# IEC 62933 — Electrical energy storage (EES) systems > IEC 62933 is the international standard series for grid-connected electrical energy storage systems: terminology, unit parameters and test methods, environmental issues and — most critically — safety, including the prevention and containment of battery thermal runaway. It works alongside UL 9540A and NFPA 855. - Source : https://industryhub.site/en/standards/iec-62933 - Body : IEC - Family : Energy Storage - Status : current - First published : 2018 - Latest revision : 2022-2023 (per part) - Edition : Part-specific - Next revision : Active maintenance - Access : Paid (IEC webstore) - Language : English ## Document structure - **IEC 62933-1** — Terminology : Defines the vocabulary of electrical energy storage — system boundary, ratings, states — used consistently across the series. - **IEC 62933-2-1** — Unit parameters and test methods : How to specify and test an EES unit: power and energy ratings, round-trip efficiency, response time, the parameters that make systems comparable. - **IEC 62933-4-1** — Environmental issues : Environmental requirements and the assessment of impacts over the storage system lifecycle. - **IEC 62933-5-1 / 5-2** — Safety requirements : Safety of EES systems: -5-1 general safety considerations, -5-2 safety for grid-integrated EES using electrochemical (battery) storage — hazard identification, thermal runaway prevention and mitigation. ## Key concepts - **EES system boundary** : An energy storage system is more than cells: it includes the battery management system (BMS), power conversion, thermal management, controls and enclosure. IEC 62933 defines the boundary so that ratings and safety claims refer to the same thing. - **BMS vs EMS** : The Battery Management System protects each cell (voltage, current, temperature, balancing); the Energy Management System decides what the asset does (frequency regulation, peak shaving, arbitrage). Safety lives mainly in the BMS; value mainly in the EMS. - **Thermal runaway & propagation** : A failing cell can self-heat, vent flammable gas and ignite — and drag neighbouring cells with it (propagation). Preventing initiation and containing propagation is the central safety problem of battery storage and the focus of IEC 62933-5-2. - **Round-trip efficiency** : Energy out divided by energy in over a charge-discharge cycle. Li-ion systems reach roughly 85-90% at the AC terminals; the losses (conversion, auxiliaries, thermal management) directly affect the business case. - **Power vs energy rating (C-rate)** : A system is defined by power (MW) and energy (MWh); their ratio is the C-rate. A high-power, short-duration system suits frequency regulation; a high-energy, long-duration one suits peak shifting. The duty drives the cell choice and the thermal design. - **UL 9540A (companion test)** : A fire-safety test method that measures cell-to-cell and unit-to-unit thermal runaway propagation. Its results feed the separation distances, ventilation and suppression required by installation codes — referenced widely even outside North America. - **NFPA 855 (companion install standard)** : The installation standard for stationary storage: setback distances, fire detection and suppression, ventilation and explosion control. It uses UL 9540A test data to size the protections — the practical counterpart to the IEC 62933 safety case. ## Making storage comparable — and safe A battery storage system is the piece that lets a grid run on variable renewables: it absorbs surplus, delivers at peak and stabilises frequency in milliseconds. **IEC 62933** is the series that makes such systems **describable** (terminology, parameters), **comparable** (test methods, efficiency) and above all **safe** — because the same energy density that makes storage useful makes a failing cell dangerous. ## What the series covers The parts split the problem: **-1** fixes the terminology and the system boundary (a storage system is cells *plus* BMS, conversion, thermal management, controls); **-2-1** defines unit parameters and test methods — power and energy ratings, **round-trip efficiency**, response time — so two systems can be compared honestly; **-4-1** addresses environmental issues across the lifecycle. ## The safety problem: thermal runaway The heart of storage safety is **thermal runaway**: a failing cell self-heats, vents flammable gas and can ignite, then drag its neighbours with it. **IEC 62933-5-2** structures the safety case for grid-integrated electrochemical storage — hazard identification, prevention of initiation, and containment of propagation. It does not stand alone. The **UL 9540A** test method measures how runaway propagates cell-to-cell and unit-to-unit, and **NFPA 855** uses that data to set separation distances, ventilation, detection and suppression. Incidents like the 2019 McMicken explosion in Arizona — where vented gas exploded as firefighters opened the door — are exactly what this stack of standards now exists to prevent. ## Grid integration A storage system that acts on the grid must speak its protocols and be cyber-secure. In practice IEC 62933 sits alongside [IEC 61850](/en/standards/iec-61850) for substation communications, [IEC 62443](/en/standards/iec-62443) for control-system cybersecurity, [IEC 61000](/en/standards/iec-61000) for electromagnetic compatibility, and [ISO 55000](/en/standards/iso-55000) for managing the asset across its life. ## Applicable industries Grid-scale battery storage (BESS), Solar-plus-storage and wind-plus-storage hybrids, Commercial and industrial behind-the-meter storage, Utilities, IPPs and system integrators ## References & further reading - [IEC 62933-5-2:2020 — safety (official)](https://webstore.iec.ch/publication/61545) - [IEC 62933-1:2018 — terminology (official)](https://webstore.iec.ch/publication/24015) - [NFPA 855 — stationary energy storage installation](https://www.nfpa.org/codes-and-standards/nfpa-855-standard-development/855)