Classification of Fires and Selection of Appropriate Extinguishing Media
Classifying the fire first determines the medium. Most lost marks come from mixing up the classes or ignoring how the fuel behaves.
The classes
- A - solid combustibles: wood, paper, textiles, cordage.
- B - flammable liquids: diesel, paint, solvents.
- C - flammable gases: LPG, methane, acetylene.
- D - combustible metals: magnesium, sodium, aluminium powder.
- E - electrical equipment (a widely used extra class, not part of the original scheme).
- F/K - cooking oils and fats.
Matching the medium
- Water cools and soaks Class A; it spreads burning liquid, conducts on live equipment, reacts violently with sodium and flashes to steam on hot fat.
- Foam blankets Class A and B; it is not a gas or metal medium.
- CO2 is clean for electrical and small liquid fires but has almost no cooling, so re-ignition is likely, and it asphyxiates in occupied enclosed spaces.
- Dry powder knocks flame down fast but leaves no blanket; expect re-ignition and damage to electronics.
- Wet chemical saponifies hot fat; sand and special powders for metals.
Worked example
A paint store is burning with thinners involved. Class B. Foam or dry powder on the fire, water spray to cool the boundary, and no jet, which would spread the thinners and push burning liquid into the bilges. Closing the store also smothers the fire.
Traps
- Calling a gas fire Class C and extinguishing it before the supply is shut - the gas keeps flowing and can form an explosive cloud.
- Using CO2 on a deep-fat fryer.
- Using water fog on live 440 V switchgear without isolating first.