Authored by: Alex Schraiber, P.E senior manager, R&D Fire Research and Development, UL Solutions and LaTanya Schwalb, principal engineer, Energy and Industrial Automation, UL Solutions, Distinguished Member of Technical Staff – William Henry Merrill Society
As battery energy storage systems (BESSs) become increasingly common in commercial and industrial applications, supporting their safe operation under severe fire conditions is critical. Two key standards — NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, and UL 9540A, the Standard Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems — play a central role in guiding installation practices and testing protocols to help mitigate fire risks.
NFPA 855: Reinforcing large-scale fire testing
The 2026 edition of NFPA 855 introduces significant revisions that emphasize the importance of large-scale fire testing. A new section, Annex G.11, outlines expectations for this test, focusing on scenarios where fire could spread from one BESS to another. This addition reflects growing concerns among authorities having jurisdiction (AHJs) and fire safety professionals about real-world failure conditions and the need for robust evaluation methods. Annex G.11 provides guidance on initiating a developed fire condition, evaluating separation distances and considering the role of active fire protection systems.
UL 9540A: Reinforcing longstanding alignment with NFPA 855
UL 9540A has long been the benchmark for assessing thermal runaway and fire propagation in BESS. Its revisions continue to reinforce longstanding alignment with NFPA 855, allowing testing protocols to reflect the latest installation safety requirements. UL 9540A, Ed. 6, updates the installation-level test, introducing a challenging fire scenario and assuming a post-deflagration condition to evaluate enclosure design, separation distances and, for indoor systems, the effectiveness of building-based fire suppression systems.
How is the post-deflagration condition established?
This condition can be determined using consensus approaches for vent area design (NFPA 68) or through explosion testing of the enclosure. These methods help ensure that the fire test realistically reflects the thermal exposure that could occur from fire growth with ventilation available through any vent areas opened after ignition of accumulated vent gases, which is critical for installations where flammable gases may accumulate.
Closing a critical data gap: Sprinkler density and ceiling height
One important outcome of these revisions is increased testing of sprinkler density and ceiling height for indoor energy storage systems (ESSs). Historically, there has been a lack of empirical data to guide sprinkler system design for these installations. The new testing protocols will help close this gap, providing AHJs and fire marshals with reliable information to support the design of sprinkler systems that properly control fires in ESS environments. This data will inform decisions on whether standard NFPA 13 criteria for Extra Hazard Group 1 remain sufficient or require adjustment for ESS-specific hazards.
Why large-scale fire testing matters
Unlike unit-level tests, large-scale fire testing simulates severe failure conditions, including ignition of flammable vent gases and developed fire scenarios. These tests help assess:
- Separation distances between systems to prevent fire spread
- Fire suppression system performance, especially for indoor installations
- Enclosure resilience under thermal stress, including structural integrity and venting behavior
For outdoor installations, the test involves at least one vertical grouping of modules (often a rack), while indoor tests require a complete module to simulate realistic stress conditions. Both scenarios aim to replicate foreseeable failure modes, such as multiple cell failures, water intrusion or battery management system (BMS) malfunctions, that could lead to uncontrolled fire growth.
Performance criteria and consistency
The revised UL 9540A methodology incorporates measurable performance criteria — such as temperature and heat flux thresholds — to establish clear pass-fail outcomes. This approach promotes consistency across manufacturers and provides AHJs with reliable data for enforcement and approval decisions. By separating fire hazard evaluation from explosion hazard evaluation, the standard establishes that each risk is addressed with appropriate rigor.
Implications for code authorities and fire marshals
- Expect more comprehensive fire testing data to support plan reviews and inspections.
- Look for UL 9540A reports that include large-scale fire test results, especially for installations with multiple BESS units.
- Be aware that sprinkler system design guidance will evolve as new data on density and ceiling height becomes available.
- Understand that venting strategies and enclosure design will play a critical role in mitigating post-deflagration hazards.
Differences between UL 9540A, Ed. 5, and UL 9540A, Ed. 6
UL 9540A, Ed. 5, has been well established as the test methodology for evaluating thermal runaway propagation and fire behavior in electrochemical BESSs. It is a sequential test method, starting with the cell-level test, moving to the module-level test, the unit-level test and finally the installation-level test. Each test contained performance criteria that must be met to determine whether the next level of test was needed. BESS manufacturers designed systems capable of meeting the performance criteria for the unit-level test, so there was no need to move to the installation-level test. Meeting the unit-level test performance criteria has contributed to more robust BESS designs that limited thermal runaway propagation within the BESS.
UL 9540A, Ed. 6, maintains the cell- and module-level tests, along with their performance criteria. However, the test protocol does not stop if the performance criteria are met. Instead, the installation-level, large-scale fire test is conducted. This test has pass-fail criteria that must be met to comply with the test.
The unit-level test is reserved for BESSs intended for residential applications. The unit-level test may also be used to evaluate an active thermal runaway propagation prevention system and whether it can remain active in the BESS during the large-scale fire test.
One final key differentiator between the installation-level test of UL 9540A, Ed. 5, and the installation-level, large-scale fire test of UL 9540A, Ed. 6, is the intentional ignition of the vented gases from batteries experiencing a thermal runaway propagation event. The ignition of vented gases establishes a fire condition and aligns with the guidance in NFPA 855, Appendix G.11.
Why two editions of UL 9540A?
We recognize that adoption of the 2026 edition of NFPA 855 will take time; in addition, we recognize that it will take time for BESS manufacturers to adapt and design to meet the pass-fail criteria for the installation-level, large-scale fire test.
After much deliberation, UL Solutions, along with UL Standards & Engagement, determined that a new edition of UL 9540A would be useful in the interim to provide clear separation between BESSs that meet the performance criteria of the unit-level test and BESSs that pass the installation-level, large-scale fire test. An effective date of Jan. 1, 2027, will be established for UL 9540A, Ed. 6. A proposal has been made to UL 9540, the Standard for Energy Storage Systems and Equipment, to require the edition of UL 9540A to be part of the marking plate label and instructions for the BESS to support both editions of UL 9540A.
Bottom line: UL 9540 continues to serve as the foundational product safety Standard for ESSs (see product category, available using Product iQ®), while UL 9540A (see product category AACD, available using Product iQ®) provides a standardized method for evaluating fire and thermal runaway behavior under extreme conditions. The latest edition of NFPA 855, and the continued alignment of UL 9540A with it, marks a significant step toward more consistent and comprehensive fire safety for BESS installations. For code authorities and fire marshals, understanding how these documents work together is essential to support continued compliance, protection of property and the safeguarding of lives.
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