Introduction
The destructive and dangerous properties of fire have led to the allocation of enormous economic resources to fire detection and suppression, fire investigation techniques and materials and product safety research. Also, the aftermath of fire often requires additional economic resourcing by insurance companies and, in times of natural fire-related disasters, by governments.
As a consequence, the many different professions that have a direct and indirect interest in fire have designed different classification systems for types of fires to reflect their particular interest. For example, when ascertaining eligibility for compensation, it is important to determine whether a fire is the result of:
• negligence
• an accident
• a malicious act
However, when public safety is an issue, it may be important to classify fires according to what was actually burned, for example
• dwellings
• commercial premises
• vehicles
• public property (schools, railway stations, churches, etc.)
• marine vessels
• parks and reserves
When fire investigation is the primary consideration, types of fire can be classified by the source of ignition, i.e. the fire cause: e.g.
• electrical
• machinery
• autoignition
• weather (lightning)
• direct ignition
It should be noted that in this final classification scheme there is no mention of intent, even in the case of a match or a cigarette lighter. This is because the effect of ignition is identical regardless of degree of intent, and a purely scientific examination for fire cause can not usually prove or refute intent. However, the presence of other evidence, such as separate and unconnected fires and/or a time-delay ignition device, will provide indisputable evidence of intent.
In this article, types of fire are classified according to the source of ignition, although on numerous occasions reference will be made to the ignition and burning characteristics of specific objects such as machinery, motor vehicles, etc. in order to illustrate a particular feature of fire.
This is therefore a rather academic analysis of fire causes. From a practical viewpoint, the various features of the development of fires must also be considered. This is made necessary by the fact that fire investigators must interpret any surviving indicators of fire development before nominating the fire origin and the ignition source.
Sources of Ignition
The fundamental types of fire according to their cause of ignition are described below.
Electrical fires
The most misunderstood of fire causes are those related to electrical origins, and this is the principal reason why some unreliable fire-cause statistics have been compiled over past years.
If electrical appliances and electricity supply are present at a fire scene, any diagnosis of an electrical cause should only be made after the following factors have been carefully considered.
Electrical energy represents a source of ignition. The simplest example is a filament-type gas stove lighter, where electrical energy heats a wire filament until it reaches the temperature necessary to ignite a gas-air mixture. Apart from this obvious example, there are three principal ways in which electricity can be attributed as the cause of ignition of fires.
Electrical sparks
Electrical sparks can ignite flammable or combustible material. This is of most concern when flammable vapor-air mixtures are present, and these mixtures are within the characteristic flammable limits that can be ignited by a relatively low-energy spark.
Significantly, any flammable or combustible material can be ignited by a spark if the energy dissipated by the spark is sufficient to raise the local temperature of the material to the fire point, or flash point, as the case may be. Where hazardous atmospheres are known to exist, intrinsically safe electrical fittings are used to insure that electrical-spark ignition does not occur.
This consideration assumes the presence of a homogenous mixture of gases which, in practice, is unusual and difficult to achieve. It should be noted here that the electrical charge responsible for the spark could be from static electricity, from alternating current electricity or from direct current electricity. The source of the electricity is not important; the significant factor is the thermal energy of the spark.
In the case of solids and liquids, the energy of the spark must be sufficient to raise the local temperature of the substance to the fire or flash point. The energy requirement for most substances is prohibitive; and therefore, fires of this type are likely to occur only in exceptional but important cases.
Electrical wires, cables, connections and components
Electrical wiring systems, electrical circuits in appliances and electrical accessories are all designed to avoid overheating from occurring in any section of the circuit. In addition to electrical safety approvals required for individual items, there are more general wiring standards that apply to domestic, commercial and industrial premises. For appliances in good condition in a building that is correctly wired and fused, the chances of an electrical fault resulting in a fire are very small.
Realistically though, not all appliances are in good condition and not all premises are correctly wired. In these cases, electrical fires can be due to:
• damaged or degraded insulation on wires or cables;
• excessive loads on power boards or outlets;
• damaged or dirty electrical contacts;
• inadequate heat dissipation.
In each case, a component or section of a circuit is heated by electrical energy until combustible material nearby is ignited. The energy available in most electrical circuits is many times greater than the energy in a spark, so ignition of combustible material close to an electrical fault is a realistic possibility.
Electrical appliances Electrical appliances can cause fires in a variety of ways. Apart from the circuit or component failures listed above, some possible causes of appliance fires are as follows:
• breakdown of transformer ballast in fluorescent lights;
• overloading of electric motors in washing machines or driers;
• overheating of battery chargers for mobile telephones,
• failure of cooling fans in appliances generating heat.
In all of these cases, the electrical energy supplied to the faulty appliance heats part of the electrical circuit to the point that nearby combustible material is ignited.
