Fire is a chemical reaction which will continue if fuel, oxygen and heat are present. To eliminate a fire one of these components must be removed. This is often expressed by means of the fire triangle shown in Figure: 1, all three corners of the triangle must be present for a fire to burn.
Fuel is found especially in the construction industry in many forms: petrol and paraffin for portable generators and heaters; bottled gas for heating and soldering. Most solvents are flammable. Rubbish also represents a source of fuel: off-cuts of wood, roofing felt, rags, empty solvent cans and discarded packaging will all provide fuel for a fire.
To eliminate fuel as a source of fire, all flammable liquids and gases should be stored correctly, usually in an outside locked store. The working environment should be kept clean by placing rags in a metal bin with a lid. Combustible waste material should be removed from the work site or burned outside under controlled conditions by a competent person.
Oxygen is all around us in the air we breathe, but can be eliminated from a small fire by smothering with a fire blanket, sand or foam. Closing doors and windows, but not locking them will limit the amount of oxygen available to a fire in a building and help to prevent it spreading.
Heat can be removed from a fire by dousing with water, but water must not be used on burning liquids since the water will spread the liquid and the fire. Some fire extinguishers have a cooling action which removes heat from the fire.
In the event of fire you should:
- raise the alarm; and immediately call the fire-fighting
- turn off machinery, gas and electricity supplies in the area of the fire;
- close doors and windows but without locking or bolting them;
- remove combustible materials and fuels away from the path of the fire, if the fire is small, and if this can be done safely;
- attack small fires with the correct extinguisher.
Only attack the fire if you can do so without endangering your own safety in any way. Always leave your own exit from the danger zone clear. Those not involved in fighting the fire should walk to a safe area or assembly point.
Fires are divided into four classes or categories:
- Class A are wood, paper and textile fires.
- Class B are liquid fires such as paint, petrol and oil.
- Class C are fires involving gas or spilled liquefied gas.
- Class D are very special types of fire involving burning metal.
Note: Electrical fires do not have a special category because, once started, they can be identified as one of the four above types. Caution: Electrical supplies at voltages above extra low voltages (ELV) – that is, above 50 V a.c. – can kill human beings and livestock and should therefore be treated with the greatest respect.
Fire extinguishers are for dealing with small fires, and different types of fire must be attacked with a different type of extinguisher. Using the wrong type of extinguisher could make matters worse. For example, water must not be used on a liquid or electrical fire.
Note: The normal procedure when dealing with electrical fires is to cut off the electrical supply and use an extinguisher which is appropriate to whatever is burning.
Figure: 2 shows the correct type of extinguisher to be used on the various categories of fire. The colour coding shown is in accordance with BS EN3: 1996.
Notice: Fire extinguishers and their applications (colour codes to BS EN3: 1996). The base colour of all fire extinguishers is red, with a different coloured flash to indicate the type.