1. Class A fires occur in wood, rubber, paper,
cloth and most plastics. The most effective type
of extinguishing agent is one using water, or
solutions containing large concentrations of water,
because the ‘‘quenching-cooling’’ effect reduces
the temperature of the burning material below its
ignition temperature. Fire extinguishers suitable for
this type of fire are designated with a classification
of “A” on the label.
2. Class B fires occur in flammable or combustible
liquids, such as petroleum products and greases.
A “blanketingsmothering” effect of an agent that
excludes oxygen or inhibits the chemical chain
reaction is most effective. Extinguishers labeled
“Class B” employ carbon dioxide, dry chemical,
Halon or foam.
3. Class C fires involve electrical equipment. The
extinguishing agent must be non-conductive.
Carbon dioxide, dry chemical and Halon are
the normal types used for electrical fires.
4. Class D fires involve combustible metals such as
aluminum, magnesium, zirconium and titanium.
The use of water and some other conventional
types of extinguishing agents are ineffective and
may cause a violent reaction. These fires can
be extinguished with specially prepared agents.
Where this hazard exists, extinguishing agents
with a D-class rating should be provided.
