Classes of Fire: A, B, C, D, F and Electrical Explained

A welder's spark lands in a skip of packaging on a Friday afternoon. The nearest extinguisher is a water one, the labourer who grabs it isn't sure it's the right type, and nobody has checked whether there's a fuel bowser ten metres away. Most site fires aren't dramatic at the start. They get worse because someone didn't know what was burning. That's what the classes of fire are for.

In the UK, the classes of fire are set by the British and European standard BS EN 2. There are five lettered classes: Class A (solid materials such as wood, paper, cardboard and textiles), Class B (flammable liquids such as petrol, diesel, paint and solvents), Class C (flammable gases such as propane, butane and acetylene), Class D (combustible metals such as magnesium, aluminium swarf and sodium) and Class F (cooking oils and fats). Fires involving live electrical equipment don't have a letter in the UK, but they're treated as their own category because of the shock risk. There is no Class E or Class K in UK practice.

Knowing the class tells you which extinguisher can be used, which must not, and what your fire risk assessment needs to say about storage and ignition sources.

What are the classes of fire?

Fire needs three things: fuel, heat and oxygen. Take one away and it goes out. The class is about the fuel, because the fuel decides how the fire behaves and how it has to be put out. Water cools burning timber well. Thrown on a burning pan of oil, it turns to steam instantly and throws burning oil across the room.

BS EN 2 groups fires by fuel so that extinguishers can be tested and labelled against them. Every UK extinguisher carries pictograms showing the classes it's rated for, and many carry a number too, such as 13A or 55B. The number reflects the size of test fire it put out under standard conditions.

Class A: solid combustible materials

Class A covers ordinary solids that burn with glowing embers: timber, paper, cardboard, fabric, many plastics, insulation and packaging. This is the most common fuel on any site or in any building.

On a construction site, think skips full of offcuts and packaging, stacked timber, pallets, formwork, protection boards and the contents of welfare cabins. In facilities management, it's stored paperwork, furniture, waste bins in bin stores and the rubbish that builds up in plant rooms and risers.

Water, foam, water mist and some powder extinguishers are rated for Class A. The best control is still housekeeping: keep waste in lidded metal containers, empty skips often and keep them away from buildings and scaffold.

Class B: flammable liquids

Class B fires involve flammable liquids or liquefiable solids: petrol, diesel, paraffin, paints, thinners, adhesives, solvents, bitumen and some resins. The vapour burns rather than the liquid itself, which is why petrol can catch from a spark some distance away.

Typical examples include the site fuel store, refuelling points, the paint and solvent cabinet and bitumen boilers, and in buildings, cleaning chemicals and generator day tanks. Anything in this group also falls under COSHH, so the COSHH regulations guide is worth reading alongside this one.

Foam, powder and CO2 extinguishers are rated for Class B. Water must not be used, because it spreads the burning liquid.

Class C: flammable gases

Class C covers fires fuelled by flammable gases: LPG (propane and butane), acetylene, natural gas and hydrogen. On site, that means gas cylinders for heaters and roofing torches, oxy-acetylene sets for cutting, and gas supplies in buildings being refurbished.

The safest way to deal with a gas fire is to isolate the supply, if that can be done safely. Putting out the flame while gas is still escaping can leave a cloud of unburnt gas that may then ignite with more force. Dry powder extinguishers are rated for Class C, but the priority is isolation, evacuation and calling 999. HSE notes that LPG has been involved in many serious fires and explosions on construction sites, particularly where leaks happen in confined areas.

Class D: combustible metals

Class D fires involve metals such as magnesium, aluminium (particularly as fine swarf or powder), titanium, sodium and lithium metal. These burn at very high temperatures and can react violently with water.

They're rare on general building sites but turn up in fabrication shops and metal cutting areas. If your site has a genuine Class D risk, it needs a specialist powder extinguisher chosen for that metal, and the people nominated to use it need specific training. Normal extinguishers can make these fires worse.

Class F: cooking oils and fats

Class F covers fires in cooking oils and fats, usually deep fat fryers and pans. It was split out from Class B because cooking oil fires reach far higher temperatures than most flammable liquids and re-ignite easily.

You'll find this risk in site canteens, welfare cabins with cookers, staff kitchens in offices, and catering units at events. Wet chemical extinguishers are designed for Class F. A fire blanket can deal with a small pan fire if the person is trained. Never use water on a fat fire.

