Containerised battery energy storage systems (BESS) have rapidly expanded in North America as utilities, businesses, and communities integrate batteries to provide grid stability, backup power, renewable energy integration, microgrid support, and hybrid energy systems
This adoption has increased the significance of safety standards such as the National Fire Protection Association Standard (NFPA) 855, as regulators and industry stakeholders seek guidelines for safely installing and operating the rising number of BESSs.
NFPA 855, formally titled “Standard for the Installation of Stationary Energy Storage Systems,” is the main fire-safety and installation standard in the US for BESS and other stationary energy storage technologies. The document specifies minimum safety requirements governing how these systems are designed, installed, operated, and protected in residential, commercial, and utility-scale environments.
The standard was established to address the unique hazards associated with large battery installations, particularly lithium-ion batteries, such as thermal runaway, flammable gas generation, and potential fire propagation between battery modules or containers.
NFPA 855 addresses these hazards with a “layers of protection” approach to hazard mitigation and fire protection. The standard expects systems to incorporate independent protections that collectively reduce the likelihood and severity of failures.
In practice, this means installations include combinations of monitoring, detection, containment, and suppression measures, involving early fire detection, gas detection, ventilation to address buildup of flammable gases, fire-suppression systems, explosion prevention and control, and design features to help prevent or limit thermal runaway propagation between cells or modules.
Containerised BESS
Using standardised container formats has become the dominant architecture in large-scale battery storage because it simplifies manufacturing, transportation, and installation, and enables systems to be scaled by adding additional containers to meet demand.
This 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.
Most utility-scale battery projects do not consist of one container, as many containerised units are deployed together in rows across a site, connected through power conversion systems and transformers to form a larger energy storage plant.
A project could include dozens or 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 thousands of megawatt-hours.
Hydrogen Accumulation
According to Geof Brazier, managing director of BS&B Safety Systems Explosion Protection Division, in the 2026 edition of NFPA 855, 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.
Brazier said, “One of the recognised safety concerns was the buildup of hydrogen and other combustible gases in containerised BESS, because hydrogen is highly flammable and can accumulate to a combustible concentration in enclosed spaces if not properly ventilated or monitored and controlled.”
Hydrogen-rich gas can be generated in certain battery failure modes or abnormal operating conditions. In some battery chemistries, 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 using 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 combination of gases that could include hydrogen, carbon monoxide, methane, and other flammable compounds.
The danger appears 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 wide flammability range and a low minimum ignition energy. In air under typical conditions, it is flammable at concentrations of 4% to 75% by volume; the lower end of this range is known as the lower flammability limit. As hydrogen is lighter than air, it tends to accumulate near the ceiling or the upper portions of a container if ventilation is inadequate; this can 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 lead to a deflagration event.
A deflagration is slower moving than a detonation, which produces a supersonic shock wave of destructive force, but still produces unacceptably high pressures in a confined structure. The expanding combustion gases press outward rapidly at high temperature and pressure, which, if not intentionally relieved, could cause the structure to suffer significant damage, and occupants or nearby individuals may be injured.
When there is around 20% hydrogen in the air, detonation events can generate powerful shock waves that travel faster than the speed of sound.
Brazier said, “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.”
The resulting deflagration or explosion could damage the container and propagate fire-driven overheating to adjacent BESS modules.
Because of these risks, Brazier says modern BESS designs emphasise early detection and layered protection strategies, including 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 releasing combustible hydrogen and other gases before an explosive concentration arises.
Sensors monitor combustible gas concentrations inside the enclosure. When elevated gas levels are detected, an actuator opens the vent flap to discharge the gases safely. Once concentrations have returned to acceptable levels, the actuator closes the flap, restoring normal operating conditions. This automated cycle repeats as needed when elevated gas levels are detected, providing continuous protection for the enclosure.
Brazier said, “An explosion prevention device doesn’t necessarily have to respond to an explosion. In this case, it responds before an explosion would occur to let the hydrogen out before it builds up into a combustible range.”
Containerised BESS are increasingly fitted with explosion vents to control pressure spikes and direct flame and gas when a thermal-runaway event creates a flammable atmosphere to ignite, and when a low concentration of combustible gas results in a deflagration.
Brazier says BS&B 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 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, directing the discharge away while avoiding discharge across egress paths.
The BS&B explosion vent type VSP-A is a breathable construction that enables 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 provides a layer of protection for enclosures exposed to deflagration and overpressure risks.
Brazier said, “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.”
Pressure relief vents of this kind are often combined with gas detection and forced ventilation systems to prevent concentrations from rising above the lower flammable limit.
Brazier said, “Explosion venting is not mandatory [in NFPA 855], but it is one of the permitted methods for achieving explosion control. 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 by evaluating the enclosure’s size and structural capacity, design strength, and the total vent area necessary to maintain internal forces within allowable limits.
Companies like BS&B Safety Systems 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.
Brazier said, “By working methodically through these parameters, the correct design approach can be established with confidence, aligning performance, safety, and compliance objectives.”
