IET EESS code of practice

The IET Code of Practice for Electrical Energy Storage Systems is practical technical guidance for people who specify, design, install, commission, operate or maintain fixed battery systems. The current publication is the third edition, published in 2024.

It covers electrochemical storage in low-voltage AC and DC installations across individual dwellings, commercial and multi-occupancy buildings and industrial applications. The IET says the third edition added updated island-mode isolation guidance, further fire-safety and battery-location guidance, revised domestic schematics and clearer diagrams.

It is guidance within a wider framework

The Code of Practice is not BS 7671 and does not replace it. BS 7671 sets requirements for the electrical installation; the Code explains how to apply safe design and installation practice to the particular characteristics of storage.

PAS 63100:2024 deals specifically with fire protection for domestic battery systems within its scope. Product standards, manufacturer instructions, network connection rules, building requirements and fire-safety duties also continue to apply.

Compliance cannot be established from a general statement that an installer “follows the IET book”. The applicable documents and their order of precedence must be identified for the system.

MCS makes the Code a direct requirement

MIS 3012:2025 is the battery installation standard for contractors certified to MCS:2025. It says qualifying systems must be designed and installed in accordance with the latest IET EESS Code of Practice.

MIS 3012 also sets an order of precedence: the latest BS 7671 comes first, then the detailed MIS requirements, then the Code of Practice. Manufacturer instructions normally apply as well, but the MIS explains what the contractor must do if those instructions conflict with its requirements.

This makes the Code more than optional reading for an MCS-certified battery installation. It forms part of the scheme’s technical requirements even though it is not legislation in its own right.

Design scope

The Code covers the system as a complete installation rather than treating the battery as a standalone appliance. Its design scope includes:

• intended operating modes, including charging, export and any backup supply;
• compatibility of batteries, power-conversion equipment and controls;
• protection against electric shock, overcurrent and thermal hazards in every mode;
• isolation and switching of every source of supply;
• earthing and fault protection when grid connected and when islanded;
• battery location, fire safety, ventilation and access;
• grid-connection and export-control requirements;
• inspection, testing and commissioning; and
• user instructions, maintenance and safe shutdown.

The handover information should identify the exact system arrangement. A generic manufacturer manual cannot show which circuits remain live during an outage, where the isolators are or how the site’s earthing and changeover have been implemented.

Island mode is a separate electrical state

A battery advertised as providing backup cannot safely be assessed only in normal grid-connected operation. When the property is islanded, the public supply is disconnected, fault current may be much lower and the installation needs a suitable earth reference and protective arrangement.

The IET’s published guidance explains that the switching, neutral-earth connection, consumer earth electrode and protective devices have to work as one coordinated design. The required arrangement depends on the equipment and installation. It should be verified by calculation and commissioning tests rather than copied from a generic diagram.

For a homeowner, the practical evidence is a system schematic, electrical certification and test results covering each intended operating mode. If backup is not commissioned or certain circuits are excluded, that should be stated clearly.

Fire location now also sits in BS 7671

BS 7671:2018+A4:2026 introduced Chapter 57 for stationary secondary batteries and directly references PAS 63100 for batteries in dwellings. The Code of Practice remains useful because it covers the wider storage design and lifecycle, not just the new wiring-regulations chapter.

As at 22 July 2026, Amendment 3 and Amendment 4 were both valid BS 7671 editions during the transition ending on 15 October 2026. The design and certificate should say which edition was used. Where Amendment 4 is selected, its newer battery requirements take precedence over older guidance.

Installation records

The installation records identify:

• the BS 7671 edition and IET Code of Practice edition used;
• whether the work is being certified under MCS and which MIS 3012 issue applies;
• the PAS 63100 assessment for a battery in a dwelling;
• the equipment combination and manufacturer approvals;
• the G98 or G99 connection route and any G100 export limit;
• the grid-connected and island-mode schematics;
• the inspection, test and commissioning records; and
• the shutdown, emergency, maintenance and warranty instructions.

A complete design shows how the documents apply to the actual system. A list of standards does not demonstrate compliance with them.

Related entries

• BS 7671 IET Wiring Regulations
• PAS 63100 domestic battery fire specification
• PAS 63100 battery fire placement
• MIS 3012 battery storage standard
• Island mode and anti-islanding
• Thermal runaway containment
• G98 notification pathway
• G99 application pathway