Camelot Unpacks UL 9540 – Part 2

In Part 1 of our Camelot Unpacks UL 9540 series, we tackled some of the most common misconceptions about this critical Battery Energy Storage System (BESS) Standard - misconceptions that can easily derail schedules, inflate costs, or cause compliance headaches. 

Now, it’s time to move from myth-busting to the nuts and bolts. In Part 2, we’ll walk through some key questions regarding the requirements baked into UL 9540, highlight when and why it’s required, and shed light on the often-misunderstood Field Listing process. Whether you’re overseeing a project, supplying equipment, or working on the financing side, this is the knowledge that keeps your BESS project both compliant and bankable. 

What does UL 9540 include?  

While no product certification is ever a perfect guarantee of safety, the UL 9540 Standard is fairly broad in its scope as it's intended for an ESS as a whole, with key tests summarized below. These tests are additional to compliance requirements related to materials, construction, software, electrical design, fire safety design, noise levels, and more. These tests are also additional to any component-level tests required. For example, UL 1973 includes about 30 different tests on the battery modules alone, covering a range of potential risks, such as overcharging, over-temperature operation, external fire exposure, and physical impacts.  

Table 1: UL 9540 Key Tests 

When is UL 9540 Listing Required? 

Compliance with UL 9540 is required under a number of major Codes, as summarized below. Note that, as of this writing, nearly all locations within the US require compliance with at least one of the Code editions noted below (or a more recent version). There are likely a few local jurisdictions not yet enforcing these Code editions but, essentially, Listing to UL 9540 is a Code requirement nearly anywhere in the US. 

Is it Acceptable to Field List a BESS to UL 9540? 

Certainly, this is quite common and widely accepted. In practice (and in Code) an ESS is "one or more devices, assembled together, capable of storing energy to supply electrical energy at a future time".  As you can see, this goes beyond simply the ESS enclosure to include the equipment facilitating connection to the broader electrical system, such as the inverter. 

Most ESS manufacturers will not have an infinite combination of their product listed with each possible DC converter, inverter, and transformer. As such, Field Listing is widely required to validate the "system" meets relevant Code requirements. 

How does Field Listing Work? 

The term "Field Listing" is a slight misnomer, as the "field" portion is only a small part of the overall review. In fact, completing the Field Listing requires considerable review of documentation and generally requires that all the components of the ESS be Listed to their own respective Standards (see summary above). The Nationally Recognized Testing Laboratory (NRTL) doing the Field Listing will review the documentation and subordinate Listing status of all the major components in order to underpin their final Field Listing.  

As you can see, a successful Field Listing requires that the ESS uses high quality components that are properly Listed, and the Field Listing is really just validating the site-specific combination of those components (and that those components have been installed/used per their Listing). Once complete, the NRTL will issue a Field Listing that applies only to that specific project or installation. Even if the exact same equipment is used again at another site, a new Field Listing is still required. 

The pathway from Code requirement to (some of) the underlying Standards is summarized in the figure below. As you can see, a simple UL 9540 Listing has a lot behind it and is a critical element in having a high quality and bankable BESS. 

Figure 1: Compliance Pathway 

Why do the Components Need to be Listed Separately for a Field Listing? 

Put simply, many of the required tests to List a BESS to UL 9540 are destructive in nature and you would not want them done to your commercial project. For example: 

• UL 9540A testing requires initiating thermal runaway (aka making the system catch fire on purpose) 

• Vibration and Impact Resistance tests may involve damaging your enclosures 

• Overcurrent and overvoltage tests require exposing the BESS to electrical conditions beyond its design 

As you can imagine, few manufacturers would be willing to honor warranties after you abuse their system in such ways. So, since we can't deliberately set projects on fire in the field, the NRTL will have to rely on the test results used to obtain other component Listings. As shown above, the DC Block is already Listed to UL 9540. In these cases, all of the most strenuous tests have already been completed and found sufficient by a NRTL and the Field Listing can really focus on the combination of components. In some cases, NRTLs may be willing to issue Field Listings based on manufacturer test reports, engineering analyses, and similar documents but this is a very risky prospect and will take considerably longer and increase the cost to the owner. Also, if the NRTL finds they don’t have sufficient basis for granting the Field Listing, they may require additional testing from the manufacturer, leaving your project in a sort of Limbo state for months, if not longer. 

So, while any combination of ESS components can theoretically be granted a Field Listing, it is far safer to ensure your ESS is a combination of already-Listed components. In particular, using a DC block that is Listed to UL 9540 in its own right is a great way to reduce the risk of significant costs and/or delays in the final Field Listing process.