UL 9540A 6th Edition: Where Does BESS Safety Really Start? — Controlling Consequences vs. Changing the Starting Point

The global expansion of the energy storage industry has been unstoppable over the past two years. Overseas markets are paying increasing attention to safety standards. Although the U.S. market is affected by legislation, many energy storage projects are still moving forward. To enter the U.S. market, you cannot avoid UL 9540 and UL 9540A.

In March this year, UL 9540A 6th Edition was officially published. The Large-Scale Fire Test (LSFT) has moved from "optional" to a de facto "market entry threshold." Industry discussions around "burning cabinets" have noticeably increased. Why is everyone paying attention? Because it is not just about technology—it also affects fire code approvals, project financing, and insurance underwriting.

Today we will discuss: What is the difference between UL 9540 and UL 9540A? Why does large-scale fire testing matter? And why, while the industry focuses heavily on "controlling consequences," we prefer to go back to where the problem starts.

01 What Is UL 9540A?

The official title of UL 9540A is Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems.

It has three characteristics:

First, it is a test method, not a certification standard. After completing UL 9540A, you receive a test report, not a certificate.

Second, its core focus is thermal runaway propagation. The core test method is: trigger thermal runaway in one cell, and observe whether the fire spreads from one cell to a module, from a module to a cabinet, and from a cabinet to an adjacent cabinet. Testing is scaled up progressively across cell, module, unit, and installation levels, while collecting data such as temperature and vent gas composition.

The unit-level test evaluates a BESS's ability to suppress thermal runaway propagation. The initiating thermal runaway condition uses the trigger conditions defined in the module-level test procedure. The 6th Edition made an important adjustment to the scope of unit-level testing: for non-residential BESS, unit-level testing is no longer mandatory, and testing can proceed directly to the installation-level Large-Scale Fire Test. However, for residential scenarios, unit-level testing is still required under the following conditions:

a) Residential electrochemical energy storage system with capacity > 20 kWh;
b) Spacing between storage units, or between storage units and doors/windows, is less than 0.9 m (3 ft);
c) Capacity > 1 kWh and planned for indoor residential installation;
d) Non-metallic enclosure;
e) Integrated with an active thermal runaway propagation suppression system (e.g., direct injection cooling system).

Third, there is no absolute "pass" or "fail." The standard itself only specifies how to test and what to test. After the test data is generated, the local fire code authority having jurisdiction (AHJ) determines whether it meets requirements based on the specific application scenario. In the industry, it is generally considered acceptable when there is no propagation between cabinets, no deflagration, and no re-ignition.

The large-scale fire test currently generating the most industry attention is based on this standard. The biggest change in the 6th Edition is making the installation-level Large-Scale Fire Test (LSFT) a mandatory threshold. NFPA 855 (2026 Edition) also explicitly references this test, requiring ESS to demonstrate that fire will not propagate between units.

02 Why Is Large-Scale Fire Testing Gaining Attention?

First, NFPA 855 explicitly requires it—there is no way around it. On the other hand, project financing and insurance underwriting rely heavily on this safety report.

The large-scale fire test burns a complete BESS cabinet, arranged according to actual project layout—or even more severely. Through nail penetration or heating/overcharge, one or several cells inside the cabinet are driven into real thermal runaway. Test personnel use thermocouples, gas sampling probes, and other monitoring equipment installed inside the cabinet to fully record the entire process.

The large-scale fire test mainly looks at four questions:

Propagation: Does fire spread to the adjacent battery cabinet?

Enclosure integrity: Is the cabinet enclosure damaged?

Explosion: Does combustible gas accumulate inside the cabinet and cause an explosion?

Re-ignition: Does the fire re-ignite after the open flame is extinguished?

Large-scale fire testing has a long cycle and high cost. From prototype preparation to test report issuance, it often takes several months, and costs are measured in millions. Therefore, thorough preparation is required before the fire test.

03 We Chose a Different Path

What mainstream industry solutions are doing is "controlling the consequences of thermal runaway." This is not wrong; thermal barriers, directed venting, and deflagration relief designs are all necessary engineering innovations.

But we asked a different question: What if we make thermal runaway itself lose the conditions to occur?

The starting point of thermal runaway is the flammable liquid electrolyte. Traditional liquid-electrolyte battery safety design assumes that "the fuel is already present" and then tries to control it. We prefer to change this premise at the cell material level.

Our full product line—the Enerbond series—uses solid-state battery cells. Compared with conventional liquid electrolyte systems, solid-state cells reduce the presence of flammable liquid electrolyte at the material level. This means the conditions for thermal runaway become harder to establish. This is not to say risk is zero, but that the probability structure has changed.

At the system level, the Enerbond series is designed to match the characteristics of the cells. For example, thermal barriers are placed between cells to physically interrupt heat conduction paths, with the goal of preventing thermal runaway from easily propagating. This physical thermal isolation design does not depend on software operation; even if the electronic system fails, it can still function independently.

04 What Does This Mean for Overseas Buyers?

First, the risk profile in insurance negotiations is more favorable. Insurers look at probability and consequences. Starting from a lower probability baseline, combined with physical thermal isolation design, is more preferred.

Second, long-term uncertainty is lower. Thermal runaway is not just a safety event; it is also an operational and reputational event. Reducing probability at the root means reducing the tail risk that affects project financing and community acceptance.