Both devices protect people from electric shock by detecting current leaking to earth and disconnecting the supply in a fraction of a second. The difference is in how much of the installation they switch off and what else they protect against.
What is an RCD?
A residual current device (RCD) compares the current flowing out on the line conductor with the current returning on neutral. If the difference exceeds its rating, usually 30mA for personal protection, it trips. In a typical “split-load” or “dual-RCD” consumer unit, one RCD protects a whole bank of circuits, with separate miniature circuit breakers (MCBs) providing overload and short-circuit protection for each circuit.
What is an RCBO?
An RCBO (residual current breaker with overcurrent protection) combines an RCD and an MCB in one device, fitted to each individual circuit. A fault on one circuit only switches off that circuit.
Side-by-side comparison
| RCD + MCBs | RCBO per circuit | |
|---|---|---|
| Earth leakage protection | Yes, shared across several circuits | Yes, per circuit |
| Overload and short-circuit protection | From separate MCBs | Built in |
| Effect of a fault | Several circuits go off, possibly including lights and fridge | Only the faulty circuit goes off |
| Nuisance tripping | More likely, as small leakages add up | Less likely |
| Cost | Lower | Higher, roughly £150–£400 more for a typical home |
Which should you choose?
For most homes we recommend a full RCBO board: it is the most resilient option, it makes fault-finding faster and cheaper, and it avoids losing lighting and freezers when a single appliance develops a fault. A dual-RCD board is still compliant and is a reasonable budget choice. For landlords and HMOs, RCBOs reduce call-outs from tenants losing power to whole floors.
What will an EICR say?
An EICR will not code a compliant dual-RCD board as a defect. However, missing RCD protection is a common observation: for socket-outlets that may supply portable equipment outdoors it is commonly coded C2, and for other circuits it may be C3. See EICR codes explained.
Planning an upgrade? See consumer unit upgrades or read rewireable fuses vs modern consumer units.