Author(s): Angelo Verzoni. Published on September 1, 2020.

'Improvements Welcome'

New reports highlight what went wrong and what can be learned from an energy storage system explosion in Arizona in 2019


Just over a year after an explosion at an energy storage system (ESS) injured four firefighters in Surprise, Arizona, two new reports shed light on what happened as well as on some of the shortcomings of the safety systems, guidelines, and procedures in place to deal with ESS incidents.

“Both documents are examples of how we can learn from past incidents to improve our codes and standards, increase the safety of our first responders, and build a safer environment,” Brian O’Connor, an engineering technical services lead at NFPA, wrote in a blog about the two reports. “It’s encouraging to see that such a collaborative approach was taken in response to this incident to determine what happened and what can be done to prevent this type of equipment failure in the future.”

The first, released on July 18, came from Arizona Public Service, the utility company that maintained the ESS that exploded. The APS report focuses on the battery technology in question, concluding that the incident most likely resulted from the internal failure of a single battery cell caused by a defect in the cell. When heat built up in the one defective cell, it spread to others—a process known as thermal runaway—and the fire suppression system installed in the ESS couldn’t stop that process, the report said. As this thermal runaway occurred, flammable gases built up inside the structure housing the batteries and eventually triggered the explosion.

The second report, released by UL on July 23, focuses on the emergency response aspects of the incident. The UL report “does a great job at addressing some of the contributing factors that led to…firefighter injuries,” O’Connor said. It points to deficiencies in responding firefighters’ preparation for the incident—including within the hazardous materials training they received, which didn’t cover ESS hazards—as well as their ability to monitor the conditions of the ESS container from a safe distance.

NFPA’s standard related to ESS—NFPA 855, Standard for the Installation of Stationary Energy Storage Systems—was first published five months after the Arizona incident, and the APS report includes recommendations for strengthening that document. For example, the standard doesn’t include requirements for preventing thermal runaway at the battery cell level; it only mentions the phenomenon in an annex section. That should change in the future, according to the report’s authors.

O’Connor hopes concerns like this will be considered as part of the standards writing process. “NFPA codes and standards are living documents that are constantly looking for ways to improve and keep up with new technology,” he said. “Recommended improvements are always welcome in the form of Public Inputs or Public Comments.” The next edition of the standard is set for release in 2023. To learn more about the schedule and process of updating the standard, visit

ANGELO VERZONI is staff writer for NFPA Journal. Top photograph: Arizona Public Service