Insights from UL 9540A, Sixth Edition, and International Testing Practices
As battery energy storage systems (BESS) move toward higher capacities and more compact layouts, a critical safety question is receiving greater attention: If a severe fire develops in one enclosure, can it spread to neighboring equipment?
Large-scale fire testing (LSFT) is therefore becoming increasingly important to manufacturers, fire departments, and authorities responsible for reviewing installation proposals.
On April 23, 2026, UL Solutions announced enhanced large-scale fire testing services based on the sixth edition of ANSI/CAN/UL 9540A. These tests evaluate how fires develop inside BESS enclosures and affect nearby equipment and buildings. The resulting data supports the review of separation distances, fire protection plans, and emergency response strategies.
In industry communications, "open-door" and "closed-door" are frequently used to describe different test conditions. From an engineering perspective, however, door position is only one part of the test configuration. Interpreting the results also requires an understanding of ignition methods, enclosure openings, fire protection system status, and the arrangement of neighboring units.
Evolving Standards: Fully Developed Fires and Post-Deflagration Conditions
According to UL Solutions, the sixth edition of UL 9540A was published on March 13, 2026. Key changes include an updated installation-level large-scale fire test method aligned with the guidance in Annex G.11 of NFPA 855, 2026 Edition, and an additional annex addressing large-scale deflagration testing of BESS enclosures.
UL Solutions' technical guidance explains that the updated installation-level test considers a post-deflagration condition when evaluating enclosure design, separation distances, and, for indoor installations, building-based fire suppression systems. Openings created by a deflagration affect ventilation and subsequent thermal exposure. This condition can be established through vent-area design methods or enclosure explosion testing.
BESS safety evaluation must therefore address several distinct hazards, including thermal runaway propagation, fully developed fires, and deflagration of flammable gases. A single observed outcome cannot demonstrate protection against every hazard.
Open-Door Testing: Assessing Fire Spread Under Open Conditions
"Open-door testing" generally describes a fire test conducted with enclosure doors open or with substantial openings in the enclosure. These openings change the paths through which air enters and hot gases escape. They may also increase direct flame exposure and radiant heat transfer to neighboring equipment.
When fire suppression is disabled and the initiating unit is allowed to develop a substantial fire, the test can help determine whether enclosure structures, insulation materials, and spatial arrangements limit fire spread to adjacent units under that specific configuration.
However, an open enclosure does not automatically represent the most severe condition for every hazard. Flame exposure under open conditions and gas accumulation or pressure hazards under closed conditions are different aspects of safety assessment.
The updated test method's emphasis on post-deflagration openings also indicates that comparisons of test severity should consider the size, location, and technical basis of those openings-not simply whether the doors are open.
Closed-Door Testing: Assessing the Actual Enclosure and Protection Configuration
"Closed-door testing" generally refers to testing with the enclosure doors closed. Closed doors do not necessarily mean a sealed enclosure, nor do they establish whether ventilation, suppression, or deflagration venting systems remain operational.
Louvers, exhaust openings, and deflagration vents also influence gas discharge, pressure changes, and fire development.
When reviewing these tests, it is essential to distinguish the initiating unit from adjacent target units and identify which protective functions participated in the test. The enclosure's ability to contain flames and the performance of ventilation or deflagration venting systems should be assessed separately using the test records.
For explosion hazards, NFPA 68 addresses protection through deflagration venting, while NFPA 69 addresses explosion prevention systems. In June 2026, UL Solutions launched related evaluation services that use system-specific data, engineering analysis, and simulation to support reviews of BESS fire and deflagration risks. The role of these standards extends beyond conducting a simulation before a closed-door fire test.
International Examples: Complementary Fire and Deflagration Testing
Fluence: Separate Evaluations of Ventilation, Deflagration, and Fire Spread
In June 2025, US-based Fluence disclosed that its Gridstack Pro 5000 system underwent two days of gas dispersion testing and three days of deflagration testing before large-scale fire testing. These evaluations examined the performance of active ventilation and deflagration venting systems, respectively.
For the subsequent fire tests, four enclosures containing batteries were arranged at minimum separation distances. The deflagration panels on the central initiating enclosure were open. Fully charged batteries were used, and the fire was allowed to develop without fire suppression or firefighting intervention.
According to the company's announcement, no enclosure-to-enclosure fire propagation occurred under the tested conditions. Importantly, the disclosure specifies open deflagration panels on the initiating enclosure; it should not be interpreted as stating that all enclosure doors were open.
Wärtsilä: Evaluating Integrated Explosion Control
In September 2025, Finland-based Wärtsilä announced that its Quantum2 system had completed three full-system tests in the United States. A flammable gas mixture representative of cell thermal runaway vent gases was used to evaluate the interaction between its Active Ignition Mitigation System and deflagration protection design.
According to the company, the tests resulted in no uncontrolled deflagrations or structural deformation. This evaluation focused on gas and deflagration risk control, providing a different type of safety evidence from tests assessing enclosure-to-enclosure propagation during a fully developed fire.
These examples illustrate how open-condition fire testing, ventilation assessments, and deflagration testing can form complementary parts of a safety validation program. They should not be treated as mutually exclusive approaches.
What Matters When Comparing Test Results?
| Assessment Factor | Open-Door or Large-Opening Conditions | Closed-Door Conditions |
|---|---|---|
| Opening configuration | Identify the location and area of open doors, deflagration panels, and other openings. | Identify ventilation openings and whether deflagration vents open or activate. |
| Fire protection intervention | Confirm whether suppression, cooling, or other protective functions participate. | Confirm the same functions; their status cannot be inferred from door position. |
| Thermal exposure | Examine flame direction and heat exposure to adjacent equipment. | Examine enclosure containment, gas discharge, and exposure after openings form. |
| Deflagration risk | Open-condition fire results cannot replace an assessment of hazards under closed conditions. | Consider gas accumulation, ventilation, and pressure-relief validation. |
| Applicability of results | Results apply to the tested product and specific installation configuration. |
Results are likewise limited by product configuration and installation conditions. |
State of charge, initiation location, the number of batteries involved, separation distances, and enclosure orientation also affect how results should be interpreted.
When evaluating a finding of "no propagation," reviewers should examine temperature records for adjacent units, the criteria used to identify thermal runaway, and the observation period. The absence of visible flames alone is insufficient to interpret the result fully.
A May 2026 publication from the American Clean Power Association (ACP) similarly emphasizes evaluating a worst-case scenario involving a fully developed fire in one enclosure and verifying that fire does not spread to nearby units installed according to the manufacturer's guidance.
Turning Test Evidence into Safer Installation Design
Successful large-scale fire testing under a particular configuration can provide valuable evidence for separation distances and fire protection design. It does not establish that a product is "absolutely safe" under all operating conditions.
Likewise, an open-door fire test alone cannot justify removing deflagration vents or other protective measures. Fire propagation and deflagration hazards require separate assessments, followed by design decisions based on applicable requirements and project conditions.
Guidance from the Australasian Fire and Emergency Service Authorities Council (AFAC) emphasizes developing an overall fire safety strategy that considers local firefighting and emergency response capabilities. System testing, site design, and emergency preparedness must work together.
For BESS manufacturers, testing laboratories, and project owners, a meaningful comparison between open-door and closed-door testing ultimately depends on traceable test conditions and measured data. Clear records of temperature, radiant heat, gas behavior, and pressure changes-together with clearly defined limits on the applicability of the findings-allow large-scale fire testing to support product improvement, installation review, and fire safety decisions.
