Fire Extinguisher Types
The range of available fire extinguisher types (their size, colour and method of operation) is so wide as to create some degree of confusion. All guidance on this area states that the equipment which could be used by the average person in a fire situation should be both suitably located and suitable for the risk. However, problems can arise when more than one type of fire risk is encountered and the pressurised person is faced with a choice of extinguisher. The wrong choice could mean fire fighting efforts are wasted or place the individual under even greater risk.
The fire extinguisher types available correspond heavily to the nature of the risk. Fires are classified into five main types, and the ability of a fire extinguisher to deal with a specific classification depends on the agent within it.
Classification of Fires
Class A – Fires that involve mainly organic solids (containing carbon based compounds) such as wood, paper, plastics etc
Class B – Fires that involve flammable liquids such as petrol, paint and oils, and liquefiable solids such as fats, waxes, greases but not including cooking oil or fats
Class C – Fires that involve gases such as butane or propane
Class D – Fires that involve certain metals such as sodium, magnesium and aluminium
Class F – Fires that involve commercial deep fat or oil fryers
Types of Fire Extinguishers
Water, when applied either as a pressurised jet or a spray, is the most effective method of quenching Class A fires. Water fire extinguishers can also be used as a spray on Class B fires, if the fire involves liquids and liquefied solids which are miscible (meaning capable of mixing) with water, such as methanol and acetone. This fire extinguisher type is ineffective on Class C fires; however water can be used to cool leaking containers.
Water extinguishers must never be used on fires containing the following:
- Electricity, as the current can flow up the water stream and electrocute the user
- Non-miscible liquids as this will make the fire worse
There are also water fire extinguishers available on the market with chemical additives included. These additives can increase the effectiveness of a water fire extinguisher and also mean the size of the canister can be reduced.
Foam is a special mixture that creates a smothering blanket over the fire which cuts off the supply of oxygen. It can safely be used on Class A fires. Foam fire extinguishers (also known as AFFF) will have an effect on some Class B fires (some types of foam break down on contact with alcohol) and Class C fires that consist of small amounts of liquefied gas.
Using a foam extinguisher requires some skill when dealing with anything but a small liquid fire, as the procedure of using one requires the blanket of foam to start at the rear of the fire and lay a foam surface across the top of the fire.
Dry powder fire extinguishers are suitable for use on Class A, B and C fires (although it is not the best choice for a Class A fire and can only be used on small liquefied gas fires within Class C). This extinguisher works by spraying a cloud over the fire, which smothers the supply of oxygen.
Dry powders can also be used on fires that involve electricity.
Class D Powder
There are specialised dry powders available on the market that use inert substances on Class D fires involving burning metals. These work by forming a crust over the burning metal and cut off the oxygen. Class D extinguishers look the same as ordinary dry powder ones but with a different label and nozzle.
Carbon Dioxide Gas
Similar to dry powder, CO2 fire extinguishers work by smothering the oxygen supply as carbon dioxide is a heavier gas. This type of extinguisher is effective on Class B fires and also on fires involving electricity as the gas is able to enter into the inside of the equipment. Carbon Dioxide extinguishers are often a popular choice in restaurant establishments as the gas will not contaminate either the cooking equipment or the food.
Wet chemical fire extinguishers are a relatively new type, designed specifically to be used on deep fat cooking fires (involving animal fat or vegetable oil). Other fire extinguishers are not particularly effective against such fires, which is why this type was created.
Agents within this type of extinguisher produce a heavy vapour that cuts off the oxygen supply to the fire. They can be used on Class A or Class B fires, and are very effective on fires that involve live electrical equipment as they interfere with the combustion reactions.
Halon is a vaporising liquid that was once widely used in fire-fighting equipment. However, Halons have been banned in many countries (including the UK) since 1994 and are in the process of being phased out in others. This is because Halon has a very high ozone-depleting capacity, and substances of this kind are considered dangerous for the environment.
However, there are currently 3 exceptions to the rule – in aircraft, military usage and in the Channel Tunnel Halon fire extinguishers are still acceptable.
This chart shows which fire extinguisher types can be used on the different fire classes:
Fire Extinguisher Colours
So that the different fire extinguisher types can be easily identified, particularly in an emergency situation, they are colour coded depending on the type of extinguishing agent inside. Currently in the UK there are two systems in operation:
- Older fire extinguishers are coded by the whole body of the unit
- Newer extinguishers have a red body with a coloured band/label
The table below shows the various fire extinguisher types and their respective colours:
|Fire extinguisher content||Colour of body or band/label|
How Fire Extinguishers Work
It is basic knowledge that for a fire to occur, three things are required – Heat, oxygen (or a similar gas) and fuel. Remove one of the three elements, and the fire will die. A fire extinguisher is designed to do just that. Different fire extinguisher types will remove different elements of the fire; however the effect is still the same.
All fire extinguisher types have some form of lever at the top of the canister, which expels the agent inside it by high pressure. For this reason to avoid accidental spillage, the levers of fire extinguishers are fitted with a pin that must be pulled out in order for the extinguisher to be used.
When the lever is compressed, this pushes into a connecting rod inside the extinguisher, which in turn presses into a spring-mounted valve, opening up the passageway to the exit nozzle. The bottom of the connecting rod has a sharp point, which when forced downwards, pierces a gas cylinder release valve inside the extinguisher. The released gas itself then puts pressure on the extinguishing agent inside the cylinder, so that it is driven upwards through a siphon tube to the exit nozzle with some considerable force. The extinguishing agent should be aimed at the fuel of the fire rather than the flames themselves in order to put the fire out.
Maintenance, Servicing and Inspection Requirements
The primary piece of legislation covering this subject is British Standard 5306-3:2009 – Fire Extinguishing Installations and Equipment on Premises. Commissioning and Maintenance of Portable Fire Extinguishers. Code of Practice.
This piece of legislation requires that regular (at least monthly) visual inspections of all fire extinguisher types should be carried out. This is a simple check to ensure that the extinguisher is where it should be, that it is unused and undamaged, has not been tampered with, is accessible and visible, the instructions on it are legible and any pressure gauges are still within the operational limits.
All Portable fire extinguisher types will also need to have a basic annual service and will need to have an overhaul every ten years (this applies only to Halon and Carbon Dioxide types) or an extended service, usually once every 5 years (this applies to all other types of fire extinguisher). An extended service includes a test discharge of the extinguisher, and an overhaul includes both a test discharge and a pressure test of the shell of the body of the extinguisher.
Fire extinguisher servicing should ideally be carried out by a specialist company.