What UL 9540A is for
Battery energy storage systems (BESS) can fail by thermal runaway: one cell overheats, vents flammable gas and can ignite its neighbours. UL 9540A gives regulators and fire departments the data they need to judge that risk. Installation codes — NFPA 855 and the International Fire Code (IFC) — rely on it when a system exceeds size or spacing limits, and UL 9540 certification of a complete system refers to it.
The four test levels
| Level | What is tested | Key data |
|---|---|---|
| Cell | A single cell forced into thermal runaway, normally by external heating | Venting and thermal runaway temperatures, gas composition, lower flammability limit (LFL), burning velocity, maximum explosion pressure (Pmax) |
| Module | Cells assembled into a module; one cell is driven into runaway | Heat release rate, gas generation and composition, external flaming, flying debris |
| Unit | A rack or cabinet of modules, with target units and walls placed alongside | Heat release rate, gas, deflagration and flying debris hazards, surface temperatures and heat flux on target units and walls, re-ignition |
| Installation | The unit installed as in the field, with its fire protection | Effectiveness of sprinklers or other fire protection and explosion protection |
Each level is required only when the previous one does not meet its criteria, so a well-designed module can shorten the programme considerably.
How we run it
- Cell level: cells are heated in a sealed pressure vessel until thermal runaway; the vent gas is analysed by gas chromatography, then tested for flammability limits (ASTM E918), burning velocity (ISO 817 Annex C) and explosion pressure (EN 15967). All three methods are in our CNAS scope.
- Module and unit levels: run under a calorimetry hood with oxygen consumption calorimetry, online gas analysis (O₂, CO, CO₂, H₂, FTIR), up to 180 thermocouple channels and water-cooled heat flux gauges.
- Larger systems: cabinets and containers are tested at our Hebei large-scale fire test site and the Jiading indoor fire hall.
UL 9540A and large-scale fire testing
UL 9540A is designed to find out whether thermal runaway propagates, so the test conditions do not always produce a sustained fire. Regulators increasingly also ask what happens when a fire does develop: does it spread to adjacent units, equipment or through a fire-rated barrier? That question is answered by a large-scale fire test, which deliberately ignites the vent gas and lets the fire develop.
Frequently asked questions
Is UL 9540A a pass/fail certification?
No. UL 9540A is a test method. It produces data — cell venting and thermal runaway temperatures, gas composition, heat release, flaming and deflagration hazards — that the authority having jurisdiction (AHJ) and the UL 9540 certifier use to decide spacing, protection and whether the system can be installed.
Do I have to test every level?
Not always. Each level is needed only if the previous one does not meet its performance criteria. For example, if the module test shows thermal runaway does not propagate beyond the initiating cell and there is no external flaming, unit-level testing may not be required. We plan the test sequence with you before the cell test.
Can the cell-level UL 9540A data be reused for UL 9540B?
Yes. The UL 9540A cell test covers the cell test required by UL 9540B, so a residential system that already has UL 9540A cell data only needs the UL 9540B fire propagation test.
Which parts of UL 9540A are within your CNAS accreditation?
Cell, module and unit levels, to ANSI/CAN/UL 9540A:2026. Installation-level tests are carried out on our sites but are not listed in our accreditation scope. Flow batteries, vented and valve-regulated lead-acid and nickel-cadmium batteries are excluded from the accredited cell test.
Can a certification body witness the test?
Yes. We are a witness laboratory for TÜV Rheinland, Intertek and CSA, and our Hebei site has a client observation room with live data and video for remote witnessing.