Introduction
According to the Electric Power Research Institute (EPRI) BESS Failure Incident Database, the global rate of battery energy storage system failures dropped a remarkable 98% per gigawatt-hour deployed between 2018 and 2024, a testament to how quickly the industry has matured. Yet beneath that encouraging statistic lies a more sobering reality. A peer-reviewed analysis published in the Journal of Loss Prevention in the Process Industries documented at least 48 fire incidents and 8 confirmed explosion events at lithium-ion BESS facilities worldwide, with delayed deflagrations such as the 2019 Surprise, Arizona event sending four firefighters to the hospital and reshaping the entire regulatory landscape. A separate joint study by EPRI, Pacific Northwest National Laboratory, and TWAICE further found that integration, assembly, and construction errors account for the largest single category of root causes, responsible for 10 of the 26 incidents where investigators could assign blame.
For engineering teams, battery integrators, and EPC contractors, those numbers tell a clear story. Fires at BESS sites are uncommon, but when they do occur, the explosion hazard is real, and the difference between a contained incident and a catastrophic one often comes down to whether a project’s deflagration analysis was rigorous, performance-based, and properly aligned with NFPA 68 and NFPA 69. As fire and life safety consultant Angelo Zandona has explained to numerous clients across the energy storage sector, choosing between deflagration venting and explosion prevention is a layered risk decision that demands transparent analysis, defensible test data, and close coordination with the Authority Having Jurisdiction. This article unpacks what NFPA 68 and NFPA 69 actually require, how they differ in approach, why most modern BESS projects benefit from a hybrid strategy, and how a properly executed deflagration analysis becomes the technical backbone of a defensible permitting package.
The Hazard at the Heart of the Standards
Before comparing the two codes, it helps to understand exactly what they are designed to control. When a lithium-ion cell enters thermal runaway, it vents a complex mixture of flammable gases including hydrogen, methane, ethylene, carbon monoxide, and other hydrocarbons. Inside the confined volume of a BESS enclosure, those gases can accumulate to concentrations within their flammable range. If an ignition source is present at that moment, a deflagration occurs, a subsonic combustion wave that produces a rapid pressure rise capable of rupturing enclosures, launching debris, and seriously injuring anyone nearby.
Research published on explosion protection for containerized BESS describes two distinct deflagration scenarios that drive code selection. Prompt deflagrations ignite within seconds of cell venting and can be addressed with passive venting strategies. Delayed deflagrations involve gas accumulation over minutes or hours before ignition, which produces severe pressure rises that overwhelm passive vent designs unless gas concentrations are actively reduced first. The 2019 Surprise, Arizona event was a delayed deflagration. The 2020 Carnegie Road incident in Liverpool involved a prompt ignition. Both required engineering controls, but the controls needed in each case were fundamentally different. This is the hazard space that NFPA 68 and NFPA 69 are designed to cover, and Angelo Zandona consistently emphasizes in his consulting work that no project team should select between them without first characterizing which scenarios are credible at the specific site.
Protection Through Deflagration Venting
NFPA 68, the Standard on Explosion Protection by Deflagration Venting, takes what fire protection engineers call a passive control approach. Rather than trying to prevent a deflagration from occurring, NFPA 68 accepts that an explosion may happen and provides design methodology for relieving the resulting pressure safely. The standard works by sizing engineered vent panels that fail at a predetermined pressure, opening a path for combustion gases and pressure to escape to a safe location, typically upward and away from occupied areas, before the enclosure ruptures uncontrollably. Critical inputs to the calculation include the fundamental burning velocity of the vented battery off-gases, the volume and geometry of the enclosure, the strength of the enclosure walls and door latches, the burst pressure of the vent itself, and the maximum pressure the enclosure can withstand without structural failure. The appeal of an NFPA 68 approach is straightforward. Once installed and certified, deflagration vents require minimal maintenance, do not depend on power or active sensors to function, and offer a reliable last line of defense even if other safety systems fail. For purpose-built BESS enclosures designed with adequate strength margins and unobstructed venting paths, this approach can be both cost-effective and highly reliable.
The limitations, however, are equally significant. Many deployed BESS units use modified shipping containers that were never engineered to withstand internal pressure, complicating vent sizing. Internal partitions, racking, and dense battery module arrangements create congested geometries that the prescriptive NFPA 68 calculations do not fully capture. And as Jensen Hughes engineers have noted in published research, the applicability of NFPA 68’s prescriptive methodology to modern ESS designs is questionable in some configurations, which is why performance-based analysis using computational fluid dynamics is increasingly used to supplement the prescriptive calculations. Angelo Zandona‘s experience with multiple developers has reinforced a key point here. NFPA 68 venting is necessary in many designs, but it is not always sufficient on its own, particularly for delayed deflagration scenarios where gases have accumulated to unusually high concentrations before ignition.
