What the 6th Edition of UL 9540A Means for Battery Storage Safety Certification

Battery energy storage systems (BESS) have become a critical pillar of modern power infrastructure. As grids strain under increasing demand, electrification, and renewable integration, energy storage has scaled rapidly, from pilot projects to multi-gigawatt-hour installations deployed near communities, data centers, and critical infrastructure.

With that growth has come heightened public scrutiny. High-profile incidents, including evacuations following fires at large-scale energy storage facilities such as Moss Landing in California, have underscored an uncomfortable reality - when battery systems fail, the consequences can extend well beyond the container or enclosure. These events have accelerated regulatory attention and forced a fundamental question to the forefront, how do we demonstrate, with confidence, that large battery systems can be designed, built, and operated safely at scale?

The publication of the 6th Edition of UL 9540A on March 13, 2026, marks a pivotal moment in answering that question. This update represents one of the most consequential shifts to date in the energy storage testing and certification ecosystem.

A Shift Toward System-Level Fire Safety

Historically, energy storage safety evaluations have focused heavily on component and subsystem behavior. While necessary, this approach has proven insufficient to address real-world fire scenarios, where cascading failures, gas accumulation, and thermal propagation can turn localized events into system-and-site-level emergencies.

The 6th Edition of UL 9540A reflects a clear evolution in regulatory thinking. It formally incorporates large-scale fire testing (LSFT) into the certification framework to demonstrate that a thermal runaway event in one energy storage system does not propagate to adjacent systems. In doing so, UL 9540A now directly addresses one of the most critical safety concerns for modern deployments: preventing escalation beyond the initial failure.

This shift aligns closely with NFPA 855-2026, which increasingly emphasizes performance-based fire safety outcomes rather than prescriptive design alone. In particular, NFPA 855-2026 §9.2.1.2.1 calls for large-scale fire testing to characterize gas composition and demonstrate non-propagation between energy storage units, an inherently system-level requirement.

Previous editions of UL 9540A referenced many of these safety concepts indirectly. The 6th Edition brings them into sharper focus by embedding spacing and propagation considerations into the certification process itself.

What the 6th Edition Does and Does Not Do

It is important to understand what this update represents, and where its boundaries remain.

The revised UL 9540A now satisfies the large-scale fire testing expectations referenced by NFPA 855 by requiring demonstration of non-propagation between BESS units. This is a meaningful and necessary step forward.

However, UL 9540A is not intended to replace performance-based engineering analysis, nor does it fully characterize all fire dynamics that authority having jurisdiction (AHJs) and fire protection engineers may require for site-specific approvals.

NFPA 855-2026 introduces the requirement that the hazard mitigation analysis (HMA) evaluate the reliability and survivability of the following critical safety components, during a thermal runaway propagation or single failure event (NFPA 855‑2026 §4.4.2.3):

a) exhaust ventilation,

b) smoke detection,

c) fire detection,

d) fire suppression,

e) combustible concentration reduction (CCR) system,

f) gas detection, and

g) explosion control and prevention 

Furthermore, NFPA 855-2026 §9.2.2.2 explicitly requires interpretation of test results by a registered fire protection engineer to verify that complete combustion of one BESS does not result in propagation to adjacent units. In practice, many engineers and AHJs continue to request additional data, such as heat release rate (HRR), heat flux, detection system response, and battery management system (BMS) behavior, to support defensible spacing, ventilation, and hazard mitigation decisions.

These data elements are not comprehensively required under UL 9540A, even in its 6th Edition.

Complementary Standards, Not Competing Ones

This is where CSA/ANSI C800:25 - Testing protocol for energy storage system reliability and quality assurance program plays a critical and complementary role. Published in CSA/ANSI C800:25, the UL9540A 6th edition revision originated from CSA Group’s custom large scale fire testing work conducted with customers and local AHJs over several years.

CSA/ANSI C800:25 provides a flexible, performance-based framework for large-scale fire testing and system characterization. It enables collection of engineering-grade data, including HRR, target measurements, detection performance, and BMS response, that directly support fire protection engineering analyses and site-specific evaluations. As stated in section 9.7.3, at a minimum, CSA/ANSI C800:25 requires the following data to be collected during an LSFT:

a) Peak heat release rate at all locations where adjacent electrical energy storage system (EESSs) or exposures are located, at the most likely locations to receive the highest heat;

b) Temperatures of the modules in adjacent EESSs;

c) Temperatures of any connections between the initiating unit and adjacent units;

d) BMS of adjacent EESS recording data;

e) Timeline of notable observations during the test including time at which fire detection would be activated/notified the critical safety systems (i.e., gas detectors and flame detectors);

f) Fire and damage impact on critical safety systems; and

g) Viability of communication pathways

In contrast, the data required by the UL9540A 6th edition LSFT includes target BESS temperatures (including temperatures of cells, modules, and interior surfaces within the target enclosures and temperatures of instrumented walls, if used in place of target enclosures), and heat flux measurements (where applicable during a test).

Rather than duplicating UL 9540A, CSA/ANSI C800:25 extends beyond certification-driven requirements to address the practical questions faced by AHJs, insurers, and engineers responsible for approving real installations in real environments.

With the release of the 6th Edition of UL 9540A, the relationship between these standards has become clearer:

• UL 9540A establishes a baseline, safety-focused certification pathway, now incorporating non-propagation via large-scale fire testing.
• CSA/ANSI C800:25 provides deeper performance characterization to support engineering judgment, AHJ review, and site-specific compliance with NFPA 855.

Used together, they form a more complete safety narrative, one that addresses both whether a system meets certification requirements and how it behaves under worse-case fire conditions.

What Comes Next

As jurisdictions begin to adopt and enforce the updated UL 9540A requirements, manufacturers will face new questions:

• How quickly will AHJs require 6th Edition compliance?
• How will existing test data be treated?
• What additional testing will be expected beyond certification?

Navigating this transition will require a clear understanding of how UL 9540A, NFPA 855, and CSA/ANSI C800:25 intersect and where each standard’s responsibilities begin and end. CSA Group is already working with customers as they chart a path through the updated certification landscape.

The 6th Edition of UL 9540A as well as CSA/ANSI C800:25 is not simply a procedural update. It signals a broader shift toward system-level fire safety accountability in the energy storage industry. Organizations that proactively engage with both certification and performance-based testing will be best positioned to demonstrate compliance, earn stakeholder confidence, and accelerate deployment.

As the energy storage sector continues to scale, safety standards must scale with it. The latest evolution of UL 9540A is an important step in that direction, but it is most effective when applied as part of a broader, integrated safety strategy. CSA Group provides testing and advisory services to develop comprehensive safety strategies for several different standards. Visit  https://www.csagroup.org/testing-certification to learn how CSA Group can help your organization achieve its safety and certification goals.

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