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New BESS Fire Rules: What UL 9540A’s 6th Edition and NFPA 855-2026 Change for Your Material Stack
On March 13, 2026, UL Solutions published the 6th Edition of UL 9540A, the test method the storage industry uses to characterize how a thermal runaway fire propagates. Weeks earlier the 2026 edition of NFPA 855, the installation standard most U.S. jurisdictions enforce, arrived with changes of its own.
Together they mark the largest shift in battery energy storage fire-safety expectations in years, and they land squarely on the passive materials inside the enclosure: the barriers, pads, gaskets and insulation that do their work whether or not any active system ever fires.
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What actually changed
Three changes matter most to anyone designing, integrating or buying storage in 2026.
Large-scale fire testing moves from optional to expected. Earlier editions of UL 9540A allowed many programs to conclude at the cell, module or unit level. The 6th Edition adds large-scale fire testing (LSFT) requirements in Section 10, in which a full BESS unit is intentionally ignited with suppression disabled to demonstrate that fire does not propagate to adjacent units. NFPA 855-2026 closes the loop from the installation side by requiring that large-scale fire testing be conducted or witnessed and reported by an approved testing laboratory. The industry is no longer content to extrapolate from a single burning module. The whole box has to prove itself.
Thermal runaway propagation prevention gets its own section. NFPA 855-2026 introduces requirements in Section 9.7.6.6 for thermal runaway propagation prevention (TRPP): active systems that detect the precursors of cell thermal runaway, such as off-gas or abnormal temperature, and act to stop propagation to adjacent cells, modules or racks. Active TRPP and passive barriers are not alternatives to one another. Detection and suppression buy time only if the barriers between cells hold while they work.
Hazard mitigation analysis becomes mandatory for most installations. A formal, site-specific analysis led by a licensed professional engineer with energy storage expertise moves from optional to required, with early coordination expected with the local authority having jurisdiction. Every claim in that analysis about how a fire behaves inside the enclosure traces back to test data, and behind that test data sits a specific, documented bill of materials.
Why new rules reach the material level
A UL 9540A report is not a certificate that travels between designs. The result belongs to the tested system: the exact cells, the exact module construction, the exact barrier materials at the exact thicknesses and positions that burned in the laboratory. Change the fire barrier between cells, thin an aerogel layer to reclaim a few millimeters of pack volume, or substitute a gasket compound, and the basis for the result changes with it.
That has two practical consequences for engineering and sourcing teams.
Materials get locked in earlier. When propagation behavior depends on the barrier stack, the barrier stack becomes part of the design freeze. Teams that treated separator pads and enclosure gaskets as commodity line items, to be re-sourced whenever a better price appeared, are finding these are now controlled components with drawings, tolerances and change-control implications.
Repeatability matters as much as the data sheet. A barrier that performed well in a test proves very little if production parts vary in thickness, density or fit. Converting quality, meaning die-cut dimensional tolerance, consistent lamination and correct adhesive coverage, quietly becomes a fire-safety variable rather than a cosmetic one.
A listing or test result attaches to the system boundary, not to any single layer inside it. Everything within the dashed line is part of what was proven.
A material can carry its own listings and ratings. Those describe the material. They do not, on their own, describe how your system behaves in a propagation test.
Where the test result actually lives
The distinction in the figure above is worth stating plainly, because it is the source of most of the confusion we hear in early conversations. A supplier can accurately tell you that a material carries a UL 94 V-0 rating at a given thickness, or that it is used widely in battery applications. A supplier cannot tell you that a pad is, by itself, compliant with UL 9540A or NFPA 855, because neither document rates individual materials. They evaluate systems and installations.
What a material supplier can meaningfully offer is a documented, repeatable part that matches what your system was tested with, along with the data sheet evidence behind it. That is the honest scope, and under the 2026 rules it is a more valuable one than it used to be.
The families doing the work
The 2026 documents do not name materials; they demand outcomes. In practice a handful of passive families carry most of the propagation-resistance load in current BESS designs.
| Location | Job under the new rules | Typical family |
|---|---|---|
| Between cells | Absorb swelling across service life while holding a thermal and flame barrier in position | Cellular-silicone fire barrier, comparable to the ProCell® position |
| Tightest gaps | Maximum thermal standoff where only a few mils exist | Polyimide-aerogel film, comparable to the AeroZero® position |
| Direct flame contact | Survive the event with no organic content to burn away | Mica barrier sheet with inorganic facing |
| Module and rack faces | Slow rack-to-rack transfer in contested thickness | Aerogel blanket, comparable to the ArmaGel® position |
| Enclosure seams and doors | Hold the sealed volume that detection depends on, through years of cycles | Closed-cell EPDM and silicone sponge; V-0 grades where specified |
| Sealed compartments | Equalize pressure through daily thermal cycling without pumping moisture past the gasket | ePTFE protective vent |
Each of these is covered in depth, with the selection factors that separate one from another, in our BESS material reference.
