What is a fire extinguisher?

As the name would suggest, a fire extinguisher is an appliance for putting out a blaze. It is a portable device, designed to assist in the safe evacuation of people from a building in a fire situation. It can be used to tackle a small fire in its early stages but the primary issue in a fire emergency is to ensure that the escape route remains open until everyone has reached a place of safety outside the building.

The Regulatory Reform (Fire Safety) Order (RRFSO) 2005 charges the responsible person(s) in control of non-domestic premises with the safety of everyone, whether employed in or visiting the building. This duty of care includes ensuring that the premises are provided with “appropriate fire fighting” equipment and that “any non-automatic fire-fighting equipment so provided is easily accessible, simple to use and indicated by signs”. A fire extinguisher is one such piece of non-automatic fire fighting equipment.

As there are different types of fires, there are different types of fire extinguisher. Using the wrong type of fire extinguisher on a blaze could make the situation much worse.

The HM Government entry level guide to interpreting the RRFSO, “A short guide to making your premises safe from fire” (June 2006), notes the four main types of portable fire extinguishers (Water, Powder, Foam and Carbon Dioxide/CO2) that are suited to tackling the different types of fire most likely to occur in non-domestic premises. It should be noted, however, that this guide does not mention the Wet Chemical extinguisher, as this is specifically designed to tackle fires involving hot fats or oils in cooking: for example, deep fat fryer fires in catering facilities.

In respect of the number of fire extinguishers that should be provided, the guide indicates there should be a minimum of one for every 200m2 of floor space, with at least one extinguisher on each floor of the premises.


What is a fire?

In order to understand the need for fire extinguishers, it is helpful to consider what a fire actually is. Three elements are required for any fire to start or be sustained:

  • Fuel (e.g. combustible solids or liquids)
  • Heat (e.g. a naked flame or electrical spark)
  • Oxygen (e.g. air supply)

Together, these elements are known as ‘the fire triangle’. If any one of these elements is missing or removed, a fire will not be able to start or, if already underway, continue to burn.

A fire extinguisher is designed to remove the heat and/or oxygen from the fire triangle: for example, by smothering the flames with a blanket of foam or powder.


What is in a fire extinguisher?

The contents of a fire extinguisher are designed to tackle a particular type of fire.

As such, there are five main types of extinguisher. Each has a red body, with a colour coded label to indicate its contents:

  • Water/Water with additive/Dry water mist (red label)
  • Powder (blue label)
  • Foam (cream label)
  • Carbon Dioxide / CO2 (black label)
  • Wet chemical (yellow label)

Water extinguishers deliver a stream of water under pressure, to dissipate the heat energy of fires through evaporation and quickly penetrate burning solid materials such as wood, paper and textiles. Water with additive extinguishers have a superior fire extinguisher rating; while still being environmentally friendly, they are safe if accidentally used on electrical equipment. Dry water mist extinguishers deliver microscopic ‘dry’ droplets of water which, as they do not sink into the burning liquid, are safe for use on fires involving liquid fats or oils, for example in deep fat fryers, found in industrial kitchens, as well as solid materials.

Powder extinguishers smother a fire by covering it in a blanket of dry powder. They do carry a risk of inhalation, however, so should not be used in confined spaces. They can be used on flammable liquid and flammable gas fires as well as solids. Specialist dry chemical powder extinguishers are required for fires involving flammable metals such as aluminium, magnesium and potassium.

Foam extinguishers create a blanket between the burning fuel and oxygen in the air; they are suitable for use on fires involving solid materials such as wood, paper, textiles, etc, and liquid flammables, such as petrol, paints and solvents.

CO2 extinguishers are suitable for use on flammable liquids and fires involving electrical equipment. They smother the fire by delivering a stream of carbon dioxide under pressure, to cut the supply of oxygen to the fire.

Wet chemical extinguishers are designed to fight fires involving combustible cooking oils and fats; a fire blanket is also recommended for such emergencies. Both smother a fire by cutting off its oxygen supply. It should be remembered that hot oil can reignite, so after stopping the blaze the oil should be left to cool in its container, e.g. deep fat fryer, with the heat source isolated.

N.B. Halon extinguishers are no longer included in the list as new halon production is not permitted in the UK.


Which fire extinguisher should I use?

Provided the fire is still small and can be reasonably fought with a fire extinguisher, the type to use will be dictated by the nature of the fire. There are five main types of fire, classified according to the fuel element:

  • Class A (solid materials: e.g. wood, textiles, paper, rubber)
  • Class B (liquids: e.g. petrol, oil, solvents, paints)
  • Class C (gases: e.g. natural gas, butane, propane)
  • Class D (metals or powdered metals: e.g. sodium, potassium)
  • Class F (cooking media: e.g. fat, oils)

The appropriate fire extinguisher to use in each case is as follows:

  • Class A – Water/Dry Water Mist, Foam
  • Class B – Powder, Foam, CO2
  • Class C – Powder
  • Class D – Powder
  • Class F – Dry Water Mist, Wet Chemical + Fire Blanket

Electrical fires are not considered to be a class on their own, as they feed on a heat supply (electricity) that should be isolated before the fire is fought. Once the supply has been cut, the flames can be tackled with an extinguisher according to the fuel that is burning, provided that the fire is still small.

If it is not possible to cut the electricity supply, it is important that a non-electricity conducting extinguishing agent is used. This can be a CO2 , foam or even a DRY water mist extinguisher. Make sure that the extinguisher has passed the 35kV dielectrical test. A powder extinguisher is also suitable for electrical fires but leaves a residue that can cause further damage to the electric equipment and rest of building. Particularly sensitive in this regard are historic buildings, churches and any environments with a concentration of electronic equipment.