On March 13, 2026, UL 9540A:2026 “Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems” (Sixth Edition) was officially released. The most significant revision incorporates the Large Scale Fire Test into the standardized framework, establishing a formal methodology for evaluating thermal runaway fire propagation at the installation level of Battery Energy Storage Systems (BESS).

Test Orientation and Purpose
The introduction of the Large Scale Fire Test is intended to establish a standardized methodology for installation-level evaluation of containerized Battery Energy Storage Systems (BESS). By simulating fire conditions within the BESS enclosure, the test aims to assess the impact of flame propagation on adjacent BESS units or nearby structures.
Core Objectives of the Test:

1. Evaluating the thermal exposure response of BESS stacks, enclosures, or assemblies (including non-battery components) under fire conditions, and determining whether fire propagates to adjacent systems.

2. Assessing the ignition risk to nearby buildings.

3. Providing data support for recommendations on separation distances within the UL 9540 certification framework.

4. Evaluating the adequacy of fire protection measures implemented in the building (e.g., sprinkler systems) to determine whether they are sufficient to control fire propagation.

Applicability
Battery Energy Storage Systems (BESS) outside the following application scenarios are not required to undergo unit-level testing; instead, they shall be subject to installation-level Large Scale Fire Tests:

1.Residential BESS with energy capacity not exceeding 20 kWh.

2. Residential BESS installed at a distance of less than 0.9 meters from doors or windows.

3. Residential BESS with energy capacity greater than 1 kWh when deployed inside dwelling units.

4. Residential BESS utilizing non-metallic enclosures.

5. BESS equipped with active thermal runaway propagation prevention systems.

For the vast majority of non-residential BESS, the Large Scale Fire Test has become the standard requirement.

Key Test Requirements
1. Authentic Replication of Installation Scenarios
The test setup must fully replicate the final installation configuration of the BESS, including module arrangement, unit configuration, enclosure or assembly construction, and the presence of target enclosures or structures adjacent to the initiating system. The spacing must strictly follow the minimum separation distances specified in the manufacturer’s installation manual.
Test Setup Layout
2. BESS State Requirements
BESS units shall be charged to their maximum operational state of charge (SOC). After resting for at least one hour, the open-circuit voltage (OCV) shall be recorded as the reference value. If the voltage drop exceeds 1% of the reference value, the unit shall be recharged prior to testing.
3. Ignition Mechanism Requirements
The ignition source must be positioned along the thermal runaway gas flow path to ensure effective ignition of the released gases. Acceptable ignition methods include the use of spark plugs, glow plugs, spark generators, or pilot flames.
4. Test Termination Criteria

(1) All flames inside and outside the initiating enclosure have been extinguished.

(2) The internal temperature of the initiating area remains below 100 °C for at least three consecutive hours.

(3) The target BESS exhibits a downward temperature trend, remaining below the performance criterion temperature for at least three consecutive hours.

(4) For systems equipped with water-based suppression, monitoring shall continue for potential re-ignition after water supply is discontinued for 12 hours.

(5) For stacked installations, an additional 24-hour observation period is required after condition (1) is satisfied to confirm no structural collapse.

MCM Reminder
The sixth edition of UL 9540A formally incorporates the Large Scale Fire Test into the standard framework, marking a substantive shift in safety evaluation of Battery Energy Storage Systems (BESS) from the individual product level to the system integration level. This transition is expected to drive the industry from a mode of passive response toward proactive verification. The test methodology provides standardized data support for the design of safety separation distances, the configuration of fire suppression systems, and the structural protection of buildings. It is anticipated to become a critical technical basis for project approval of energy storage deployments in the North American market.
Facing the transformation, MCM offers clients end-to-end technical interpretation, ranging from cell-level exhaust temperature measurement to installation-level fire propagation criteria, enabling enterprises to precisely grasp the testing requirements at every tier. We recognize that in order to gain a competitive edge in the intense international market, companies must go beyond merely meeting the baseline requirements of standards. It is essential to embed the principles of safety, performance, and reliability deeply into product design from the outset, thereby effectively avoiding repeated evaluations and unnecessary cost overruns during the certification stage.
If you wish to evaluate the compliance pathway of your energy storage products under UL 9540A:2026, please contact MCM. We will provide you with clear, actionable compliance support and partner with you to unlock greater possibilities.