UL 9540A: A Practical Guide to Large-Scale Fire Testing for Safer ESS and BESS

Types of energy storage systems, including energy storage systems (ESS) and battery energy storage systems (BESS), are growing quickly across utility, commercial, and residential applications.

As deployments scale, understanding how energy storage works and how these systems behave under failure conditions—especially fire—becomes critical for safe design and approval.

Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems (UL 9540A) is the primary test method used to evaluate thermal runaway fire propagation in ESS and BESS.

It generates the data that engineers, designers, and authorities having jurisdiction (AHJs) rely on to assess risk, set safety measures, and approve system installations.

This guide explains how UL 9540A testing works in practice, including test levels, execution, data interpretation, and design implications.

It focuses on large-scale fire testing and approval-relevant evidence, and does not cover industrial lithium battery product-safety certification frameworks.

Key Points

• UL 9540A is a fire-propagation test method—not a certification—that AHJs and codes like Standard for the Installation of Stationary Energy Storage Systems (NFPA 855) and International Fire Code (IFC) rely on to evaluate ESS/BESS placement, setbacks, ventilation, and suppression.
• The method uses four escalating levels (cell, module, unit, installation) to initiate thermal runaway and measure heat release, gas composition, temperatures, and flame spread.
• Test data directly informs design: engineers adjust spacing, barriers, vent paths, and suppression based on measured heat, gas behavior, and temperature rise.
• Energy Storage Systems and Equipment (UL 9540) and UL 9540A (fire-propagation data) serve complementary roles, supporting both equipment certification and installation-level evaluation.
• A forthcoming 2025 revision will refine initiation methods, instrumentation, and reporting, making clear documentation of triggers and measurements increasingly important.

Why UL 9540A Matters

UL 9540A testing forces worst-case conditions in a controlled way.

It triggers thermal runaway on purpose, then measures heat, gas, flame, and spread. That data shows if a design contains the event or if it propagates beyond the initiating cell.

AHJs rely on these results to make approval decisions and set distances from walls and exits.

Fire codes including NFPA 855 and IFC cite this method when reviewing ESS and BESS for indoor and outdoor use.

The test series ties performance to clear thresholds, like wall temperature rise and external flaming.

It also documents gas make-up that can affect room safety. This evidence helps resolve risk questions early and informs design choices such as ventilation, suppression, and layout.

Broader approval workflows, permitting steps, and documentation requirements are covered in the energy compliance guide.

Test Levels and Prep

UL 9540A uses four sequential levels of Large-Scale Fire Testing.

A system advances only when the prior level shows concerning behavior, which means many designs never reach Level 4. The structure and triggers are consistent across labs.

• Level 1: Cell. A single cell is driven into thermal runaway. Vent gases are captured to measure volume and composition.
• Level 2: Module. One cell in a production-intent module is initiated to see if fire propagates within the module.
• Level 3: Unit. A full ESS unit with multiple modules is tested with suppression disabled to observe external effects.
• Level 4: Installation. A complete room or field layout is tested with suppression active to assess room-level impacts.

Samples are production-intent hardware.

That means the same cells, module design, bus bars, casing, and vent paths a customer would see. Worst-case conditions are set. Suppression is off until installation-level testing. Enclosures are oriented for maximum thermal and flame stress.

Instruments record heat release rate (HRR), surface and wall temperatures, and gas composition.

Gas flammability is evaluated using Standard Practice for Determining Limits of Flammability of Chemicals at Elevated Temperature and Pressure (ASTM E918), which determines if the vented mix can ignite under test conditions. Battery Module Testing adds thermocouples in and around the module to watch temperature spread.

Thermal runaway is initiated by methods such as a thin-film heater, nail penetration, overcharge, or over-discharge.

The lab selects the worst credible trigger for the chemistry and construction.

Testing stops when data shows no further propagation risk at the current level. If a module contains the event, there is no need to run a unit or installation test.

That saves time and focuses engineering on what matters.

How UL 9540A Tests Run

A Thermal Runaway Fire Evaluation begins with selecting the initiation method that reflects a credible failure.

For prismatic cells in rigid modules, external heating is common. For cylindrical packs, nail penetration or controlled overcharge may be used.

The lab then sets instrumentation to capture key data:

• HRR: measured with oxygen consumption calorimetry or comparable methods
• Gas sampling: collected near relief ports to analyze vented mixtures
• Thermocouples: track temperatures across cells, modules, units, and target walls
• Visual recording: cameras document flame height, duration, and spread

The trigger is executed in a controlled bay.