Misuse of appliances Despite the increasing use of safety features, including thermal cutout switches, appliances such as electric radiators continue to cause fires when they are placed inappropriately, e.g. too close to curtains. The fire which can result in this way would, in many jurisdictions, be classified as an electrical fire. However, they are more properly classified as ‘appliance misuse’ because the term ‘electrical fire’ implies an electrical fault or failure of some type.
Weather
A significant subclass of fires is those caused by natural phenomena. The best documented in this category are fires caused by lightning. These are common and are a recognized problem in forest management. It is not difficult to comprehend that lightning striking a tree, particularly a dead tree, can heat the tree sufficiently for it to ignite. In a summer thunderstorm moving across south eastern Australia, there can be tens, even hundreds, of lightning strikes in forested areas, resulting in many fires.
Lightning strikes on structures occur frequently. Lightning rods or conductors that safely carry the electrical current to earth without damaging the structure protect most tall buildings. Houses and commercial premises are rarely struck by lightning because there are usually many taller and thus more vulnerable targets such as telephone poles or trees in the immediate vicinity. However, a lightning strike on a house or other building could cause a fire or fires in addition to structural damage. There are several factors, which may point to the possibility of lightning as an ignition source:
• a recent thunderstorm with significant electrical activity;
• the affected structure is in an exposed and/or elevated position;
• physical damage to an upper exterior portion of the premises, possibly due to a chimney or aerial.
Apart from lightning, sunlight and rain both have the potential to cause fires in a strictly limited sense. Sunlight can be focused with a lens to cause intense heat and fire and, if the focal point coincides with combustible material, a fire may result. Reports of wildfires (bushfires) being ignited by the sun shining through broken bottles are exaggerated to the point of being untrue. There have, however, been cases of the sun shining through vases or other objects on window ledges and igniting furniture.
Both sun and rain can play roles in the ignition of fires in haystacks and silos through self-heating (often incorrectly referred to as ‘spontaneous combustion’). This requires particular conditions of temperature and humidity (see below). Nevertheless, if an agricultural district has been drenched, it may be that there is a sudden outbreak of haystack fires. This might be attributed to the average moisture level of the haystacks increasing to the point where self-heating becomes possible. This type of fire is classified as a ‘self-heating fire’, but is mentioned here for completeness because of the obvious indirect connection to weather.
Many other climatic conditions can cause fires. Almost any extreme condition can lead indirectly to fires through adverse effects on wiring, cables or machinery.
Machinery
Fires can be caused by machinery. As has been established, for the ignition of flammable or combustible material, that material must be heated to the fire point. Under certain conditions, it is possible for mechanical action to cause a fire by heating combustible material.
An example of ignition by mechanical action is the historic use of the fire bow. The bow is used to rapidly rotate a stick that has one end rotating in a hollow in a wooden plate. Friction causes heating which eventually results in the ignition of finely divided cellulosic material.
A more contemporary example is a fire in a wheel casing. Here, a damaged bearing leads to greatly increased friction and the heat generated can ignite grease and hydraulic fluid in the hub.
Motor vehicles can be ignited as a result of mechanical failure. Many parts, particularly the exhaust manifold, operate at high temperatures. Failure of fuel or hydraulic lines in the engine compartment can result in fuel or hydraulic fluid being sprayed over a high temperature area such as the manifold. If the temperature of the manifold is above the fire point of the fluid, a fire will result.
Fires started by machinery are not necessarily the result of a fault in the machinery itself, but may be due to the method of operation or installation. Many fixed plant items, refrigerators for example, require specified clearances from walls and the floor to allow adequate ventilation for cooling purposes. Failure to comply with these requirements can lead to overheating and result in a fire.
Autoignition
The complex phenomenon of autoignition, or self-heating as it is more properly known, is often incorrectly referred to as ‘spontaneous combustion’. There is nothing spontaneous about self-heating, and the principles of chemistry and biochemistry that underlie it are well understood. It is both predictable and avoidable.
Materials which are easily oxidized will, in many instances, have self-heating properties. The spontaneous ignition of haystacks is an example of biological heating. If the hay is in the required moisture range, significant bacterial activity can develop, causing a rise in temperature. A moderate increase in temperature can promote further bacterial growth, with a consequential rise in temperature.
However, the limit to temperature increase as a result of bacterial activity is around 70°C, the upper limit for most bacterial growth.
At or above this temperature, chemical oxidation can further increase the temperature, which can then lead to eventual ignition. Whether the temperature rises to this level depends on the balance between biological/chemical heat production within the stack and heat losses from the bales as a result of conduction and convection. When the heat production exceeds the heat loss, the temperature will rise and the process accelerates. If the heat losses increase (for example, when the external temperature drops significantly), heat loss may exceed heat production and the temperature falls. The probability that the stack will ignite ‘spontaneously’ is then reduced.
