# UL 9540A: Test method for evaluating thermal runaway fire propagation in battery energy storage systems

> UL 9540A is the test method that shows how a battery energy storage system behaves when a cell goes into thermal runaway, at cell, module, unit and installation level. The 6th edition, published on 13 March 2026, adds large-scale fire testing.

Source: https://batteryfirebreak.com/standards/ul-9540a/
Updated: 2026-11-16
Publisher: Battery Firebreak. Licence: CC BY 4.0.

UL 9540A is a test method, not a pass/fail standard. It forces cells into thermal runaway and measures what happens at up to four levels: cell, module, unit and installation. The data (gas volume and composition, heat release, flame spread, whether propagation stops) are used by fire codes such as NFPA 855 and by fire authorities to decide spacing, ventilation, explosion control and suppression. The current 6th edition was published on 13 March 2026 and adds large-scale fire testing (LSFT) at installation level.

## How the four levels work

| Level | What is tested | Main outputs |
|---|---|---|
| Cell | Single cell forced into runaway, usually by external heating | Runaway onset temperature; vent gas volume and composition (including hydrogen); lower flammability limit of the gas |
| Module | One module with one cell (or more) forced into runaway | Whether runaway propagates within the module; heat and gas release |
| Unit | Full unit (e.g. rack or cabinet) with an initiating module, next to target units | Propagation to adjacent units; surface temperatures; flaming outside the unit |
| Installation | Unit in a representative installation, with and without fire protection | Effectiveness of suppression and other installation-level protection |

Testing moves to the next level only if the previous one shows a hazard. For example, if cell-level gas is not flammable, or module-level runaway does not propagate, later levels can be reduced.

## What changed in the 6th edition (2026)

The 6th edition was published on 13 March 2026. Its main addition is large-scale fire testing (LSFT): a test of whether a fire in one energy storage unit spreads to adjacent units and nearby structures, and how ventilation and gas release affect fire behaviour. Earlier editions did not explicitly include LSFT. Sources link the change to NFPA 855; check the edition of NFPA 855 adopted in your jurisdiction. Commentators describe UL 9540A differently: Intertek calls it a data-generating test method rather than a certification standard, while other comments describe LSFT as part of the certification process. Rely on the official text for that question.

## Earlier change: the 5th edition (2025)

The 5th edition (March 2025) added clearer criteria for cell-to-cell propagation, high-temperature test methods for different chemistries and protocols for applications such as rooftop and open-garage installations, according to UL Solutions.

## UL 9540A vs UL 9540

- UL 9540 is the safety standard for energy storage systems and equipment. A product is listed to it.
- UL 9540A is the test method for thermal-runaway fire propagation. Its data are often required as part of UL 9540 certification and by fire codes for installation approval.

## What it means for barrier materials

Module-level results decide whether a system needs extra installation-level protection. A barrier design that stops propagation at module level can greatly simplify site requirements such as spacing and suppression. That makes cell-to-cell and module-to-module barriers a direct cost lever for BESS integrators, not just a safety extra.

## Sources


- Energy-Storage.news, "UL9540A: New edition of key BESS fire safety standard 'establishes new precedent'" (2026): energy-storage.news <https://www.energy-storage.news/ul9540a-new-edition-of-key-bess-fire-safety-standard-establishes-new-precedent/>
- Intertek, "Understanding the 2026 update to UL 9540A" (23 March 2026): intertek.com <https://www.intertek.com/blog/2026/03-23-understanding-the-2026-update-to-ul-9540a/>
- UL Solutions, large-scale fire testing for BESS (April 2026): ul.com <https://www.ul.com/news/ul-solutions-launches-large-scale-fire-testing-battery-energy-storage-systems>
- UL Solutions, "UL Solutions Enhances Battery Energy Storage System Safety Test Methods…" (April 2025): ul.com <https://www.ul.com/news/ul-solutions-enhances-battery-energy-storage-system-safety-test-methods-address-industry>
- UL 9540A 2025 updates (presentation, hosted by Santa Fe County): santafecountynm.gov <https://www.santafecountynm.gov/images/UL9540A-2025_updates.pdf>
- Solar Power World (April 2025): solarpowerworldonline.com <https://www.solarpowerworldonline.com/2025/04/ul-enhances-9540a-battery-testing-to-better-address-ess-industry-of-the-future/>
- Official standard: UL Standards & Engagement <https://www.shopulstandards.com/> (search "9540A")