Electrical fires (no letter in the UK)

Electrical equipment isn't a fuel in itself. The fire is usually in the insulation, casing or nearby materials, but the live supply adds a shock risk for whoever tackles it.

Typical sources include temporary distribution boards, extension leads run under welfare cabins, overloaded chargers, heaters, server rooms and plant rooms. HSE's construction guidance points to CO2 extinguishers for electrical fires. Where it's safe to do so, isolate the power first, which can turn the fire into an ordinary Class A or B fire. Electrical teams will find more on the controls in our RAMS for electricians.

What about lithium-ion batteries?

Lithium-ion battery fires, from cordless tools, e-bikes and e-scooters, don't sit neatly in any class. A damaged or overcharged battery can go into thermal runaway and re-ignite after it seems to be out. The practical controls are about charging: use manufacturers' chargers, don't charge unattended overnight in site cabins, keep charging areas clear of combustibles, and take damaged batteries out of use. If a battery is on fire, get people out and call 999.

Why the class matters on a construction site

Construction sites mix fuel classes in a way few buildings do. On one mid-sized refurbishment you could easily have:

• Class A: timber, packaging, insulation, skips and protection materials
• Class B: diesel for plant and generators, paints, solvents and adhesives
• Class C: LPG for heaters and roofing torches, and oxy-acetylene sets
• Electrical: temporary supplies, chargers and heaters in welfare cabins
• Class F: a cooker or fryer in the site canteen

Each of those needs the right extinguisher at the right fire point, and the people nearby need to know which is which. HSE's general fire safety page for construction says extinguishers should be at identified fire points around the site and suitable for the kind of fire likely there. It gives water for wood, paper and cloth, dry powder or foam for flammable liquids, and CO2 for electrical.

Hot works are where the classes collide. Welding, cutting, grinding and torch-on roofing all produce heat or sparks, often close to Class A materials and sometimes near Class B or C fuels. HSE's guidance on process fire risks lists clearing combustibles from the area, having suitable extinguishers and keeping a careful watch throughout the work, and says a permit to work system can help on larger projects. Our guide to RAMS for hot works covers what that method statement needs, and the permit to work guide explains how permits tie it together.

Temporary site accommodation deserves its own mention. Cabins combine Class A contents, electrical heaters and chargers, and sometimes a cooker. HSE's guidance HSG168, Fire safety in construction, covers the siting and construction of site accommodation in more detail.

Examples from security and facilities management

Facilities management has a different but just as mixed pattern.

• Bin stores and waste compounds: mostly Class A, often next to the building, and a common target for arson.
• Plant rooms: electrical switchgear, generators with Class B fuel, and sometimes stored materials that shouldn't be there.
• Staff kitchens and restaurants: Class F risk from fryers and Class A from packaging.
• Cleaning cupboards: Class B solvents and aerosols sitting next to Class A paper stock.

For security teams, fire is often discovered by the person on patrol. A mobile patrol officer finding smoke from a bin store at 3am needs to know whether it's safe to act, who to call and what's stored nearby. Our FM risk assessment guide covers how to build these into site-specific assessments.

Only tackle a fire if you're trained and it's safe

Knowing the classes of fire isn't permission to fight one. Fire and rescue services and HSE guidance are consistent on this: raise the alarm first, and only use an extinguisher if you've been trained, the fire is small, you know what's burning, and you have a clear escape route behind you. If any of that isn't true, get out, close doors behind you if you can, and call 999.

The Fire Safety Order requires the responsible person to nominate competent people to carry out fire-fighting measures and to make sure they're properly trained. A blog post or poster doesn't replace that training, though a toolbox talk is a good way to remind a crew which extinguisher is at which fire point before hot works start.

What the law requires

In England and Wales, the main law is the Regulatory Reform (Fire Safety) Order 2005, usually called the Fire Safety Order. It places duties on the responsible person, which can be the employer, owner, landlord, occupier or anyone else in control of the premises, such as a facilities manager or principal contractor.

The gov.uk guidance says the responsible person must carry out a fire risk assessment and review it regularly, tell staff about the risks identified, put in place and maintain appropriate fire safety measures, plan for an emergency, and give staff information, fire safety instruction and training. It also says you must keep a written record of the fire risk assessment.

