RCBO vs MCB vs RCD: Consumer Unit Protection Explained
Three initialisms cover most of what a consumer unit does, and they are easy to muddle. MCB, RCD and RCBO each protect against a different danger, and knowing which is which explains why modern boards look the way they do. An MCB (miniature circuit breaker) protects against overload and short-circuit too much current. An RCD (residual current device) protects against earth leakage, the fault that causes electric shock and fires. An RCBO (residual current breaker with overload) combines both in one device, protecting a single circuit against overload, short-circuit and earth leakage. Under IS 10101, most circuits need 30mA RCD protection, and populating a board with individual RCBOs is now the common way to provide it.
Compared at a Glance
| Device | Overload / Short-Circuit | Earth Leakage (Shock) | Scope |
|---|---|---|---|
| MCB | Yes | No | One circuit |
| RCD | No | Yes | One or several circuits |
| RCBO | Yes | Yes | One circuit |
An RCBO is the only device on this table that covers both fault types on a single circuit that's why it's increasingly the default choice for new boards.
MCB: Overcurrent Protection
An MCB trips when a circuit draws more current than it should a sustained overload, or a short-circuit fault. It protects the cable and the circuit from overheating. What it does not do is detect current leaking to earth, so an MCB alone will not protect a person from a shock.
RCD: Shock Protection
An RCD watches the balance between the live and neutral currents. If some current is leaking away through a fault, or through a person the balance is upset and the RCD trips fast, typically at 30mA, before the leakage can be lethal. RCDs are the device that protects against electric shock and many electrical fires, but a plain RCD does not provide overload protection on its own.
RCBO: Both in One
An RCBO combines an MCB and an RCD in a single module, giving one circuit overload, short-circuit and earth-leakage protection together. The advantage over a shared RCD is selectivity a fault on one circuit trips only that circuit's RCBO, instead of taking out every circuit sharing an RCD. That is why RCBO-populated boards are now common.
- Overload and short-circuit protection, same as an MCB
- 30mA earth-leakage protection, same as an RCD
- Both protections on a single dedicated circuit
- A fault trips only the affected circuit, not the whole board
- No nuisance tripping of unrelated circuits sharing an RCD
- The standard building block for fully RCBO-populated boards
What IS 10101 Expects
IS 10101 requires 30mA RCD protection for most circuits - socket outlets, circuits in domestic premises and cables in walls, among others. That protection can be provided by RCDs covering groups of circuits or, increasingly, by an RCBO per circuit for better discrimination. For additional guidance on electrical safety and RCD protection in Irish homes, see Safe Electric’s Home Safety guidance. WEW stocks MCBs, RCDs, RCBOs and consumer units; see our consumer unit selection guide, and the board must be designed and certified by a registered electrician.
Why Choose an RCBO-Populated Board?
| Differentiator | Why It Matters |
|---|---|
| Per-circuit protection | Overload and earth-leakage cover on every circuit individually |
| Fault selectivity | Only the faulty circuit trips the rest of the board stays live |
| Simplified diagnosis | The tripped RCBO tells you exactly which circuit has the fault |
| IS 10101 aligned | Meets the 30mA RCD requirement circuit-by-circuit |
| Future-proof boards | Easier to add or modify circuits without re-balancing shared RCDs |
Frequently Asked Questions
-
What is the difference between an RCBO and an MCB?
An MCB only protects against overload and short-circuit. An RCBO does that and adds earth-leakage (RCD) protection in the same device, so a single circuit is protected against overcurrent and shock together. -
Do I need RCBOs or is an RCD enough?
Both satisfy the requirement for 30mA protection, but RCBOs give better selectivity: a fault trips only the affected circuit, not everything on a shared RCD. Many modern boards use an RCBO per circuit for that reason. -
Is RCD protection required in Ireland?
Yes. IS 10101 requires 30mA RCD protection for most circuits, including socket outlets and cables concealed in walls. It can be delivered by RCDs or RCBOs, designed and certified by a registered electrician.
Source it at WEW
WEW (Waveform Electrical Wholesale) is your electrical wholesaler and lighting expert in Meath, supplying trade and homeowners across the region. Call in to the counter or the lighting showroom for stock, advice and everything referenced in this guide.