Anyone who knows why things burn also knows how to prevent a fire and what to put it out with. This lesson explains the basics of combustion, the classes of fire and the main dangers in a fire.
The fire triangle
Combustion needs three things at once:
- a combustible material: paper, wood, textiles, plastics, oil, petrol, gas,
- oxygen: it is in the air, usually in sufficient amounts,
- a source of ignition: a flame, a spark, a hot surface, an electrical short circuit, friction or static electricity.
If you remove any one of them, the fire goes out or never starts. Prevention and firefighting both rest on this:
- cooling (water) takes away heat,
- smothering (foam, a blanket, CO₂) separates the fuel from oxygen,
- removing the fuel (a closed gas valve, a switched-off fuel supply) takes away what feeds the fire.
Classes of fire
Fires are divided into classes according to what is burning. You need to know them because each extinguisher suits them differently:
- class A: solid organic materials such as wood, paper, textiles, straw. They burn with a flame and smoulder.
- class B: flammable liquids or substances that melt when they burn, for example petrol, varnish, solvents, wax, some plastics.
- class C: flammable gases, for example natural gas, propane-butane, acetylene.
- class D: flammable metals, for example magnesium, powdered aluminium, sodium.
- class F: fats and oils in cooking, for example a deep fryer or a frying pan in a kitchen.
Electricity is not a class of fire. What burns is always insulation, plastic or some other material. Beware of the current, though: while equipment is live, do not put it out with water or foam. Extinguishers are marked precisely for use on live equipment.
How a fire spreads
A fire spreads by:
- radiation: heat travels through the air and ignites things nearby,
- convection: hot gases and flames rise and carry the fire further, for example up a staircase or a shaft,
- conduction: heat travels through metal, pipes or a wall,
- falling burning parts, which ignite things below.
A fire grows very quickly. A small flame can become a fire filling a whole room in a few minutes. Closed doors slow this down, which is why it is right to close them when you leave.
Smoke is the most dangerous
Far more people are endangered in a fire by smoke than by the flames themselves:
- it contains poisonous gases, such as carbon monoxide, which has no smell and causes loss of consciousness,
- it is hot and burns the lungs,
- it reduces visibility. You will not see the way or the signs,
- it rises, so there is more air near the floor and it is easier to breathe there.
Do not enter smoke-filled areas. In smoke, move low, keep close to the wall and breathe through a cloth if you have one. A cloth does not filter out poisonous gases, however. The best protection is to leave in time.
Special dangers
- Flammable liquids and gases. Vapours of flammable liquids spread along the ground and can ignite far from the source. A mixture of gas and air can explode.
- Dust. Fine combustible dust (wood, flour, metals) in the air can explode.
- Gas cylinders. A heated gas cylinder can burst. Do not go near it.
- Batteries and accumulators. Lithium batteries can catch fire when damaged or overheated, and are hard to put out. Do not charge them unattended and do not use damaged ones.
- Oil in a pan. Never put it out with water. Water evaporates in the oil at once, throws the burning fat about and the flames flare up.
Summary
- A fire needs fuel, oxygen and a source of ignition. Take away one of them and the fire goes out.
- Classes: A solids, B liquids, C gases, D metals, F fats and oils.
- A fire grows quickly and the greatest danger is smoke.
- In smoke, move low, and do not enter smoke-filled areas.