Today we cover another branch of safety of Battery Energy Storage Systems (BESS), that is explosion prevention in mitigation. I always thought you can either end with a fire or with an explosion, and boy I was wrong... but we will go back to this later. Now I bring on Dr. Lorenz Boeck (REMBE) and Nick Bartlett (Atar Fire) to unpack how gas released during thermal runaway turns a container into a deflagration hazard, and what it takes to design systems that actually manage the pressure, flame, and fallout. This is a tour through real incident learnings, rigorous lab data, and the evolving standards that now shape best practice.

We start with the fundamentals: from the overview given by NFPA855, why modern BESS enclosures—with higher energy density and less free volume—see faster pressure rise, how gas composition varies by cell and manufacturer, and why stratification matters when lighter hydrogen-rich mixtures sit above heavier electrolyte vapors. From there, we translate UL 9540A outputs—gas quantity, composition, flammability limits, burning velocity—into engineering decisions. NFPA 69’s prevention path typically relies on gas detection and mechanical ventilation designed to keep concentrations below 25% LFL, validated with CFD to capture obstructions, sensor placement, fan ramp, and louver timing. NFPA 68’s mitigation path kicks in if ignition happens, with certified vent panels sized to the actual reactivity and geometry, relieving pressure and directing flame away from exposures.

A major takeaway: the latest NFPA 855 now often pushes for both prevention and protection. Even with active ventilation, partial-volume deflagration hazards remain, especially as cell capacities rise and gas volumes scale up. We dig into venting trade-offs—roof vs sidewall, snow and hail loading, heat flux to back-to-back units—and how targeted sidewall venting can deflect flame upward while reducing weather vulnerabilities. Perhaps most critical, we talk about late deflagrations observed hours into large-scale fire tests, when changing ventilation conditions allow pockets to ignite. Active systems aren’t built to operate throughout a long fire, so passive venting becomes essential during and after ignition.

Whether you’re a fire engineer, AHJ, insurer, or developer, this conversation connects the dots between lab data, CFD, and field realities. You’ll leave with a clearer view of how to apply UL 9540A, NFPA 68, NFPA 69, and NFPA 855 in a world of stacked containers and supersized cells—plus where training can shorten your learning curve. 

If you are interested by the course given by colleagues in Lund in January 2026 - here it is: https://www.atarfire.com/event-details/nfpa-855-8-hour-training-lund-university

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