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Battery energy storage systems (BESS) can turn a localized cell failure into a fire, an explosion hazard, or a prolonged emergency. The initiating event is usually thermal runaway: a battery cell releases intense heat and flammable gases, which can ignite, heat neighboring cells, and propagate through modules or enclosures. A data center’s actual risk depends on the installed chemistry, module and rack design, enclosure, cooling, ventilation, separation, detection, suppression, and maintenance—not on the words “lithium-ion” alone.
Large-scale testing shows why configuration matters. In a 2026 FM evaluation of a Delta UPS battery system for Equinix, the original design allowed propagation and produced an approximately 700 kW peak heat-release rate. Design changes that reduced heat transfer and improved sprinkler cooling confined runaway to the initiating module in a retest. Those are configuration-specific results, not a failure-rate statistic or a guarantee for every BESS.
How a battery failure becomes a fire
Thermal runaway starts the chain
Thermal runaway is a self-heating reaction in which a cell’s temperature rises faster than heat can be removed. Internal short circuits, manufacturing defects, overcharge, physical damage, high ambient temperature, or failures in battery-management controls can initiate it. As the separator and electrolyte break down, the cell releases heat and combustible decomposition products. The failed cell can vent, rupture, or eject hot material.
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FM describes lithium-ion batteries as both ignition sources and fuel sources. Its technical report states: “LIBs can act as both ignition and fuel sources, and fires involving them may lead to significant property damage and complicate firefighting efforts.” A hot cell can ignite nearby materials, while the energy stored in surrounding cells supplies additional heat and fuel.
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Vent gases can ignite or deflagrate
Thermal runaway produces gases that may include flammable hydrocarbons, carbon monoxide, hydrogen, and other decomposition products. The exact mixture depends on cell chemistry, state of charge, temperature, and enclosure conditions. If gases collect in a cabinet, battery room, ceiling void, or adjacent space, an ignition source can cause a flash fire or deflagration. Pressure can damage doors, walls, cable penetrations, and other equipment even when the initial battery fire is small.
Gas accumulation is therefore a separate hazard from visible flame. A system can have dangerous concentrations before smoke or flame reaches a detector. Ventilation, gas detection, pressure relief, and explosion-control design must be evaluated together; simply adding a smoke detector does not remove the gas hazard.
Heat transfer drives propagation
Cells are packed closely to deliver power in a practical footprint. Heat from one runaway cell can pass through busbars, module frames, cooling plates, insulation, and enclosure air to neighboring cells. Once another cell reaches its runaway threshold, the event can progress from cell to module, rack, cabinet, or an adjacent unit. The amount of delay and the boundary of propagation are properties of the tested installation, not just the cell chemistry.
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Firefighting can be difficult
Water may cool exposed surfaces without reaching the cells that are generating heat. Enclosures can shield modules from sprinkler discharge, and a battery can reignite after flames appear to be out if heat remains trapped inside. Damaged cells may continue venting, creating toxic or flammable atmospheres. Responders also face electrical shock, arc-flash, structural, and re-ignition hazards. A data center’s emergency plan must address access, isolation, ventilation, runoff, and re-entry rather than treating a BESS like an ordinary office fire.
What the FM large-scale UPS test demonstrates
FM’s collaborative evaluation with Delta and Equinix examined a specific UPS battery configuration. The original configuration experienced thermal-runaway propagation from one module to adjacent modules and units. The test reached an approximately 700 kW peak heat-release rate. Sprinklers activated, but the enclosure restricted water access and reduced cooling effectiveness; surface temperatures on adjacent modules exceeded 500 °F (260 °C). The system had previously demonstrated UL 9540A compliance, yet it failed the FM large-scale fire test.
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That result does not mean UL 9540A is ineffective or that every certified system will fail. It shows that a product-level or specified test result cannot be generalized beyond its tested arrangement, and that enclosure geometry, sprinkler access, cooling paths, and spacing can change the outcome.
FM then retested a modified system. Changes intended to reduce heat transfer and improve sprinkler cooling confined thermal runaway to the initiating module. Neighboring modules remained below 212 °F (100 °C). These temperatures and boundaries apply to that modified configuration under the stated test conditions; they are not a universal performance promise for redesigned lithium-ion systems.
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|---|---|---|
| Original UPS configuration | Propagation to adjacent modules and units; approximately 700 kW peak heat release | Closely packed equipment can spread heat despite sprinkler activation. |
| Original enclosure and sprinklers | Adjacent-module surfaces exceeded 500 °F (260 °C); enclosure limited water access and cooling | Suppression performance depends on whether water can reach the heat source. |
| Modified configuration | Runaway confined to the initiating module; neighboring modules below 212 °F (100 °C) | Heat-transfer controls and cooling details can change the propagation boundary. |
Use the FM findings as a design lesson: ask for evidence on the exact cabinet, module spacing, sprinkler arrangement, state of charge, ventilation, and room layout proposed for your site. Do not convert one test into a probability that a BESS will or will not burn.