Protection Through Explosion Prevention
NFPA 69, the Standard on Explosion Prevention Systems, takes a fundamentally different approach. Rather than preparing the enclosure to survive a deflagration, NFPA 69 aims to prevent the deflagration from occurring in the first place by ensuring that flammable gas concentrations never reach ignitable levels. The most common NFPA 69 strategy used in BESS applications is combustible concentration reduction through mechanical exhaust ventilation. The system uses continuous gas detection sensors monitoring the enclosure interior, and when concentrations approach a defined threshold, typically 25 percent of the lower flammability limit, exhaust fans activate to dilute the atmosphere and maintain it below ignitable levels. Other NFPA 69 strategies include inerting with non-flammable gases, oxidant concentration reduction, explosion suppression with chemical agents, and explosion isolation using flame arrestors and isolation valves.
Active NFPA 69 systems offer compelling advantages for BESS installations. They can address delayed deflagration scenarios that passive venting cannot reliably handle. They provide continuous monitoring data that integrates with site supervisory systems and Emergency Response Plans. And they can be tuned to specific battery chemistries and gas release profiles characterized through UL 9540A testing. The trade-offs are real, though. Active systems require power, maintenance, calibration, and periodic testing of sensors and actuators. They depend on engineering assumptions about how a thermal runaway event will unfold, and field incidents have shown that real-world failure progressions can differ meaningfully from those modeled in standardized testing. As one published analysis of explosion control for ESS notes, the design of NFPA 69 mechanical exhaust systems is typically based on limited test data and engineering analysis using assumptions for the progression of the failure event, which leaves room for ambiguity that careful documentation must address.
Why Most Modern Projects Use Both
For engineering teams approaching their first BESS deflagration analysis, the temptation is often to treat NFPA 68 and NFPA 69 as either-or alternatives. The fire codes do permit either standalone, with NFPA 855 calling for explosion control through one or the other. But increasingly, leading consultants and insurers are advocating for a layered, defense-in-depth approach that uses both.
Active NFPA 69 systems handle the delayed deflagration scenario by keeping gas concentrations diluted, addressing the most consequential ignition pathway documented in real-world incidents. Passive NFPA 68 venting then provides backup protection if the active system underperforms, fails entirely, or encounters an event that develops faster than the gas detection threshold can respond to. As the CSA Group has emphasized in recent publications, when used in tandem these standards create a more resilient defense-in-depth strategy that improves outcomes for battery operators, firefighters, inspectors, and surrounding communities.
This is the approach Angelo Zandona has consistently recommended for utility-scale projects, particularly those located near occupied buildings, transmission infrastructure, or areas where AHJ scrutiny is high. The cost differential between a single-strategy and dual-strategy approach is modest in the context of overall project capital, while the risk reduction and permitting benefits are substantial.
What a Comprehensive Deflagration Analysis Contains
A defensible deflagration analysis is far more than a vent-sizing calculation or a fan capacity worksheet. It is an integrated engineering document that ties hazard characterization, code interpretation, equipment specification, and operational considerations into a single coherent narrative.
The analysis begins with a detailed characterization of the battery hazard, drawing on UL 9540A test data for the specific cells and modules in use. This includes the volume of gas released per cell during thermal runaway, the burning velocity and explosivity parameters of that gas mixture, the lower flammability limit, the gas release rate over time, and the propagation behavior between adjacent cells and modules. Without this empirical foundation, every downstream calculation rests on assumption rather than evidence.
The analysis then defines the credible deflagration scenarios for the specific installation, distinguishing between prompt and delayed ignition events and identifying which scenarios drive the design. It evaluates the enclosure geometry, including volume, internal congestion, vent path obstructions, and structural strength, since these parameters determine what venting or ventilation rates are achievable.
For NFPA 68 designs, the document specifies vent area, burst pressure, location, and discharge orientation, with calculations demonstrating that the reduced pressure during a deflagration remains below the enclosure’s structural capacity. For NFPA 69 designs, it specifies sensor placement and threshold settings, exhaust capacity and runtime, power and reliability provisions, and maintenance protocols. For hybrid designs, it documents how the two systems interact and what redundancy each provides.
The analysis closes with operational considerations, including alarm logic that integrates with the Emergency Response Plan, lockout procedures during maintenance, and the boundaries of safe approach for emergency responders, ensuring fire departments understand what they will encounter on the worst day the facility might face.
