Theories of Fire & Fire Extinguishment

What Is Fire?

Fire is a chemical reaction in which oxygen is combined with a gaseous or vaporous fuel. Note that, even if the fuel is a solid (eg wood) or a liquid (eg petrol) it is the vapours given off when the fuel is heated that burn. This rapid oxidation produces heat and light (flames). Fire can usually take place only when these three elements are present:

  • Oxygen
  • Fuel
  • Heat (energy)

These 3 elements make up what is commonly called the 'Fire Triangle.'


  • Oxygen is usually readily available. It makes up 21% of the air we breathe.


  • Solid combustibles like paper, furniture, clothing and plastics.
  • Flammable liquids like petrol, oils, kerosene, paints, solvents and cooking oils / fats.
  • Flammable gases like natural gas, LPG, acetylene.


  • The heat given off by the oxidation reaction sustains the fire once it is established. But first, a heat source is required to produce ignition.

Ignition sources include:

  • Heating and cooking appliances.
  • Faulty electrical equipment.
  • Cigarettes, lighters and matches.
  • Friction.

Chemical Chain Reaction:

Research has added a fourth side to the fire triangle concept resulting in the development of a new model called the 'Fire Tetrahedron.' The fourth element involved in the combustion process is referred to as the 'chemical chain reaction'. Specific chemical chain reactions between fuel and oxygen molecules are essential to sustain a fire once it has begun.

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Fire Extinguishment

Essentially, fires are extinguished by taking away one or more of the elements in the fire tetrahedron.

This can be achieved by:

  • Removal or separation of unburnt FUEL (eg turn off the gas).
  • Removal or dilution of the OXYGEN supply (eg smothering the fire with a fire blanket or an inert gas).
  • Removal of the HEAT of the oxidation reaction (eg spraying the fuel with water).
  • Inhibiting the CHAIN REACTION by modifying the combustion chemistry.

Sound fire prevention practices are based upon the principle of keeping fuel and ignition sources apart.

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Classes Of Fire

Fires are classified according to the type of fuel involved. Not all extinguishing agents are compatible with all types of fuel (eg water used on a flammable liquid fire is likely to increase the rate of burning dramatically and to disperse the fuel to cover a greater area).

Thus, if the wrong type of extinguisher is selected the fire situation can be made worse, often threatening your own personal safety. Some extinguishers are simply more effective than others on particular classes of fire.

The various types of fires are classified as follows:

Class A

Fires involving carbonaceous solids, such as wood, cloth, paper, rubber and plastics. Class A does not include flammable metals (see Class D).


Class B

Fires involving flammable and combustible liquids.


Class C

Fires involving combustible gases.


Class D

Fires involving certain combustible metals, including potassium, sodium, & magnesium. Specialist advice should be sought.


Class F

Fires involving cooking oils and fats.

Most fire extinguishers have a pictograph label identifying the type of fuels that may be extinguished.

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Electrical hazards

Where a fire, involving an electrical hazard, can be expected, the extinguisher must be electrically non-conductive, in addition to having the correct classification for the materials involved. The marking of '(E)' on the fire extinguisher indicates the extinguisher is safe for extinguishing a fire involving energised electrical equipment.

For further advice ring 8204 3611 or email us.

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