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RCBO vs MCB: Key Differences and When to Use Each One
Updated October 2026 · Meteor Electrical guides · 8 min read
Quick answer: Use an MCB where a circuit only needs overload and short-circuit protection and is already covered by an upstream 30 mA RCD. Use an RCBO where you want each circuit to have its own overcurrent and earth leakage protection, so a fault only switches off that one circuit. For most new UK and Irish boards, all-RCBO is now the popular choice.
RCBO vs MCB is one of the most common questions when planning a new consumer unit or adding a circuit. Both devices protect a single circuit and look almost identical, but they do different jobs. This guide puts them side by side, explains when to use each, and compares split-load and full RCBO boards so you can choose with confidence.
In this guide
RCBO vs MCB: what is the difference?
The short version: an MCB (miniature circuit breaker) protects the cable from too much current, while an RCBO (residual current breaker with overcurrent protection) does that and protects people from electric shock by detecting current leaking to earth. In effect, an RCBO is an MCB and an RCD built into one device.
| Feature | MCB | RCBO |
|---|---|---|
| What it protects against | Overload and short circuit | Overload, short circuit and earth leakage |
| Shock protection on its own | No, needs an upstream RCD | Yes, usually 30 mA |
| Standard | BS EN 60898-1 | BS EN 61009-1 |
| Circuits covered | One | One |
| What goes off on an earth fault | Every circuit on the shared RCD | Only the faulty circuit |
| Cost per way | Lower | Higher, but no separate RCDs needed |
| Typical width | One module | Usually one module (some two) |
| Trip curves | B, C and D | Usually B and C |
| Fault-finding | Harder on shared RCD circuits | Easier, the tripped device points to the circuit |
Neither device is "safer" in every situation. A circuit on an MCB behind a working 30 mA RCD has shock protection too. The real differences are how much of the installation goes off when something trips, how easy faults are to find, and what the board costs.
What does an MCB do?
An MCB switches off a circuit automatically when the current is too high. It has two trip mechanisms:
- Thermal trip: a bimetal strip bends when a moderate overload carries on for a while, such as too many heaters on one socket circuit.
- Magnetic trip: a coil trips the breaker almost instantly when a large fault current flows, such as a short circuit between line and neutral.
What an MCB cannot do is detect a small current leaking to earth, for example through damaged insulation or a person. A few tens of milliamps through the body can be dangerous, yet that is far too little to trip a 32 A breaker. That is why, in a modern domestic board, MCBs normally sit behind an RCD. Browse our MCBs for consumer units.
What does an RCBO do?
An RCBO contains the same thermal and magnetic protection as an MCB, plus a residual current sensor. The sensor compares the current flowing out on the line conductor with the current returning on the neutral. If the difference reaches the trip rating, usually 30 mA, the RCBO disconnects that circuit within a fraction of a second.
Because the leakage protection is built into each circuit, a fault on the kitchen sockets only trips the kitchen sockets. Every RCBO also has a test button, which should be pressed regularly as the label on your consumer unit advises. For a deeper explanation of how they work, see our guide to RCBOs.
Pro tip: Check the RCD type as well as the rating. Type A is the usual choice for domestic circuits, as it also detects pulsating DC leakage from modern electronics. BS 7671 now restricts Type AC, and some EV chargers and inverters need Type A, F or B as their instructions specify.
When should you use an MCB?
MCBs are still widely used and are perfectly compliant in the right place. They make sense for:
- Circuits on the RCD side of a split-load board. The upstream RCD provides the shock protection, and the MCB handles overload and short circuit.
- Circuits that do not need 30 mA RCD protection, where BS 7671 allows it and the designer has confirmed it, such as some sub-main feeds to another board that has its own protection.
- Commercial and industrial loads that need a Type D curve, since D-curve RCBOs are uncommon.
- Like-for-like replacement of a faulty MCB in an existing board, using the same make and range.
Fitting a 30 mA RCBO on a circuit that already sits behind a 30 mA RCD adds little benefit, because the two devices cannot discriminate and the RCD may trip first. On that side of the board, an MCB is usually the right part.
When should you use an RCBO?
Choose an RCBO wherever you want individual earth leakage protection on a circuit. Common examples are:
- New consumer units and rewires, where an all-RCBO board keeps every circuit independent.
- Circuits where an outage matters, such as fridges, freezers, alarms, home offices and medical or care equipment.
- Higher-risk locations, including bathrooms, kitchens and outdoor sockets, garden lighting, sheds and garages.
- EV chargers, heat pumps and solar or battery systems, which often need their own protective device with a specific RCD type.
- Adding a circuit to the non-RCD side of an older split-load board, where an RCBO gives the new circuit the 30 mA protection it needs.
RCBOs also help with nuisance tripping. Every appliance leaks a little current to earth, and on a shared RCD the leakage from several circuits adds together. Spreading circuits across individual RCBOs keeps each one well below its trip level. Shop our RCBOs.
Split-load vs full RCBO boards: which is better?
The RCBO vs MCB decision is really a choice of board layout. These are the three common arrangements in UK and Irish homes:
| Board layout | How it works | Pros | Cons |
|---|---|---|---|
| Dual-RCD split-load | Main switch, two 30 mA RCDs, each feeding a bank of MCBs | Lowest hardware cost; simple and familiar | One earth fault trips half the circuits; leakage from several circuits adds up on each RCD |
| High-integrity | Two RCD banks with MCBs, plus RCBOs on selected ways | Key circuits (freezer, alarm, EV charger) stay independent | Larger board; needs careful planning of which circuit goes where |
| All-RCBO | Main switch with an RCBO on every circuit | Each fault only affects its own circuit; easy fault-finding; flexible for future circuits | Higher cost per way; check RCBO type and width suit the board |
A dual-RCD split-load board can still meet BS 7671 if the circuits are arranged carefully, for example by splitting the lighting across both RCDs so one fault never leaves the house in darkness. The regulations require the installation to be divided to avoid danger and minimise inconvenience in the event of a fault, and an all-RCBO board is the simplest way to achieve that. If you are replacing a board, our consumer unit upgrade guide covers the wider choices, including surge protection.
