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NFPA 921 Sections 12-2.1 through 12-6
Cause Determination

[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 temperature.

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 machinery.

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 this guide.

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 them.

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 cause.

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 sequence.

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. 2786 (1993).]

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.

For more information, contact:
The NFPA Library at (617) 984-7445 or e-mail

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 its entirety.

Used by permission.

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