Products containing drying or semidrying oils, such as linseed, cottonseed, sunflower seed and perilla oils are susceptible to self-heating through chemical oxidation and polymerization. This can give rise to the well-documented occurrence of fires in oily rags.
Also, finely divided, readily oxidized materials such as charcoal and coal dust also have the potential for self-heating. This is the cause of many serious industrial accidents in silos, bunkers and processing plants where high levels of dusts are produced.
Direct ignition
Direct ignition is a term that has evolved to classify simple fire causes that do not involve a series of events. The most common cause of direct ignition is the simple application of a flame due to a match or cigarette lighter to a combustible material. This is distinct from, say, a fire due to an electrical cause where some electrical malfunction was the primary event and the fire was a secondary or resulting event.
Direct ignition shares the same basic principles as other types of ignition. A heat source is applied to flammable or combustible material. For that material to ignite, it must be heated to the flash point or fire point. This may take a fraction of a second, as in the case of a flammable liquid, or several minutes, as in the case of a combustible solid such as wood or plastic.
Match or cigarette lighter Whether a match or a cigarette lighter can cause ignition is dependent on the heat transfer associated with the ignition process. Heat generated by the flame is transferred in part to the material to be ignited. However, if heat is lost by radiation and conduction, only a portion of the heat generated initially serves to increase the temperature of the material.
Heat absorbed by the material to be ignited will raise the temperature towards the flash point or fire point, at a rate that is dependent on the thermal capacity of the material. Material directly exposed to the flame can be oxidized immediately, with heat generated by this reaction contributing towards the overall temperature rise. If the flash point or fire point is reached over sufficient of the material before the heat source, i.e. match or lighter, is exhausted, the generated heat may exceed heat losses, at which point the material is regarded as having ignited.
Once the material has ignited, the combustion reaction is, by definition, a self-sustaining reaction. The development of the fire from this point depends on the nature of the material that has been ignited and on factors such as ventilation. It is not dependent on the method or process of ignition.
Time-delay ignition devices Time-delay ignition devices are occasionally used by arsonists in order to ensure that they are not present when the fire occurs and to allow time for them to establish an alibi in the event that they come under suspicion.
Although fires caused in this way appear to be another ‘type of fire’, in effect, they can be classified according to the ignition source as can any other fire. The construction of time-delay ignition devices is only restricted by imagination and opportunity. They range from simple devices based on burning candles or cigarettes to sophisticated devices which depend on chemical reactions, electrical appliances, mechanical apparatus (such as mouse-traps), etc. In some instances, items such as clocks and electrical timers have been used to determine the ignition time. Electrical items which can be activated by a telephone call have also been used to initiate ignition.
The most significant feature of fires caused by the ignition of time-delay ignition devices, is the irrefutable evidence of intent which, at least from a purely scientific point of view, is unusual.
Fire Development
The preceding discussion is only of academic interest unless the consequences of ignition sources on sub-sequent fire development are understood. This understanding is crucial to the determination of the fire origin and the fire cause, i.e. the identification of the ‘type of fire’. Fire development, postignition, can be classified in the following way.
After the initial ignition phase, fire enters a second stage known as the incipient phase. The establishment of a self-sustaining chemical reaction characterizes this phase, i.e. unsupported burning occurs. However, although the fire has not yet developed to the point where flaming combustion or even the products of combustion are apparent, there may be sufficient smoke to trigger smoke detectors and there is an increase in the heat generated by the fire. This heat serves to increase the temperature of the burning material, but at this stage does not make a significant contribution to the temperature of the surroundings.
The next stage is emergent smoldering, where the fire grows from a barely self-sustaining reaction to visible flames. Combustion products reach significant levels, and smoke is apparent. The heat loss from the burning material, and the heat generated by the flames causes an increase in the temperature of the surroundings in addition to that of the fuel.
The fourth stage is open burning, where there is a relatively rapid increase in heat output and obvious open flames. As the growing fire heats and ignites nearby material, the room or compartment temperature increases markedly, particularly in the ceiling layer. This layer becomes a source of radiant heat energy, contributing to the rate of temperature rise. There is an adequate supply of air for combustion, and growth of the fire is limited only by the supply of fuel.
If the amount of air in the room or compartment is sufficient, open burning can progress to flashover. For this phenomenon to occur, the heat released, particularly radiant heat from the upper smoke layer, must be sufficient to raise the temperature of all the exposed combustible material in the room to the fire point, at which point all the combustible material in the room will ignite.
The final stage is air- or oxygen-controlled burning. In this stage, the combustion reactions occuring in the room are limited by the lowered proportion of oxygen available. The effect of this restriction is that flames will diminish, possibly to a level of smoldering and, although the temperature may continue to rise, the rate of temperature rise is much reduced. The introduction of additional air/oxygen will return the fire to the open burning situation until the additional oxygen has been consumed.
