Fires are classified by what is burning, and extinguishers are chosen to match. Getting the pairing right matters because several combinations are actively dangerous: water on live electrics, water on burning fat, and powder in a small occupied room are each a worse outcome than doing nothing and closing the door.
The classes of fire
Class A is ordinary combustible solids: paper, wood, textiles, most rubbish. Class B is flammable liquids. Class C is flammable gases. Class D is metals. Class F is cooking oils and fats. Electrical fire is not a class of its own in the GB scheme, because electricity is an ignition source rather than a fuel, but extinguishers are still marked for whether they are safe on live equipment.
What each type covers
Water: class A. Foam: A and B, and it also cools. Dry powder: A, B and C, and special powders for class D. Carbon dioxide: B and safe on live electrical equipment. Wet chemical: F, and usually A as well. That table is the whole of the matching problem, and it is why most buildings carry two or three types rather than one.
Where the combinations go wrong
Water or foam on live electrical equipment conducts back to the person holding it. Water on a fat fryer flashes to steam and throws burning oil across the room. Powder indoors takes visibility to nothing in seconds and is an inhalation hazard. CO2 on a class A fire will knock down the flame and let it reignite, because it does not cool. Each of those is a predictable outcome of grabbing the nearest unit.
Special cases worth knowing
Class D metal fires need a specific powder and a specific technique, and the wrong agent can react violently: they belong to workshops and laboratories that know they have the risk. Lithium battery fires are their own emerging problem and are not reliably dealt with by any of the standard five, which is why storage and charging arrangements matter more there than extinguisher choice.
Matching the building
The assessment asks what is present. An office: water or foam plus CO2 near the electrics. A commercial kitchen: adds wet chemical by the fryer. A workshop with solvents: foam and CO2, powder if it can be sited outside the occupied space. A plant room with gas: powder, sited so somebody can use it from the doorway rather than from inside.
Questions people ask about fire extinguisher types and uses
What are the types of fire extinguisher in the UK?
Water, foam, dry powder, carbon dioxide and wet chemical, identified by a red body with a coloured band or panel.
What is a wet chemical extinguisher used for?
Class F fires, meaning cooking oils and fats, which is why commercial kitchens with fryers carry one. It usually covers class A as well.
What are the classes of fire?
A for combustible solids, B for flammable liquids, C for flammable gases, D for metals and F for cooking oils and fats. Electrical risk is marked separately rather than classed.
Which extinguisher is safe on live electrical equipment?
Carbon dioxide is the usual choice because it leaves no residue and does not conduct. Some dry powder units are also marked for electrical use.