The fire triangle and the classes of fire, explained

Every fire needs three things at once. Take away any one and it dies. That single idea explains every extinguisher on the wall — and why using the wrong one is dangerous.

The fire triangle

A fire needs heat, fuel and oxygen. Remove any side of the triangle and the fire goes out. Every method of firefighting is just one of those three removals:

• Cooling — take away the heat. Water does this.
• Smothering — take away the oxygen. Foam, CO2 and a fire blanket do this.
• Starving — take away the fuel. Turning off a gas supply does this.

There's a fourth element in the fuller model — the fire tetrahedron adds the chemical chain reaction. Dry powder and wet chemical work partly by interrupting that reaction.

The classes of fire

Under BS EN 2, fires are grouped by what's burning — and that's what decides which extinguisher is safe:

• Class A — solids: wood, paper, fabric.
• Class B — flammable liquids: petrol, oils, solvents.
• Class C — flammable gases.
• Class D — metals (specialist powder only).
• Class F — cooking oils and fats.

There's no "Class E" in the UK. Electrical fires aren't a class — an extinguisher safe near live electrics carries an electrical spark symbol instead.

How each extinguisher attacks the triangle

• Water cools — Class A only. Never on liquids or electrics.
• Foam smothers and cools — A and B.
• CO2 smothers — great for electrical and B, but it doesn't cool, so a fire can reignite.
• Dry powder interrupts the chain reaction — versatile, but wrecks visibility indoors.
• Wet chemical reacts with hot oil to form a sealing layer — the Class F specialist for kitchens.

This is why matching matters: put water on a chip-pan fire (Class F) or live electrics and you don't remove a side of the triangle — you make things violently worse. Want the colour code? See our guide to fire extinguisher colours and types.

This guide is for learning and applies to England & Wales. Always work from the current regulations and your own fire risk assessment; Scotland and Northern Ireland have equivalent regimes.