BS 7671 Amendment 3: What Changed and Why It Matters

BS 7671:2018+A3:2024 is the third amendment to the 18th Edition of the IET Wiring Regulations. It was issued by the Institution of Engineering and Technology (IET) on 31 July 2024 as a free PDF supplement. Unlike a new edition of the standard, Amendment 3 is a targeted addition — it does not replace the existing brown book (BS 7671:2018+A2:2022) but adds to it.

The amendment addresses a specific technical gap that became increasingly important as the UK transitioned to renewable energy sources and embedded generation. With more homes and commercial premises installing solar PV arrays, battery energy storage systems (BESS), and other forms of distributed generation, the electrical installation now commonly includes multiple sources of supply. This creates the possibility of current flowing in both directions through protective and switching devices — a scenario that the previous edition of BS 7671 did not explicitly address.

The core of Amendment 3 is a new regulation — Reg 530.3.201 (Chapter 53 of BS 7671) — together with two new definitions: "bidirectional protective device" and "unidirectional protective device". Reg 530.3.201 sets out the requirement to select and install protective devices appropriate to the direction of current flow they will experience. The amendment also includes associated guidance notes to help designers and installers understand the practical implications.

For electricians, the key message is straightforward: if you work on any installation that has, or may have in the future, more than one source of supply (solar PV, battery storage, generator, EV vehicle-to-grid), you need to understand this amendment and ensure the devices you select are suitable for the direction of current flow they will experience.

The Amendment 3 addition is Regulation 530.3.201, sitting within Chapter 53 of BS 7671 (Selection and Erection of Equipment). A3:2024 also introduces two new definitions — "bidirectional protective device" and "unidirectional protective device" — to support the regulation's application. Reg 530.3.201 requires that selection and erection of protective equipment shall take account of the appropriate use of either a unidirectional or a bidirectional protective device, depending on whether current can flow in one direction only or in both directions through the device.

The regulation does not mandate that all devices in every installation must be bidirectional. It is targeted: only where the installation topology creates the possibility of reverse current flow does the requirement apply. A standard domestic installation with a single DNO supply and no embedded generation is unaffected.

A bidirectional protective device can safely interrupt fault current flowing in either direction through the device. Most miniature circuit breakers (MCBs) conforming to BS EN 60898-1 are inherently bidirectional — they use a thermal-magnetic trip mechanism that responds to overcurrent regardless of the direction of flow.

You need bidirectional devices in the following scenarios:

The good news is that most standard MCBs used in UK consumer units are already bidirectional. The issue arises more commonly with moulded case circuit breakers (MCCBs), isolators, and certain specialist switching devices that may only be rated for unidirectional operation.

A unidirectional protective device is designed to interrupt current flowing in one direction only — typically from the supply (line) terminal to the load terminal. If fault current flows in the reverse direction through a unidirectional device, it may not trip correctly. In the worst case, the device could fail to disconnect the fault at all, leading to:

The practical lesson is simple: before installing any protective or switching device in a circuit that includes embedded generation, check the manufacturer documentation to confirm whether the device is bidirectional or unidirectional. If the datasheet does not explicitly state bidirectional capability, assume it is unidirectional and either choose a different device or contact the manufacturer for clarification.

For the majority of day-to-day domestic electrical work — socket additions, lighting circuits, consumer unit changes on single-supply installations — Amendment 3 does not change your working practices. The requirement is specifically triggered by installations with multiple sources of supply.

However, the proportion of installations with embedded generation is growing rapidly. The UK government's net zero targets, the Smart Export Guarantee (SEG), and falling solar PV prices mean that more domestic and commercial properties are adding solar, battery storage, and V2G EV charging every year. As an electrician, you will increasingly encounter these installations, and you need to be prepared.

If you are studying for your C&G 2391 inspection and testing qualification, expect questions on Amendment 3 to appear in exam papers. The examiners will be looking for your understanding of when bidirectional devices are required and how to identify installations that are affected.

The driving force behind Amendment 3 is the rapid growth of solar PV and battery storage installations in the UK. When a solar PV system is connected to a domestic consumer unit via an AC-coupled inverter, the inverter generates AC current that flows back through the consumer unit and into the grid. This reverse current flow passes through the main switch, the meter tails, and potentially through other protective devices depending on the connection arrangement.

Battery energy storage systems add another layer of complexity. A BESS can charge from the grid (drawing current from the supply), charge from solar PV (drawing current from the inverter), and discharge to the installation (supplying current to loads). The direction of current flow through the protective devices changes dynamically depending on whether the battery is charging, discharging, or idle.

AI circuit designer for solar PV installations

Elec-Mate's AI circuit designer handles bidirectional device selection automatically. Describe the installation — solar PV capacity, battery storage…

Try it free for 7 days

Amendment 3 has direct implications for how you complete electrical certificates. When issuing an Electrical Installation Certificate (EIC) for a new installation or alteration that includes embedded generation, you must confirm that the design complies with Reg 530.3.201 (A3:2024) — the bidirectional device selection requirement.

On the EIC, the design section should note that bidirectional current flow has been considered and that all protective and switching devices in the circuit path are suitable. If you are using Elec-Mate to produce the certificate, the app prompts you to record embedded generation details and flags any device compatibility issues.

When carrying out periodic inspection and testing (EICR), you should check whether the existing installation includes any embedded generation. If it does, verify that the protective devices are suitable for bidirectional operation. If a unidirectional device is found in a bidirectional circuit path, record it as a C2 (Potentially Dangerous) observation referencing Reg 530.3.201 (A3:2024).

Remember that Amendment 3 is not retrospective. An installation that was compliant with BS 7671:2018+A2:2022 at the time of installation is still compliant. However, if you identify a genuine safety issue (a unidirectional device in a bidirectional circuit path that could fail to disconnect a fault), that should be recorded as a defect regardless of the regulation edition in force at the time of installation.

Keeping track of regulation changes is one of the biggest challenges for working electricians. Amendment 3 is a relatively small change, but it is easy to miss — and getting it wrong on a solar PV or battery storage installation could result in a serious safety issue and a defective certificate.

Stay ahead of regulation changes

Elec-Mate updates its regulation references, AI agents, and training content with every BS 7671 amendment.

Try it free for 7 days