RCBO vs RCD vs MCB — What's the Difference? (London Electrician's Guide)

Yes, by default. BS 7671 Amendment 4:2026 expects 30 mA additional protection on essentially every final circuit in a dwelling, with very narrow exceptions (some industrial control circuits, certain medical-locations setups). In practice, in a domestic London property, every circuit gets a 30 mA Type A RCBO.

Sometimes. The existing board must be a current-manufacturer model (Hager, Wylex, MK, Crabtree etc.) with a compatible RCBO range still in production, and the busbar must accept the wider RCBO module. Older boards (pre-2010), discontinued manufacturers, and any board where the manufacturer's compatible RCBO is no longer made will be cheaper to replace outright than to retrofit.

TT installations (where the earth electrode is local rather than supplied by the DNO) have always required 30 mA RCD protection on every circuit, because earth-fault loop impedance is too high to rely on overcurrent devices for disconnection. RCBO-per-circuit was effectively mandatory on TT systems long before A4:2026 — the amendment harmonises TN-S, TN-C-S and TT requirements around the same standard.

When several circuits share one RCD, every circuit contributes a small amount of background earth leakage from electronic loads (LED drivers, switch-mode power supplies, IT equipment, white goods). The leakage sums against a single 30 mA threshold. A fault on any one circuit, or a fridge starting at the wrong moment, pushes the total above 30 mA and the whole bank trips — including circuits with no actual fault.

30 mA is the residual current threshold at which the device must operate within 40 ms — the level set to prevent ventricular fibrillation in the average adult. Higher-sensitivity (10 mA, 6 mA) devices exist for special locations like medical and bathroom shaver outlets; lower-sensitivity (100 mA, 300 mA) devices are used for fire-protection rather than personal shock protection.

Type AC detects only AC fault current — adequate for resistive and conventional inductive loads. Type A also detects pulsating DC — required for induction hobs, EV chargers, modern LED drivers, variable-speed appliances. Type B detects smooth DC — required for some three-phase variable-speed drives and certain EV charger topologies. Type A is the domestic default under A4:2026; Type B is specialist.

AFDD is mandatory under A4:2026 on socket-outlet circuits in HMOs, residential care premises, schools and student accommodation. For owner-occupied dwellings it is recommended on bedroom and lounge socket circuits — where most domestic electrical fires originate. Adding AFDD at the point of board change is £35 per circuit; retrofitting later is typically £85.

An MCB protects only the cable from overcurrent (overload or short circuit). It does not detect earth leakage and offers no personal-shock protection. An RCBO does the same overcurrent job as an MCB and adds 30 mA earth-leakage detection in the same module. Under modern BS 7671 you cannot install an MCB on a circuit that requires additional protection — you fit an RCBO.