Introduction to Fire Safety Management

fl ammable limits
or 
fl ammable/
explosive range
; these issues will be addressed in the 
explosion element later in this chapter.
7.3
Classifi cation of fi re
Fires are classifi ed in terms of their types. BSEN 2 iden-
tifi es fi ve classifi cations. The classifi cations assist in 
recognising not only the type of fi re but also the type 
of extinguishing medium that will need to be utilised to 
extinguish or suppress the fi re.
Class A
– fi res that involve solid materials, usually 
of an organic nature, such as wood, cardboard, paper, 
hardboard, soft furnishings such as carpets and curtains, 
in which combustion normally takes places with the 
formation of glowing embers.
Vapour 
density
Hydrogen 1 
Lighter than air
Methane 8
Ammonia 8.5
Air 14.4
Carbon dioxide 
22 
Heavier than air
Sulphur dioxide 
32
Chlorine 35.5
Table 7.1
Vapour 
density
}
}
Figure 7.11
Class ‘A’ fi res
Class B
– fi res that involve liquids such as petrol, 
paraffi n, white spirit, thinners, varnish and paints or 
liquefi able solids such as candles (wax) and fats.
Figure 7.12
Class ‘B’ fi res
Some of the more energetic particles escape.
Figure 7.10
Vapour particles escaping from the surface 
of a substance
The vapour pressure and density affect the substances’ 
rate of reaction (the amount and rate of atoms/molecules 
escaping) and where/what level the gas will be found 
(hydrogen sulphide heavier than air, methane lighter than 
air). This information will enable a variety of differing 
control measures to be considered.

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