UL 9540A - Battery Design

UL 9540A is the test method for evaluating thermal runaway fire propagation in battery energy storage systems (BESS). It is not a product certification standard — it is a data-driven test methodology used to demonstrate how a thermal runaway event in one ESS behaves, and whether it propagates to adjacent systems or causes fire, explosion, or toxic gas hazards.

Published by: UL Solutions (Underwriters Laboratories)
Current edition: 6th Edition — published March 2026, effective January 2027
Full title: Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems

Purpose and Scope

UL 9540A provides fire and safety data used to support compliance with installation codes, primarily NFPA 855 and the International Fire Code (IFC). The test results inform Authorities Having Jurisdiction (AHJs) on acceptable installation configurations, required fire protection measures, and system spacing and siting. Without UL 9540A data, large BESS projects cannot receive AHJ approval in most US states.

The standard applies to stationary battery energy storage systems and covers lithium-ion chemistries used in residential, commercial, and grid-scale applications.

Test Levels

Testing progresses through four levels, each providing data for the next:

Cell level — thermal runaway initiation, gas generation, heat release of a single cell
Module level — propagation behaviour across a battery module; mandatory for all BESS
Unit level — propagation within a full ESS unit; mandatory for residential BESS; optional for non-residential from 6th Edition
Installation level — large-scale fire test (LSFT) demonstrating that thermal runaway in one unit does not propagate to adjacent units; now formally incorporated in 6th Edition

6th Edition (2026) — Key Changes

Unit level test no longer required for non-residential BESS — replaced by the installation-level large-scale fire test
Section 10 revised to incorporate a clear large-scale fire test (LSFT) methodology aligned with NFPA 855 Annex G.11
Annex C added — large-scale deflagration test for BESS enclosures
More precise measurement of hydrogen and hydrocarbon gas accumulation and venting behaviour
Stronger assessment of explosion risk in enclosed or semi-confined environments
Tighter alignment with NFPA 855 (2026 edition)

Relationship to UL 9540

UL 9540A (test method) and UL 9540 (certification standard) are closely related but distinct. UL 9540 is the product safety standard for energy storage systems; UL 9540A provides the fire propagation test data that supports UL 9540 listing and AHJ code compliance. A system may be UL 9540 listed but still require UL 9540A test data for installation approval under local fire codes.

Who Requires It

NFPA 855 — directly references UL 9540A for fire testing of representative ESS
International Fire Code (IFC) — references UL 9540A for installation approval
AHJs across most US states — required for siting and installation permits on large BESS projects

Related Standards

NFPA 855 — installation code for stationary storage systems (references UL 9540A)
UL 9540 — product safety and certification standard for ESS
UL 1973 — safety standard for batteries in stationary and light EV applications
IEC 62619 — IEC equivalent safety standard for stationary lithium-ion batteries

Disclaimer: this is not a definitive guide, just an indication and it is important that you always go back to the source for legislation.