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UL 9540 & UL 9540A: What Designers Need to Know

UL 9540 & UL 9540A:  What Designers Need to Know

UL 9540A and UL 9540 were both created to improve safety for energy storage systems (ESS) and equipment. However, there are some key differences.

UL 9540A is a test method (NOT a certification!) for evaluating fire and explosion risk from thermal runaway events for battery energy storage systems.  It results only in a comprehensive report of raw data (not a listing or certification of any kind).  The results are often used by manufacturers to prove code compliance to local code authorities, insurers, and end-users. 

UL 9540 is a safety standard (that can result in a certification or “listed” status). It specifies size and installation requirements for the design and construction of energy storage systems (ESS).  Some higher-risk, complex ESS systems are required to demonstrate satisfactory UL 9540A test results before becoming UL 9540 certified, while lower-risk, less-complex ESS systems are not.

Summary of UL 9540A Testing Process: The UL 9540A test method consists of incremental tests performed at the Cell, Module, Unit, and Installation levels.  Data is gathered on such characteristics as:

  • heat release rate during a thermal runaway event,
  • gas composition during a thermal runaway event,
  • flammability of gases released,
  • rate of fire spread to adjacent components.

Benefits of Satisfactory UL 9540A Test Results:  Satisfactory test results can potentially result in an increased project scope with reduced project costs.  Manufacturers can request code authority approval to install LARGER individual ESS units with SMALLER separation distances (than the 2021 IFC and NFPA 855 allow for), based on proof of satisfactory UL 9540A test performance. 

The data can also be used to demonstrate compliance and achieve UL 9540 listing status for the system.  Again, not all UL 9540-certified products need to be tested under UL 9540A to achieve certification, but the higher-risk and larger ESS systems do.  For instance, ESS larger than 50 KWh (or with separations less than three feet) cannot be listed in the second edition of UL 9540 without complying with appropriate UL 9540A fire test performance requirements.


Electrification design is exciting to be a part of but is also rapidly evolving.  Materials suppliers and fabricators are keeping pace with developments and have more thermal management products available and ready to be custom-converted than ever before. For instance:

Electrical Insulation for UL 94 V-0 dielectric safety barriers between battery and electrical components.

Thermal Interface Materials conduct heat away from the battery and towards a heat sink; AND/OR from a cooling plate towards a battery.

Thermal Control Products like thermal runaway barriers or liquid cooling wraps for batteries.

Thermal Insulation to line the inside of Battery Energy Storage System (BESS) enclosures and compartments.

Fabricated Foam and Rubber to act as UL 50 Gasket enclosure seals or as a compression pad.

Fabricated Plastics to provide UL 94V-0 flame-retardant machined or thermoformed ductwork for cooling fans.

Specialty Tapes are made of conductive foil (for EMI control), specialty Polyimide (for dielectric insulation), or Polyester (used in Assembly).

Additional Resources:

For a helpful table outlining typical thermal control products available to electrification equipment, e-mobility and EVs, electrified battery/charger systems, and energy storage system (ESS) application designers, please visit our electrifications products page. also recommends the following additional resources to designers to fully understand the scope of UL 9540A:

PNNL 2010:2016, Protocol for Uniformly Measuring and Expressing the Performance of Energy Storage System
IEC 62933-2-1, Electrical energy storage (EES) systems - Part 2-1: Unit parameters and testing methods - General specification
IEC 61427-2, Secondary cells and batteries for renewable energy storage - General requirements and methods of test - Part 2: On-grid applications
IEC 62620, Secondary cells and batteries containing alkaline or other non-acid electrolytes – Secondary lithium cells and batteries for use in industrial applications
IEC CDV 62932-2-1, Flow Battery Systems for Stationary applications - Part 2-1 Performance general requirement & method of test (under development at the time of this publication)
IEC 1679, Recommended Practice for the Characterization and Evaluation of Emerging Energy Storage Technologies in Stationary Applications
IEEE 1661, Guide for Test and Evaluation of Lead Acid Batteries Used in Photovoltaic (PV) Hybrid Power Systems
 • NFPA 855, Standard for the Installation of Stationary Energy Storage Systems (ESS)

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