Suppression, separation and design limits

Detection and ventilation aim to catch a failure early and stop gas from accumulating. Suppression, separation and limits on stored energy deal with what happens if a failure becomes a fire. This lesson explains what fire suppression systems can and cannot do against battery fires, how separation and fire-rated construction contain a fire, how codes limit the energy in a space, and why these design limits matter to the people who operate and maintain battery rooms. HSE-323, Fire Suppression and Liquid Cooling Hazards, covers the suppression systems themselves, including clean agent discharge hazards and pre-action sprinklers; this lesson focuses on their role and limits for batteries.

What suppression can and cannot do

Fire suppression for batteries has two different goals that are easy to confuse. The first is to extinguish or control burning materials: plastics, cables, packaging and the gases and electrolyte released by cells. The second is to stop thermal runaway from propagating to more cells. Most suppression systems are good at the first goal and poor at the second.

Clean agent systems, which flood a room with a gaseous agent such as a halocarbon or an inert gas, are common in data centers because they leave no residue and do not damage electronics. They can extinguish flames from burning vent gas and surrounding materials. They do not remove heat from cells in runaway, and they do not prevent cells from venting more flammable gas. Testing and incident investigations, including the McMicken analysis, have shown that a clean agent can put out visible flames while thermal runaway continues to spread inside the modules, and that the unburned gas can then accumulate to an explosive concentration. Extinguishing the flames without stopping the gas can make the explosion hazard worse. The agent concentration also decays over time as it leaks from the room, while runaway can continue for hours.

Water is the most effective agent for cooling cells and slowing propagation, because of its high heat capacity and heat of vaporization. Sprinklers, water mist and deluge systems can cool exposed modules and surrounding materials and prevent a fire from spreading beyond the battery. Water has limitations too: it reaches cells inside steel modules and cabinets poorly, it can cause electrical shorts in batteries and other equipment, large volumes may be needed for long periods, and the run-off can be contaminated. Despite these drawbacks, NFPA 855 and fire codes generally require automatic sprinkler protection or an approved alternative for battery rooms, and fire services typically use water to cool battery fires.

Because no suppression system reliably stops thermal runaway in sealed modules, the design relies heavily on preventing propagation within the battery (Lesson 7), on detection and isolation (Lesson 8) and on separation and fire-rated construction to contain whatever fire does develop. The 2022 Pangyo investigation recommended exactly these measures: suppression suitable for lithium-ion fires and separation of batteries from other electrical equipment.

Separation and fire-rated construction

Separation keeps a battery fire from spreading to other batteries, to other equipment and to the rest of the building. It works at several scales.

• Within a room, groups of battery racks or cabinets are separated by spacing or by barriers so that a fire in one group does not spread to the next. Fire codes and NFPA 855 have historically limited the energy in each group and required minimum spacing between groups, unless large-scale fire testing shows that closer spacing is acceptable.
• At the room level, battery rooms are separated from the rest of the building by fire-rated walls, floors and ceilings, typically with fire-rated doors and with penetrations for cables and ducts fire-stopped to maintain the rating.
• At the building level, the location of battery rooms matters: on an exterior wall for venting and fire service access, away from exits and from critical spaces such as other halls, network rooms or the opposite side of a 2N power system.
• Outdoors, BESS containers are separated from buildings, property lines, each other and exits by distances set by the code or justified by testing.

For a data center, separation has a reliability purpose as well as a safety one. A battery fire that spreads to the UPS modules, switchgear or opposite-side batteries can take down both power paths, as the Pangyo and Daejeon fires did to the services they supported. Locating A-side and B-side batteries in separate fire compartments is a basic resilience measure.

Limits on stored energy

Fire codes and NFPA 855 limit how much energy can be stored in a single fire area or room, and they set threshold quantities below which the energy storage provisions do not apply. Recent editions of the International Fire Code, for example, set a maximum stored energy for lithium-ion systems in a fire area (600 kWh in the 2021 and 2024 editions) and permit larger quantities when large-scale fire testing and the fire code official approve them. NFPA 855 has similar provisions. Limits are expressed in kilowatt-hours and differ by chemistry and by occupancy.

