BS ISO 3941:2026 Introduces Class L for Lithium-ion Battery Fires

The latest edition of BS ISO 3941:2026, Classification of fires, has been published with a significant update: the introduction of Class L, a new fire category specifically for lithium-ion (Li-ion) battery fires.

The revision reflects the rapid growth in the use of Li-ion batteries throughout the built environment and wider society. From large-scale battery energy storage systems (BESS) and electric vehicles to e-bikes, scooters and everyday portable devices, Li-ion technology is now embedded across multiple sectors. As deployment has accelerated, so too has recognition of the distinct fire risks associated with this electrochemical technology.

Why a new classification was needed

BS ISO 3941 categorises fires according to the type of material undergoing combustion. The addition of Class L acknowledges that Li-ion battery fires present characteristics that differ significantly from traditional Class A (solid materials), Class B (flammable liquids), or electrical fires.

Class L applies to fires involving Li-ion cells and batteries where no metallic lithium is present. The classification was introduced because Li-ion batteries exhibit a number of behaviours in fire scenarios that require specific consideration, including:

• High energy density, which can result in rapid heat release and accelerated fire growth;

• Thermal runaway, where failure in one cell can propagate quickly to adjacent cells;

• Release of toxic and flammable gases during venting;

• Explosion risk where vented gases accumulate in confined spaces;

• Limited physical access due to battery pack design and construction;

• Projectile hazards, with cells or burning electrolyte expelled during failure; and

• Stranded electrical energy, creating the potential for delayed re-ignition even after initial extinguishment.

These combined factors can produce fire behaviour that is more complex and potentially more hazardous than conventional fire classes covered by earlier versions of the standard.

Implications for fire protection and risk management

As the standard notes, the new classification is particularly relevant to the specification of fire protection and firefighting equipment. It provides clearer guidance for those responsible for assessing, managing and insuring fire risk in environments where Li-ion batteries are present.

This is especially important for facilities such as:

• Battery energy storage systems (BESS);

• EV charging infrastructure;

• Commercial warehousing and distribution centres;

• Industrial premises;

• Residential and office buildings; and

• Transport hubs and depots.

The introduction of Class L means designers and duty holders must give dedicated consideration to detection and suppression strategies tailored to Li-ion risks. This includes planning for potential re-ignition, managing residual electrical energy, and updating emergency response procedures accordingly.

Impact on fire risk assessments

Fire risk assessments will now need to explicitly identify the presence of Li-ion batteries, detailing their type, quantity, storage conditions and charging arrangements. With the widespread adoption of Li-ion-powered devices across almost every sector, Class L risks are likely to be relevant in a broad range of premises.

The publication of BS ISO 3941:2026 marks a significant step in aligning fire classification standards with modern technologies, ensuring that evolving battery risks are clearly defined and appropriately managed.

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