RCBO vs MCB vs RCD — What's Actually in Your Fuse Board

MCB — Miniature Circuit Breaker. Protects against overcurrent: overload (too many appliances drawing current on one circuit) and short circuit (live touches neutral or earth). An MCB will NOT protect a human against electric shock. Trip rating is in amps — typically 6A for lighting, 16A for radial sockets, 32A for ring final circuits.

RCD — Residual Current Device. Detects an imbalance between live and neutral, which indicates current leaking to earth — for example through a person touching a live conductor. Trips in under 40 milliseconds at 30mA. An RCD does NOT protect against overload or short circuit. Trip rating is in milliamps.

RCBO — Residual Current Breaker with Overcurrent protection. One device, one circuit, both protections. The modern default. Slightly more expensive per pole but eliminates the entire family of nuisance-tripping problems caused by shared RCDs.

You plug a hairdryer into a faulty extension lead. The damaged inner cable arcs to the earth conductor. The RCD (or RCBO) detects the imbalance and trips in 25ms — fast enough that you feel a jolt but suffer no lasting harm. The MCB does nothing here — the current is well under the overload threshold.

You plug a kettle, microwave, and toaster into the same kitchen socket via an adaptor. Total load 13A on a 13A spur. The MCB or RCBO trips on overload after 30–60 seconds. The RCD does nothing — there is no earth leakage. The protection has just stopped your kitchen flex catching fire.

A nail through a wall pierces a buried cable, shorting live to neutral inside the plasterboard. The MCB or RCBO trips instantly on short-circuit current — milliseconds. The RCD may also trip if there is enough earth-leakage in the resulting fault arc. Without overcurrent protection, the cable would melt and ignite before anything else stopped it.

A washing machine motor degrades and starts leaking 35mA to earth through damp internal insulation. An RCD or RCBO trips. An MCB-only circuit just keeps running until someone touches the chassis — at which point the chassis becomes the path to earth.

BS 7671:2018 requires that final circuits up to 32A serving socket outlets used by ordinary persons have 30mA RCD protection. Amendment 4 (published April 2026, mandatory for new work from October 2026) extends this further — virtually all circuits in domestic premises now require RCD-class protection at the consumer unit.

The standard also requires discrimination — a fault on a downstream circuit must trip the downstream device, not the main switch. With shared RCDs covering 4–6 circuits each, a fault on one circuit can take out a whole bank. With RCBOs per circuit, only the faulty circuit drops.

For Amendment 4 new-build and rewires, individual RCBO per circuit is now the default specification for any installation a serious contractor will warranty.

A dual-RCD board splits the circuits across two RCDs — typically 6 circuits each. It is the cheap-and-cheerful 2015-era specification. The problem: when one RCD trips, half the house goes off, and you cannot easily tell which circuit caused the fault.

An all-RCBO board has one RCBO per circuit. Cost difference at install: roughly £150–£300 on a typical 10-way consumer unit. Benefit: a fault on the bathroom circuit takes out the bathroom only. The fridge keeps the freezer cold, the router keeps the alarm online, the lights keep the stairs safe.

For HMOs, all-RCBO is now effectively mandatory in spirit if not in regulation — a single shared RCD that takes out half a property serving multiple tenants is a serviceability nightmare and increasingly flagged as a C3 (or C2 in shared sleeping accommodation) on EICRs.

Type AC RCDs detect AC fault current only. Type A RCDs detect AC plus DC components. Modern appliances — LED drivers, EV chargers, induction hobs, computer PSUs, home batteries — generate DC fault components that can blind a Type AC device, preventing it from tripping when it should.

Amendment 4 effectively retires Type AC for general use in new installations. Any new consumer unit serving circuits that may carry DC fault current (which now includes basically all socket and lighting circuits in a modern home) should be specified with Type A or higher.

On EICRs, a Type AC RCD on a circuit clearly capable of DC fault current — induction kitchen, EV charger, dedicated comms cabinet — is now routinely coded C2. Type AC on a domestic lighting-only circuit still tends to attract C3 (improvement recommended) but the trend is upward.

Triggers for a full fuse-board upgrade: any current EICR with multiple C2 codes against the consumer unit itself; any plastic or wooden consumer unit (metal-clad has been required since the 2016 amendment); any rewireable fuses; any board pre-dating the dual-RCD era and with no RCD protection on circuits serving wet areas.

A typical London fuse board upgrade — new metal-clad consumer unit, all-RCBO, Type A devices, SPD included — runs £650–£950 for a 10–12 way board fitted, certified, and registered. Add £80–£140 for an integrated SPD if not standard.

Upgrade at the same visit as an EICR remediation and you collapse two compliance trips into one. The EICR is then re-issued against the new board, satisfactory on day one, with a fresh 5-year clock.

Specifying for today's load without thinking about tomorrow's. A 10-way consumer unit is fine for a property with one EV charger, no heat pump, no solar. Add a heat pump in 2027 and a battery in 2028 and you are out of ways — meaning either a tandem board, a sub-board, or a complete swap. Add 4 spare ways at the original install to absorb future loads.

Buying the cheapest board on the market. Hager, Wylex, MK Sentry, and Schneider Square D dominate UK domestic installs. They are not all the same. The cheaper imports often have inferior contact reliability and shorter spare-parts availability. A £45 saving on the board itself buys you future headaches.

Ignoring SPD (Surge Protection Device). Amendment 4 makes SPD effectively mandatory in most domestic installations from October 2026. Fit it now at install (£80–£140 incremental) rather than retrofit at the next EICR cycle.

Misunderstanding selectivity at the main switch. A standard 100A main switch in a domestic consumer unit is not RCD-protected. Adding a 100A 30mA RCD as a main switch creates a single point of failure for the whole house — bad design. Use per-circuit RCBOs instead, and let the main switch be a simple non-RCD device.

Arc Fault Detection Devices (AFDDs) detect series and parallel arcing — the precursor to most domestic electrical fires. They are not RCDs and they are not MCBs; they protect against a different fault class. From Amendment 4 (mandatory for new work from October 2026), AFDDs are required on socket circuits in higher-risk premises including HMOs, care homes, and residential properties in tall buildings.

A modern board can combine RCBO + AFDD functionality in a single 1-module device, which keeps the board footprint compact. Hager AFDD-RCBOs, Schneider iC60 AFDD, and Wylex NHXSAFDD are the units most commonly fitted in London at the moment.

AFDDs trip on real arcing events but are designed to ignore the brief arcs of normal switching (light switches, motor start, contactor close). False trips on modern AFDDs are rare — early-2010s versions had a reputation that the 2024-vintage devices have largely cleared up.

Cost: AFDD-RCBOs add £18–£35 per circuit over plain Type A RCBOs. For a 10-way board, an all-AFDD specification adds £180–£350 to the install. For HMOs and other higher-risk applications where AFDDs are now expected, the cost is unavoidable — and a property that already has AFDDs on the circuit register reads as future-proofed at the next EICR cycle.