Introduction to Health and Safety at Work

Class B
– fi res which involve liquids or liquefi ed solids 
such as paints, oils or fats. These can be further subdiv-
ided into:
Class B1
– fi res which involve liquids that are soluble 
in water such as methanol. They can be extinguished 
by carbon dioxide, dry powder, water spray, light 
water and vaporizing liquids
Class B2
– fi res which involve liquids not soluble 
in water, such as petrol and oil. They can be extin-
guished by using foam, carbon dioxide, dry powder, 
light water and vaporizing liquid
Class C
– fi res which involve gases such as natural 
Methods of extinction gas or liquefi ed gases such as 
butane or propane. They can be extinguished using 
foam or dry powder in conjunction with water to cool 
any containers involved or nearby
Class D
– fi res which involve metals such as aluminium 
or magnesium. Special dry powder extinguishers are 
required to extinguish these fi res, which may contain 
powdered graphite or talc
Class F
– fi res which involve high temperature cooking 
oils or fats in large catering establishments or restaurants
Electrical fi res
– fi res involving electrical equipment or 
circuitry do not constitute a fi re class on their own, as 
electricity is a source of ignition that will feed a fi re until 
switched off or isolated. But there are some pieces of 
equipment that can store, within capacitors, lethal volt-
ages even when isolated. Extinguishers specifi cally 
designed for electrical use like carbon dioxide or dry pow-
der should always be used for this type of fi re hazard.
Fire extinguishers are usually designed to tackle one 
or more class of fi re. This is discussed later.
13.6 
Principles of heat transmission 
and fi re spread
Fire transmits heat in several ways, which need to be 
understood in order to prevent, plan escape from, and 
fi ght fi res. Heat can be transmitted by convection, con-
duction, radiation and direct burning (Figure 13.3).