Glossary · Consumer units and protection
Key points
- An MCB protects against overcurrent only. An RCBO adds protection against current leaking to earth.
- With MCBs, shock protection comes from an RCD that covers a group of circuits.
- With an RCBO on every circuit, a fault switches off that circuit and leaves the others on.
- NICEIC says most, if not all, circuits in a home should be RCD-protected.
- Fitting or changing either is work for a registered electrician.
How MCBs and RCBOs work
A circuit breaker (MCB) protects a circuit against overcurrent and short circuit. It does not detect current leaking to earth. The IET says protection from electric shock comes from an RCD, and that RCDs are rated in milliamps, so they are thousands of times more sensitive than circuit breakers or fuses.[3]IET, Consumer units: safe fuse-box upgrades for modern homes
An RCBO, in the IET’s words, is a residual current circuit breaker with overload protection.[1]IET Wiring Matters, Which RCD Type? It watches for leakage like an RCD and protects against overload like an MCB. The result is one device that does both jobs for one circuit. The RCD vs RCBO page covers the other half of this comparison.
MCB and RCBO side by side
| MCB | RCBO | |
|---|---|---|
| Overload and short circuit protection | Yes | Yes |
| Earth leakage protection for people | No | Yes |
| Needs an RCD alongside it | Yes | No |
| What a fault switches off | The circuit, or the whole RCD group first | That circuit only |
A consumer unit built with MCBs gets its shock protection from one or two RCDs placed over groups of circuits. NICEIC says a dual RCD unit divides the circuits between two RCDs, with a circuit breaker on each circuit.[2]NICEIC, Consumer units and fuse boxes: installation and replacement A fault on one circuit can switch off every circuit on that RCD.
NICEIC describes the alternative as a high integrity unit. Each circuit has either a circuit breaker or an RCBO, and no circuits are combined under one RCD. A fault then affects only the circuit it is on.[2]NICEIC, Consumer units and fuse boxes: installation and replacement A split-load type mixes the two: RCBOs on the circuits that benefit most, and shared RCDs with MCBs on the rest.[2]NICEIC, Consumer units and fuse boxes: installation and replacement
How many circuits need RCD protection
NICEIC says most, if not all, circuits in a home should be RCD-protected to cut the risk of shock.[2]NICEIC, Consumer units and fuse boxes: installation and replacement The IET adds that all luminaires in a home on an AC final circuit need additional protection by a 30 mA RCD. This applies to new lighting circuits and to existing ones that are significantly altered.[4]IET, BS 7671 FAQs: Consumer Units and Protective Devices So the choice in a modern board is usually not whether to have RCD protection. It is how to arrange it.
Example
Hannah has a consumer unit with MCBs on every circuit and two RCDs above them. Her downstairs sockets and her bathroom lights share one RCD. A fault in a hairdryer trips it and both go dark at once.
If she moved to RCBOs on those circuits, the same fault would take out the downstairs sockets only. The bathroom lights would stay on. The cost and the layout of the box would change, and a registered electrician would advise on both.
What people get wrong
The common assumption is that anything marked as a breaker protects against shock. An MCB does not. It protects the circuit. Only an RCD or an RCBO protects a person from earth leakage.
The second mistake is to think RCBOs are required everywhere. The sources describe RCBOs as one way to arrange RCD protection. NICEIC lists several layouts, including dual RCD units with MCBs.[2]NICEIC, Consumer units and fuse boxes: installation and replacement The right one depends on the home.
The bottom line
An MCB guards a circuit against overload and short circuit. An RCBO does that and guards people against earth leakage as well, on a single circuit. Most circuits in a home need RCD protection, and the choice is how to arrange it. A registered electrician should make that choice, because work in a consumer unit is not for the householder. See when to replace a consumer unit for when a change is worth considering.
Common questions
Does an MCB protect against electric shock?
No. An MCB protects the circuit from overload and short circuit. Protection from shock comes from an RCD, which the IET says is thousands of times more sensitive than a circuit breaker or fuse.
Is an RCBO just an MCB with an RCD built in?
In effect, yes. The IET defines the RCBO as a residual current circuit breaker with overload protection, so it does the work of an RCD and a circuit breaker in one device for one circuit.
Do all circuits in a home need RCD protection?
Most do. NICEIC says most, if not all, circuits in a home should be RCD-protected. The IET adds that new and significantly altered lighting circuits in a home need a 30 mA RCD.
Which is better for a home?
RCBOs limit a fault to one circuit. NICEIC describes this as the main benefit of its high integrity layout. Which layout suits your wiring is a decision for a registered electrician.
Sources
We cite the original source for every fact: legislation, GOV.UK, HSE, the IET, registration bodies and manufacturers' own documents. Each one shows the date we last checked it.
- IET Wiring Matters. Which RCD Type?. Checked 11 November 2026. ↩
- NICEIC. Consumer units and fuse boxes: installation and replacement. Checked 11 November 2026. ↩
- IET. Consumer units: safe fuse-box upgrades for modern homes. Checked 11 November 2026. ↩
- IET. BS 7671 FAQs: Consumer Units and Protective Devices. Checked 11 November 2026. ↩
