As battery energy storage systems (BESS) grow in capacity and energy density, fire safety has become one of the most important considerations for developers, manufacturers, EPC contractors and authorities having jurisdiction (AHJs). In 2026, three important standards — ISO 3941:2026, UL 9540A 6th Edition and NFPA 855:2026 — are reshaping how lithium-ion battery fire risks are classified, tested and managed.

Together, these standards create a clearer safety framework for modern containerized BESS projects.


ISO 3941:2026: Recognizing Lithium-Ion Battery Fires

ISO 3941 provides an international system for classifying fires according to the nature of the fuel. The 2026 edition is particularly relevant to the energy storage industry because it recognizes Class L for lithium-ion battery fires.

This distinction is important because lithium-ion battery fires behave differently from conventional fires. A battery failure can involve thermal runaway, high-temperature vent gases, jet flames, toxic emissions, re-ignition and propagation from cell to cell.

Class L should also not be confused with Class D fires involving combustible metals. Typical BESS technologies such as LFP and NMC use lithium-ion cells rather than metallic lithium.

For the BESS industry, the new classification reinforces an important principle: lithium-ion battery fire protection requires dedicated risk assessment and mitigation strategies rather than simply applying conventional fire-extinguishing concepts.


UL 9540A 6th Edition: Moving Toward Large-Scale Fire Testing

Published on March 13, 2026, UL 9540A 6th Edition represents a major development in BESS fire testing.

UL 9540A evaluates thermal runaway and associated fire and explosion hazards. Cell-level testing examines thermal runaway characteristics and vent-gas flammability, while module-level testing evaluates cell-to-cell propagation, heat release, gas release and potential ignition or deflagration hazards.

The major change in the sixth edition is the stronger focus on Installation-Level Large-Scale Fire Testing.

For non-residential BESS, the testing sequence generally progresses from cell and module testing toward installation-level large-scale testing. During the large-scale test, gases released from batteries undergoing thermal runaway are intentionally ignited to establish a developed fire condition.

The test can therefore evaluate critical questions:

  • Can fire propagate from one BESS enclosure to another?
  • Is the specified separation distance sufficient?
  • Can the enclosure withstand severe fire conditions?
  • Are nearby equipment and structures adequately protected?
  • Are fire suppression and protection measures effective?

This moves BESS fire evaluation closer to realistic worst-case installation scenarios.


NFPA 855:2026: Safer Installation of Stationary Energy Storage Systems

While UL 9540A provides the test methodology, NFPA 855 addresses how stationary energy storage systems should be installed and protected.

NFPA 855:2026 strengthens its focus on fire and large-scale fire testing. Annex G.11 provides guidance for evaluating scenarios in which fire could spread between BESS units.

For containerized BESS projects, NFPA 855 considerations can influence container spacing, fire detection, gas detection, ventilation, explosion protection, emergency shutdown, fire suppression and emergency response planning.

UL 9540A test results can consequently become critical evidence when determining acceptable installation configurations and separation distances.


What Do the 2026 Standards Mean for BESS Manufacturers?

The three standards can be understood as one connected safety framework:

ISO 3941:2026 → Fire Classification

UL 9540A 6th Edition → Fire and Thermal Runaway Testing

NFPA 855:2026 → ESS Installation and Fire Safety

For BESS manufacturers, compliance is increasingly moving beyond simply installing fire detectors and suppression equipment. Safety must be engineered into the complete system, including battery racks, enclosure design, thermal management systems (TMS), battery management systems (BMS), fire suppression systems (FSS), combustible gas detection, ventilation and explosion protection.

As energy density continues to increase, validated system-level fire performance will become increasingly important.


TLS Energy and TLS Offshore Containers International provide containerized BESS solutions for global energy storage projects. By integrating battery racks, liquid cooling, BMS, fire detection and suppression, gas detection, ventilation, electrical systems and customized container enclosures, TLS supports customers in developing safer and more reliable energy storage systems for international markets.

The direction of the 2026 standards is clear: future BESS safety will depend not only on preventing thermal runaway, but also on demonstrating how the complete energy storage system performs when a serious fire actually occurs.



ISO 3941:2026 official standard page · UL 9540A 6th Edition overview · NFPA 855:2026 official preview

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