Unit 10 B Fire investigation, explore the cause, phase and behaviour of fire.
Natural fires:
Different types of fuels will catch fire at different temperatures. For any material to turn into
gas, it requires a certain amount of heat energy. Then it takes additional heat to trigger a
reaction with oxygen. Heat produced will vary depending on the fuels molecules, the most
flammable fuels consist of hydrocarbons, these easily recombine with oxygen, forming water
or carbon dioxides. Log fires will depend on the materials used, these fires are designed to
release heat. The fuel used to assist the fire will depend on its behaviour. If the fire is located
near other fuels or intense weather conditions, the fire will behave much more intensely, due
to the additional fuel added.
A natural fire is a fire started without human intervention, often these fires will occur due to
natural disasters, commonly droughts, wind and lightning. The most common example of a
natural fire is wildfires. Wildfires can occur at any given time and can occur anywhere. The
risk of wildfires increases massively in extreme weather conditions such as storms and
droughts. There are many factors which can affect a wildfire. One factor would be the size,
the time taken for a fuel to catch fire and combust will correspond with the size of the fuel.
Larger fuels take longer to absorb heat energy to its ignition temperature, whereas the
smaller pieces of wood will ignite much faster as it heats up quicker.
Another factor which can affect wildfires is the surface area. If the surface area of the fuel is
much greater, the oxygen in the air will collide with the surface. Combustion reactions will be
much faster when increased oxygen collides per second with the fuel. By breaking up fuels,
you can increase the surface area. By chopping wood into smaller pieces, it will burn much
quicker. KIndling, is a common way to start a fire, kindling uses small pieces of wood, by
criss-crossing the pieces, the surface area is much greater and more oxygen is getting in.
Powders have the greatest surface area, meaning they have the fastest reaction rate.
The heat produced is the quantity of energy released and the speed at which the fuel burns.
Different fuel compounds will react differently with oxygen, the different compounds have
different heat energy release. Consequently, the reaction with the surrounding oxygen may
happen quickly or slowly. The amount of fuel available to combust is a fuel load, the greater
the fuel load, the more intense the fires energy output will be. The moisture content is
another factor which can lead to wildfires, fuels need to be dry enough to burn, when a fuel
has little to no moisture, the more likely it will ignite and burn.
There are many weather factors which can lead to a wildfire. Relative humidity is the amount
of moisture in the air, If the moisture in the air is low, the fuels will dry. HIgh relative humidity
results in the fuels absorbing moisture, making it harder to ignite. Wind is a big factor in
determining a fire spread. It affects the rate of oxygen supply to the burning fuel, tilting the
flame forward supply so the uncombusted fuels receive energy (radiation and convection), at
a high speed. Another factor is rain, the absorption of moisture is dependent on the size of
the fuel. Fine fuels absorb moisture quicker than coarse fuels, and so the lack of rain will
affect the drying process of fuels.
, Accidental fire:
An accidental fire involves anything not proven to be a deliberate human act, to ignite and
spread a fire into an area, where there shouldn’t be a fire. Often, it starts by a deliberate
ignition but an accidental spread of the fire. An example of this would be a rubbish fire
spreading by a gust of wind, the spread of the fire was accidental, but the ignition was
deliberate. Another example of a deliberate fire would be cigarettes. When a person lights a
cigarette, they are intentionally igniting it, to smoke the cigarette. This would be considered
an intentional / deliberate ignition, although they don’t intend on a fire spreading. If a person
lit the cigarette and then threw it on the ground after using it, not completely extinguishing it,
this could cause the ground surface (eg. grass) to ignite. Whilst they intended to ignite the
cigarette they had no intention for it to spread.
Undetermined fire:
Undetermined fires are cases which can’t be proven the fires classification is left
undetermined. Whilst all undetermined cases can be proven after extensive investigation, in
most circumstances it will remain unproven. All investigators should remain unbiased when
determining a case, if one factor may suggest the fires cause but the others don’t, the case
will remain undetermined until substantial evidence is gathered to prove it.
Deliberate fires: (incendiary fires / arson)
Deliberate fires are the act of starting a fire with malicious intent. Often buildings, boats and
vehicles are the most common target of deliberate fires. Arson fires are a common type of
deliberate fire. Often arson involves liquids such as diesel or petrol. In addition, to suggest
arson, forced entry may be visible such as broken windows. The burning patterns of fuels
and localised burning patterns can also suggest arson. There are many reasons a person
may commit arson, commonly, people commit arson to generally vandalise, often this is
more common in younger people, often teens. Another reason is to collect monetary gain
through a person's home insurance policy. Another reason is to seek revenge on another
person using arson. Lastly, an arsonist may have pyromania, however this is very rare.
Flammable liquids such as petrol, diesel and kerosene are commonly used to accelerate a
fire. The use of accelerants (flammable substances), can be identified as each different
substance has a different burn pattern. Multiple seats of fire and hydrocarbon vapours using
sniffer dogs or detectors can be used to assist this identification. Often, flammable liquid
containers can be found at the scene, especially if the liquid can be easily obtained. Some
flammable liquids can be present at the scene for innocent purposes, it is vital to determine
whether the accelerants were stored on site prior to the fire. An example of how an
accelerant is at the scene for innocent purposes could be the liquid stored in the garage.
Other types of accelerants could be newspapers, this could be intentionally laid out.
If an incendiary device was used to ignite a fire, evidence of the device may be found in the
debris. Furthermore, if numerous devices were used, they could be found intact if they failed
to detonate. Signs of forced entry into a premises can suggest arson, this includes: broken
windows, tools at the scene and even disabled alarms. Commonly, the removal of items from
