Class L Fires Explained: BS ISO 3941:2026 Guide

For decades, UK fire classifications have been reassuringly stable. Class A for solids, B for flammable liquids, C for gases, D for metals, F for cooking oils. Every extinguisher label, every fire risk assessment and every staff training session has been built on that framework.

In 2026 it changed. The British Standards Institution published BS ISO 3941:2026, which replaces the 2007 edition and introduces a brand new classification: Class L, created specifically for fires involving lithium-ion cells and batteries.

If your building has staff charging e-bikes, a warehouse full of pallet-truck batteries, a bench of cordless tool chargers, EV charge points in the car park or a battery storage unit alongside your solar array, this affects you. Here is what Class L actually means, what it does and does not require of you legally, and what a competent fire provider should now be checking on your site.

What Is a Class L Fire?

Class L covers fires involving lithium-ion cells and batteries where no metallic lithium is present. That last clause matters, and it is the detail most summaries get wrong.

Fires involving metallic lithium (the reactive metal itself) remain Class D, alongside magnesium, sodium and other combustible metals. Class L is for the rechargeable lithium-ion cells found in everyday equipment: e-bikes, e-scooters, laptops, power tools, forklifts, mobility scooters, electric vehicles and battery energy storage systems.

The reason it needed its own category is that a lithium-ion fire behaves unlike anything else on the list. It is driven by thermal runaway, a self-sustaining chain of exothermic reactions within the cell that can generate intense heat and release flammable and toxic gases. Once it starts, it produces:

• Extremely rapid heat release, with cell temperatures capable of exceeding 400°C within seconds
• Toxic and flammable gas venting, including hydrogen fluoride, before any visible flame appears
• Cell-to-cell propagation, where one failing cell heats its neighbours until the whole pack is involved
• Delayed re-ignition, sometimes hours after the fire appears to be out

That final characteristic is why a lithium-ion fire is so dangerous in a commercial setting. A pack that has been knocked down, declared safe and moved into a bin store can reignite long after everyone has gone home.

The UK Fire Classifications, Updated

One common point of confusion is worth clearing up. There is no "Class E" for electrical fires in the UK. Electricity is an ignition source, not a fuel, so fires involving live electrical equipment are handled through extinguisher suitability ratings rather than a letter class. We cover that distinction in more detail in our guide to fire extinguishers for electrical fires, and the full colour-coding system in fire extinguisher colours explained.

Does Class L Create a New Legal Duty?

No. It is worth being precise about this, because a fair amount of the commentary published since BS ISO 3941:2026 has implied otherwise.

A British Standard is not legislation. BS ISO 3941:2026 defines and classifies; it does not impose duties on building owners. No new statutory instrument accompanied it, and nobody is going to be prosecuted for failing to have a Class L extinguisher on the wall.

The duty that does apply is the one that has been there since 2005. Under the Regulatory Reform (Fire Safety) Order, the Responsible Person must ensure the fire risk assessment remains valid, and must review it if there is reason to suspect it is no longer valid or if there has been a significant change to the premises or the activities carried on there.

For most UK businesses, lithium-ion is exactly that kind of change. The batteries arrived gradually: a delivery rider's e-bike here, a new fleet of cordless tools there, EV charge points added to the car park two years ago. In a great many buildings the fire risk assessment has never been updated to acknowledge them. The new classification simply removes any remaining excuse for treating a lithium-ion fire as an ordinary electrical one.

What Class L Does and Does Not Mean

Because a good deal of the coverage since publication has blurred the two, it is worth setting out the distinction plainly.

What Class L does not mean

• It does not mean every lithium-ion battery on site now requires a dedicated extinguisher next to it.
• It does not make your existing extinguishers non-compliant.
• It does not create a new statutory duty. No legislation accompanied the standard.
• It does not specify extinguisher types. BS ISO 3941:2026 classifies fires; it does not certify equipment, and there is currently no Class L rating within the portable extinguisher standard.
• It does not mean Class D agents should be used on lithium-ion batteries. Those are formulated for burning metal and are a different proposition entirely.

What Class L does mean

• Lithium-ion battery fires now have a distinct classification, separate from Class D metal fires and from ordinary electrical incidents.
• Fire risk assessments have a recognised category in which to record lithium-ion as a specific hazard, rather than folding it into something it does not resemble.
• Businesses storing, charging or operating meaningful quantities of lithium-ion batteries have a clear reason to look at how those fires actually behave.
• Detection, separation, charging arrangements, storage, suppression and emergency procedures should all be considered against the real risk on the site, rather than against a product category.

The practical takeaway is that this is a fire strategy question before it is a purchasing question. Anyone using the new classification to sell you equipment on the basis that you are now non-compliant has the relationship the wrong way round.

