MCBs, RCDs and RCBOs Explained: Which Circuit Protection Does Your Install Need?

MCBs, RCDs, and RCBOs all protect circuits — but they do different jobs, and speccing the wrong one causes callbacks or fails inspection. Here is a straight breakdown for electricians: what each device does, when to use an MCB versus an RCBO, and how the 18th Edition affects which you fit.

MCB (Miniature Circuit Breaker): Overcurrent Protection Only

An MCB protects against overload and short circuit. It trips and isolates the circuit when either occurs. What it does not do: it provides no protection against earth leakage or electric shock. If a live conductor contacts a person but there is no short circuit, the MCB will not trip. That is the RCD's job.

The standard domestic spec is Type B, which trips at 3 to 5 times rated current. This suits most residential loads: lighting circuits, ring finals, immersion heaters. Type C (5 to 10 times rated current) is for motors and commercial inrush loads. On domestic work, Type B is almost always the correct spec.

The Axiom 10A SP Type B MCB and Axiom 32A SP Type B MCB cover the ratings you will use on most domestic lighting and ring final circuits. MK equivalents are also stocked for boards that need MK compatibility.

RCD (Residual Current Device): Earth Leakage Protection Only

An RCD monitors the balance between live and neutral current. If more than 30mA is leaking to earth — indicating a fault or shock path — it trips in under 40 milliseconds. That is fast enough to prevent electrocution in most fault conditions.

What an RCD does not do: it provides no protection against overload or short circuit. An RCD is not a substitute for an MCB. In a split-load consumer unit, both work together: the RCD protects banks of circuits against earth leakage, the MCBs protect individual circuits against overcurrent.

The Axiom 40A DP RCD 30mA is the standard 2-module device for split-load boards. An 80A version is available where higher load totals are required.

RCBO (Residual Current Breaker with Overcurrent): Both Functions, One Module

An RCBO combines MCB and RCD protection in a single module — overcurrent and earth leakage protection for one circuit, one device. Under BS 7671:2018 (18th Edition), individual circuit RCD protection is the preferred approach for new domestic installations. RCBOs are the standard way to achieve this cleanly.

The key practical benefit: if a fault develops on one circuit, only that RCBO trips. With a split-load board using RCDs, a single fault takes out all circuits on that RCD — half the house goes dark, and the client cannot identify which circuit caused the problem. RCBOs eliminate nuisance tripping of unrelated circuits entirely, which is a significant advantage for occupied properties.

The Axiom RCBO 32A SP Type A Mini is a 1-module device — important when working within consumer unit space constraints. Also available in 6A, 16A, and 20A for lighting and smaller circuits.

Which to Fit: A Practical Guide

• New domestic installation (18th Edition): Individual RCBOs per circuit is the preferred approach. More modules, marginally higher materials cost per circuit, but cleaner, future-proof, and no nuisance tripping.
• Split-load consumer unit retrofit: MCBs on each circuit with RCDs protecting each bank. Acceptable under 18th Edition as long as all circuits have RCD coverage. Lower cost per module.
• Adding a circuit to an existing board: Match what is already fitted. RCBO board — add an RCBO. Split-load with RCDs — add an MCB on the correct RCD side.
• Garage, shed, or outbuilding sub-board: Any socket circuit feeding outdoors or a detached structure needs 30mA RCD protection. An RCBO is the cleanest solution for one or two circuits where a full split-load board is not warranted.

Type A vs Type AC: Why It Matters

Modern inverter-driven appliances — EV chargers, heat pumps, solar inverters — can produce DC fault currents that a standard Type AC RCD will not detect. Type A devices detect both AC and pulsating DC fault currents and are required by the 18th Edition for circuits feeding these loads. The Axiom RCBOs stocked at APM are Type A — the correct spec for any modern domestic installation.

Same-Day Collection — Acton, West London

APM Electricals stocks Axiom and MK MCBs, RCDs, and RCBOs at our trade counter in Acton. Browse the full circuit breaker and RCBO range or pick up same day.

APM Electricals
24 Western Avenue, Acton, London W3 7TZ
Tel: 020 8702 8080
www.apmi.uk