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Cell, Module, Compartment: Layering Fire Barriers in a Li-ion UPS, and Who Owns the UL 9540A Result
Thermal-runaway containment is a layering problem, and the two ways UPS programs get it wrong are mirror images. In the first, the barrier tiers are never drawn: containment is assumed to live somewhere else (the BMS, the room suppression), the module design closes without a cell-to-cell or module barrier, and adding one after the geometry is frozen becomes the expensive retrofit.
In the second, a drawing note claims "UL 9540A barrier" for a die-cut sheet, and the review stalls, because UL 9540A evaluates the tested assembly, not a material. This article walks the three-tier barrier architecture the parent application page teaches, which material family carries each tier, and the standards language that keeps the drawing set honest: material classes per TDS, system standards by designation, and the listing with the tested assembly.
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The design assumption: one cell will fail
Containment design starts from a deliberately pessimistic premise: assume a single cell goes into thermal runaway, the self-accelerating exothermic failure mode of lithium-ion chemistry, and engineer the layers so the event stays local. That premise is what UL 9540A characterizes: it is the test method for evaluating thermal-runaway fire propagation in battery energy storage systems, run at cell, module, unit, and installation tiers on the tested assembly.
The materials below are the converter-side ingredients of that strategy. They are chosen early, drawn as parts, and documented like everything else in a battery program, because in this application the paperwork is part of the part: a correct material with an undocumented class, or a drawing that claims a system listing for a component, costs more schedule at review than any cutting error.
Tier 1: cell-to-cell, the mica layer
The first tier sits between adjacent cells: mica fire-barrier sheet in the EV battery fire-barrier grade, thin and inorganic, die-cut to the cell footprint so it packages inside the stack without stealing the compression pad's room. Mica earns this position because it is mineral, not polymer: it does not contribute fuel, and it stands up to the local temperatures of a cell event.
The cautious language is part of this material's spec: the barrier supports a thermal-runaway strategy that is evaluated on the tested assembly per UL 9540A; H-O supplies the converted layer and its documentation, and the system designer owns the evaluation.
Practically, tier 1 is also a packaging negotiation with layer 1 of the stack, the PORON compression pads that hold the cells in their preload window (methods per ASTM D3574); deciding both at the same time is what keeps the gap budget honest.
Tier 2: module-level, the ProCell firewall
The second tier contains a module-scale event: ProCell EV Firewall Barrier as walls, lids, and dividers, the engineered fire barrier crossing over from EV practice into stationary duty. Tier 2 is where the never-drawn failure usually happens, because module walls feel like structure rather than safety layers, and the barrier function gets discovered missing when the 9540A plan is written.
Decide the tier early and draw it as parts: a module wall that is also a fire barrier is a different drawing callout, with its own material documentation, than a module wall that is only sheet metal.
Tier 3: module-to-module and compartment, the aerogel layer
The third tier slows heat between modules and compartments: ArmaGel HT and HTL flexible aerogel blankets, which buy the most temperature drop per millimeter of any layer in the stack (thermal conductivity per ASTM C177, surface burning per ASTM E84, and the aerogel-insulation standard ASTM C1728 cited per TDS). Aerogel shows up twice in a UPS cabinet: as the compartment barrier in the containment strategy, and as plain thermal insulation on warm internal surfaces.
Converting matters here: aerogel sheds fines when cut, so edge sealing and kitting are part of the part, not an installation afterthought.
The standards ladder, by designation
Four designations frame the program, and each attaches at a different tier. UL 1973 is the battery-system standard for stationary and motive auxiliary applications. UL 9540 is the energy-storage-system tier above it. UL 9540A is the test method that characterizes thermal-runaway fire propagation. NFPA 855 is the installation standard, the spacing, capacity, and documentation tier the AHJ reviews.
None of these belongs to a die-cut part, which is why the parent page's drawing-note discipline is worth quoting: write material classes (UL 94) on the part callouts, cite system standards by designation, and never let a drawing imply that a pad or sheet is "UL 9540 certified."
