BS 7671:2018+A4:2026
RCD, RCBO and AFDD types explained
Choosing the right residual current device is no longer a "any 30 mA RCD will do" job. Modern loads produce DC and high-frequency leakage that older Type AC devices simply can't see. Here's how Paul selects RCD, RCBO and AFDD types under BS 7671:2018+A4:2026.
RCD types by residual waveform
Type AC
Detects: Sinusoidal AC residual currents only.
Historically the default. Now considered inadequate for most modern loads because so many appliances have electronic front ends. BS 7671 restricts its use — do not select Type AC for a new circuit without justification.
Type A
Detects: AC residual currents and pulsating DC residual currents up to about 6 mA smooth DC.
The new practical default for most domestic and commercial final circuits — sockets, lighting, immersion, most white goods and Class I appliances with switched-mode power supplies.
Type F
Detects: As Type A, plus higher-frequency and composite waveforms from single-phase variable-speed drives.
Circuits feeding single-phase inverter-driven equipment — many modern washing machines, tumble dryers, dishwashers and single-phase VSDs.
Type B
Detects: AC, pulsating DC, and smooth DC residual currents.
Mandatory (or a Type A with a DC-fault detection device) for most Mode 3 & 4 EV charge points — see Section 722. Also required by many three-phase VSDs and DC-coupled PV / battery systems where smooth DC leakage is possible.
RCBOs — combined MCB and RCD protection
An RCBO (Residual Current Breaker with Overcurrent protection) does two jobs in one device: it protects the circuit against overload and short-circuit like a normal MCB,and it trips on earth leakage like an RCD. That means a fault on one circuit only knocks out that circuit, not every circuit downstream of a shared RCD.
- Typical rating
- 6 A – 45 A, B or C curve, 30 mA residual sensitivity, single-pole or 1P+N. Higher ratings and Type A/F/B versions are available for EV chargers, solar PV and three-phase loads.
- Where Paul uses them
- All domestic circuits — excluding sub-mains where a suitably protected cable is already installed — and most industrial circuits, although some exclusions exist depending on the installation design.
- Why they matter
- RCBOs give discriminated protection. A kettle fault in the kitchen won't plunge the fridge, freezer, broadband and alarm into darkness. They are now the default choice for most new consumer unit upgrades.
How UK consumer units have evolved
The layout of the fuse board itself has changed almost as much as the devices inside it. Understanding which generation is on the wall helps explain why a single fault can take out half a house — or just one circuit.
1. Split-load consumer units (late 1980s – early 2000s)
A single 30 mA RCD protected one bank of MCBs — typically sockets and anything feeding outdoors — while the other bank (lights, cooker, immersion, alarms) ran on MCBs alone with no earth-leakage protection. Better than nothing, but a fault on any protected circuit tripped the whole RCD side.
2. Dual-RCD boards (BS 7671:2008, 17th Edition)
Amendment 1 of the 17th Edition effectively required additional protection by 30 mA RCD on most final circuits. Boards were split into two RCD banks so a fault only knocked out half the circuits — but lights and sockets in the same room were deliberately placed on different RCDs so you were never left completely in the dark.
3. High-integrity dual-RCD boards
An evolution of the dual-RCD design with three neutral bars: two RCD-protected and one dedicated to a non-RCD section. Fire alarms, burglar alarms, freezers and other circuits where nuisance tripping is unacceptable could sit on RCBOs in the non-RCD section, keeping them independent of the two shared RCDs.
4. All-RCBO boards (BS 7671:2018 onwards — today's default)
Every final circuit gets its own RCBO — full discrimination, so a fault on one circuit only affects that circuit. This is what Paul fits on new installations and consumer unit changes, typically with Type A or Type F RCBOs and, where required by BS 7671 Amendment 2, AFDDs as well.
AFDDs — Arc Fault Detection Devices
A modern AFDD is an MCB, an RCBO and an arc fault detection device all rolled into one single-module unit. On top of tripping for overload, short circuit and earth leakage, it also detects the high-frequency signature of series and parallel arc faults — loose terminals, damaged cables, rodent-chewed insulation — before they cause fire. Under BS 7671 Amendment 2 (2022), AFDDs are required for single-phase AC final circuits ≤32 A supplying socket-outlets in:
- Higher-risk residential buildings (HRRB) as defined in Building Regulations,
- Houses in multiple occupation (HMOs),
- Purpose-built student accommodation, and
- Care homes.
For all other premises — including ordinary domestic — AFDDs are recommended. Paul routinely offers them on rewires and consumer unit changes because they close a real fire-risk gap that MCBs and RCDs can't.