The Fire Triangle and Classes of Fire
Three ingredients, and why the class determines the extinguishing agent.
12 min
Fire requires fuel, oxygen and heat in the right proportions, sustained by a chemical chain reaction. Removing any one stops the fire, which is exactly what each extinguishing method does: cooling removes heat, smothering removes oxygen, starving removes fuel, and some agents interrupt the chain reaction chemically.
Classes of fire
| Class | Fuel | Suitable agents |
|---|---|---|
| A | Solids — wood, paper, textiles, plastics | Water, foam, dry powder |
| B | Flammable liquids — petrol, solvents, oils | Foam, CO₂, dry powder |
| C | Flammable gases — propane, butane, methane | Dry powder — but isolate the gas supply first |
| D | Metals — magnesium, sodium, aluminium swarf | Specialist metal powder only |
| F (or K) | Cooking oils and fats | Wet chemical only |
| Electrical | Not a fuel class — live equipment involved | CO₂ or dry powder; never water or foam |
The two that kill
Water on a cooking oil fire causes a violent steam explosion that throws burning oil across the room. And extinguishing a gas fire without isolating the supply allows unburned gas to accumulate and find another ignition source, producing an explosion far worse than the fire. Isolate first; if you cannot isolate, let it burn and cool the surroundings.