FIRE AND ARSON INVESTIGATION
Fire continues to be the most costly public safety problem today. By efficiently and accurately identifying the cause of fires, investigators can make a substantial contribution to reducing the terrible loses associated with fires.
Principles of Fire
Fire is an exothermic oxidation reaction that proceeds at such a rate that it generates detectable heat and light. In order for a fire to occur, several conditions must exist:
Combustible fuel must be present.
An oxidizer (i.e. the oxygen in air) must be available in sufficient quantity.
Energy as some means of ignition (e.g. heat) must be applied.
The fuel and the oxidizer must interact in a self-sustaining chain reaction.
The first three elements listed have long been described as the fire triangle, but the fourth must also be present if the fire created is to be continuous (self-sustaining). Removal of any one of the four elements results in extinguishment of the fire. Fire generates much heat and raises the temperature of the reacting components, thus increasing the rate of reaction. This is the key to the fourth element of fire the chain reaction.
Heat is transferred in three ways:
Conduction is the transfer of heat energy through a material by direct contact between its moving molecules. Heat always travels from hot areas of a solid to cold ones by conduction.
Convection is the dominant heat transfer mechanism in fires, accounting for the most heat movement, spread and damage patterns. When heated, a fluid expands, becomes less dense, and is displaced by the heavier colder fluid. In fires, the gaseous products of combustion, along with the surrounding air are heated, expanding and moving upward at a rapid rate.
All matter which contains heat, radiates this heat constantly in the form of infrared radiation. Radiant heat is a major factor that makes firefighting difficult due to its effects on fire fighters. All surfaces exposed to a fire will absorb hear and rise in temperature. When this temperature reaches the ignition point of the material itself, it will burst into flame. Charred areas inconsistent with the ordinary fire spread are commonly due to heat radiation.
In a real fire, flames are also spread by direct flame impingement. Direct flame contact is a combination of all three basic mechanisms.
The Nature and Behavior of Fire
A normal fire in a structure goes through three fairly predictable stages of development. These stages vary in duration (sometimes dramatically) with the circumstances of ignition, fuel load, and ventilation.
(see Leading Edge, Vol. 3, no 1 'Motor Vehicle Fires' for a full account of these stages.)
Determination of the ignition source of fire cause is fundamental to the investigation in terms of establishing liability for social and economic losses. Fire causes can be viewed in terms of three categories:
Natural Ignition Sources
Natural causes, sometimes termed 'acts-of-God', refer to such sources as lightning, spontaneous combustion, and intervention by various animal species. Natural disasters such as earthquakes, and wind also occasionally cause serious fires.
Accidental Ignition Sources
Accidental sources of ignition are numerous. Primary ignitors include matches, lighters, sparks (electrical and mechanical), hot objects, friction, radiant heat (e.g. through pyrolysis) and chemical reactions. Many of these sources are associated with appliances and/or building services.
It has been estimated that arson fires account for as much as one third of all reported structure fires. The crime of arson has basically three elements.
There has been a burning of property.
The burning is shown to be started with malice, that is, with the specific intent of destroying property. This is often accomplished by establishing a motive.
The burning is incendiary in nature. There must be proof of the existence of an effective incendiary device. Proof must be accomplished by showing how all possible natural or accidental causes have been considered and ruled out.
Studies show that by far the most common arson set is the direct pouring of a flammable liquid with ignition by match, which is found in almost two thirds of all cases. Delayed ignition devices are only involved in approximately 10% of arson cases.
Every fire deserves careful and thorough investigation. The origin and cause should be identified so that it can possibly be prevented from occurring again. A methodical and analytical approach to the investigation is critical to a successful determination of the cause of a fire.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2