The Permitting and Insurance Reality
Beyond pure engineering, deflagration analyses serve an essential business function. They are the document that AHJs, insurers, and increasingly community stakeholders use to assess whether a proposed BESS project meets a defensible safety standard. Many local AHJs reviewing a BESS application do not have in-house expertise in deflagration venting equations, gas detection thresholds, or combustion dynamics. As CSA Group has noted, deflagration test reports are highly technical documents filled with references to venting equations, gas volumes, burst pressures, and combustion dynamics, and not every AHJ has the in-house expertise to properly interpret them. A clearly written, transparently sourced analysis dramatically increases the probability of a smooth permitting process. A poorly constructed one can trigger months of back-and-forth, mandatory peer review, or outright rejection.
Insurance underwriters are increasingly requiring deflagration analyses as a condition of coverage, with some carriers mandating third-party verification before binding policies. Higher premiums, coverage exclusions, or refusals can result from inadequate documentation, and these insurance outcomes flow directly into project economics. Angelo Zandona has frequently observed that the most successful BESS projects engage qualified deflagration analysis expertise during preliminary engineering rather than at the permitting submission stage. By that late point, fundamental design decisions have already constrained what protection strategies are feasible, and retrofitting venting infrastructure or active prevention systems into a finalized layout is dramatically more expensive than building it in from the start.
Conclusion
The choice between NFPA 68 deflagration venting and NFPA 69 explosion prevention is one of the most consequential technical decisions a BESS project team will make. Each standard reflects decades of process safety experience, each addresses real and credible hazards, and each carries trade-offs in cost, complexity, and operational implications. For most modern utility-scale installations, the engineering and economic case favors a layered approach that combines passive venting with active prevention, providing defense-in-depth against both prompt and delayed deflagration scenarios.
For developers, EPC contractors, and battery manufacturers entering this space, the guidance Angelo Zandona offers is consistent. Engage qualified deflagration analysis expertise early. Demand UL 9540A test data from cell suppliers. Treat the deflagration analysis as an integrated document with the Hazard Mitigation Analysis, Water Supply Analysis, FMEA, and Emergency Response Plan rather than a standalone calculation. And recognize that the cost of rigorous explosion protection during initial design is a fraction of the cost of a permitting setback, an insurance dispute, or, worst of all, an actual incident with inadequate controls. Done right, a comprehensive deflagration analysis becomes the foundation that allows clean energy projects to scale safely and earn the trust of regulators, communities, and the responders who would arrive on the worst day of the facility’s life.
FAQs
Is a deflagration analysis required for every BESS project?
ANS: NFPA 855 requires explosion control measures for stationary energy storage systems above defined energy capacity thresholds, which captures essentially all commercial and utility-scale lithium-ion installations. The deflagration analysis is the engineering document that specifies how those requirements are met.
What is the practical difference between NFPA 68 and NFPA 69?
ANS: NFPA 68 is reactive and passive, sizing engineered vents that relieve pressure if a deflagration occurs. NFPA 69 is proactive and active, preventing the deflagration from happening by keeping flammable gas concentrations below ignitable levels through detection and mechanical exhaust or other prevention strategies.
How does UL 9540A testing factor into the analysis?
ANS: UL 9540A is the empirical foundation for any credible deflagration analysis. It provides the gas release volumes, burning velocities, lower flammability limits, and propagation behavior that drive both NFPA 68 vent sizing and NFPA 69 ventilation rate calculations. Without UL 9540A data specific to the cells and modules being installed, the analysis rests on generic assumptions that AHJs increasingly will not accept.
Can deflagration protection be retrofitted after construction?
ANS: Technically yes, but at significant cost and disruption. Adding deflagration vents requires structural modifications and re-evaluation of enclosure strength. Retrofitting NFPA 69 ventilation systems requires new gas detection, ducting, and power provisions. Both typically require taking the facility offline during installation, directly affecting revenue, which is why early engagement with deflagration analysis expertise is far more economical than late-stage retrofits.
How does battery chemistry influence the deflagration analysis?
ANS: Different lithium-ion chemistries produce different gas release profiles during thermal runaway, with varying volumes, compositions, and burning velocities. Lithium iron phosphate (LFP) chemistries generally release smaller volumes of less reactive gases than nickel manganese cobalt (NMC) chemistries, which typically translates to less demanding deflagration protection requirements. However, the analysis must always be based on chemistry-specific UL 9540A test data rather than chemistry-class generalizations, since cell design, electrolyte formulation, and module configuration all materially affect the deflagration hazard.