The honest sentence, the one that passes review, is that the materials support designs evaluated to those standards, and the listing belongs to the tested assembly.
Putting it on the drawing set
- Name the tiers early: cell-to-cell, module, compartment, decided before the geometry freezes, because retrofitting a barrier into a finished module design is the expensive version of the same spec.
- Draw barriers as parts: each tier gets a part number, a material family, and a footprint, not a note that says "fire protection by others."
- Write the citation language correctly: "supports designs evaluated per UL 9540A" on the barrier callouts; UL 94 classes cited per grade TDS at the specified thickness.
- Keep the gap budget visible: tier 1 shares space with the compression pads; the cell maker's preload window and the barrier thickness are negotiated together.
- Kit the documentation with the parts: lot-code TDS records per material, shipped with the kit, because the system evaluation will ask for exactly that.
Frequently asked questions
Can a die-cut barrier sheet be "UL 9540A certified"?
No. UL 9540A is a test method that evaluates thermal-runaway fire propagation on the tested assembly, at cell, module, unit, and installation tiers; it does not list component materials. A barrier material carries its own documentation (its TDS, its test methods, any UL 94 class on the rated grades) and supports a design evaluated under 9540A. Drawing notes that claim the system result for a component are the ones that stall reviews.
Do I need all three tiers in every UPS design?
The tier count belongs to the system designer and the evaluation plan; the parent page's guidance is that a containment strategy usually layers two or three tiers, and that the expensive failure is deciding late. The honest engineering sequence is to name the tiers at concept, draw them as parts, and let the 9540A work confirm or adjust the strategy on the tested assembly rather than discovering a missing layer at review.
Why mica between cells instead of more aerogel everywhere?
Different jobs. Mica is thin, mineral, and dimensionally crisp at cell-footprint scale: it adds barrier function without stealing the millimeters the compression pads need. Aerogel is the thermal-isolation specialist: more temperature drop per millimeter, best spent between modules and compartments where there is room for a blanket. ProCell carries the structural module-wall tier between them. Layering by tier is what lets each material do the one thing it is best at.
What documentation ships with the barrier parts?
Lot-code TDS records per material, certificates of conformance on request, and material traceability through our ISO 9001:2015 certified quality management system. Kitted module sets keep each layer's paperwork attached to the kit, which is the format system-level evaluations want to see. What H-O does not supply is the system evaluation itself: the 9540A work and the listing belong to the tested assembly and its owner.
Planning a barrier strategy?
Send the module geometry, the tier plan (or "help us draw one"), and the gap budget. Engineering will return a layer-by-layer material direction with the citation language written the way reviews expect.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks. MOQ varies by material and part.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- By designation, listing with the tested assembly: UL 9540A (thermal-runaway fire propagation test method, at cell / module / unit / installation tiers); UL 1973 (batteries for stationary applications); UL 9540 (energy storage systems); NFPA 855 (installation of stationary energy storage systems).
- Material-level methods per TDS: UL 94 (V-0 listings on rated grades, per grade and thickness); ASTM C177 / C1728 / E84 (aerogel blanket data); ASTM D3574 (PORON CFD methods).
- Maker technical data sheets: mica fire-barrier grade; ProCell EV Firewall Barrier; ArmaGel HT / HTL; PORON 4701 series; kSil V-0 and RS-series silicone sponge; RE-series EPDM.
This article explains barrier-tier architecture and standards language qualitatively for specification purposes. System standards (UL 1973, UL 9540, UL 9540A, NFPA 855) are cited by designation only; they evaluate systems and installations, and the listing belongs to the tested assembly. No fire-resistance duration, propagation result, or rating is stated or implied for any material; per-grade data belongs to the maker's current technical data sheet.
H-O converts materials and supplies their documentation; it does not manufacture cells, design battery systems, or certify systems. All parts are custom manufactured to order; minimum production run quantities vary by material and part.