BESS safety: thermal runaway, codes, and the 2026 standards reset
NFPA 855, UL 9540A, and IEC 62933 standards have all received material updates in 2025–2026 reflecting lessons from utility-scale BESS thermal runaway incidents. New requirements include mandatory unit-level cell-monitoring, exclusion zones, and post-incident dispatch protocols. Compliance is now a baseline expectation, not a competitive differentiator.
In 50 words: NFPA 855, UL 9540A, and IEC 62933 received material updates in 2025–2026 reflecting lessons from utility-scale BESS thermal runaway incidents. New requirements include unit-level cell monitoring, exclusion zones, and post-incident protocols. Standards compliance is now baseline expectation, not a differentiator — buyers need it explicit in procurement contracts.
What changed
The three primary BESS safety standards underwent material revisions in 2025–2026:
- NFPA 855 (2026 edition) — mandatory exclusion zones, cell-level monitoring, post-incident dispatch protocols
- UL 9540A (5th edition) — expanded large-scale fire testing scope, propagation testing to neighbouring units
- IEC 62933 (Part 5-2 update) — alignment with NFPA 855 on safety architecture
Why it changed
The standards updates reflect lessons from several utility-scale BESS thermal incidents in 2023–2025:
- Moss Landing thermal runaway events (multiple, 2021–2024)
- Several Australian large-scale BESS incidents
- Korean ESS incident series triggering KEMS standard
The pattern in nearly all incidents: failure of a single cell or module led to thermal propagation due to insufficient compartmentalisation, monitoring gaps, or first-responder protocol gaps.
Practical impact for developers
For new utility-scale BESS projects commissioning in 2026 onwards, baseline expectations are now:
- Unit-level cell monitoring — every cell, voltage and temperature
- Compartmentalisation — physical separation preventing propagation between cabinets
- Exclusion zones — published distances to nearest buildings, infrastructure
- Emergency response plans — pre-coordinated with local fire departments
- First-responder training — supplier-provided to local fire authorities pre-commissioning
- Post-incident dispatch protocol — defined cooldown period, when to re-energise, when to replace
Impact on procurement
For BESS buyers:
- Explicitly require NFPA 855 / UL 9540A compliance in procurement RFPs
- Require post-incident demonstration of compartmentalisation (manufacturer evidence)
- Verify first-responder training included in supplier scope
- Lock cell monitoring and BMS firmware update protocols in contracts
What to watch next
The next standards cycle is expected to address grid-forming BESS specifically. Grid-forming systems' control complexity introduces new failure modes that the current standards don't explicitly cover. Watch IEC working group output in H2 2026.
Researched and drafted with AI assistance; reviewed and edited by the named editor within 24 hours of draft.