Two claims to be careful about
Flame ratings are grade- and thickness-specific. A family name is not a rating. UL 94 V-0 applies to a particular grade at a particular thickness, and it should be confirmed on the manufacturer’s data sheet for the exact material and gauge you intend to cut, not inferred from a product line.
Compliance belongs to the tested system. UL 9540, UL 9540A, NFPA 855, and the ingress and enclosure ratings that accompany them describe systems and installations. When a component is described as compliant with one of them on its own, that is a claim worth asking a follow-up question about.
How H-O Products helps
H-O Products is a die-cutter and converter, not a test laboratory and not a battery integrator. What we contribute to a program working toward these requirements is the part itself: fire-barrier, insulation, thermal interface and gasket materials cut, slit, laminated and kitted to your print, repeatably, as an ISO 9001:2015 certified organization, so that the parts running in production match the parts that were in the system when it earned its result.
In practice that looks like tight dimensional tolerances on compressible materials, consistent multi-layer laminations where a stack-up is converted as a single part rather than assembled by hand on the line, adhesive applied per drawing, and traceable material lots. When a substitution is proposed, we would rather raise it with your engineering team than quietly ship a different construction.
Three questions to ask now
Which edition is your program targeting? Adoption timelines vary by jurisdiction and by customer. Some projects will be permitted against earlier editions for a while yet, but new utility and data-center solicitations are already referencing the 2026 documents.
Is your barrier stack documented as a controlled design? Every layer between cells, between modules and across rack faces should exist on a drawing with material, thickness, tolerance and adhesive called out. That drawing is what your hazard mitigation analysis and your test report ultimately rest on.
Can your supply chain hold that spec? Ask converters how they control thickness and dimensional tolerance on compressible materials, how lots are traced, and what happens procedurally when a substitution is proposed. Under the 2026 rules a quiet material substitution is not a purchasing decision. It is a design change.
Frequently asked questions
Does UL 9540A certify a material?
No. UL 9540A is a test method that characterizes thermal runaway fire propagation in cells, modules, units and, in the 6th Edition, at large scale. Its output is data about a system, not a certification of any individual material inside it. Individual materials carry their own separate ratings, such as UL 94 flame class at a stated thickness.
What is the difference between UL 9540 and UL 9540A?
UL 9540 is the safety standard used to list energy storage systems and equipment. UL 9540A is the test method that generates fire propagation data, which is then used to inform installation decisions under codes such as NFPA 855 and to support the hazard mitigation analysis.
If we change a barrier material, do we have to retest?
That determination belongs to your test laboratory, your listing body and the authority having jurisdiction, not to a materials supplier. What we can say is that the barrier stack is part of what was evaluated, so material changes in that stack should be treated as design changes and reviewed rather than handled as routine substitutions.
What should we send to get a quote on a barrier or gasket part?
A drawing or dimensioned sketch, the material family or the problem if the family is still open, the available gap or thickness, the temperature and voltage the part lives with, and the fire-safety context, meaning whether the part sits inside a tested barrier design and at what test level. With those in hand an engineering review can usually confirm quickly whether the direction is right.
Have a BESS barrier stack reviewed
Send the location inside the enclosure, the available gap, and the UL 9540A test level you are designing to. Our engineering team will confirm whether the family you have in mind is the right direction and quote the die-cut part against your print.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job.
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Related reading
Related articleThermal isolation vs. fire protection: layering a barrier stackRead →
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT). H-O Products is a die-cutter and converter; we do not perform fire testing or issue listings.
References
- UL Solutions. UL 9540A, Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, 6th Edition, published March 13, 2026.
- NFPA. NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, 2026 edition, including Section 9.7.6.6 thermal runaway propagation prevention requirements.
- UL Solutions. UL 9540, Standard for Energy Storage Systems and Equipment.
- UL Solutions. UL 94, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances.
Product names referenced are trademarks of their respective owners. H-O Products converts these materials and makes no independent certification claims on their behalf. Flame ratings such as UL 94 are grade- and thickness-specific; confirm on the manufacturer's current data sheet.
Code and standard summaries here are informational and reflect published editions as of August 2026. Adoption varies by jurisdiction. Confirm current requirements with your test laboratory and the authority having jurisdiction.