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Battery energy storage systems (BESS) used for data-center backup can create a chain of hazards: a cell or module may enter thermal runaway, release heat and flammable gases, ignite those gases, and transfer enough heat to neighboring equipment to extend the fire. An enclosure can also restrict sprinkler access, while accumulated vent gas can produce a deflagration or explosion. The actual risk depends on the cells, rack and enclosure design, cooling, ventilation, controls, room layout, fire protection, installation quality, and maintenance of the specific system.
Evidence from a 2026 FM evaluation of a Delta UPS battery system for Equinix shows why a product certificate alone is not a complete fire strategy. In the original configuration, runaway propagated to adjacent modules and produced an approximately 700 kW peak heat release rate. After design changes intended to reduce heat transfer and improve sprinkler cooling, a retest confined runaway to the initiating module. Those results describe the tested configuration and conditions; they are not a failure rate or a universal prediction for every lithium-ion installation.
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What makes a data-center BESS fire different
A backup battery installation combines a large amount of stored chemical energy with tightly packed equipment, continuous electrical availability, occupied or valuable buildings, and a requirement to keep critical loads operating. Lithium-ion batteries can therefore be both the ignition source and the fuel. FM’s technical report summarizes the consequence: “LIBs can act as both ignition and fuel sources, and fires involving them may lead to significant property damage and complicate firefighting efforts.”
Data-center operators also face a difficult choice during an event. Keeping a battery-backed load energized can sustain an electrical hazard, but abruptly disconnecting equipment can affect cooling, networking, or controlled shutdown. Emergency procedures must be written for the installed system, coordinated with responders, and exercised before an incident.
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No reliable, directly attributable statistic establishes how often BESS fires occur in data centers. An incident account or a laboratory test can explain mechanisms and controls, but it cannot be converted into a probability for every site.
How the fire and explosion sequence develops
1. A cell enters thermal runaway
Thermal runaway is a self-heating failure in which a cell generates heat faster than it can shed it. Triggers can include internal defects, mechanical damage, overcharge, an external short circuit, high temperature, or abuse during installation or operation. Once the cell’s internal reactions accelerate, temperatures rise rapidly and the cell can vent.
Battery-management controls can detect abnormal voltage, current, or temperature and may isolate a circuit. They cannot guarantee that a damaged cell will not run away, so detection and isolation are layers of protection rather than proof that an event is impossible.
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2. Heat and vent gas leave the cell
A failing cell can release hot gases, smoke, electrolyte vapor, and ejecta. The composition and quantity vary with cell chemistry, format, state of charge, and the failure mode. Vent gas can be hot enough to ignite when it meets an arc, a hot surface, a relay, or another ignition source. If gas remains below its flammable range initially, continued release can bring a confined enclosure or room into the flammable range later.
3. Ignition creates a fire or deflagration hazard
Ignited vent gas can produce a jet flame or a rapidly burning cloud. In a room or enclosure, pressure can rise faster than normal ventilation can remove the gas. The result may be a deflagration that damages doors, panels, cable routes, or nearby equipment. The July 2020 UL Research Institutes account of a near-miss ESS explosion in Surprise, Arizona, emphasizes responder hazards and the need for ESS-specific training, remotely accessible gas monitoring, explosion-prevention protection, and full-scale testing research. That incident is a response case, not evidence of how frequently data-center BESS events occur.
4. Heat transfers to neighboring modules
Radiant heat, hot gases, direct contact, and conductive paths through racks or busbars can raise adjacent cells to their own runaway threshold. Closely packed modules and metal enclosures can make this transfer faster. Once several modules are involved, fire load, smoke production, electrical isolation, and cooling demands increase sharply.
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5. Fire protection may not cool the source
Sprinklers can control surrounding combustibles, but water may have limited access to the hottest cells inside a sealed cabinet. In FM’s original test, sprinklers activated, yet the enclosure restricted water access and cooling effectiveness. Adjacent-module surface temperatures exceeded 500 °F (260 °C), and runaway propagated to neighboring modules and units.
