Uninterruptible power supply (UPS) technologies have been used in various applications for many years to support continued operation of key loads during interruptions of power from the grid. These systems have been used in many different locations to provide additional immunity from grid interruptions interfering with the operation of defined loads. UPS systems are often used to protect computers, computer facilities and telecommunication equipment. With the recent evolution of new energy technologies, energy storage systems (ESS) have proliferated rapidly. ESS, particularly those using battery technologies, are typically supplied by renewable sources such as solar or wind power and enable the storage of energy produced by these sources for use at different times. 

The current US ANSI standard for UPS is UL 1778, the Standard for Uninterruptible Power Systems. and CSA-C22.2 No. 107.3 for Canada. UL 9540, the Standard for Energy Storage Systems and Equipment, is the American and Canadian national standard for ESS. While both the mature UPS products and the rapidly evolving ESS produced have some commonality in technical solutions, operations and installation, there are important differences. This paper will review the critical differentiations, outline the applicable product safety requirements associated with each and summarize how codes are evolving in addressing both types of installations.

Introducing UPS

Formation

A UPS system is an electrical system designed to provide instantaneous temporary alternating current-based power for critical loads in the event of electric grid failure or other mains power source failure modes. The UPS is sized to provide an instantaneous continuation of a predetermined amount of power for a specific duration. This allows a secondary power source, e.g., a generator, to come online and continue with power backup. The UPS may safely shut down non-essential loads while continuing to provide power to more important equipment loads. UPS systems have been providing this critical support for various applications for many years. A UPS will utilize stored energy from an integrated energy source. This is typically battery bank, supercapacitor or the mechanical movement of a flywheel as an energy source.

A typical UPS using a battery bank for its supply consists of the following main components:

Rectifier/charger – This UPS section takes the AC mains supply, rectifies it and produces a DC voltage used to charge the batteries.

• Inverter – In the event of a mains supply failure, the inverter will convert the DC power stored in the batteries into clean AC power output suitable for the supported equipment.

• Transfer switch – An automatic and instantaneous switching device that transfers power from various sources, e.g. mains, UPS inverter and generator, to a critical load.

• Battery bank – Stores the energy needed for the UPS to perform its intended function.

Current standards for UPS systems

• The current U.S. ANSI standard for UPS is UL 1778/C22.2 No. 107.3, the Standard for Uninterruptible Power Systems, which define a UPS as “a combination of converters, switches, and energy storage devices (such as batteries) constituting a power system for maintaining continuity of power to a load in case of input power failure.”

• Under development are new editions of IEC 62040-1 and IEC 62477-1. UL/CSA 62040-1 (using UL/CSA 62477-1 as a reference Standard) will be harmonized with these standards.

Introducing energy storage systems (ESS)

ESSs are gaining traction as the answer to a number of challenges facing availability and

reliability in today’s energy market. ESS, particularly those using battery technologies, help mitigate the variable availability of renewable sources such as solar or wind power. ESS are a source of reliable power during peak usage times and can assist with load management, power fluctuations and other grid-related functions. ESS are used for utility, commercial, industrial and residential applications.

Current standards for ESS

UL 9540, the Standard for Energy Storage Systems and Equipment, is the American and Canadian national standard for ESS.

• First published in 2016, UL 9540 includes multiple technologies for ESS including battery energy storage systems (BESS). UL 9540 also covers other storage technologies: mechanical ESS, e.g., flywheel storage paired with a generator, chemical ESS, e.g., hydrogen storage paired with a fuel cell system, and thermal ESS, e.g., latent heat storage paired with a generator.
• UL 9540, its second edition defines an energy storage system as “Equipment that receives energy and then provides a means to store that energy in some form for later use in order to supply electrical energy when needed.” The second edition of UL 9540 further requires that a BESS be subjected to UL 9540A, the Standard Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, if required to meet exceptions in the codes.
• UL 9540 is currently in its third edition.