Where the Risk Actually Sits in a Commercial Building

Lithium-ion risk is rarely where people expect it. In our experience surveying commercial sites across London, Kent and Suffolk, these are the areas that most often go unassessed:

• Staff e-bikes and e-scooters: frequently charged in reception areas, corridors, stairwells and under desks. In other words, directly on the means of escape.
• Warehouse and logistics equipment: pallet trucks, forklifts, MEWPs and floor scrubbers, often on charge overnight in an unoccupied, undetected part of the building.
• Tool charging benches: a dozen cordless batteries on a single bank of chargers, usually on a wooden bench, usually surrounded by combustibles.
• IT and UPS installations: laptop trolleys in training rooms, and lithium UPS units that have quietly replaced older lead-acid systems in comms rooms.
• EV charge points: particularly in enclosed or underground car parks, where smoke logging and access for firefighting are already difficult.
• Battery energy storage systems: increasingly common alongside commercial solar, and often installed with more attention to the electrical design than the fire strategy.
• Damaged and end-of-life batteries: the single most overlooked item. A swollen or impact-damaged pack awaiting disposal, sitting in a general waste store, is one of the highest-risk objects on most sites.

Why Standard Extinguishers Do Not Work on Class L Fires

This is the part that surprises most facilities managers. The extinguishers already on your wall were selected for fuel fires, and a lithium-ion fire does not behave like one. The heat is generated by reactions inside the cell itself, so removing the flame at the surface does not necessarily stop the event.

An important caveat that reputable suppliers will tell you and less reputable ones will not: no portable extinguisher reliably "puts out" a lithium-ion battery fire. AVD and water mist units are cooling and containment tools. They buy time to evacuate, prevent the fire spreading to the surrounding building, and give the fire service something less severe to arrive at. Anyone selling you an extinguisher on the promise that it will extinguish a thermal runaway event is overselling it.

The practical implication is that fire strategy for lithium-ion has to be weighted heavily towards prevention, separation and early detection rather than suppression. Where the batteries are charged matters far more than what is hanging on the wall next to them.

Detection: Why a Standard Smoke Detector May Be Too Late

A cell entering thermal runaway can vent electrolyte vapour before it produces conventional smoke. Conventional smoke detection may therefore not give the earliest possible warning of a developing battery failure, particularly where batteries are enclosed, or charging in concentrated quantities, or sited in a large open space with detection at high level. How quickly a detector responds depends on the cell chemistry, the enclosure, ventilation, detector siting and how the fire develops, so this is a question to work through for the specific area rather than a universal rule.

Options worth reviewing with your fire alarm provider include:

• Detector siting and type: multi-sensor detection positioned over charging areas rather than relying on general area coverage.
• Aspirating smoke detection (ASD): actively samples air and offers very early warning, well suited to dedicated charging rooms and battery stores.
• Off-gas detection: a developing technology that senses the specific electrolyte vapours released before smoke or flame, giving the earliest possible alert.
• System category review: if charging has been introduced into an area covered only by an L-category system designed for life safety along escape routes, the detection coverage may no longer suit how the building is actually used.

That last point ties directly into the current fire alarm standard. BS 5839-1:2025 places more emphasis on selecting a system category based on how a building is genuinely used rather than defaulting to a familiar option. A building that has quietly become a battery charging facility is a textbook case for reassessment.

Your Lithium-Ion Charging Area Checklist

Drawing on National Fire Chiefs Council and London Fire Brigade guidance for Responsible Persons, these are the controls that make the biggest difference:

• Never charge on an escape route. No e-bikes, e-scooters or battery packs in corridors, stairwells, lobbies or final exit routes. This is the single most important control.
• Designate a charging area away from the main building where possible, ideally externally or in a fire-separated compartment with its own detection.
• Keep combustibles clear: no cardboard, packaging, textiles or waste within the charging zone.
• Charge on non-combustible surfaces, not wooden benches or carpet.
• Do not charge overnight or unattended where it can be avoided, and never in unoccupied buildings without detection.
• Use manufacturer-supplied chargers only. A large proportion of UK e-bike fires involve incompatible or aftermarket chargers and conversion kits.
• Quarantine damaged batteries immediately: swollen, dented, punctured or water-damaged packs go outside, away from the building, in a non-combustible container, pending specialist disposal.
• Provide appropriate signage and staff instruction so people know where charging is permitted and what a failing battery looks and smells like.
• Record it in the fire risk assessment: quantity, chemistry, storage location, charging arrangements and controls.

Installing the Risk and Protecting Against It

Sygma occupies a slightly unusual position here. We install EV charging and solar and battery storage systems as well as delivering fire risk assessments, fire alarm systems and extinguisher provision. We are on both sides of this risk, which shapes how we design.

In practice, that means a few things we would want addressed on any installation involving significant lithium-ion capacity:

• Siting EV charge points and BESS units with fire separation and firefighting access considered at the design stage, not retrofitted after commissioning
• Clearly labelled, accessible electrical isolation so the fire service can make an installation safe on arrival
• Detection coverage extended over new charging infrastructure as part of the installation, rather than left as a separate job for someone else
• An updated fire risk assessment issued alongside the electrical certification, so the paperwork actually reflects the building as it now stands
• Extinguisher provision reviewed for the specific risk, with honest advice about what portable equipment can and cannot achieve

If nobody has looked at your building since the batteries arrived, the assessment on file is describing a building that no longer exists.

Frequently Asked Questions About Class L Fires

References and Verification Sources

• BS ISO 3941:2026, Classification of fires (British Standards Institution, 2026)
• GOV.UK: E-cycle and e-scooter batteries: managing fire risk for premises
• London Fire Brigade: Guidance for Responsible Persons: e-bike and e-scooter safety
• The Regulatory Reform (Fire Safety) Order 2005
• BS 5839-1:2025, Fire detection and fire alarm systems for buildings: Code of practice