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Unit 10 Assignment B - Behaviour of Fire
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Unit 10Behaviour of Fire
Assignment B
The behaviour of fire and the way it spreads and the different phases is crucial within a fire
investigation to determine how the fire started, the way it has spread, and how the plume
patterns created can tell us how the surroundings have been affected by the flames.
Causes of a Fire
Fires can be caused by a variety of events, such as accidental, natural and deliberate fires.
Accidental fires are usually done with no ill intent, and are typically from sources such as
candles that have been unattended, or electrics malfunctioning. It can also be from other
things such as smoking, and appliances such as toasters and hair dryers/straighteners.
When using an electrical fire as an example, electrical fires can be caused by things such as
overheated wiring. This causes the insulation around a wire to degrade, insulation is
important for electrical wiring as it ensures the restriction of the electrical current (1), and that
there isn’t direct contact between exterior materials and the conductor of the wire, when this
isn’t here the risk of short circuiting within a wire is increased. The resistance increases
within the wire due to the increased heat. It also may compromise surrounding areas due to
increase in the residual heat. Sparks cause fires when the electrons within a spark are
moving at a high energy and land on a flammable material. This leads to the start of a fast
fire, with pyrolysis materials being generated. This is due to the increase of temperature from
the electrical currents. It is stated that ‘13% of residential fires are due to equipment,
appliances or electrical malfunctions.’ (2) (Mein, S. 2019) Flames may spread differently
depending on the fuel that ignited the flame initially, and may be intense or slow-burning. For
example, an electrical fire usually happens spontaneously and rapidly.
Natural fires can be through things such as lightning, and droughts causing issues such as
leaves that are dry and heat. Natural fires can also be a result of spontaneous combustion,
spontaneous combustion occurs when there is a fire as a result of no spark or source of
ignition. It is when heat cannot leave materials and an exothermic reaction is present,
leaving heat to build within a material. This can accelerate reactions until the flame point of
the material is reached and it ignites (3). Hot lightning has a voltage that occurs over a
longer time span. Its voltage is a high electrical current that can heat the Earth around it to
‘27,760 degrees Celsius’ (4), it can also contribute towards wildfires.
Natural fires tend to be self-sustaining Figure 1 - Heat
due to the vegetation present, and fire Release Rate vs.
behaviour may change depending on
the weather outside. Other things
such as whether a fire occurs on a hill
etc. can influence its movement and
direction.
Every year the fire police within
Nottingham are at the scene of
around ‘2,000 deliberate fires’ (5).
, They can be caused through things such as bin fires, and fires being set around dry areas
like fields and haystacks. There are usually signs of a deliberate fire such as if certain
accelerants have been used, the fire patterns present, and the presence of char. There may
also be multiple origin points. Deliberate fires are usually rapidly developing due to
accelerants being used.
Phases of Fire
1. During the incipient phase (which is the ignition/beginning
stages of a fire) the flames are relatively controlled. They
tend to be below ceiling level (6) and are usually around
the point/s of origin. During this phase, the room may
heat up due to increased gas and particle energy, leading
to a growth of pyrolysis products due to the radiant heat
and a light grey smoke will be present. During the
incipient phase, it is still relatively easy to evacuate a
building, due to visibility, and the flame can still be
extinguished using water and fire extinguishers. There
are factors as to whether the flame will then spread and
develop into the growth phase, these are whether the fire
Figure 2 -
has easy access to oxygen, how many flammable materials are
nearby, and whether there is anyone to extinguish the fire. This
phase has the least amount of heat release rate as shown in Figure 1.
2. The growth phase is the second stage of a flame, this is when a ‘positive heat
feedback loop’ is generated (7). This is when the heat generated is enough to
combust more materials. During this phase, pyrolysis
increases and things such as cracks on windows and
stains appear. It may be a problem if a window or house
is semi-detached to another property as the flame may
spread over. As well as this, patterns begin to form
across the ceiling, spreading heat products horizontally,
and hot gases also circulate from the ceiling to the rest of
the building. During this stage, it is vital to evacuate
anyone within a building, as this is the stage before Figure 3 - Growth
flashover occurs - when enough heat is generated for materials/fuels
to spontaneously ignite due to an increase in pyrolysis products vapour and heat
build up. Smoke becomes thicker and also starts reducing visibility, however it is still
above the flames and the structural integrity of the building may start to be affected.
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