Comparing ESS with UPS

Functions and dimension

An ESS is similar in construction to a UPS but differs in its usage. Like UPS, ESS includes an energy storage mechanism such as batteries, power conversion equipment, e.g., inverters, and various other electronics and controls. Unlike the UPS, however, an ESS may operate in parallel with the grid, which results in greater cycling of the system than a UPS would ever experience. An ESS can collaborate interactively with the grid or in a standalone mode, or both, depending upon the type of power conversion system employed. An ESS may even work as UPS functionality. Like UPS, ESS can come in a variety of sizes from a small residential system that is less than 20 kWh of energy to utility applications using multi-megawatt energy container systems with multiple battery racks within the container

Chemical composition and safety

The typical battery chemistries used in UPS have always been lead-acid or nickel-cadmium batteries. Unlike UPS, BESS uses technologies such as lithium-ion batteries from the beginning because lithium-ion batteries have better cycle performance and higher energy density, which can provide more energy in a smaller physical footprint. Lithium-ion batteries also have much lower maintenance requirements than traditional battery technologies. But currently, lithium-ion batteries are also increasingly used in UPS applications.

However, a serious accident in Arizona in 2019 involving an ESS used in utility applications resulted in serious injuries to several first responders and attracted the attention of various stakeholders, including regulators and insurance agencies. To ensure that this growing field is not hampered by avoidable safety incidents, appropriate specifications and standards need to be developed for ESS. To encourage the development of appropriate safety specifications and standards for ESS, the U.S. Department of Energy (DOE) launched the first annual forum on ESS Safety and Reliability in 2015.

The first DOE ESS Forum contributed to a large amount of work on ESS specifications and standards. The most noteworthy is the development of NEC No. 706 and the development of NFPA 855, a standard for stationary energy storage system installations, which directly affects the standard for stationary battery systems in ICC IFC and NFPA 1. Today, NEC and NFPA 855 have also been updated for 2023 versions.

Current status of ESS and UPS standards

The goal of all rules and standards development activities is to adequately address the security of these systems. Unfortunately, current standards have created some confusion in the industry.

1.NFPA 855. The key document affecting the installation of BESS and UPS is the 2020 version of NFPA 855, Standard for the Installation of Stationary Energy Storage Systems. NFPA 855 defines energy storage as “an assembly of one or more devices capable of storing energy for future supply to local electrical loads, utility grids, or grid support.” This definition includes applications for UPS and ESS. Additionally, NFPA 855 and fire codes require ESSs to be evaluated and certified to UL 9540. However, UL 1778 has always been the traditional product safety standard for UPS. The system has been independently evaluated for compliance with applicable safety requirements and supports safe installation. Therefore, the requirement of UL 9540 has caused some confusion in the industry.

2. UL 9540A. UL 9540A requires starting from the battery level and testing step by step until passing the installation level. These requirements result in UPS systems being subject to marketing standards that were not required in the past.

3.UL 1973. UL 1973 is the battery system safety standard for ESS and UPS. However, the UL 1973-2018 version does not include testing provisions for lead-acid batteries, which is also a challenge for UPS systems using traditional battery technology such as lead-acid batteries.

Summary

Currently, both the NEC (National Electrical Code) and NFPA 855 are clarifying these definitions.

• For example, the 2023 version of NFPA 855 clarifies that specific lead-acid and nickel-cadmium batteries (600 V or less) are listed in UL 1973.
• In addition, lead-acid battery systems certified and marked according to UL 1778 do not need to be certified according to UL 9540 when used as a backup power supply.

In order to solve the problem of the lack of test standards for lead-acid and nickel-cadmium batteries in UL 1973, Appendix H (Evaluate alternatives to valve-regulated or vented lead-acid or nickel-cadmium batteries) was specifically added to the third edition of UL 1973 released in February 2022.

These changes represent a positive development in order to differentiate the safe installation requirements of UPS and ESS. Further work includes updating NEC Article 480 to better address installation requirements for technologies other than lead-acid and nickel-cadmium. Additionally, the NFPA 855 standard needs to be further updated to provide greater clarity on fire protection regulations, particularly regarding the various technologies used in stationary applications, whether they are UPS or ESS.

The author hopes that continued changes will improve the safety of the industry, regardless of whether a traditional UPS or ESS is used. As we see energy storage solutions proliferate in significant and rapid ways, addressing the intrinsic safety of products is critical to unlocking safety innovation and meeting society’s needs.