Class L – A new classification of fire

BS ISO 3941:2026 Classification of fires

The publication of BS ISO 3941:2026 Classification of fires is the first significant structural update to fire classification in many years, reflecting the increased presence of lithium-ion battery risks in today’s modern premises.

The introduction of Class L clarifies how these risks should be categorised within fire risk assessments. Although extinguisher fire ratings under BS EN 3 remain unchanged, the update strengthens the expectation that lithium-ion battery hazards are properly identified and addressed where present.

BS ISO 3941:2026 sets out the internationally aligned fire classification framework adopted in the UK. It introduces Class L alongside the existing fire classes:

• Class A – Solid combustibles (wood, paper, textiles, plastics)
• Class B – Flammable liquids (petrol, solvents, paints)
• Class C – Flammable gases (propane, butane, methane)
• Class D – Combustible metals (magnesium, lithium metal, sodium)
• Class F – Cooking oils and fats (deep fat fryers)
• Class L – Fires involving lithium-ion cells and battery packs (where no metallic lithium is present) Note: the introduction of Class L does not change existing extinguisher fire ratings under BS EN 3.

Why Lithium-Ion battery fires require their own Class L classification

Lithium-ion batteries are now embedded in everyday life, from handheld devices and power tools to e-bikes, e-scooters, warehouse equipment and electric vehicles. With that growth has come increased fire incidents across residential, commercial and industrial settings.

Lithium-ion battery fires are fundamentally different from traditional fuel-based fires.

Key characteristics include:

Thermal runaway: A self-sustaining chemical reaction within the battery cell that generates extreme heat.
Re-ignition risk: Even after apparent extinguishment.
High energy density: Resulting in rapid fire growth and intense heat release.
Toxicity: Production of toxic and potentially flammable off gases.
Oxygen-independent combustion: Unlike Class A fires, they do not rely on atmospheric oxygen in the same way.

These factors mean that conventional suppression strategies are not always appropriate or effective. The formal recognition of Class L acknowledges that lithium-ion battery fires require specific understanding and response strategies.

For more information on lithium-ion battery hazards, see our news article https://resfire.co.uk/london-fire-brigade-calls-for-urgent-e-bike-and-e-scooter-legislation/

Fire Risk Assessments and Class L fire risks

It is essential that premises Fire Risk Assessments (FRA) are reviewed in accordance with the Regulatory Reform (Fire Safety) Order 2005 (RRO) as soon as lithium-ion battery risks become present. Where lithium-ion battery hazards are present, they should be clearly identified within the premises FRA. Failure to adequately assess such risks could result in an incomplete assessment under Article 9 of the RRO. While British Standards are not law, enforcing authorities and courts may refer to them when considering whether a Responsible Person has acted reasonably and proportionately.

Lithium-ion battery fires behave differently from traditional combustible materials or flammable liquids and in particular, thermal runaway can lead to rapid heat release, re-ignition and complex fire development, hence the importance of identifying these risks in an FRA.

Previously, lithium-ion battery fires were typically considered under existing fire classes depending on the materials involved. The introduction of Class L provides clearer categorisation for risk assessment and fire safety planning purposes.

It does not invalidate existing classifications but clarifies how lithium-ion battery hazards should be described where present.