Short answer: 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 5th edition was released in March 2025.
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 5th edition (2025)
According to UL Solutions, the 2025 edition adds clearer criteria for cell-to-cell propagation, high-temperature test methods for different battery chemistries, and protocols for newer applications such as rooftop and open-garage installations. Separate work on a large-scale fire test for NFPA 855 has been under way in the UL 9540A technical committee. Check the current status before relying on it.
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
- UL Solutions, "UL Solutions Enhances Battery Energy Storage System Safety Test Methods…" (April 2025): ul.com
- UL 9540A 2025 updates (presentation, hosted by Santa Fe County): santafecountynm.gov
- Solar Power World (April 2025): solarpowerworldonline.com
- Official standard: UL Standards & Engagement (search "9540A")