BS 7671 Amendment 2:2022 (the 18th Edition, second amendment) substantially extended the scope of mandatory RCD protection. What was once limited to specific locations — bathrooms, kitchens, gardens — now applies to virtually every final circuit in new domestic installations, and to many circuits in commercial premises. Understanding exactly which circuits must be protected, and what type of RCD to use, is essential for compliant design.
What an RCD does
A residual current device monitors the difference between the current flowing in the line conductor and the current returning in the neutral conductor. In a healthy circuit these are equal. If a person touches a live conductor, some current flows to earth through the person rather than returning via neutral — creating a residual current. The RCD detects this imbalance and disconnects the circuit within milliseconds.
For personal protection, the trip threshold is 30 mA and the device must disconnect within 40 ms at that current (per BS EN 61008). At 150 mA (five times the threshold), the device must trip in less than 40 ms — ensuring disconnection before a potentially fatal cardiac event.
Types of RCD
The type of RCD matters because modern loads produce DC components in their earth leakage currents that can blind certain RCD types.
- Type AC — detects sinusoidal AC residual currents only. Not suitable for circuits feeding electronic equipment, variable-speed drives, EV chargers, or switch-mode power supplies. Being phased out of domestic use.
- Type A — detects AC and pulsating DC residual currents. Required for most modern circuits per BS 7671 Regulation 531.3.3. Covers the vast majority of domestic and light commercial loads.
- Type F — detects AC, pulsating DC, and composite high-frequency currents. Required for circuits feeding variable frequency drives and some HVAC equipment.
- Type B — detects all of the above plus smooth DC residual currents. Required for EV charging equipment (Mode 3 AC charging and Mode 4 DC charging), certain medical equipment, and some industrial drives.
Regulation 531.3.3 now generally requires Type A or better for any circuit supplying equipment likely to produce DC components. In practice, this means specifying Type A as the minimum for any circuit connected to modern electronic equipment, and Type B or F where specifically required.
Circuits requiring 30 mA RCD protection under BS 7671:2018+A2:2022
Domestic premises
Regulation 411.3.4 now requires 30 mA RCD protection for all final circuits in a domestic premises, with the following key categories:
- All socket outletsup to and including 32 A, unless installed in a designated medical location or another exempted location (Regulation 411.3.4(i)).
- Mobile equipment outdoors — any socket outlet that may be used for equipment outside the equipotential zone.
- Cables buried in a wall or partitionat a depth less than 50 mm, unless the cable is protected by an earthed metallic cover or is a MICC cable (Regulation 522.6.202). This catches most domestic wiring run in studwork or plastered walls.
- Cables buried in a floor or ceilingat a depth less than 50 mm from the surface (Regulation 522.6.203).
- Lighting circuits— Regulation 411.3.4(vii) now requires 30 mA protection for all lighting circuits in domestic premises (not just bathrooms and kitchens). This is the most significant change in Amendment 2.
- Electric vehicle charging equipment requires at minimum a Type A RCD; a Type B or EV-specific device is required where the charging equipment does not have one built in.
Non-domestic premises
In commercial and industrial premises, Regulation 411.3.3 requires 30 mA protection for:
- Socket outlets rated up to and including 32 A where the location presents a shock risk (all socket outlets accessible to the public, all socket outlets in rooms used for medical procedures, etc.).
- Luminaires in bathroom zones and swimming pool zones.
- Equipment in high-risk locations as defined by the local authority or enforcing authority.
Exemptions
Not every circuit in a non-domestic installation requires 30 mA RCD protection. Where a documented risk assessment demonstrates that RCD protection is inappropriate — for example, where loss of supply would create a greater risk than the shock hazard (certain industrial processes, medical life-support equipment) — it may be omitted provided alternative protective measures are in place. Any exemption must be recorded in the installation schedule.
Socket outlets connected to a named user (a dedicated workstation in a server room, for example) are lower risk than general-purpose sockets because the person using them is identifiable and can be trained. Some designers omit RCDs from such circuits, but this requires a documented justification.
RCD protection for cables in walls
The rule about cables buried less than 50 mm deep catches almost all domestic installations. The only alternatives to RCD protection for such cables are:
- Running the cable at a depth of 50 mm or more (impractical in most domestic construction).
- Using a cable with an earthed metallic sheath (MICC, SWA) where the sheath provides mechanical protection.
- Running the cable in earthed conduit or trunking that provides equivalent mechanical protection.
For a standard domestic rewire using twin-and-earth cable run in the plaster, RCD protection is effectively mandatory for every circuit.
Split-load boards and RCBO-only boards
Two common approaches to implementing RCD protection:
A split-load consumer unit has one main switch and two RCCBs, each protecting a group of MCBs. This provides the minimum required protection but means a single fault takes out all circuits on that RCD — a real problem in a kitchen. Also, split-load boards are no longer sufficient where all lighting circuits require protection, because you need to ensure no single RCD trip removes all lighting.
An RCBO-only board fits an RCBO (combined MCB and RCD) in each way. Every circuit has independent protection — a fault on one circuit trips only that circuit. Higher initial cost, but significantly better in practice. For new domestic installations post-Amendment 2, RCBO boards are increasingly the standard choice.
Testing RCDs on site
Every RCD must be tested after installation per BS 7671 Chapter 64 and BS EN 61557. The test sequence involves applying a residual current of 50% of the rated trip threshold (the device must not trip), then 100% (must trip within 300 ms for non-S-type), then at 5× rated current (must trip within 40 ms). Record all results on the electrical installation certificate.
For related disconnection time calculations, see our guide on earth fault loop impedance, which explains how Zs determines whether your protective device will disconnect in time. The earthing conductor calculator sizes the protective conductor that works with your RCD to complete the fault path.