BS 7671 18th Edition: A Working Guide
BS 7671 is the UK standard for electrical installations. Knowing how it is structured — and where the numbers actually live — turns a 500-page book into a fast on-site reference.
BS 7671, published by the IET and BSI, is the national standard every UK electrical installation is designed and tested against. It is not law in itself, but it is the accepted way of meeting the legal duties in the Electricity at Work Regulations and Building Regulations Part P.
This guide walks through how the 18th Edition (2018, incorporating Amendment 2:2022) is laid out, what each part covers, and how to navigate to the regulation or table you need without reading it cover to cover.
Key takeaways
- BS 7671 is split into seven parts plus appendices — Parts 4 (protection), 5 (selection) and 6 (verification) do most of the day-to-day work.
- Part 7 holds the special-location rules (bathrooms, pools, EV, solar) that modify the general requirements.
- The appendices carry the numbers you actually use — cable ratings (Appendix 4), max Zs and correction factors.
- Compliance with BS 7671 is how you demonstrate you have met the Electricity at Work Regulations.
- Amendment 2:2022 brought AFDDs, prosumer installations and revised model certificate forms.
How the standard is structured
BS 7671 is organised into seven parts. Part 1 sets scope and object. Part 2 is definitions. Part 3 is assessment of general characteristics. Part 4 covers protection for safety — shock, thermal effects, overcurrent and voltage disturbances. Part 5 covers selection and erection of equipment. Part 6 is inspection and testing. Part 7 covers special installations and locations.
The regulation numbering follows the part, so a regulation like 411.3.3 lives in Part 4, Chapter 41, on automatic disconnection. Once you know the part a topic belongs to, you can jump straight to it.
The fastest route in
Use the index at the back to find a topic, then the regulation number tells you which part and chapter it sits in — you rarely need to read a whole chapter.
The parts you use most
In practice most site work touches three parts. Part 4 gives you the protection rules — disconnection times, RCD requirements, and where SPDs and AFDDs apply. Part 5 tells you what to select and how to install it, including earthing and bonding conductor sizes. Part 6 gives you the inspection schedule and the testing sequence.
Part 7 then modifies the general rules for special locations — a bathroom (Section 701), a swimming pool (702), an EV charging point (722) or a solar PV installation (712). If you are working in one of those, always check Part 7 as well as the general rules.
Where the numbers live: the appendices
The appendices are where the working figures sit. Appendix 3 has time/current characteristics and maximum Zs values for protective devices. Appendix 4 has the tabulated cable current-carrying capacities, installation reference methods and correction factors. Appendix 6 holds the model certificate and report forms.
Most calculations you do on a job — cable sizing, volt drop, maximum Zs — pull their inputs from these appendices, which is why they are the most thumbed pages in the book.
Skip the table lookups
TradePlanr's calculators build the Appendix 4 correction factors and Appendix 3 Zs limits in, so you enter the job details and get the compliant answer without flicking between tables.
Keeping current with amendments
BS 7671 is updated by amendments rather than full re-issues. The 18th Edition landed in 2018, Amendment 1 was a corrigendum, and Amendment 2:2022 introduced AFDD requirements, prosumer (Section 712) installations and new model forms.
Amendment 4:2026 is the next significant change. Working to the current edition matters because your certification declares the installation complies with a specific version of the standard.
Frequently asked questions
Is BS 7671 a legal requirement?
Not directly. BS 7671 is a British Standard, not statute. But it is the recognised way of meeting the legal duties in the Electricity at Work Regulations 1989 and Building Regulations, so working to it is how you demonstrate compliance.
What edition is current?
The 18th Edition (BS 7671:2018) incorporating Amendment 2:2022 is current, with Amendment 4:2026 following. Always certify to the edition and amendment in force when the work is done.
Where do I find maximum Zs values?
Maximum earth fault loop impedance values for protective devices are in Appendix 3, and the tabulated cable ratings and correction factors are in Appendix 4.
From guidance to action
Related guides
BS 7671 Amendment 4:2026: What’s Changing
The headline changes in Amendment 4:2026 — AFDD scope, model forms, earthing and what to check before it lands.
Special Locations (Part 7) Explained
Why Part 7 exists, the locations it covers, and how bathroom zones and supplementary bonding work.
Consumer Unit Regulations (BS 7671)
The regulations behind consumer units — non-combustible enclosures, RCD/SPD/AFDD provision and RCBO boards.