Suppression remains disabled through unit-level testing to reveal propagation behavior. Teams document external flaming, casing changes, debris, and any ignition of adjacent targets.

At the unit and installation levels, a target wall is instrumented to record temperature rise.

Safety gating between levels applies.

If cell-level results show non-flammable vent gases per ASTM E918 and no inducible thermal runaway, testing can end. If a module contains the event without propagation, unit testing may not be needed.

For installation-level testing, suppression and ventilation operate as designed. The layout reflects a real site plan, allowing teams to evaluate room-scale behavior under test conditions.

Detailed test documentation, sample preparation, and evidence requirements are covered in the IEC 62619 guide on secondary lithium cells and batteries for industrial applications. 

Interpreting UL 9540A Data

Technical Articles on UL 9540A tend to start with HRR curves.

A lower, shorter peak means less energy released. The area under the curve shows total heat. Comparing HRR across levels reveals whether a module or unit dampens or amplifies the event.

Gas composition is next.

Samples are evaluated for flammability using ASTM E918. A non-flammable mix under test conditions lowers the chance of secondary explosions. If hydrogen or carbon monoxide dominate, the narrative shifts to venting, dilution, and ignition control.

Containment and propagation indicators come from temperature traces and visual records:

• Temperature spikes: indicate potential spread to adjacent cells or modules
• External flaming: assessed by duration and reach
• Material response: casing changes, debris, or structural impact
• Field indicators: tools like cheesecloth tests help flag flame ejection in smaller systems

At the unit level, key pass/fail criteria include a target wall temperature rise below 97°C and no external flaming beyond the unit.

These thresholds and the four-level structure are detailed in the UL 9540A test method. Installation-level results consider room-scale behavior with active suppression.

Presenting a data-driven narrative for AHJs means stitching the pieces together.

Show how the module contains the event, how gases remain non-flammable, how the wall stays below threshold, and why setbacks or added suppression complete the margin.

Clear plots, photos, and concise summaries help officials decide.

Designing for UL 9540A

Consulting engineering for ESS turns test findings into practical design changes.

• If HRR stays high at the module level, teams may add thermal barriers between cell groups or adjust spacing to slow heat transfer.
• If exterior flaming occurs, enclosure vents and flame arrestors can redirect and attenuate jets.

Module layout and enclosure design play a central role.

Teams may add intumescent layers or ceramic wraps around high-risk zones to contain failure at the smallest level and limit propagation.

Venting and deflagration management address gas behavior. If tests show flammable mixtures, designs route gases to safer zones and reduce pooling.

Non-sparking materials and smooth duct paths can lower ignition risk, while deflectors keep hot jets away from cables and adjacent units.

Material choices shape outcomes. Noncombustible enclosures and cable insulation with higher ignition temperatures improve margins.

Sensor strategies have limits—gas detection can confirm an event but may not capture evolving conditions in enclosed spaces, so placement and integration with shutdown logic must be considered carefully.

Suppression integration closes the loop.

While it is disabled until installation-level testing, results guide nozzle placement, activation sequencing, and coordination with ventilation.

Early design reviews and alignment with test criteria can reduce iteration between lab runs, lower compliance cost, and support smoother approvals.

UL 9540 vs. 9540A

UL 9540 ≠ UL 9540A.

• UL 9540 is a safety standard for ESS.
  
  • A UL 9540 listing is a certification that the equipment meets that product safety standard after evaluation by a recognized lab.
• UL 9540A is a test method that produces fire propagation data.
  
  • It is not a certification or listing. It explains how systems behave during thermal runaway and provides evidence used by code officials.

Both matter in practice.

Residential deployments often rely on UL 9540 listings and, where codes require, UL 9540A data to support indoor placement. Commercial projects use UL 9540A to refine setbacks, ventilation, and suppression at the site level.

Broader comparisons across battery safety and certification standards are covered in the IEC 62619 guide.

2025 Updates to UL 9540A

Teams tracking revisions to UL 9540A:2025 should expect clarifications on initiation methods, instrument placement, and reporting.

Draft discussions also point to refinements for installation-level variants, including room layouts and egress evaluation.

Programs can be future-proof by documenting trigger rationale, instrument calibration, and gas analysis methods clearly. That makes reports easier to compare across labs as guidance evolves.