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FM evaluated a Delta UPS battery system used by Equinix. In the original configuration, thermal runaway in one module propagated to adjacent modules and units. The measured peak heat release rate was approximately 700 kW. FM also reported that the tested system failed its large-scale fire test despite prior UL 9540A compliance.
The system was then changed to reduce heat transfer and improve sprinkler cooling. In the retest, thermal runaway remained confined to the initiating module, and neighboring modules stayed below 212 °F (100 °C). This is valuable evidence that enclosure geometry, module spacing, cooling paths, and sprinkler access can materially change the outcome.
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It is not a guarantee for all redesigned systems. A result belongs to the exact cells, modules, cabinet, controls, suppression arrangement, state of charge, room, and test conditions evaluated. A different installation can have different propagation boundaries or vent behavior even when it uses the same broad battery chemistry.
UL 9540, UL 9540A, and NFPA 855: different jobs
| Document | Primary role | What it tells a project team |
|---|---|---|
| UL 9540 | Equipment and energy-storage-system safety standard | Whether the evaluated system and its equipment meet the standard’s product-safety requirements within the stated scope. |
| UL 9540A | Thermal-runaway fire-propagation test method | How the tested cell, module, unit, and installation respond to fire and thermal runaway, including propagation and fire behavior at the tested scale. |
| NFPA 855 | Installation standard for stationary energy storage systems | Installation provisions addressing items such as detection, suppression, explosion control, exhaust ventilation, gas detection, separation, and thermal-runaway protection. |
UL describes UL 9540 as the foundational product-safety standard for energy-storage systems and equipment, while UL 9540A evaluates thermal-runaway fire behavior. Passing one does not make the other unnecessary. The FM result illustrates the distinction: a system with prior UL 9540A compliance still failed FM’s particular large-scale test in its original configuration.
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Edition 6 of UL 9540A
UL reports that the sixth edition of UL 9540A was published on March 13, 2026, with an effective date of January 1, 2027. Its installation-level large-scale fire test considers developed fire conditions, vent-gas ignition, separation distance, enclosure performance, and, for indoor systems, the performance of the building’s fire-suppression system. A specification issued before the effective date may reference an earlier edition, so the contract, certification documents, and local code should be checked together.
UL’s FAQ states that the 2026 NFPA 855 and the 2024 International Fire Code require UL 9540A testing in specified circumstances. “Specified circumstances” matters: the requirement depends on system type, location, adopted code text, and the authority having jurisdiction.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Build a site-specific fire strategy
Engage a competent local fire engineer and battery-system specialists while the design is still changeable. The team should review the manufacturer’s test and certification scope, then map it to the proposed room, cabinet arrangement, electrical topology, ventilation, and emergency procedures.
Configuration and siting
- Record cell chemistry, module format, rack density, state-of-charge limits, cabinet construction, clearances, and whether the system is indoors, outdoors, or in a detached building.
- Confirm fire-resisting walls, floors, doors, cable penetrations, busway routes, and separation from generators, fuel, transformers, offices, and critical IT spaces.
- Check that the tested enclosure, spacing, ventilation assumptions, and suppression arrangement match the installation drawings. A certificate for a different cabinet or room does not establish equivalence.
Detection and controls
- Use layered detection: battery-management alarms, temperature and smoke detection, and gas detection where the hazard assessment calls for it.
- Make alarms remotely visible to the operations center and responders. Define thresholds, automatic isolation actions, escalation, and a controlled shutdown sequence.
- Keep monitoring available when the room is inaccessible or power is compromised, and document how alarms are acknowledged and investigated.
Suppression and cooling
- Verify sprinkler or other suppression design against the cabinet’s obstruction, discharge pattern, water demand, drainage, and expected cooling access.
- Ask test laboratories or the equipment supplier to identify propagation limits and neighboring-module temperatures under the tested conditions.
- Do not assume that extinguishing visible flames has cooled damaged cells; re-ignition and extended observation may be necessary.
Ventilation, gas control, and explosion protection
- Determine how vent gas is detected, diluted, exhausted, and prevented from accumulating in high or enclosed spaces.
