed from an analysis of the data collected in an investigation must stand
the challenge of reasonable examination. (See Chapter 2.) [See
Daubert v. Merrel Dow Pharmaceuticals, Inc., 509 U.S. 579, 113 S.Ct.
Ultimately, the decision as to the level of confidence in data collected
in the investigation or any hypothesis drawn from an analysis of the data
rests with the investigator. The final opinion is only as good as the quality
of the data used in reaching that opinion. If the confidence level of the
opinion is only possible or suspected, the cause should be listed as undetermined.
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Taken from NFPA 921Guide for Fire and Explosion Investigations
1998 Edition, copyright © National Fire Protection Association,
1998. This material is not the complete and official position of the NFPA
on the referenced subject, which is represented only by the standard in
Used by permission.