UL 9540A Edition 6: Comprehensive Interpretation of the Large Scale Fire Test (LSFT)

— The Highest Safety Threshold for Battery Energy Storage System Fire Risk Assessment

1. Background: Why Perform a Large Scale Fire Test?

Driven by explosive growth in the global energy storage market, the fire safety of Battery Energy Storage Systems (BESS) has become an urgent industry priority. Under extreme conditions such as overheating, overcharging, or mechanical damage, lithium-ion batteries can undergo thermal runaway—releasing significant volumes of flammable gases and triggering severe fires. If a fire propagates to adjacent battery cabinets or surrounding structures, the consequences can be catastrophic.

On March 13, 2026, Underwriters Laboratories (UL) officially published the 6th edition of ANSI/CAN/UL 9540A. The core updates in this revision focus heavily on the Installation Level – Large Scale Fire Test (LSFT). Serving as the ultimate benchmark for evaluating the fire safety performance of an energy storage system under real-world installation scenarios, the LSFT is also mandated by NFPA 855:2026 (Standard for the Installation of Stationary Energy Storage Systems).

In simple terms, an LSFT involves constructing a BESS array in a laboratory that closely mimics actual field installation, deliberately initiating a Fully Developed Fire Condition within one of the enclosures, and observing whether the fire spreads to adjacent battery enclosures or surrounding building structures. The data collected scientifically validates minimum separation distances, fire protection plans, and overall system design integrity.

2. Panoramic View of UL 9540A Test Hierarchy

To appreciate the role of the Large Scale Fire Test, one must first understand UL 9540A's four-level testing hierarchy. Moving progressively from micro to macro levels, each level provides critical foundational data for the subsequent level:

Table 1: Overview of the UL 9540A Four-Level Test Hierarchy

3. Core Objectives of LSFT

According to Chapter 10 of UL 9540A Edition 6, the Large Scale Fire Test is designed to address four key engineering and safety questions:

• Thermal Exposure Assessment: When a fully developed fire occurs inside a BESS unit, what is the severity of thermal exposure to adjacent or nearby systems? Is there a risk of secondary thermal runaway or ignition?
• Structural Fire Risk: Does the fire pose an ignition threat to nearby building structures (e.g., combustible walls or roofs)?
• Separation Distance Verification: Are the manufacturer's specified minimum installation separation distances sufficient to prevent fire propagation?
• Fire Protection Plan Validation: Are active fire protection measures in the building design (e.g., automatic sprinkler systems) effective at controlling fire spread?

4. Test Method Details

4.1 Test Environment Requirements

LSFT mandates strict environmental controls to ensure data reliability and test repeatability. For outdoor testing, the following conditions must be met:

• Wind speed ≤19.3km/h ( ≈5.4 m/s), utilizing wind screens if necessary
• Ambient temperature ≥ 0℃ (32°F)
• Relative humidity < 90%
• Zero precipitation during testing
• Controlled management of vegetation and combustible materials within the test perimeter

Indoor testing utilizes a standardized configuration for non-dedicated buildings, featuring flat ceilings of fixed height and test walls positioned at corners. This layout prevents hot gas layers from being enclosed by the walls, enabling the extrapolation of data to larger room volumes. Additionally, indoor setups permit active fire suppression systems (typically water-based sprinklers) to assess their suppression efficacy.

4.2 Test Layout

The test layout must represent the final field installation configuration of the BESS. The primary principle requires that the Enclosure of Origin be fully populated with battery modules and auxiliary equipment, while surrounding Target Enclosures and/or target walls are positioned at the manufacturer's minimum specified separation distances. The figures below illustrate a typical indoor ground-mounted test configuration: 

Figure 1: Indoor Floor-Mounted LSFT Test Layout — Elevation View

Figure 2: Indoor Floor-Mounted LSFT Test Layout — Plan View (with Fire Sprinkler System Schema)

In the figures, the red area indicates the fire initiation location (fully populated, initiation location of developed fire condition), the blue area represents the remaining portions of the Enclosure of Origin, and the yellow area denotes the instrumented Target Enclosure. The letters A, B, C, and D represent the minimum allowable separation distances in each direction.

Target enclosures may be provided in three forms: a fully populated physical unit, a representative surrogate sample, or an instrumented target wall structure. Thermocouples shall be arranged in a grid pattern with spacings not exceeding 0.61m (2 ft), covering cell surfaces, module surfaces, or interior enclosure surfaces on the target unit, depending on the actual interior equipment configuration of the target enclosure.

4.3 Fire Initiation Methods

Standard LSFT requires establishing a Fully Developed Fire Condition within the Enclosure of Origin. This is triggered using one of three standardized methods:

• Heater Board + Auxiliary Ignition Source: A heater board drives cells into thermal runaway, paired with an ignition source (spark plug, igniter, or pilot flame) positioned along the gas venting path to ensure flammable gas combustion.
• Electrical/Mechanical Abuse + Auxiliary Ignition Source: Thermal runaway is induced via overcharge, external short circuit, or mechanical abuse, combined with a gas ignition source.
• Gas Burner: A diffusion, premixed, or semi-premixed gas burner acts directly as both the heat source and ignition source.

