A Class B fire is a fire involving flammable liquids or liquefiable solids, such as petrol, diesel or oils. Tackle it with an extinguisher that carries a B rating on its label (for example 34B) – typically foam, CO2 or dry powder – and never with a water-only extinguisher. In the UK, fires are grouped into five classes under BS EN 2: A, B, C, D and F. Fires involving electrical equipment are treated separately because live electricity changes which extinguishing agents are safe to use.
This guide explains each class, shows which extinguisher types suit which fire, and answers common questions such as which extinguisher you should not use on flammable liquids and what to use on cooking oil.
The UK classes of fire at a glance
Every fire extinguisher made to BS EN 3-7 carries a fire rating on its label, such as 13A, 34B or 15F. The letter tells you the class of fire the extinguisher has been tested against; the number indicates its tested performance. Always check the label – the table below is a general guide only.
| Class | Fuel | Typical examples | Suitable extinguishers | Do not use |
|---|---|---|---|---|
| Class A | Solid materials, usually organic, that burn with glowing embers | Wood, paper, textiles | Water, water with additive, foam, dry powder (other types, such as water mist, may also carry an A rating – check the label) | – |
| Class B | Flammable liquids and liquefiable solids | Petrol, diesel, oils | Foam, CO2, dry powder with a B rating | Water-only extinguishers |
| Class C | Flammable gases | Gas supplies and cylinders | Isolate the gas supply first if safe; dry powder is the type usually suited to gas fires – seek specialist advice | Do not put out a gas flame while gas is still escaping |
| Class D | Metals | Specialist metal fire risks | Special Class D powder, chosen for the specific metal – seek specialist advice | Any extinguisher without a Class D rating |
| Class F | Cooking oils and fats in cooking appliances | Deep-fat fryers, chip pans | Wet chemical extinguisher with an F rating; light-duty fire blanket for small fires in containers | Water, foam |
| Electrical (no class) | Live electrical equipment | Distribution boards, computers, machinery | CO2; dry powder; water-based types only if dielectrically tested and marked as suitable | Standard water extinguishers |
Class A fires: solid materials
Class A fires involve ordinary combustible solids such as wood, paper and textiles. Class A materials are present in almost every building. Water and water extinguishers with additives cool the burning material and are the traditional choice. Foam extinguishers and dry powder extinguishers also carry A ratings.
Class B fires: flammable liquids
A Class B fire involves liquids or liquefiable solids – petrol, diesel, oils and similar. These fires are common risks in workshops, garages, plant rooms, and anywhere fuels or solvents are stored or used.
Which extinguishers work on a Class B fire?
- Foam – suitable for Class A and B fires and particularly suited to liquid fires such as petrol and diesel. Government guidance notes that foam should not be used on free-flowing liquid fires unless the operator has been specially trained, as it can spread the fire.
- CO2 – carbon dioxide extinguishers carry B ratings and are also the usual choice where electrical equipment is present.
- Dry powder – works on most classes of fire and gives good knock-down, but it does not cool the fire, so it can re-ignite. Powder reduces visibility and is not generally suitable for enclosed spaces.
Which extinguisher should you not use on flammable liquids?
Do not use a water (water-only) extinguisher on flammable liquids. Water extinguishers are suitable for Class A fires only; on burning liquids they can spread the fire. Some water extinguishers with additives are also rated for Class B – if so, a B rating will be shown on the label. As a rule, never use an extinguisher on a liquid fire unless its label shows a B rating.
Class C fires: flammable gases
Class C fires involve gases. The most important step is to shut off the gas supply if it is safe to do so. Putting out the flame while gas is still escaping allows unburnt gas to build up, which could re-ignite. Dry powder extinguishers are the type most commonly suited to gas fires, but the approach depends on the risk, so seek specialist advice and follow your fire risk assessment.
Class D fires: metals
Class D fires involve burning metals and are a specialist risk. Standard extinguishers are not designed for them. Special Class D powder extinguishers are used, with the size and type chosen according to the specific risk. Always seek specialist advice for Class D risks.
Class F fires: cooking oils and fats
A Class F fire involves vegetable or animal fats in cooking appliances, such as deep-fat fryers.
What fire extinguisher should you use on cooking oil?
In commercial kitchens, the recognised choice is a wet chemical extinguisher with an F rating, which is particularly suitable for catering premises with deep-fat fryers. Foam extinguishers are not suitable for deep-fat fryers or chip pans. Light-duty fire blankets are suitable for small fires in containers of cooking oil or fat, and for fires involving clothing.
At home, fire and rescue services advise that you should never throw water on a pan of burning oil, and should not use an extinguisher on it because the force of the discharge can spread the fire. Turn off the heat only if it is safe to do so, leave the room, close the door, get everyone out and call 999.
Electrical fires
There is no "Class E" in the UK classification. Fires involving electrical equipment are treated separately because live electricity makes some extinguishing agents dangerous to use. Disconnect the power if it is possible and safe to do so. Our guide on which fire extinguisher to use on electrical fires covers this in more detail.
- CO2 is particularly suitable for fires involving electrical equipment, as it extinguishes the fire without causing further damage in most cases (some electronic equipment can still be affected).
- Dry powder can be used on electrical fires, but it is likely to leave the equipment unusable.
- Standard water extinguishers are not suitable for live electrical equipment. Some water-based models (such as certain water mist extinguishers) pass the dielectric test in BS EN 3-7; only use them near electrics if the label says so.
Choosing extinguishers for your premises
Most workplaces need a combination of extinguishers matched to the fire classes present. Government guidance suggests that, for Class A risks, one water-based extinguisher for roughly every 200m² of floor space, with at least two per floor, will normally be adequate – with further types added for other risks such as flammable liquids, electrical equipment or cooking. Detailed selection and positioning guidance is in BS 5306-8. To identify each type at a glance, see our guide to fire extinguisher colours and types.
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Frequently asked questions
What is a Class B fire?
A Class B fire involves flammable liquids or liquefiable solids, such as petrol, diesel or oils. Use an extinguisher with a B rating – usually foam, CO2 or dry powder.
Which extinguisher should you not use on flammable liquids?
A water-only extinguisher. Water extinguishers are for Class A fires and are not suitable for burning liquids or live electrical equipment.
What is a Class F fire?
A Class F fire involves cooking oils and fats in cooking appliances, such as deep-fat fryers. Wet chemical extinguishers with an F rating are designed for these fires.
How many classes of fire are there?
Five under BS EN 2: A (solids), B (liquids), C (gases), D (metals) and F (cooking oils and fats). Electrical fires are treated separately rather than as a class.
Can I use a fire blanket on a chip pan fire?
Light-duty fire blankets are suitable for small fires in containers of cooking oil or fat. At home, fire services advise not to tackle a pan fire yourself: turn off the heat if it is safe, get out, close the door and call 999.
This guide is general information; follow your fire risk assessment and a competent person's advice.