What Is an RCBO? How It Works and RCBO Types Explained
RCBOs are now a familiar sight in consumer units and distribution boards because they give each final circuit its own residual-current and overcurrent protection. That makes faults easier to isolate and can reduce the number of healthy circuits lost when one circuit trips.
What does RCBO stand for?
RCBO means residual current circuit breaker with overcurrent protection. It protects an individual circuit against three broad fault conditions:
- Residual current: current flowing along an unintended path, potentially through earth.
- Overload: current above the circuit’s designed continuous capacity.
- Short circuit: a high fault current caused by conductors making unintended contact.
An RCBO performs the combined role otherwise provided by an RCCB and an MCB. It is not interchangeable with any breaker that happens to fit the same DIN rail: ratings, terminal arrangements and consumer-unit compatibility all matter.
How does an RCBO work?
The residual-current section monitors the balance of current in the live conductors. If current leaves the intended path, an imbalance appears and the RCBO trips when its operating threshold and time characteristics are met.
The overcurrent section responds separately to overloads and short circuits. Its thermal element handles sustained overload current, while its magnetic element provides rapid operation for high fault current. The exact characteristics are defined by the product standard and the manufacturer’s data.
RCBO vs RCD and MCB
| Device | Residual-current protection | Overload and short-circuit protection | Typical arrangement |
|---|---|---|---|
| RCCB / “RCD” | Yes | No | One device may protect a group of circuits, with separate MCBs. |
| MCB | No | Yes | Protects one circuit against overcurrent. |
| RCBO | Yes | Yes | Provides both functions for one circuit. |
Individual RCBOs can improve circuit separation: a fault on one circuit does not automatically remove power from every circuit that would otherwise share an upstream RCCB. They can also make fault finding more direct. Cost, board space, system design and manufacturer compatibility still need consideration.
For the full device comparison, read RCD vs MCB vs RCBO, or browse our RCBO collection.
What are the different RCBO types?
“RCBO type” can refer to two different characteristics. Keeping them separate avoids one of the most common specification mistakes.
| Characteristic | Common markings | What it describes |
|---|---|---|
| Residual-current type | Type AC, A, F or B | The residual-current waveforms the RCBO is designed to detect. |
| Overcurrent trip curve | B, C or D curve | The instantaneous magnetic operating range and tolerance of starting or inrush current. |
| Residual sensitivity | For example, 30 mA | The rated residual operating current, shown as IΔn. |
| Current rating | For example, 6 A, 16 A, 32 A or 40 A | The rated current of the protected circuit, subject to the circuit design. |
A Type B RCBO and a B-curve RCBO are not the same description. “Type B” refers to residual-current detection, including smooth DC within the device’s specified range. “B curve” refers to the overcurrent trip characteristic. A product can, for example, be a Type A residual-current device with a B-curve overcurrent characteristic.
Our guides to B-curve RCBOs and C-curve RCBOs explain the overcurrent side in more detail. For electronic loads and unwanted tripping, see high-immunity RCBOs.
Single-module, two-module and pole arrangements
RCBOs are available in several physical and switching formats. A single-module RCBO can save board space, while wider devices may provide different pole arrangements, isolation characteristics or electronics. Module width alone does not confirm what conductors are switched.
- 1P+N RCBO: normally protects the line conductor and switches the neutral, with the exact arrangement shown on the device diagram.
- Two-pole RCBO: provides protection and/or switching across both live conductors as defined by the product.
- Four-pole RCBO: used in suitable three-phase and neutral applications where the design calls for a four-pole device.
- Flying neutral or switched-neutral designs: connection arrangements vary, so the diagram and board system must be checked.
Some electronic RCBOs also have a defined supply and load direction. Bidirectional applications such as generation and storage require devices specifically confirmed as suitable by the manufacturer.
What should be checked when selecting an RCBO?
- Rated current and cable protection: matched to the design current, conductor capacity and installation method.
- Trip curve: selected with load inrush and required fault-current operation in mind.
- Residual-current type: suitable for the waveforms the connected equipment can produce.
- Residual sensitivity: selected for the required protection purpose.
- Breaking capacity: adequate for the prospective fault current at the point of installation.
- Poles, neutral arrangement and directionality: confirmed from product markings and data.
- Consumer-unit compatibility: use the manufacturer-approved device range, busbar and enclosure system.
Application labels such as “EV RCBO” or “solar RCBO” are useful starting points, but the electrical characteristics and equipment instructions decide suitability. For a common domestic rating, see our guide to 32 A RCBO applications.
RCBO FAQs
Is an RCBO better than an RCD?
They describe different arrangements. An RCBO gives one circuit residual-current and overcurrent protection. An RCCB can provide residual-current protection to a group of circuits when paired with separate overcurrent devices. The better arrangement depends on the design.
Does an RCBO need an MCB?
No separate downstream MCB is normally required for the same circuit because the RCBO already provides overcurrent protection. Upstream protection and coordination still form part of the installation design.
What is a 30 mA RCBO?
It is an RCBO with a rated residual operating current of 30 mA. The circuit current is shown separately, such as 16 A or 32 A.
Can any RCBO fit any consumer unit?
No. Similar dimensions do not prove system compatibility. Use devices, busbars and enclosures approved by the relevant manufacturer.
Shop RCBOs by current rating, curve, residual-current type and pole arrangement, or compare the wider circuit protection range. Product data should always be checked against the board system and circuit design.
This guide is for general product-selection information. Electrical work should be designed, installed, tested and certified by a competent person in accordance with the current edition of BS 7671 and relevant manufacturer instructions.