Principles of Fire

The basic principles of fire and combustion can be represented by the fire triangle:

For fire to exist, three things must be present:

Fuel – a combustible material or substance that is consumed during the combustion process. In a typical workplace, fuels can include paper and cardboard, wood and soft furnishings, structural materials, petrol and diesel fuels, butane, acetylene and other gases, solvents and other chemicals.

Oxygen – consumed during combustion when it is chemically combined with the fuel. Oxygen is present in air at a concentration of 21%.

During a fire oxygen can also come from other sources, including certain oxygen-rich chemicals (usually called oxidising agents), such as ammonium nitrate.

Sources of ignition (heat) – a heat or ignition source is essential to start the combustion process. Once combustion has started it generates its own heat which is usually sufficient to keep the fire burning (in other words once the fire starts the heat source can be removed and the fire stays alight). Some examples will be described later in this element.

Once a fire has started it will produce heat, a flame (the zone where oxygen and flammable vapours are chemically combining in the combustion process) and smoke. The exact composition of the smoke will vary but typically smoke is made up of hot combustion gases such as carbon monoxide (CO) and carbon dioxide (CO2) and small particles (soot).

The fire triangle is useful for two reasons:

Fire prevention – if the three elements are kept apart fire cannot start.

Fire-fighting – if one of the elements is removed the fire will go out.