Wednesday 20th April 2022
Unit 25 B: Explore the cause, phases, and behaviour of fire.
PASS
Describe and discuss the different causes of fire, (accidental, natural, and deliberate)
how they behave and their phases (incipient, growth, free burning, and smouldering.)
Include graphical data to show the correlation between the Heat release rate (HRR) and
the phases of a fire.
The fuel has a big impact on how a fire behaves. Other elements or variables may include the
location of the fuel and its proximity to other fuels, the weather specifically wind and relative
humidity, oxygen concentration, and, in the situation of outdoor fires, the terrain form.
Ignition - Aboutforensics.co.uk. (2019).
This happens when all conditions are present to start a fire, for example this can be from producing
smouldering or flaming fire. This will be induced by the addition of eat to fuel in the air, caused by
several sources for instance: exothermic chemical reactions, friction, solar radiation and finally
electricity.
Temperature needed for the ignition to occur depends on the fuel - Aboutforensics.co.uk. (2019).
The least temperature at which a fuel favour is temporarily ignited in air by an external ignition
source is known as the flash point. Nevertheless, this does not guarantee that combustion will
continue and that a fire would result.
The flame, also known as the fire point, is the lowest temperature at which sufficient vapour is
created to allow combustion to proceed.
A substance's flash and flame points can be measured by placing a tiny quantity of sample in an
airtight container, gradually increasing its temperature while adding an ignition source on a periodic
basis, and then measuring the point at which the flash and flame points are attained.
The spontaneous ignition temperature, also known as the auto-ignition point, is the lowest
temperature at which a material will ignite without the help of an external source of ignition. This is
determined by heating a sample, examining the material's core temperature, and recording the
temperature at which spontaneous ignition occurs.
When a substance is heated to its flash point, flame point, or spontaneous ignition temperature, it
ignites.
Smouldering - Aboutforensics.co.uk. (2019).
Not all types of fire produce flames. Smouldering is a form of flameless combustion which occurs at
the surface of the material in cellulosic substances that can form a solid char.
The presence of a smouldering fire is characterised by extremely localised burning and the
production of thick, tarry smoke.
, The surface temperature can be linked to the colour of the smouldering. For example, dark red
surfaces suggest a temperature of 500-600 oC, whereas a white surface indicates temperatures more
than 1400oC.
The rate of propagation is dependent on the material burning and the amount of oxygen available.
Only low concentrations of oxygen are required for smouldering combustion, but if sufficient oxygen
is supplied, smouldering fires can then produce flames.
Free burning phrase
This is where objects in a burning phase or structures burn in the presence of sufficient oxygen. The
rate of burning is affected by the amount of supplying oxygen. For example, the burning will be
decelerated to a minimum if there is a low concentration of oxygen, this will therefore, be the
opposite if it was vice versa.
Spontaneous combustion - Aboutforensics.co.uk. (2019).
The unexpected igniting of a substance without the use of an external ignition source such as a flame
or spark is referred to as spontaneous combustion. The effect is caused by exothermic chemical
processes within the material, which release heat. When materials are placed together, heat cannot
properly disperse, and the temperature within the substance rises.
Chemical processes speed up as the temperature rises, creating even more heat. The temperature
can rise until the material's flame point is achieved, resulting in ignition. The apparent source of the
fire in spontaneous combustion is usually the centre of the material, since heat is dispersed more
rapidly from the surface, resulting in the centre attaining the maximum temperature. Oil-soaked
rags, sawdust, and hay piles have all been known to suddenly combust.
House fires - Fires caused by electricity - Aboutforensics.co.uk. (2019).
Any material resistance will be encountered as an electrical current travel through it, causing heat to
be produced.
Electrical wiring is typically manufactured and placed in such a way that any heat generated is
minimal and quickly dispersed.
On rare circumstances, the heat produced can reach temperatures high enough to trigger ignite.
Electricity is a common source of unintentional fires, especially when an electrical arc occurs.
When two conductors come into contact after the cable's insulation is compromised, an electrical
arc erupts. Overheating, overloading, mechanical damage, or manufacturing faults are some of the
causes of this damage. Heat will be unable to disperse if the cable becomes too hot, potentially due
to wire coiling, and the insulation will melt, enabling conductors to contact. When more electricity is
taken via a cable than it is intended to handle, such as when too many plugs are placed into one
socket, overloading occurs. This can also happen when the wrong fuses or cable diameters are used.
