Battery storage standard UL 9540A updated with large-scale fire testing

Battery storage standard adds large-scale fire testing

UL Standards & Solutions has published the sixth edition of UL 9540A, adding large-scale fire testing requirements to the standard for evaluating thermal runaway fire propagation in battery energy storage systems.

The new edition was published on 13 March and is described it as a key standard for battery energy storage systems (BESS), including lithium-ion systems.

The standard is titled UL 9540A, Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, and is identified by UL as the American and Canadian national standard for assessing fire propagation related to thermal runaway events in energy storage systems (ESS).

UL said testing to the standard is an essential element of due diligence when the design or installation conditions of an ESS exceed the limits set by National Fire Protection Association (NFPA) 855, NFPA 1, the International Fire Code (IFC) or the International Residential Code (IRC).

UL 9540A is the only consensus standard explicitly cited in NFPA 855 for large-scale fire testing and the only national standard in the US and Canada for fire safety testing methods for battery ESS.

UL also said it offers testing aligned with both the fifth and sixth editions of UL 9540A.

Battery storage testing and CSA Group response

CSA Group published comment from Dana Parmenter, Commercial Vice President, Industrial at CSA Group, on what the sixth edition may mean for manufacturers and authorities having jurisdiction.

Parmenter said: “The release of the 6th Edition of UL 9540A in March 2026 establishes a new precedent in the energy storage system (ESS) testing and certification ecosystem.

“This change will raise many questions for manufacturers around enforcement timelines, jurisdictional adoption, and how updated requirements may be interpreted by AHJs.

“This edition expands testing expectations and introduces large scale fire (LSFT) testing requirements within the certification process.

“Section 10 requires LSFT to demonstrate that fire will not propagate between ESS units.

“This change is part of a larger trend across ESS standards, placing greater emphasis on LSFT as it relates to system level safety.”

CSA Group said NFPA 855 Section 9.2.1.2.1 requires large-scale fire testing to characterise gas composition and demonstrate non-propagation between ESS units, which involves gas analysis and calorimetric data such as heat release rate (HRR).

It added that previous editions of UL 9540A did not explicitly incorporate large-scale fire testing, and said the sixth edition now addresses spacing and fire propagation as safety considerations within the certification framework.

CSA Group said CSA/ANSI C800:25 continues to address performance-based characterisation, including heat release rate, target unit measurements such as heat flux, operation of detection systems, battery management system data, and a framework for acquiring data for fire protection engineer analysis.

Complementary roles in ESS approvals

CSA Group said NFPA 855 Section 9.2.2.2 requires interpretation of test results by a registered fire protection engineer, and added that many engineers recommend HRR and other measurements in CSA/ANSI C800:25 to support their analysis.

It said CSA/ANSI C800:25 can be used alongside UL 9540A to provide additional performance and engineering-based data for site-specific installation requirements, engineering approvals and AHJ review.

Parmenter added: “UL 9540A and CSA/ANSI C800:25 now serve more distinct, yet complementary, roles within the ESS testing and approvals process.

“Looking ahead, both standards may be applied to support demonstration of alignment with NFPA 855.

“UL’s recent revisions to UL 9540A expand the scope of safety focused requirements by incorporating certain performance-based considerations, such as those informing spacing and propagation mitigation, into the certification framework.

“CSA/ANSI C800:25 continues to provide additional performance and engineering-based characterization, including data elements that remain optional or out of scope under UL 9540A.”