- Evaluate ignition-source control, pressure relief, explosion-prevention equipment, equipment shutdown, and safe responder access as one system.
- Coordinate ventilation operation with fire detection and suppression. A fan sequence that spreads smoke or supplies oxygen can worsen an event.
Response, inspection, and maintenance
- Give the fire department current plans showing battery rooms, disconnects, access routes, ventilation controls, gas-monitoring points, and hazards that remain energized.
- Provide ESS-focused training and drills; the UL Research Institutes incident account specifically highlights the need for responder training and remotely accessible gas monitoring.
- Inspect cells, racks, cooling, detectors, suppression equipment, seals, doors, penetrations, firmware, and alarm communications on a documented schedule.
- Quarantine damaged, overheated, flooded, or impact-affected modules under the manufacturer’s and fire engineer’s procedures. Do not return them to service solely because an alarm cleared.
How to compare BESS designs or proposals
Use documented, installation-level evidence rather than ranking vendors or chemistries by reputation. Request the following for every candidate:
- Cell and module test reports, including the initiating failure and state of charge.
- UL 9540 certification scope and the exact equipment covered.
- UL 9540A edition, test scale, installation configuration, propagation boundary, vent behavior, and separation assumptions.
- Large-scale fire-test data, including heat release, neighboring-module temperatures, duration, re-ignition observations, and suppression performance.
- Cabinet construction, cooling paths, pressure relief, gas detection, ventilation rates, and explosion-control assumptions.
- Required clearances, room construction, drainage, emergency isolation, inspection intervals, and maintenance responsibilities.
Reject comparisons that present a cell-level result as proof of installation safety, omit the tested enclosure, or quote a test outcome without its edition and conditions. The relevant question is not “Which chemistry is safest?” in isolation; it is “What happens in this complete installation when one module fails, and how does the design keep that event from becoming a room fire or explosion?”
Common mistakes that increase risk
- Equating certification with immunity: UL 9540 certification and UL 9540A testing address defined scopes; neither removes the need for correct installation and upkeep.
- Copying a test layout: Moving cabinets closer, changing ceiling height, adding cable trays, or substituting suppression equipment can invalidate the assumptions behind a result.
- Ignoring gas after flame knockdown: A quiet cabinet can still contain hot cells or a flammable atmosphere. Follow the system’s re-entry and ventilation procedure.
- Designing only for firefighters: Remote isolation, readable status information, and safe access protect operators before responders arrive.
- Using the wrong code edition: Confirm the adopted NFPA 855, International Fire Code, electrical code, and local amendments with the authority having jurisdiction.
- Treating maintenance as optional: Failed sensors, blocked vents, degraded cooling, or uncommunicated firmware changes can remove the layers the design depends on.
Questions to ask before energizing a data-center BESS
- What exact cell, module, rack, cabinet, and room configuration was tested?
- Which UL 9540 and UL 9540A editions apply, and do the certificates cover the equipment being installed?
- What happened to neighboring modules in the large-scale test, and what were their peak temperatures?
- How are vent gas, smoke, heat, and pressure detected and controlled?
- Can operators isolate the system remotely, and what loads or safety systems remain energized?
- Have the fire department, insurer, facilities team, and authority having jurisdiction reviewed the same drawings and emergency plan?
- Who owns inspection, alarm testing, software updates, module replacement, and post-event decisions?
Bottom line
A data-center BESS can turn a single cell failure into a fire or explosion hazard through thermal runaway, flammable vent gas, ignition, heat transfer, and propagation. The FM UPS evaluation shows both sides of the problem: an original configuration propagated runaway despite sprinkler activation and prior UL 9540A compliance, while changes to heat transfer and sprinkler cooling confined the event in a retest. Treat that result as configuration-specific evidence. A defensible project combines the correct NFPA 855 edition and local adoption requirements with installation-level UL 9540A evidence, engineered detection, suppression, ventilation and gas control, compartmentation, responder planning, and disciplined maintenance.
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