UL Standards & Solutions published the 6th Edition of UL 9540A on March 13, 2026, formally embedding large-scale fire testing (LSFT) into the certification framework for battery energy storage systems (BESS)-a shift with direct consequences for project timelines, procurement decisions, and insurance underwriting across the utility-scale storage sector.
Background
UL 9540A is the Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems. UL identifies it as the American and Canadian national standard for assessing fire propagation related to thermal runaway events in energy storage systems. It serves as the benchmark for evaluating safety-particularly thermal runaway and propagation risks-and is therefore critical to the bankability of energy storage technologies.
Previous editions of UL 9540A did not explicitly incorporate large-scale fire testing. Industry emphasis on LSFT had been growing, with many manufacturers conducting such tests voluntarily as a best practice for roughly a year before this testing became mandatory in the 2026 edition of NFPA 855. The new standard update aligns the certification process with that regulatory direction.
Battery energy storage systems are now core infrastructure across utilities, commercial facilities, and distributed energy networks, with deployments increasing in both size and complexity. Thermal runaway events remain a known risk tied to battery chemistry and system design, with incidents affecting projects, operations, and public safety.
Key Changes and Implications
The 6th Edition of UL 9540A reflects a clear evolution in regulatory thinking. It formally incorporates large-scale fire testing into the certification framework to demonstrate that a thermal runaway event in one energy storage system does not propagate to adjacent systems.
Dana Parmenter, commercial VP of industrial at testing and certification provider CSA Group, said the release "establishes a new precedent in the energy storage system testing and certification ecosystem." She added that the edition "expands testing expectations and introduces LSFT requirements within the certification process," with Section 10 requiring LSFT "to demonstrate that fire will not propagate between ESS units."
The new edition includes clearer criteria for evaluating cell-to-cell propagation and requires results from previous tests at module, unit, and installation level. It also introduces test criteria for non-lithium technologies, including sodium-ion and flow batteries.
Gas accumulation and explosion risk rank among the most critical hazards during a thermal runaway event. The 6th Edition enhances how these risks are measured, including more precise measurement of hydrogen and hydrocarbon gases, improved evaluation of gas accumulation and venting behavior, and stronger assessment of explosion risk in enclosed or semi-confined environments.
UL Solutions introduced an upgraded version of its LSFT services on April 24, 2026, designed to give code officials and fire service personnel more precise insights into how battery fires originate, develop, and propagate. The updated methodology expands the scope of installation-level, large-scale fire testing, enabling UL Solutions to better simulate fire initiation within an energy storage enclosure and its potential impact on adjacent equipment or structures.
For the insurance market, the stakes are material. Commercial property insurers routinely require UL 9540 listing and UL 9540A test data as preconditions for coverage on facilities with BESS-without them, projects risk being uninsurable or facing premiums that undermine financial viability. The 6th Edition raises the data threshold insurers and authorities having jurisdiction (AHJs) can demand before binding coverage or granting permits.
Parmenter noted the update "will raise many questions for manufacturers around enforcement timelines, jurisdictional adoption, and how updated requirements may be interpreted by authorities having jurisdiction."
Outlook
Manufacturers now face increased pressure to integrate UL 9540A considerations earlier in the product development lifecycle. This shift demands a more proactive approach to system design, with a clear focus on large-scale fire behavior and real-world failure scenarios. Organizations must also prepare for more rigorous, data-intensive testing programs requiring deeper validation of system performance, mitigation strategies, and overall safety under complex conditions.
While UL 9540A now addresses NFPA 855's safety testing requirements, authorities often request additional performance-based data for site-specific evaluations. CSA Group's CSA/ANSI C800:25 standard includes additional measurements such as heat release rate and gas composition analysis, which registered fire protection engineers frequently recommend to support their analysis. The two standards now serve distinct yet complementary roles within the testing and approvals process.
For related coverage, see our earlier analysis: New U.S. Safety Framework for Long-Duration Storage: How Fire Incidents Are Reshaping NFPA and UL Standards.