The increase in burning during this stage will decrease the supply of air in the room. In some situations ventilation will maintain the required airflow, or the fire may breach a window or door and thus ensure adequate ventilation for continued development. If the proportion of oxygen available to the fire is limited, the fire will revert from open burning to smoldering. This may occur before the fire reaches flashover, in which case flashover may not eventuate.
Although fires generally follow this series of events, the source of ignition can have a significant influence on each of these stages and particularly the time required to complete the early stages.
• A fire that is ignited by self-heating processes or by a relatively low-power electrical fault can take a long time to ignite and may remain in the incipient stage for a considerable time.
• A smoldering fire, such as, but not necessarily confined to, a fire started by self-heating, may move from the emergent smoldering stage to the final air-controlled smoldering stage without entering the open burning phase.
• Progression from emergent smoldering to open burning/flashover to air-controlled smoldering is usually dependent on both the nature of the fuel and the amount of ventilation. Increasing the ventilation initially increases the rate of burning and hence heat output and increases the likelihood of reaching flashover. Beyond a certain upper limit, increasing ventilation has the effect of removing heat from the room and effectively preventing the flashover temperatures being reached.
• A fire involving the ignition of flammable liquid will progress through the incipient stage in an extremely short time, possibly fractions of a second, to reach the open burning stage.
Fire development in the latter circumstances may follow several courses. If the heat generated by the burning liquid is insufficient to ignite the carpet or other combustible materials on, or near where it was sited, the fire might simply cease with the exhaustion of the fuel supply or the air supply. This might result in relatively light burning, isolated areas of scorching or in burn trails in flooring, which is otherwise undamaged.
If ignition of the fuel/air mixture does generate sufficient heat to ignite available combustible materials, this ignition will probably occur at many points on the perimeter of the liquid pool or trail. Any fire that develops from this type of ignition can show comparable levels of damage throughout the premises. Trails related to the spread of flammable liquid are likely to be less distinctive and will certainly become more difficult to distinguish as the fire develops to flashover or oxygen-controlled burning.
Although the fuel vapor/air mixture must be within the explosive limits for ignition to occur, in some instances a substantial proportion of the mixture is within these limits and an ignition known as deflagration can occur. The flame front originating at the point of ignition passes through the mixture at speeds approaching the speed of sound, and significant overpressure may be generated. The subsequent damage can range from minor window glass damage to complete destruction of substantial buildings. Stoichio-metric or lean mixtures are more likely to cause major explosions whereas rich mixtures are likely to cause extensive burning with comparatively minor explosion damage.
Although the concepts and principles of fire development remain scientifically valid for all fires, this discussion on fire development applies primarily to fires in compartments. In the case of bushfires (wildfires), concepts such as flash over and ventilation are either irrelevant or insignificant, and the dominating influences on fire development are fuel condition and atmospheric conditions, particularly wind velocity, humidity and ambient temperature.
As in all fire-cause investigations, it is the correct interpretation of the fire travel and development indicators that allows a logical fire cause to be nominated. In the case of bushfires, fuel load, wind speed and wind direction are the primary considerations in the interpretation of fire travel.
Having determined the starting point of the fire, many ignition sources (and therefore, ‘types of fire’) should be considered. In fact, all the ignition sources mentioned above have been known to be the cause of bushfires. In the case of electrical causes, the formation of sparks caused by clashing electrical conductors (power lines) in high winds is a commonly diagnosed ‘type of fire’. Sparks caused by farm machinery, camp fires and acts of arson are all other common ways in which bushfires have started.
By far, however, the most common ignition source for bushfires is lightning strikes. This fire cause is characterized by physical disruption to a tree or other object through which the lightning has ‘earthed’. Evidence of a lightning strike is sometimes visible but in other situations, does not survive the ensuing fire.
Whether these types of fire are ‘electrical’ or ‘weather’ is academic.
Summary
The basis for the classification of fires outlined above is only one approach. However, it does have the advantage of avoiding any reference to the diagnosis of intent, which with the exception of certain specific cases, is not a scientific conclusion but a forensic outcome. It must be appreciated that the types of fire discussed are simple examples. In fact, a fire may be a combination of several types (for example, a spark generated by a damaged wheel bearing could ignite a pool of flammable liquid) or, due to environmental factors, might not develop precisely in the manner described (or prescribed). For this reason, the experienced fire investigator will avoid complying with a rigorous classification scheme for fires and will understand that the classification of a fire is achieved after the cause and origin of the fire have been determined, rather than before. A thorough understanding of the various types of ignition and the consequential fire development is crucial to this determination.
