WHITE PAPER

Enhancing Energy
Storage System Safety:
Evolving Large-Scale Fire Test Methods
and Regulatory Demands

LET’S GO

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Table of contents Abstract 03

Introduction 04

NFPA 855 and key code requirements for ESS 05

Illustration of installation restrictions for lithium-ion BESS 06

How UL 9540, UL 9540A and NFPA 855 coordinate for ESS safety 07

UL 9540A testing levels 08

Regulatory demand and response for enhanced large-scale fire 09

testing for nonresidential BESS

Key notes in the proposals for enhanced large-scale fire testing 10

(LSFT) for nonresidential BESS

Conclusion 11

Energy storage safety support from UL Solutions 12

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Abstract
This white paper underscores the safety codes
and standards related to energy storage systems
(ESS), including NFPA 855; ANSI/CAN/UL 9540, the
Standard for Safety of Energy Storage Systems and
Equipment; and ANSI/CAN/UL 9540A, the Standard
for Test Method for Evaluating Thermal Runaway Fire
Propagation in Battery Energy Storage Systems. UL
9540A has been referred to as the large-scale fire
test (LSFT) by NFPA 855 to evaluate the thermal
runaway fire hazards of ESS. While LSFT methods in
applicable safety standards continue to be refined
through expert collaboration in the consensus
process, other methodologies have been introduced.
These other methods are not intended or recognized
for use in satisfying the established safety code
and safety standard requirements in North America.
Because these other methods are different from and
incompatible with the established safety requirements
in the national standards for the United States (U.S.)
and Canada, they introduce confusion into the ESS
market and create risks for ESS manufacturers when
misused to address essential safety requirements.

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Introduction
Sampling of energy storage system fire incidents in 2024

Location Event date Description

The integration of energy storage systems (ESS) into

One of several lithium-ion containers was observed to be smoking. The fire department
the energy infrastructure is becoming increasingly
Neermoor, Germany April 27, 2024 consulted with the operator and opened the container, resulting in an explosion.
prevalent, driven by various factors, including the
Two firefighters were injured. The fire was extinguished in ten hours.
escalating demand for renewable energy and the
necessity for grid stability. Because of the overall
The energy storage facility was involved in a fire, and water was pumped into the building’s fire
growth in ESS deployment, the occurrence of fire suppression system to extinguish it. A 600-foot safety barrier was maintained for over twenty two hours
San Diego, CA, U.S. May 15, 2024
incidents related to energy storage systems worldwide due to air monitors showing high levels of hydrogen. The fire was declared extinguished the next day
has also increased, with over 40 reported incidents but reignited several times until the fire department left the scene nearly seventeen days later.
since 2011, including six fires in 2024 alone [1]. The
safety of ESS in the event of thermal runaway and A Battery Energy Storage System BESS fire occurred in an industrial area,
Santa Ana, CA, U.S. July 17, 2024
subsequent fire propagation has emerged as a primary leading to a one-hour evacuation in the area due to smoke.

concern for manufacturers, installers and regulatory

One of twenty four containers caught fire. Businesses adjacent to the
bodies across the globe. Escondido, CA, U.S. Sept. 5, 2024
substation or within approximately 0.25 mi were evacuated.

An explosion and fire occurred in a lithium-ion BESS at a data center. Four water jets and
Singapore Sept.10, 2024 the sprinkler system were used to contain the fire, and an unmanned firefighting robot was
used to cool the batteries. The fire was declared to be under control after two days.

[1] BESS Failure Incident Database - EPRI Storage Wiki

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NFPA 855 and key code

requirements for ESS
The Standard for the Installation of Stationary Energy Storage Systems, NFPA 855, issued by the National
Fire Protection Association (NFPA), sets forth comprehensive requirements for energy storage systems,
including installation, commissioning and maintenance. NFPA 855 mandates that ESS be listed to the
safety requirements of UL 9540 (outlined below) and imposes numerous installation restrictions for battery
energy storage systems (BESS) to mitigate fire hazards, including but not limited to the following:

• Maximum BESS size: 50 killowatt-hours (kWh) • Smoke and fire detection: mandatory
• Minimum separation distance: 3 feet (ft) • Explosion control: explosion prevention per
• Maximum stored energy: 600 kWh NFPA 69, deflagration protection per NFPA 68
(for lithium-ion BESS within a fire area) • Minimum clearance to windows and
• Fire barriers: 2-hour fire rating doors for wall mounted: 5 ft
• Minimum sprinkler density: • Minimum clearance to egress: 10 ft
0.3 gallon per minute per square foot (gpm/ft2)

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Illustration of installation restrictions

BESS that exceed installation limits in NFPA 855 require
additional evaluation relative to fire hazards. For example,

for lithium-ion BESS

for nonresidential ESS configurations with individual
units either spaced less than 3 feet apart or having a
capacity larger than 50 kWh, large-scale fire testing in
accordance with UL 9540A is required. This testing is
essential to assess the safety of these ESS installations
and supports manufacturers in obtaining installation
approval from the relevant code authorities.

