Amendment 3 (A3:2024) to BS 7671 introduced Regulation 530.3.201, requiring designers and installers to take account of whether a unidirectional or bidirectional protective device is appropriate — and to follow any orientation marking on the device. This guide explains what it means, which installations are affected, and how to comply.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Regulation 530.3.201, introduced by BS 7671:2018+A3:2024, requires that the selection and erection of equipment for protection takes account of the appropriate use of either a unidirectional or a bidirectional protective device. It matters for solar PV, battery storage and V2G EV chargers, where current can flow back towards the supply. A NOTE adds that some devices are marked (in/out, line/load, arrows) to indicate direction.
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Regulation 530.3.201 is a new regulation introduced by Amendment 3 (A3:2024) to BS 7671:2018. It addresses a specific safety concern: the selection and erection of protective devices in installations where electrical current can flow in both directions through those devices.
The regulation requires that those selecting and erecting protective equipment take account of the appropriate use of either a unidirectional or a bidirectional protective device. The designer or installer shall determine which directional characteristic is appropriate for the application. Where a device is marked to indicate its orientation (for example "in"/"out", "line"/"load", or arrows), the installer shall connect it in accordance with that marking.
This regulation was added because the UK is seeing rapid growth in domestic and commercial installations that include embedded generation (solar PV), battery energy storage systems (BESS), and vehicle-to-grid (V2G) EV chargers. All of these technologies can cause current to flow "backwards" through devices that were historically designed only for unidirectional current flow.
Prior to Amendment 3 (A3:2024), there was no explicit regulation in BS 7671 requiring designers and installers to consider the directional characteristics of protective devices. Electricians relied on manufacturer guidance and good practice, but there was no regulatory framework mandating it. Regulation 530.3.201 fills this gap.
“Selection and erection of equipment for protection shall take account of appropriate use of either a unidirectional protective device or a bidirectional protective device.”
NOTE: Product standards as listed in Appendix I, for some protective devices including RCCBs, RCBOs, circuit-breakers and AFDDs, require these devices to be marked to indicate if they are unidirectional — e.g. “in” and “out”, “line” and “load”, or arrows.
The NOTE to Regulation 530.3.201 states that product standards (listed in Appendix I of BS 7671) for some protective devices — including RCCBs, RCBOs, circuit-breakers, and AFDDs — require those devices to be marked to indicate if they are unidirectional (e.g. “in”/“out”, “line”/“load”, or arrows). Where such marking is present, connect the device in line with it. Before signing off a solar PV, battery storage, or V2G installation, check every protective device in the bidirectional current path for orientation markings and confirm they are installed accordingly.
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BS 7671:2018+A3:2024 was issued on 31 July 2024 by the IET. It is a supplementary amendment to the base standard (BS 7671:2018, commonly known as the 18th Edition) and to Amendment 2 (A2:2022). A3:2024 is a small, focused document available as a free PDF from the IET website — it is not a new book. Its primary addition is Regulation 530.3.201 (bidirectional and unidirectional protective devices), together with two supporting definitions.
BS 7671:2018+A4:2026 (Amendment 4) followed in 2026. Amendment 4 is the current amendment and, among other changes, redrafted Reg 421.1.7 on arc fault detection devices (AFDDs), inserted a new Reg 411.6.5, and added a new Regulation group 419 for situations where automatic disconnection in accordance with Regulation 411.3.2 is not feasible. The full current citation for the standard is BS 7671:2018+A4:2026.
The current full citation for the standard is BS 7671:2018+A4:2026. When referencing the standard on certificates and documentation, use this full citation to confirm compliance with the latest amendment. The 18th Edition guide covers the base standard in detail.
Amendment 4 (A4:2026) extends beyond the bidirectional device rules introduced by A3:2024. Key additions in A4:2026 include:
See the full A4:2026 changes guide for a complete overview.
Understanding the difference between bidirectional and unidirectional switching devices is essential for compliance with Regulation 530.3.201. BS 7671 Part 2 (Definitions) defines both terms by reference to how the manufacturer intends the supply to be connected:
Unidirectional protective device. A protective device where it is intended by the manufacturer that a source of supply is only connected to one defined set of connection terminals.
Bidirectional protective device. A protective device where it is intended by the manufacturer that a source of supply is connected to either or both sets of connection terminals.
Designed to interrupt current flowing in one direction only — from supply to load. The internal arc-quenching mechanism is optimised for this direction of current flow.
Standard MCBs, most RCBOs, and many isolators are unidirectional. They will function as a switch in either direction, but the arc-interruption capability may not be guaranteed for reverse current flow. Under a high-energy fault in the reverse direction, the device may fail to extinguish the arc safely.
Designed and tested to interrupt current flowing in either direction — from supply to load and from load to supply. The arc-quenching mechanism works equally well regardless of current direction.
Bidirectional devices are specifically tested to IEC standards for both directions of current flow and are marked as suitable for bidirectional operation. They may carry a specific symbol or marking on the rating plate.
Not all devices clearly state whether they are bidirectional or unidirectional on their front plate. Check the manufacturer's technical datasheet or installation instructions for the specific device. If in doubt, contact the manufacturer. Major manufacturers (Hager, Schneider, Siemens, ABB) now clearly state bidirectional suitability in their product documentation.
Solar PV installations are the most common application of Regulation 530.3.201 in domestic work. A solar PV system with a grid-tied inverter generates AC power that can flow in two directions: to the property's loads (self-consumption) and to the grid (export).
The key devices that experience bidirectional current in a typical domestic grid-tied solar PV installation are set out below:
When specifying protective devices for a solar PV installation, confirm with the device manufacturer that each device in the bidirectional current path is rated for bidirectional operation. Document this on the installation certificate.
Battery energy storage systems (BESS) create bidirectional power flow because the battery charges from the grid or PV system (consuming power) and discharges to supply loads or export to the grid (generating power). This makes every switching device in the battery circuit path subject to Regulation 530.3.201.
The battery storage system's AC connection to the consumer unit or distribution board carries current in both directions during normal operation — not just under fault conditions. This is a key distinction from some solar PV installations where reverse current flow occurs only during export. Battery systems routinely cycle between charge and discharge multiple times per day.
Hybrid inverters that combine solar PV and battery storage in a single unit are particularly relevant — the single AC output circuit carries bidirectional current from both the PV generation and the battery discharge.
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Achieving compliance with Regulation 530.3.201 is straightforward in practice, but it requires awareness and a change in procurement habits. Here is a practical workflow:
Elec-Mate solar PV and battery storage certificates include fields for confirming bidirectional device selection per Regulation 530.3.201.
Try it free for 7 daysMajor switchgear manufacturers now produce devices specified for prosumer (generation + storage) installations. Use the table below as a starting checklist, then confirm bidirectional suitability against the manufacturer's datasheet for the specific catalogue number before you order.
Manufacturer names and ranges change frequently — always verify the current datasheet for the exact catalogue number rather than relying on a brand having a suitable device in the past.
When issuing an EIC for an installation with bidirectional power flow, the certificate should reference compliance with BS 7671:2018+A4:2026 (not just BS 7671:2018 or A2:2022). This confirms that the installation has been designed and installed in accordance with the latest amendment, including Regulation 530.3.201.
The certificate should specifically note:
For EICRs on existing installations with solar PV or battery storage, check whether the installed devices are bidirectional-rated. If unidirectional devices are found in the bidirectional current path, raise a C3 observation recommending replacement with bidirectional-rated devices.
Elec-Mate certificates for solar PV, battery storage, and EV installations automatically reference BS 7671:2018+A4:2026.
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