Explosion Prevention and Protection for BESS Enclosures
Containerized battery energy storage systems (BESS) have expanded rapidly in North America as utilities, businesses, and communities integrate batteries for grid stability, backup power, renewable energy integration, microgrid support, and hybrid energy systems.
This widespread adoption is one reason safety standards such as NFPA 855 have become more significant, as regulators and industry stakeholders seek consistent guidelines for safely installing and operating the expanding number of battery energy storage systems.
The National Fire Protection Association Standard 855, formally titled “Standard for the Installation of Stationary Energy Storage Systems,” is the primary fire-safety and installation standard in the United States for BESS and other stationary energy storage technologies. The document establishes minimum safety requirements governing how these systems must be designed, installed, operated, and protected in residential, commercial, and utility-scale settings.
The standard exists because large battery installations introduce unique hazards, particularly with lithium-ion batteries, including thermal runaway, flammable gas generation, and potential fire propagation between battery modules or containers.
NFPA 855 addresses these hazards by requiring a “layers of protection” approach to hazard mitigation and fire protection. Rather than relying on a single safeguard, the standard expects systems to incorporate several independent protections that collectively reduce the likelihood and severity of failures.
In practice, this means installations typically include combinations of monitoring, detection, containment, and suppression measures. These can involve early fire detection, gas detection, ventilation to address buildup of flammable gases, fire-suppression systems, explosion prevention and control, and design features intended to prevent or limit thermal runaway propagation between battery cells or modules.
Containerized BESS
The use of standardized container formats has become the dominant architecture in large scale battery storage because it simplifies manufacturing, transportation, and installation. It also allows systems to be scaled easily by adding additional containers to meet the desired energy demand.
This approach allows manufacturers to package batteries, cooling systems, power electronics, and safety equipment into a modular unit that can be transported by truck, rail, or ship and installed quickly at a site.
However, most utility-scale battery projects do not consist of just one container. Instead, many containerized units are deployed together in rows across a site, connected through power conversion systems and transformers to form a much larger energy storage plant.
A single project might include dozens or even hundreds of containers arranged in arrays. The combined capacity of these installations can range from tens of megawatt-hours for small grid support projects to hundreds or even thousands of megawatt-hours at the largest facilities.
Hydrogen Accumulation
According to Geof Brazier, Managing Director of BS&B Safety Systems Explosion Protection Division, in the most recent 2026 edition of NFPA 855, several new requirements were introduced related to battery system hazards and protection strategies, including expanded hazard-mitigation analysis, additional testing expectations, and stronger provisions related to fire and explosion risk management in large battery installations.
“One of the recognized safety concerns was the buildup of hydrogen and other combustible gases in containerized BESS, because hydrogen is highly flammable and can accumulate to a combustible concentration in enclosed spaces if not properly ventilated or monitored and controlled,” says Brazier.
Hydrogen rich gas can be generated during certain battery failure modes or abnormal operating conditions. In some battery chemistries, even traditional lead-acid batteries, hydrogen is produced as a normal byproduct during charging through electrolysis of water in the electrolyte and typically in small, easily ventilated quantities.
In BESS installations that use lithium-ion batteries, hydrogen and other combustible gases can be generated during thermal runaway or internal battery damage. When lithium-ion cells overheat or fail, chemical decomposition of the electrolyte and other cell components can produce a mixture of gases that may include hydrogen, carbon monoxide, methane, and other flammable compounds.
The danger arises when hydrogen or other flammable gases accumulate in an enclosed space and are then ignited by electrical equipment, static discharge, or other ignition sources.
Hydrogen has a very wide flammability range and a low minimum ignition energy. In air under typical conditions, it is flammable at concentrations of approximately 4% to 75% by volume; the lower end of this range, about 4%, is known as the lower flammability limit. Because it is lighter than air, hydrogen tends to accumulate near the ceiling or the upper portions of a container if ventilation is inadequate. This can further increase hydrogen concentrations in those upper areas.
