Thermal runaway protection, suppression & fire acceptance
Published on September 20, 2026
When sourcing a battery energy storage system, fire compliance often decides whether a project gets approved and commissioned at all. This guide explains what thermal runaway is, which layers of protection stop an incident from spreading, how to choose a suppression agent, and which standards apply.
1. Thermal runaway: the core safety issue
Thermal runaway is a chain reaction inside a cell — triggered by internal short circuit, overcharge, mechanical damage or excessive heat — where temperature rises rapidly and can ignite neighbouring cells. Its characteristics:
Fast propagation: a single cell can spread to a whole module within minutes.
Very high temperature: internal temperatures reach several hundred degrees, beyond what ordinary extinguishing can cool quickly.
Re-ignition risk: even after flames are out, internal reactions may continue.
So the goal is not "put out one fire" — it is detect early, isolate early, and keep cooling.
2. The three layers of protection
Layer 1 — cell and pack level
Stable chemistry: LiFePO4 (LFP) is thermally more stable than NMC and is the mainstream choice for storage.
BMS: monitors voltage, current and temperature; alarms or disconnects on over-voltage, over-current and over-temperature.
Cell balancing: reduces the risk of individual cells being over-charged or over-discharged.
Layer 2 — enclosure level detection and suppression
Combustible gas detection: detects gases released early — before any flame — giving a much earlier warning than smoke detectors.
Heat and smoke detection: supplementary means.
Automatic suppression: releases the agent automatically once abnormal conditions are detected.
Exhaust and explosion relief: removes flammable gases and prevents enclosure rupture.
Layer 3 — system level fire protection
Fire separation: required spacing between containers and between the storage area and other buildings.
Alarm and interlocking: connect to the site fire system for audible/visual alarm and power shutdown.
Fire department water connection: an external inlet allowing prolonged cooling — the final line of defence.
3. Choosing a suppression agent
Novec 1230 (FK-5-1-12): non-conductive, leaves no residue, high efficiency — the mainstream choice for in-cabinet automatic suppression.
Aerosol: low cost and compact, suitable where space is limited, but leaves a small amount of residue.
Water mist: excellent cooling and re-ignition control, but needs a water supply and piping — usually for large containers or fixed installations.
FM-200 (HFC-227ea): a proven gaseous agent, though it can decompose at high temperature.
Practical recommendation: automatic in-cabinet suppression (Novec 1230 or aerosol) combined with an external fire-service water connection is the most common configuration for C&I and container projects.
4. Main international standards
NFPA 855 (US): installation standard for stationary ESS — widely referenced for separation distances, ventilation and suppression.
UL 9540 / UL 9540A: system safety certification and thermal runaway propagation test method.
IEC 62933 series: international framework for safety and environmental requirements of ESS.
Local fire codes: many countries impose additional mandatory requirements — confirm before design and acceptance.
5. Fire acceptance essentials
Separation distances and fire access routes comply with local code.
Gas detection, alarm, suppression, exhaust and relief interlocks tested.
Suppression units within validity period and correct pressure.
Fire-service water connection and clear warning signage provided.
Complete fire design documentation and product certification reports available.
6. How to judge a supplier's fire provisions
Cells: Grade-A LiFePO4 with full BMS protection.
Detection: combustible gas detection present (smoke detection alone is not enough).
Suppression: agent type, coverage and zoning per enclosure.
Certification: third-party UL 9540A thermal runaway test report available.
Documentation: supplier can provide fire design notes and support acceptance.
Need fire design documentation and certificates? We provide product fire design notes, UL/IEC test reports and support for project fire acceptance.