The initiation location is determined through a structural and thermal review to identify the location capable of generating maximum thermal stress. Typically, this is selected at the lowest or second-lowest vertical module position near the outer boundary of the enclosure—where heat accumulation is most severe and ventilation is restricted.

4.4 Criteria for a Developed Fire Condition

The standard establishes clear benchmarks to confirm a "fully developed fire condition":

• Outdoor Installations: Involves thermal runaway and active fire propagating across at least one entire vertical column (rack) of modules within the Enclosure of Origin.
• Indoor Installations: Involves at least one full module experiencing complete thermal runaway propagation and the ignition of all combustible materials within that module.

5. Testing for Stacked Energy Storage Systems

As energy density increases, stacked BESS configurations have become increasingly prevalent. Edition 6 introduces specific test requirements for multi-tier stacked BESS installations:

Figure 3: Schematic Layout for Stacked BESS Large Scale Fire Testing

Stacked testing requires placing target enclosures both adjacent to and directly above the Enclosure of Origin. The complete stack must be installed at its maximum rated mechanical load capacity, including all mounting hardware and support brackets.

During the test, thermal exposure on the underside of the upper unit must be monitored. If requested by the client, the entire vertical stack can be evaluated as a single Enclosure of Origin (applicable when fire is anticipated to propagate vertically within the stack structure).

6. Pass/Fail Criteria: Detailed Performance Metrics

LSFT pass/fail criteria are categorized into outdoor and indoor installations, covering temperature limits, thermal runaway confinement, and structural stability.

Table 2: Comparison of LSFT Performance Criteria (Outdoor vs. Indoor)

Note on Allowable Temperature Tolerances: All temperature measurements are evaluated using a 60-second moving average to eliminate transient spikes. Furthermore, for thermocouples installed on module or interior enclosure surfaces, the standard provides an allowance: measured values may exceed the limit for up to 5 minutes total cumulative duration (continuous or intermittent), provided they do not exceed 120% of the threshold. This provision reflects practical engineering considerations.

Table 3: Key Performance Criteria for Outdoor LSFT Installations

Table 4: Key Performance Criteria for Indoor LSFT Installations

7. Test Termination Conditions

An LSFT is an extended, high-intensity test. Under normal conditions, the test is concluded when all of the following conditions are satisfied:

1. Open flames inside and outside the Enclosure of Origin are completely extinguished.
2. After the fully developed fire condition is established, the temperature inside the Enclosure of Origin remains below 100°C (212°F) for a continuous 3 hours.
3. Target BESS temperatures exhibit a downward trend and remain below criteria limits for at least 3 consecutive hours.
4. If a water-based fire suppression system is activated, a 12-hour observation period must follow after shutting off the water supply to rule out re-ignition.
5. Stacked configurations require an additional 24-hour observation period to ensure long-term structural stability.

If thermal runaway propagates to a target enclosure, the stacked structure collapses, temperature limits are breached, or personnel safety is threatened, the test is terminated prematurely and deemed a failure.

8. Key Highlights of Edition 6 Updates

• Harmonization with NFPA 855:2026: Formally links LSFT into NFPA 855 as a mandatory compliance requirement, establishing a cohesive "test-to-code" framework.
• Clarified Exemption Criteria: Explicitly defines applicable conditions under which Level 3 (Unit Level) testing may be bypassed or exempted.
• Standardized Stacked Testing: Introduces precise testing procedures and evaluation criteria tailored for multi-tier stacked BESS.
• Refined Performance Limits: Incorporates a 110% multiplier and a 5-minute grace tolerance for surface temperature criteria, aligning closer with practical engineering dynamics.
• Enhanced Re-ignition Monitoring: Mandates a 12-hour post-suppression observation period for indoor systems to verify the long-term containment capability of fire protection systems.
• Comprehensive Reporting Mandates: Requires full-process data trace records from physical test assembly to final conclusions.

9. Conclusion

The Large Scale Fire Test (LSFT) represents the highest technical standard for fire safety assessment of battery energy storage systems globally. Moving beyond theoretical calculations or small-scale bench testing, it uses full-scale physical trials to address a fundamental safety challenge:

"When a fire actually breaks out inside your energy storage system, will it propagate to adjacent units or surrounding structures?"

For BESS manufacturers, integrators, and project developers, mastering the nuances of LSFT is essential—not only for regulatory compliance and market access, but as a critical pathway to advancing the intrinsic safety of energy storage products.

— This article is written with reference to the ANSI/CAN/UL 9540A-2026 (6th Edition) standard. —