In the lower ranges when hydrogen comprises less than 20% of the mixture in air by volume, an ignition can cause a deflagration event.
Unlike a detonation, which produces a supersonic shock wave of great destructive force, a deflagration is slower moving but still produces unacceptably high pressures in a confined structure. The expanding combustion gases press outward rapidly at high temperature and pressure and, if not intentionally relieved, the structure can suffer significant damage, and occupants or nearby individuals may be seriously injured.
When the percentage of hydrogen in the air is around 20%, detonation events can generate powerful shock waves that travel faster than the speed of sound.
“When you get into the higher percentages, you are dealing with explosions that can transition to an unprotectable detonation, so it is important to do the utmost to reduce the level of hydrogen accumulation in the container so the conditions for an explosion do not arise,” says Brazier.
The resulting deflagration or explosion may not only damage the container but may propagate fire driven overheating to adjacent BESS modules.
Because of these risks, Brazier says modern BESS designs emphasize early detection and layered protection strategies. These include monitoring battery temperature and voltage to detect failures early, detecting flammable gases before they reach hazardous concentrations, and providing controlled ventilation or explosion relief to prevent pressure buildup.
BS&B Safety Systems’ VSP Actuated Ventilation System is an NFPA 69 explosion prevention device designed to protect BESS enclosures by actively releasing combustible hydrogen and other accumulated gases before an explosive concentration arises.
Sensors continuously monitor combustible gas concentrations inside the enclosure. When elevated gas levels are detected, an actuator opens the vent flap to safely discharge the gases. Once concentrations return to acceptable levels, the actuator closes the flap, and normal operating conditions are restored. This automated cycle repeats as needed whenever elevated gas levels are detected, providing continuous protection for the enclosure.
“An explosion prevention device doesn’t necessarily have to respond to an explosion,” explains Brazier. “In this case, it responds before an explosion would occur to let the hydrogen out before it builds up into a combustible range.”
Containerized BESS are also increasingly fitted with explosion vents to control the pressure spikes and direct flame and gas when a thermal-runaway event causes a flammable atmosphere to ignite and a low concentration of combustible gas results in a deflagration.
Brazier says BS&B specifically designed its BESS-Saf™ as a family of explosion and pressure relief vents with BESS enclosure dynamics in mind. The vents support controlled pressure relief to help mitigate explosion risk resulting from thermal runaway and gas generation.
The low-burst-pressure explosion vent panels can be mounted on the container roof or upper exterior walls. In the event of a deflagration or explosion, the panel opens and vents to the open atmosphere, directing the discharge away with attention to avoidance of discharge across egress paths being essential.
The BS&B explosion vent type VSP-A is a breathable construction that permits combustible gases to pass through the device under normal operating conditions while providing a barrier from rain, snow and other climatic influences.
Flame-Free versions incorporate a flame arrester rated for hydrogen and other gas deflagration conditions with an explosion vent. This combination provides a reliable layer of protection for enclosures exposed to deflagration and overpressure risks.
“If hydrogen or other gases accumulate and a deflagration arises, the explosion vent opens to relieve overpressure while the integrated flame arrester quenches the flame front to mitigate the release of flame to the atmosphere,” says Brazier.
Pressure relief vents of this kind are often combined with gas detection and forced ventilation systems to keep concentrations below the lower flammable limit.
“Explosion venting is not mandatory [in NFPA 855], but it is one of the permitted methods for achieving explosion control,” explains Brazier. “Because venting is often a comparatively economical solution, it receives significant attention and is frequently viewed as the preferred cost-effective approach.”
According to Brazier, vent selection is determined through an evaluation of the enclosure’s size and structural capacity, the design strength, and the total vent area necessary to maintain internal forces within allowable limits.
Companies like BS&B Safety Systems are able to provide technical guidance throughout the specification process to help identify the appropriate explosion vent configurations, and materials to support an effective venting strategy aligned with applicable codes and standards.
“By working methodically through these parameters, the correct design approach can be established with confidence, aligning performance, safety, and compliance objectives,” says Brazier.
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