UL 9540, UL 9540A, and NFPA 855 are different tools
These designations are often mentioned together but perform different functions.
| Document | Role | How to use it |
|---|---|---|
| UL 9540 | Equipment and energy-storage-system safety standard | Confirm that the complete equipment is certified within a defined scope, including the batteries, power-conversion equipment, controls, and interfaces covered by the listing. |
| UL 9540A | Test method for evaluating thermal-runaway fire and explosion behavior | Review cell, module, unit, and installation-level results, including propagation, vent-gas ignition, enclosure behavior, and separation assumptions. |
| NFPA 855 | Installation standard for stationary energy storage systems | Use its provisions for detection, suppression, explosion control, exhaust ventilation, gas detection, thermal runaway, spacing, and related fire-protection measures, subject to local adoption and amendments. |
UL describes UL 9540 as the foundational product-safety standard, while UL 9540A evaluates fire behavior during thermal runaway. Certification to UL 9540 does not replace installation-level fire analysis, and a UL 9540A report is not the same thing as a product-safety listing.
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Which editions and codes apply in 2026 and 2027?
NFPA identifies the 2026 edition of NFPA 855 as current. The enforceable requirement at a project is determined by the authority having jurisdiction (AHJ), including the edition adopted by the state, city, or other jurisdiction and any local amendments. A contract, permit, insurer, or utility may impose additional conditions.
UL’s installation guidance says the 2026 NFPA 855 and the 2024 International Fire Code require UL 9540A testing in specified circumstances. UL also reports that the sixth edition of UL 9540A was published on March 13, 2026, with an effective date of January 1, 2027. During the transition, verify which edition the AHJ and project documents recognize; do not describe Edition 6 as binding everywhere before local adoption.
For a permit package, record the adopted NFPA 855 and fire-code editions, the AHJ’s interpretation, the UL 9540 listing scope, the UL 9540A edition used for the proposed configuration, and any insurer or owner requirements. This record prevents a test performed to one edition from being presented as proof of compliance with another.
A site-specific fire-risk assessment workflow
- Define the installation. Document cell chemistry, manufacturer and model, nominal and maximum state of charge, number of cells and modules, rack or cabinet arrangement, indoor or outdoor location, room volume, ceiling height, cable routes, and proximity to occupied or mission-critical spaces.
- Identify credible initiating events. Review electrical faults, overcharge, cooling failure, mechanical damage, water ingress, firmware or sensor faults, maintenance errors, and external fires. Include single-cell, single-module, and multi-rack scenarios.
- Obtain the complete test record. Request UL 9540A cell-, module-, unit-, and installation-level results where applicable. Confirm the tested enclosure, spacing, ventilation, state of charge, suppression, gas detection, and temperature or pressure measurements match the proposed design.
- Map propagation boundaries. Determine whether runaway is expected to remain within a cell, module, cabinet, room, or outdoor array. Use the result to set fire-resisting separation, protected penetrations, and the distance from UPS rooms, generators, fuel, servers, and evacuation routes.
- Design detection and control layers. Coordinate early warning, smoke and heat detection, off-gas or gas detection, battery-management alarms, automatic shutdown, remote isolation, ventilation control, and alarm transmission to the control room.
- Engineer suppression and cooling. Verify that the selected system can cool the actual modules and enclosure, not merely the room air. Analyze sprinkler discharge, water access, duration, drainage, electrical isolation, and the possibility of re-ignition.
- Address explosion and ventilation hazards. Calculate how vent gases could move and accumulate. Specify exhaust paths, gas monitoring, pressure relief, ignition-source control, and explosion-prevention measures appropriate to the enclosure and room. Do not assume normal HVAC will provide safe dilution.
- Plan emergency operations. Give responders drawings, isolation points, gas and electrical hazards, access restrictions, runoff controls, expected re-ignition behavior, and conditions for safe entry. Include remotely accessible monitoring where responders may not be able to approach the equipment.
- Verify construction and commissioning. Inspect clearances, firestopping, sensors, cable penetrations, sprinkler coverage, ventilation controls, alarms, and shutdown interlocks. Commission the system under the approved sequence before placing it in service.
