Introduction to Fire Safety Management

Introduction to Fire Safety Management
Class C – fi res that involve gases such as LPG (i.e.
butane, propane) or those involving natural gas.
Figure 7.13 Class ‘C’ fi res
Class D – fi res that involve metals such as sodium,
lithium, manganese and aluminium when in the form of
swarf or powder.
Figure 7.14 Class ‘D’ fi res
Class F – fi res that involve cooking mediums such
as vegetable or animal oil and fats in cooking appliances.
Such fi res are particularly diffi cult to extinguish as they
retain considerable heat allowing the chemical reaction to
restart.
Figure 7.15 Class ‘F’ fi res
It is worthy of note that there is no classifi cation for
electrical fi res; this is due to the fact that electricity
does not actually involve any fuels which can be extinguished.
Fires that involve electrical circuits and appliances,
cables, etc. can either be extinguished with a
non-conductive extinguishing medium, or the supply can
be isolated and the actual material (given its own ‘class’)
can be extinguished.
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Figure 7.16 Boxes containing matches being stored
adjacent to a light fi tting
7.4 Principles of fi re spread
Once a fi re has started and there is suffi cient fuel and
oxygen to sustain it, there are three recognised ways in
which it can spread within the building: convection, conduction
and radiation. There are also the effects of direct
burning or heat transfer to take into account.
7.4.1 Convection
Due to its properties hot air rises; this can be seen graphically
when smoke from a bonfi re rises and disperses
within the atmosphere or a fi re is started within a grate
and rises up through the chimney.
The convection process begins when combust ible
materials are subject to excessive levels of heat and
they give off a vapour which in turn ignites. When these
vapours are heated they expand and become less dense
than air. As they rise they leave an area of low pressure
which is replaced instantly by cooler unheated air. This
fresh air is then mixed with the vapour and heated,
assisting in the development of greater temperatures. The
process is cyclic, continuing to support the fi re process.
Convection is the most common cause of fi re
spread within buildings and structures. During a fi re
hot gases and vapours (smoke and heated air) will rise
vertically through stairwells, lift shafts and service risers
to the highest level available. They then form a layer at
that height, from which they spread out horizontally until
checked.
As the temperature of the smoke and now the
toxic gas/vapour layer increases, heat is radiated back
downwards and may ignite other combustible mater ials
in the vicinity. Modern research also shows that the