Reading a UL 9540A Report Properly: Which Values Matter for Thermal Barrier Design

UL 9540A is not a product certificate but a test method: it characterises how a battery system behaves during thermal runaway. The outcome is a data report — not a pass/fail verdict. That is precisely where its value for thermal barrier design lies: the report supplies the input parameters needed to dimension fire barriers, wall assemblies and insulation layers, rather than falling back on blanket standard assumptions.

The test programme is structured in four stages: cell level, module level, unit level (rack or cabinet) and installation level. Each stage answers different questions. Anyone evaluating a report should first check up to which stage testing was carried out — and whether the tested configuration matches the installation actually being planned. A module report based on a different cell batch, a different enclosure or a different arrangement offers only limited validity for the specific plant.

The cell-level test establishes whether and at what temperature the cell goes into thermal runaway, when the safety vent opens (venting temperature) and which gas mixture is released in the process. FTIR analysis quantifies hydrogen, carbon monoxide, carbon dioxide and hydrocarbons, among others. The safety-related characteristics are then determined from the reconstituted gas mixture.

• Lower and upper flammability limits (LFL/UFL) — at room temperature and at venting temperature
• Maximum explosion pressure Pmax of the vent gas
• Laminar burning velocity of the gas mixture
• Gas volume per cell or relative to capacity
• Venting and thermal runaway temperature of the cell

For compartmentation these values are doubly relevant: they govern the design of explosion venting and ventilation to NFPA 68/69 — and thus the pressure and temperature loads that walls, linings and penetration seals must withstand in the event of an incident.

At module and unit level, thermal runaway is deliberately initiated and the response of the complete assembly is measured. This is where the values that directly size a fire barrier are found: the heat release rate with its peak value and time profile, the total energy released, the gas release rate, and the question of whether thermal runaway propagates to neighbouring cells and modules.

Particularly valuable for designers of thermal barriers are the measurements taken at the target surfaces: heat flux on instrumented wall segments and adjacent units, plus surface temperatures at defined measuring points. These values show what thermal load a partition wall or protective cladding actually receives at a given distance — and for how long. From this, the choice of insulation material, layer thickness and the permissible temperature on the unexposed side can be derived.

The report does not replace fire protection design; it feeds it. In practice, a structured evaluation has proven effective: first check the propagation statement (does the event remain confined to cell, module or unit?), then extract the thermal loads on adjacent surfaces, and finally hand the gas characteristics over to the explosion protection design.

• Propagation behaviour: the basis for separation distances and for whether reduced spacings can be justified under NFPA 855
• Heat flux and wall temperatures: input parameters for fire resistance, insulation build-up and barrier material selection
• Gas volume, LFL, Pmax and burning velocity: the basis for explosion vent areas and ventilation design
• Secondary fire phenomena: flame lengths, sparking and burning particles define protection zones in front of the barrier

If individual values are missing, or testing only went as far as cell level, the design must take a conservative approach — with larger separation distances, higher fire resistance and a more robust lining. A complete report up to unit level therefore also pays off economically: it allows a well-founded, often leaner design instead of blanket safety margins.