These limits matter to operations as well as to design. Adding cabinets to an existing battery room to increase runtime, replacing lead-acid batteries with lithium-ion in the same room, or storing spare modules in the room can push the stored energy past the limit or change the hazard the room was designed for. Each of these is a change that needs review under management of change, including the fire protection engineer and, where required, the authority having jurisdiction.

The program below is a simple check of a proposed battery room layout against illustrative limits for energy per group, spacing between groups and total energy in the room. The values are for teaching only and do not represent any particular code edition; real compliance is determined by the adopted code, NFPA 855, the test data and the authority having jurisdiction.

# Teaching check of a battery room layout against ILLUSTRATIVE limits.
# Real limits come from the adopted fire code, NFPA 855, test data and the AHJ.
GROUP_LIMIT_KWH = 50       # illustrative energy per group without LSFT approval
MIN_SPACING_M = 0.9        # illustrative spacing between groups
ROOM_LIMIT_KWH = 600       # illustrative maximum energy in the fire area

groups = [  # (name, kWh, spacing to next group in m)
    ("G1", 48, 1.0), ("G2", 48, 1.0), ("G3", 52, 0.6), ("G4", 48, None),
]
total = 0
for name, kwh, gap in groups:
    total += kwh
    issues = []
    if kwh > GROUP_LIMIT_KWH:
        issues.append(f"group energy {kwh} kWh exceeds {GROUP_LIMIT_KWH} kWh")
    if gap is not None and gap < MIN_SPACING_M:
        issues.append(f"spacing {gap} m below {MIN_SPACING_M} m")
    print(f"{name}: {kwh} kWh " + ("OK" if not issues else "REVIEW: " + "; ".join(issues)))
print(f"Total {total} kWh: " + ("within" if total <= ROOM_LIMIT_KWH else "over") + " room limit")
spare_modules = 4 * 5.1
print(f"Storing {spare_modules:.1f} kWh of spare modules in the room raises the total to {total + spare_modules:.1f} kWh")

G1: 48 kWh OK
G2: 48 kWh OK
G3: 52 kWh REVIEW: group energy 52 kWh exceeds 50 kWh; spacing 0.6 m below 0.9 m
G4: 48 kWh OK
Total 196 kWh: within room limit
Storing 20.4 kWh of spare modules in the room raises the total to 216.4 kWh

The check flags group G3, which is slightly above the illustrative group limit and too close to its neighbor, and it shows how storing spare modules in the room adds to the total. In practice, a review would ask whether large-scale fire test results support the proposed arrangement and would keep spares in a separate, appropriate storage area.

Large-scale fire testing and closer spacing

Large-scale fire testing, now explicitly required by NFPA 855 (2026) and included in the sixth edition of UL 9540A, tests a complete battery unit by forcing an internal failure and observing whether fire spreads to adjacent units placed at the intended spacing. If testing shows that fire does not spread, the authority having jurisdiction may approve closer spacing or larger groups than the prescriptive limits. For data centers, where floor space is valuable, this is an important path. It also means that the as-installed arrangement must match the tested arrangement: changing cabinet spacing, adding units, removing barriers or mixing products can invalidate the basis of approval.

Limits on location

Codes also restrict where battery rooms may be located: limits on height above grade and depth below grade, because fire service access and smoke removal are harder there, requirements for exterior wall access for some systems, and restrictions near exits. The Pangyo fire started in a battery room on the third basement level, where firefighting access was difficult and the fire affected power equipment in the same room. Battery rooms in basements or high in a building may be allowed with additional protection, but the location affects how the fire service can respond.

Design limits and the people who work there

Design limits are written into drawings and fire protection reports, but they are kept or broken by day-to-day decisions on site. The table below lists common ways operations can undermine a battery room's design basis.