AFDD vs RCD vs RCBO — When Each Actually Matters

MCB (Miniature Circuit Breaker) — overcurrent (overload or short circuit). Trips a circuit when too much current flows.

RCD (Residual Current Device) — earth leakage. Trips when current flowing out doesn't equal current flowing back, indicating leakage to earth (e.g. through a person).

RCBO — MCB + RCD in one device. Both functions on a single circuit.

AFDD (Arc Fault Detection Device) — arc faults. Detects the high-frequency electrical signature of an arcing fault from damaged cable, loose terminal, or punctured wire.

Every domestic socket circuit — 30 mA RCD protection mandatory. Has been since the 17th Edition (2008).

Every lighting circuit in domestic — 30 mA RCD mandatory from 18th Edition (2018).

Outdoor sockets, garage circuits — 30 mA RCD mandatory.

Bathroom circuits — 30 mA RCD plus supplementary bonding requirements.

Mobile equipment (caravan hookups, marinas) — 30 mA RCD mandatory.

Type AC RCDs only detect alternating fault current. Adequate for old appliances.

Type A RCDs detect AC plus pulsating DC fault current. Required where modern appliances (induction hobs, EV chargers, LED drivers, certain motor controls) create DC components.

BS 7671:2018 Amendment 4 (April 2026) makes Type A protection mandatory where DC fault current is foreseeable. In practice: every domestic socket circuit and every EV charger circuit.

Older boards with Type AC RCDs are not retrospectively coded C2 — they're typically C3 (improvement recommended). New installs from October 2026 must be Type A.

HMOs — AFDD mandatory on socket circuits from October 2026.

Residential care and schools — same.

High-fire-risk premises (timber-framed, wooden floors, listed buildings) — recommended.

Standard domestic owner-occupier — not mandatory but recommended.

AFDD cost is dropping fast — £35-60 per device in 2026, down from £80-100 a few years ago. For HMO upgrades, factor 1 AFDD per socket circuit.

Split-load RCD board — fuse board has 2 RCDs, each protecting half the circuits. Cheap. Problem: any fault on one circuit trips half the board.

Full RCBO board — every circuit on its own RCBO. More expensive but only the faulty circuit trips. Modern best practice.

Cost difference for a 12-way board: split-load £180-220, full RCBO £280-380. Worth the £80-160 upgrade for any landlord install or any property with vulnerable occupants.

BS 7671 doesn't mandate RCBO over split-load, but coding works in favour of RCBO — fewer 'reduced fault protection' findings on the EICR.

Hager and Schneider both ship combined RCBO + AFDD modules — a single device handling MCB, RCD and AFDD on one circuit.

Cost: £55-80 per device. Saves space (one DIN module per circuit) and simplifies layout.

For HMO new builds from October 2026, this is becoming the standard spec — every socket circuit on a combined RCBO + AFDD.

For domestic owner-occupier, the cost is hard to justify yet. Type A RCBO + standalone AFDD on the highest-risk circuit (typically the kitchen) is the cost-effective approach.