The Fire Code, NFPA 1 issued by National Fire Protection

1. Each Unit Max. 50 kWh
Association (NFPA), the International Fire Code (IFC) and
2. Min. 3ft between units, from
International Residential Code (IRC), published by the
units to wall
3
International Code Council, also have requirements for safe
3. Max. stored energy: 600kWh
for Li-ion BESS ESS installations. The NFPA 1, IFC and IRC requirements for
2
ESS are harmonized with NFPA 855, referencing both UL
9540 and UL 9540A for product safety and fire mitigation.
1

Note: The 2023 edition of NFPA 855 renames

“large-scale fire testing” to “fire and explosion
test,” while the test method according to UL 9540A
remains unchanged. However, the 2024 edition of
the IFC continues to use the term “large-scale fire
testing” and cites UL 9540A for assessing fire hazards
when those installation limits are exceeded.

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How UL 9540, UL 9540A and NFPA 855

coordinate for ESS safety
UL 9540, the Standard for Energy Storage Systems and UL 9540A, the Standard for Test Method for Evaluating As bi-national U.S. and Canadian consensus standards,
Equipment is the nationally adopted standard for the United Thermal Runaway Fire Propagation in Battery Energy UL 9540 and UL 9540A are the result of a balanced
States and Canada. This standard assesses overall product Storage Systems, was developed to establish standard test and open collaborative process involving industry,
safety, covering electrical, mechanical, environmental methodologies for assessing the hazards associated with professional organizations, government agencies,
and functional safety. Furthermore, the standard addresses fire propagation associated with thermal runaway events manufacturers and other expert stakeholders.
risks associated with component interactions and in the batteries of ESS. UL 9540A is the American and
In the ESS sector, there is a coordinated synergy between
compatibility, the operation and maintenance of ESS, Canadian National Standard for this important safety issue,
the UL 9540, UL 9540A and the NFPA 855 standards and
as well as the integration of ESS with electrical grids. and testing to these national standard requirements is an
the applicable codes. UL 9540 provides product safety
As previously stated, safety codes require ESS to comply important element of due diligence. UL 9540A provides a
criteria for ESS, UL 9540A provides methodologies for testing
with the requirements of UL 9540. For ESS installations methodology to test the system’s safety-related behavior
thermal runaway and fire spread in energy storage systems,
exceeding NFPA 855 limits, UL 9540 also mandates when the design or installation conditions of an ESS
while NFPA 855 addresses the installation and similar use
large-scale fire testing in accordance with UL 9540A. exceed the limits set by NFPA 855, NFPA 1, IFC or IRC.
requirements. Though these standards focus on different
aspects, they complement each other, collectively enhancing
the safety of ESS. The applicable codes address electrical, fire
and other safety requirements that are enforced by regulatory
authorities in their jurisdiction and rely on compliance with
the standards to demonstrate safety of the ESS installations.

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UL 9540A
Cell-level test Unit-level test
Cell-level testing evaluates the thermal Unit-level testing evaluates the likelihood of fire spread

testing levels
runaway characteristics of the cell as well as between modules, the unit’s heat and gas release
the composition and flammability of the gases. rates, and the potential for deflagration or re-ignition.

UL 9540A was first published in November 2017 and was

updated to a consensus standard in its fourth edition in 2019.
It encompasses four levels of testing. For lithium-ion battery
energy storage systems, the first three levels of testing,
from cell level to unit level, are generally required. If flames
or other noncompliance indicators are observed at the
unit-level test, the installation-level test is needed.

UL 9540A has achieved international adoption,

with ESS installation regulations in Singapore, Malaysia
and the Australian state of Victoria referencing UL
9540A to permit specific installation scenarios.

Module-level test Installation-level test

Module-level testing evaluates the tendency Installation-level mainly evaluates the
of cell thermal runaway propagation, the heat effectiveness of the fire protection system, as
and gas release rate of the module, and the well as the heat and gas release rate of the system
potential danger of ignition or deflagration. and the danger of deflagration or re-ignition.
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Regulatory demand and response for

enhanced large-scale fire testing
for nonresidential BESS
With the increasing number of ESS safety incidents, be easily used by manufacturers for testing and It is important to note that UL
global concern for ESS safety has heightened, by code authorities for oversight. The consensus
especially for large-scale BESS. Some regulatory testing methodology is expected to be integrated
9540A remains the only consensus
bodies have called for enhanced large-scale fire into the UL 9540A Standard, and the guidance standard explicitly cited in NFPA
testing of nonresidential BESS, seeking to evaluate on the test objectives is expected to be included 855 for large-scale fire testing and
how fire spreads within and between units following in the Annex of the 2026 edition of NFPA 855.
a developed fire condition within the initiating
the only national standard in the
Meanwhile, some organizations have introduced
unit. This enhanced testing is to provide robust
their own large-scale fire testing procedures for
U.S. and Canada for fire safety
data to support firefighting strategies and rescue
BESS and are promoting their testing services. It is testing methods for BESS.
operations for large-scale BESS incidents.
important to note that UL 9540A remains the only
In response to this need, the Technical Committee consensus standard explicitly cited in NFPA 855
responsible for UL 9540A has established a for large-scale fire testing and the only national
dedicated task group focused on large-scale fire standard in the U.S. and Canada for fire safety
testing (LSFT) for ESS. This task group has worked testing methods for BESS. Testing to standards that
in tandem with the NFPA Technical Committee, have not been adopted on the national level, have
conducting multiple discussions and formulating a not been developed based on the consensus of all
preliminary testing procedure. Their unified objective interested parties and do not aim to ensure safety
is to create clear guidance and a consensus- may lead to rejection of ESS installations by relevant
based standard test method for LSFT that could regulatory bodies and subsequent issues with failure
to comply with identified national safety standards.
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Key notes in the proposals for