- Maintain the protection system. Track cell and module alarms, temperature trends, capacity changes, cooling performance, detector calibration, suppression valves, ventilation fans, firmware, and incident records. Reassess after a cabinet change, software update, capacity expansion, or room alteration.
Protection measures that work as layers
Detection and early warning
Use battery-management alarms together with smoke, heat, and—where justified by the hazard analysis—off-gas or flammable-gas detection. Alarm thresholds should provide time to isolate equipment and evacuate before concentrations or temperatures become dangerous. Send alarms to a continuously staffed location and make status available remotely to responders.
Compartmentation and separation
Fire-resisting rooms, barriers, rated doors, protected cable penetrations, and physical spacing limit heat and flame transfer. The barrier must account for pressure, vent direction, and maintenance openings, not just nominal wall ratings. Keep BESS cabinets away from critical servers, fuel systems, egress paths, and air intakes unless the fire-engineered design specifically addresses those interfaces.
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Cooling and suppression
Suppression is not a substitute for cooling. Design for the geometry that was tested, including obstructions below sprinklers, cabinet roofs, drip shields, and drainage. Confirm water supply reliability, duration, and post-discharge access. If an alternative agent is proposed, require evidence that it controls the battery’s heat and does not merely extinguish visible flames.
Ventilation and gas control
Ventilation should direct gases away from people and ignition sources while preserving required fire and smoke separations. Gas detection, fan controls, dampers, and pressure relief need a defined failure mode. A design that exhausts gases into a neighboring data hall, intake, or enclosed service corridor can move the hazard instead of removing it.
Inspection and maintenance
Protection degrades when filters clog, fans fail, sensors drift, sprinklers are obstructed, or battery-management alarms are ignored. Establish inspection intervals, functional tests, spare-parts requirements, training, and a process for taking a cabinet out of service after an alarm or impact. UL guidance emphasizes that listed and tested products still depend on correct specification, installation, and upkeep.
How to compare BESS proposals without guessing
Do not rank vendors or chemistries from a marketing label. Compare documented evidence for the exact system and installation assumptions.
| Evidence to request | Questions to ask |
|---|---|
| Cell and module tests | What runaway triggers, heat release, gas composition, and propagation were measured? |
| Installation-level large-scale test | Was the complete cabinet or array tested, and to which UL 9540A edition? |
| Propagation boundary | Did the event stop at a cell, module, cabinet, room, or array? How was that boundary measured? |
| Enclosure and vent behavior | Where do gases and pressure go, and what happens if vents or fans fail? |
| Separation distances | What clearances were used in the test, and are they achievable after construction and maintenance access are included? |
| Suppression performance | Did sprinklers or another system reach the modules, for how long, and under what water supply and obstruction conditions? |
| Detection and gas controls | Which sensors, alarm thresholds, remote monitoring, ventilation rates, and explosion controls were part of the tested design? |
| Listing and installation scope | Does the UL 9540 certification cover the proposed battery, power-conversion equipment, controls, and configuration? |
If a supplier provides only a certificate number or a small-scale test summary, ask for the configuration details and limitations before treating it as evidence for a data-center room or outdoor array.
What the Surprise, Arizona incident tells responders
A 2020 UL Research Institutes account of an energy-storage-system deflagration in Surprise, Arizona highlights responder hazards associated with accumulated gases and delayed ignition. It recommends ESS-specific training, remotely accessible gas monitoring, explosion-prevention protection, and additional full-scale testing research.
This incident is a response and design case study, not evidence of how often BESS fires occur in data centers. Published material does not establish a reliable, directly attributable frequency statistic for data-center BESS fires. General fire or outage statistics should not be substituted for that missing number.
Quick Recap
Common mistakes to avoid
- “UL 9540 listed” means fireproof. A safety listing does not eliminate thermal runaway or prove that the installed room will contain it.
- “UL 9540A tested” means every arrangement is safe. Results apply to the tested configuration, edition, operating conditions, and installation assumptions.
- “The sprinklers activated, so the fire was controlled.” In FM’s original test, sprinklers activated while enclosure limitations reduced cooling and adjacent modules exceeded 500 °F (260 °C).
- “Lithium-ion” is a complete risk description. Cell chemistry, state of charge, packaging, controls, spacing, enclosure, and ventilation materially change behavior.
- “Indoor HVAC is enough for vent gas.” Normal comfort ventilation may not detect, dilute, or safely exhaust flammable gases.
- “A one-time commissioning check is sufficient.” Protection depends on ongoing detector, fan, sprinkler, battery-management, software, and housekeeping maintenance.
- “A code edition is universal.” NFPA 855 and fire-code requirements depend on adoption, amendments, the AHJ, and the project’s applicable date.
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