RCD, RCCB, RCBO and MCB: What's the Difference?
These four acronyms cause more confusion than almost anything else in a distribution board. They all protect circuits, but each one guards against a different kind of fault, and using the wrong one leaves gaps in protection. This guide explains what each device does, how they differ, and where each fits, so you can read a board layout with confidence.
Start here: protection devices do one of two jobs
Almost every device in a consumer unit exists to do one (or both) of these:
- Overcurrent protection. Stops too much current flowing, which covers overloads (a circuit drawing more than its rating) and short circuits (a sudden, very high fault current). This protects the cable and the equipment.
- Earth-fault (residual current) protection. Detects current leaking to earth, for example through a person or through damaged insulation, and disconnects fast enough to prevent electric shock and fire.
Keep those two jobs in mind and the four devices below fall into place.
MCB (Miniature Circuit Breaker)
An MCB provides overcurrent protection only. It trips when a circuit draws more current than its rating (overload) or when a short circuit occurs. It does the job a fuse used to do, but it is resettable.
MCBs are rated by current (for example 6, 10, 16, 20, 32 A) and by tripping curve (B, C and D), which sets how quickly they react to inrush. An MCB does not protect a person against electric shock, because it does not detect earth leakage.
Browse miniature circuit breakers (MCB).
RCD (Residual Current Device)
RCD is the umbrella term for any device that detects residual current, the small imbalance between the live and neutral conductors that appears when current leaks to earth. When that imbalance exceeds the rating (commonly 30 mA for shock protection), the RCD disconnects in a fraction of a second.
An RCD on its own does not protect against overload or short circuit. It comes in two main physical forms, and this is where RCCB and RCBO come in.
RCCB (Residual Current Circuit Breaker)
An RCCB is an RCD with no overcurrent protection built in. It protects against earth faults only, usually for a group of circuits at once. Because it does not handle overcurrent, it is installed together with MCBs that protect the individual circuits.
RCCBs are classified by type (AC, A, B and F) according to the kind of residual current they can detect. See the section below.
Browse RCCBs.
RCBO (Residual Current Breaker with Overcurrent protection)
An RCBO combines an RCD and an MCB in a single device. It provides earth-fault protection and overcurrent protection for one circuit. The big advantage is selectivity: a fault on one circuit trips only that circuit, instead of taking out every circuit sharing a single RCCB. The trade-off is higher cost per circuit and, in some designs, more width in the board.
Browse RCBOs.
Quick comparison
| Device | Overcurrent | Earth leakage | Typically protects |
|---|---|---|---|
| MCB | Yes | No | A single circuit, overcurrent only |
| RCCB | No | Yes | A group of circuits, earth faults only |
| RCBO | Yes | Yes | A single circuit, full protection |
For higher current ratings beyond the range of MCBs, the equivalent overcurrent device is the moulded case circuit breaker (MCCB).
RCD types: AC, A, B and F
The type of an RCD or RCCB describes which residual currents it can reliably detect:
- Type AC detects standard alternating residual currents only.
- Type A also detects pulsating DC residual currents, which modern electronics, EV chargers and induction appliances can produce. Type A is now the common baseline for circuits feeding electronic loads.
- Type F and Type B extend detection further for frequency-controlled equipment and smooth DC residual currents.
As a general rule, circuits feeding modern electronic loads call for at least Type A rather than Type AC.
How they work together in a board
There is no single correct device, only the right combination for the installation. Common patterns are an RCCB protecting a bank of MCBs, or an RCBO on each circuit for the best fault isolation. The design depends on the installation and must follow the wiring regulations and standards that apply in your country (for example the relevant parts of the IEC 60364 series, with MCBs to IEC 60898 and RCCBs and RCBOs to IEC 61008 and IEC 61009). Circuit design and device selection should always be carried out by a qualified electrician.
If you want the combination chosen, assembled and tested for you, our pre-wired distribution boards arrive ready to install with genuine Hager enclosures and Legrand protection inside.
In short
- MCB: overcurrent only.
- RCCB: earth leakage only, for a group of circuits.
- RCBO: both, for a single circuit.
- RCD is the family name that covers RCCBs and RCBOs.
Get the protection right and a board is safe, selective and easy to fault-find. Browse the full range of circuit protection devices at Gabby Electric, or talk to our team about a build to suit your project.