enhanced large-scale fire test (LSFT)
for nonresidential BESS
The LSFT assesses the thermal risks from fires within The test setup should reflect the final installed configuration The compliance criteria for LSFT demonstrate the
BESS and the potential for fire to spread to nearby of BESS, ancillary equipment and targets, representing critical fire safety concerns regarding the spread
units or occupancy areas. It provides quantitative data any immediate surrounding enclosure or buildings. of fire to other BESS and surrounding areas.
on thermal exposure but is not designed to evaluate Fire protection systems installed in accordance with • The fire shall not propagate to the
flammable gas accumulation prevention, deflagration applicable codes and standards may be installed and adjacent BESS during the testing.
mitigation or the reliability of safety systems. The test active in all enclosures in the test. Active thermal • The surface temperature of modules within the
excludes residential BESS less than 20 kWh in capacity. runaway mitigation technologies can be incorporated adjacent BESS units shall not exceed the temperature
in all adjacent enclosures but will be deactivated in at which thermally initiated cell venting occurs,
Before performing the LSFT, cell-level testing should be
the enclosure or rack of origin. Ventilation paths in the according to UL 9540A cell-level testing.
conducted according to UL 9540A to characterize cell-
enclosure should be opened if they are anticipated to be • Ignition of combustible materials on or in any
level thermal runaway performance and gas flammability.
opened due to a deflagration in the enclosure of origin. adjacent enclosure shall not occur during testing.
A burner or heater plus ignitor is to be used as one of the • Venting or thermal runaway of any cells
ignition methods, to cause overheating of the BESS unit of in adjacent BESS shall not occur.
origin, resulting in a developed fire condition. During the test,
This comprehensive fire test, developed through
data on battery reaction, fire spread, and the temperatures
a consensus process with expert involvement,
of the BESS and adjacent targets will be captured.
demonstrates that the BESS design and spacings will
not contribute to a cascading, hazardous fire event.

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Conclusion
As the deployment of ESS accelerates globally, Through our expertise in safety research, testing
increasing their safety and reliability becomes more and certification of ESS, UL Solutions collaborates
critical. The establishment of safety standards with the NFPA on the consensus standard for LSFT,
such as NFPA 855, UL 9540 and UL 9540A reflects helping to minimize risks, protect consumer interests
the industry’s collective commitment to safer and advance the safe and orderly growth of the ESS
ESS design and operation. Manufacturers also market. The updates to and international adoption
obtain significant benefits from demonstrating of UL 9540A, along with the response to calls for
due diligence through conformance to published enhanced large-scale fire testing, further reinforce
national standards in the U.S. and Canada rather this commitment. Through ongoing collaboration, we
than other documents that do not fully address the can collectively increase the resilience of our energy
requirements of safety codes and regulations. infrastructure and the safety of communities worldwide.

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Energy storage We provide the following services to global

battery and energy storage manufacturers:

safety support • UL 1973 energy storage battery

testing and certification

from UL Solutions
• UL 9540 energy storage system
safety testing and certification
• UL 9540A large-scale fire testing reports
• Customized large-scale fire testing reports
As a pioneer in the field of battery and energy storage
• NFPA 855, NFPA 68, NFPA 69
safety, UL Solutions has extensive experience in
compliance assessments
safety research, testing and certification of batteries
and energy storage systems. We also work closely
with the code authority community, the NFPA, UL Solutions has extensive testing experience in
the fire service, and other experts dedicated LSFT, having assisted numerous customers globally in
to collaboration for safer and more responsible designing test methods and conducting experiments.
deployment of ESS technology. UL Solutions leverages We serve our customers with a scientifically rigorous
our extensive industry connections and experience approach, applying our deep expertise in ESS safety
to help customers understand and appropriately to help ESS manufacturers complete a comprehensive
navigate the latest and most comprehensive safety assessment before deployment, boost
ESS codes and regulatory requirements. customers’ confidence that hazards have been
successfully addressed, and provide thorough reports
that promote ready acceptance by regulatory authorities
across geographies. Our objective is to minimize the
safety risks associated with energy storage products,
safeguard the interests of energy storage customers
and foster safer systematic growth of the ESS market.

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