Article 13 of the Order deals with fire-fighting and fire detection. It requires premises, where necessary, to have appropriate fire-fighting equipment, and says what's appropriate depends on things like the size and use of the premises, the equipment in it, and the physical and chemical properties of substances likely to be there. That last point is the classes of fire in legal language. Non-automatic equipment such as extinguishers must be easy to get to, simple to use and shown by signs.

On construction sites, the Construction (Design and Management) Regulations 2015 add specific duties. Regulation 29 covers the prevention of risk from fire, and regulation 32 requires suitable and sufficient fire-fighting equipment and fire detection and alarm systems where necessary, maintained and tested, with workers instructed in their use so far as is reasonably practicable. Underneath it all sits the general duty of care in the Health and Safety at Work etc. Act 1974.

Scotland and Northern Ireland have their own laws. In Scotland, it's the Fire (Scotland) Act 2005 and the Fire Safety (Scotland) Regulations 2006. In Northern Ireland, it's the Fire and Rescue Services (Northern Ireland) Order 2006 and the Fire Safety Regulations (Northern Ireland) 2010.

How to put classes of fire into your risk assessment

A fire risk assessment that just says "fire extinguishers provided" won't help anyone. Here's a practical approach.

1. List the fuels on site or in the building by area, and give each one its class. Diesel store: Class B. Timber and packaging: Class A. LPG cage: Class C. Canteen fryer: Class F. Distribution boards: electrical.
2. List the ignition sources near each one: hot works, heaters, smoking areas, electrical equipment, vehicles and arson risk.
3. Set out the controls: storage distances, lidded bins, fuel store locations, hot work permits, charging rules and housekeeping checks.
4. Match the extinguishers to the classes at each fire point, and record who is trained to use them.
5. Brief it. Put the fire points and their extinguisher types on the site plan and in the induction.
6. Review it as the job changes. A site's fire risks at groundworks are very different from those at fit-out.

Our construction fire risk assessment guide walks through that process for sites in more detail.

How swiftRMS helps

swiftRMS is an AI tool that drafts RAMS for construction, security and FM teams. Describe the task, such as torch-on roofing next to a timber-framed extension, and it drafts a risk assessment and method statement that picks up likely fire hazards, suggests controls and cites the relevant legislation, including the Fire Safety Order and CDM 2015 where they apply.

The draft is a starting point. A competent person must review, edit and approve it before it's used, because only someone who knows the site can confirm the fuels, the fire points and the extinguishers that are actually there. You can see the kind of output it produces on our RAMS examples page.

If you'd like to see a draft for your own task, you can try a free RAMS preview with no signup. If it's useful, there's a 14-day free trial and no card is needed.

Frequently Asked Questions

How many classes of fire are there in the UK?

There are five lettered classes under BS EN 2: A (solids), B (flammable liquids), C (flammable gases), D (combustible metals) and F (cooking oils and fats). Electrical fires are treated as a separate category but don't have a letter.

Is there a Class E fire in the UK?

No. Some websites use Class E for electrical fires, but that isn't part of BS EN 2. In the UK, electrical fires aren't given a letter because electricity is a source of ignition and shock rather than a fuel. Isolate the supply if it's safe to do so, and CO2 is the usual extinguisher for live electrical equipment.

What class of fire is diesel?

Diesel is a flammable liquid, so it's Class B. The same applies to petrol, paraffin, paints, thinners and most solvents. Foam, powder and CO2 extinguishers are rated for Class B. Water isn't.

What class of fire is LPG or propane?

LPG, propane and butane are flammable gases, so they're Class C. The priority with a gas fire is to isolate the supply if it's safe to do so, get people away and call 999. Dry powder extinguishers are rated for Class C.

Why is there a separate class for cooking oil?

Cooking oil and fat fires burn much hotter than most flammable liquids and can re-ignite after the flames are knocked down. BS EN 2 gives them their own class, F, and wet chemical extinguishers are designed for them. Water must never be used on a fat fire.

Should my team fight a fire themselves?

Only people who've been trained, and only if the fire is small, they know what's burning and they have a clear way out. The first job is always to raise the alarm. If there's any doubt, evacuate and call 999.

Official Resources

• Workplace fire safety: your responsibilities (gov.uk)
• Regulatory Reform (Fire Safety) Order 2005 (legislation.gov.uk)
• General fire safety in construction (HSE)
• Process fire risks in construction (HSE)
• HSG168 Fire safety in construction (HSE)
• Construction (Design and Management) Regulations 2015 (legislation.gov.uk)