BESS Fire Safety: Thermal Runaway, NFPA 855 and UL 9540A

Author: Fikret KIR – FK SOLUTION Fire Engineering & Consultancy

Content type: English Knowledge Centre / Technical Article

Battery Energy Storage Systems require a layered fire and explosion safety strategy. A thermal runaway event can generate heat, flammable and toxic gases, cell-to-cell propagation, open flame, deflagration and re-ignition. A single suppression agent cannot be assumed to address all of these mechanisms.

The 2026 edition of NFPA 855 places increased emphasis on fire and large-scale fire testing for representative ESS in specified situations. UL Solutions states that UL 9540A is the test method referenced by NFPA 855 for fire and explosion testing, and that the 6th Edition of UL 9540A was published on March 13, 2026. The 6th Edition expands installation-level large-scale fire testing and evaluates factors including fire-protection effectiveness, heat and gas release, deflagration and re-ignition.

This makes test-data interpretation a critical engineering task. A UL 9540A report should be compared with the exact project configuration: cell chemistry, rack arrangement, enclosure, ventilation, active protection, separation and site geometry.

The BESS Failure Chain

• Cell abnormality
• Off-gas generation
• Thermal runaway
• Module/rack propagation
• Flame and heat release
• Flammable gas accumulation
• Deflagration risk
• Re-ignition after apparent extinguishment

What UL 9540A Contributes

• Cell-level thermal runaway characteristics and gas data
• Module-level propagation behaviour
• Installation-level large-scale fire behaviour
• Fire-protection system effectiveness
• Heat and gas release information
• Deflagration / re-ignition related observations

Layered Protection Strategy

• BMS and electrical protection
• Thermal management
• Early gas/smoke/temperature detection
• Propagation barriers and separation
• Ventilation / gas management
• Suppression and cooling
• Fire-water and fire-service strategy
• Pressure/deflagration management
• Post-fire monitoring and re-ignition management
• Contaminated-water/drainage planning

Frequently Asked Questions

Does UL 9540A certify a BESS installation?

UL 9540A is a fire-propagation test method. Product certification and installation approval are separate concepts.

Is clean-agent suppression enough for a BESS enclosure?

Not necessarily. It may not address thermal runaway heat generation, propagation, flammable-gas hazards or re-ignition on its own.

Can water be used?

Water can be valuable for cooling and limiting propagation in many scenarios, but the strategy should be based on the product, enclosure, test data and project requirements.

Why is large-scale testing important?

It provides more representative evidence of installation-level fire behaviour, exposure, separation and active fire-protection performance.