Electrical Energy Storage Systems (EESS) provide storage of electrical energy so that it can be used later. EESS may be installed for a variety of reasons, for example increasing the 'self-consumption' of buildings fitted with renewable energy systems; arbitrage services; ancillary services and providing a back-up or alternative power supply. This Code of Practice looks at EESS applications and provides information for practitioners to specify safely and effectively, design, install, commission, operate and maintain a system. The scope of this Code of Practice includes EESS intended for fixed installation applications including: individual dwellings, commercial applications, including multi-occupancy buildings and multi-occupancy residential buildings, industrial applications, and covers: electrochemical energy storage systems in electrical installations, integration into low voltage (LV) power systems (AC and DC) and, systems aligned with existing standards, regulations, and guidance. This third edition looks to build on the success of the previous edition, bringing it up to date with the latest developments in the electrical energy storage market. Additions to the third edition include: An update to the requirements for island mode isolators, Further guidance on fire safety and the location of batteries within an installation, Updates to schematics for domestic use, General update to the presentation of diagrams.