What are the cost and space trade-offs?
Cost
An RCBO costs noticeably more than an MCB for each circuit. A split-load board saves money because two RCDs cover all the circuits, so on a board with many ways the all-RCBO option has the higher hardware cost. Against that, you save the RCDs themselves, and fault-finding call-outs are often quicker because the tripped device points straight to the faulty circuit.
Space
Most current RCBOs are single-module devices, the same 18 mm width as an MCB. An all-RCBO board does not need the two-module RCDs, so it can need fewer ways for the same number of circuits. Some RCBOs, particularly double-pole or older designs, take two modules or have a flying neutral lead, so check the exact device before choosing an enclosure.
Compatibility
Always use MCBs and RCBOs from the same manufacturer and range as the consumer unit, for example Garo devices in a Garo board. The board and its devices are tested together as an assembly, and mixing brands can affect the fit, the busbar connection and the tested ratings.
Safety: Work inside a consumer unit is dangerous, because the incoming tails stay live even with the main switch off. Changing protective devices must meet BS 7671 (I.S. 10101 in the Republic of Ireland) and may be notifiable. Use a qualified, registered electrician.
RCBO and MCB trip curves: B, C and D
Both MCBs and RCBOs carry a letter and a number, such as B32 or C16. The number is the rated current in amps and the letter is the trip curve, which sets how big a surge the device tolerates before its magnetic trip operates.
| Trip curve | Instant (magnetic) trip at | Typical circuits |
|---|---|---|
| Type B | 3–5× rated current (In) | Most domestic circuits: sockets, lighting, showers, cookers, immersion heaters |
| Type C | 5–10× In | Motors, pumps, fans, large banks of LED lighting, small transformers |
| Type D | 10–20× In | Welders, large motors and transformers with very high inrush (mainly MCBs) |
A Type C device is not "stronger" than a Type B. Overload protection is the same; it simply tolerates a bigger start-up surge. The trade-off is that it needs a lower earth fault loop impedance to disconnect in time, so never swap a B for a C to stop nuisance tripping without an electrician checking the circuit. For a full breakdown of ratings and curves, read our MCB buying guide or our overview of the different types of circuit breaker.
Key takeaways
- An MCB protects against overload and short circuit; an RCBO adds 30 mA earth leakage protection in the same single-circuit device.
- Use MCBs behind an RCD in a split-load board, for Type D loads and for like-for-like replacements.
- Use RCBOs for new boards, critical circuits, higher-risk locations, EV chargers and new circuits on non-RCD ways.
- All-RCBO boards cost more per way but keep every circuit independent and make faults easier to find.
- Match the trip curve (usually Type B in homes), the RCD type (usually Type A) and the board manufacturer.
RCBO vs MCB: making the right choice
In the RCBO vs MCB comparison, the MCB is the economical choice where an upstream RCD already gives shock protection, and the RCBO is the better choice wherever you want each circuit independently protected. For new installations, an all-RCBO board is increasingly the default, while MCBs remain the right part for split-load boards and like-for-like replacements. Whichever you choose, match the device to the board and have the work tested and certified. If you are planning a new board, see how to install a consumer unit.
Shop MCBs, RCBOs and consumer units
MCBs, RCBOs, RCDs and boards from leading brands including Garo. Trade prices, next working day delivery across NI & ROI on in-stock orders before 5pm.
Shop RCBOs Shop MCBsNeed help choosing? Call our team on 028 8675 1515 (048 from ROI), Monday to Friday, 8am–5pm.
Looking for a whole board or shared RCD protection? Browse our consumer units and RCDs.
FAQs
What is the difference between an RCBO and an MCB?
An MCB protects one circuit against overload and short circuit only. An RCBO does the same job and also detects earth leakage, usually at 30 mA, so it gives additional protection against electric shock. Both cover a single circuit, but an MCB needs an upstream RCD to provide shock protection.
Is an RCBO better than an MCB?
An RCBO is not always better, but it does more. It combines overcurrent and earth leakage protection, so a fault only switches off its own circuit. An MCB behind a shared RCD can still meet BS 7671, but one earth fault can then switch off several circuits at once.
Can I replace an MCB with an RCBO?
Often yes, as long as the RCBO is from the same manufacturer and range as the consumer unit and suits the circuit rating and trip curve. On the RCD-protected side of a split-load board it adds little, because the upstream 30 mA RCD may trip first. Have an electrician do the work.
Do I need RCBOs on every circuit?
No. BS 7671 requires 30 mA RCD protection on most domestic final circuits, but that protection can come from RCDs covering groups of MCBs or from individual RCBOs. Many electricians now choose an all-RCBO board for new work because it limits disruption and makes faults easier to trace.
Does an RCBO take up more space than an MCB?
Most modern RCBOs are single-module devices, the same width as an MCB. Some older or double-pole RCBOs take two modules. Because an all-RCBO board needs no separate RCDs, it can need fewer ways than a dual-RCD board with the same number of circuits, but check the exact devices.
Should I choose a Type B or Type C RCBO?
Choose Type B for most domestic circuits, including sockets, lighting, showers and cookers. Type C tolerates higher start-up currents from motors and large lighting loads, but it needs a lower earth fault loop impedance to disconnect in time. An electrician should confirm the curve against the circuit.