An RCBO in a single-phase system combines the functions of an RCD (Residual Current Device) and an MCB (Miniature Circuit Breaker) into a single unit. It provides protection against earth leakage (residual current) and overcurrents (such as short circuits and overloads) for individual circuits.

RCBOs are commonly used in consumer units (fuse boxes) in single-phase electrical installations to provide enhanced protection for circuits supplying sockets, lighting, appliances, and other electrical loads.

RCBOs offer the advantage of combined protection in a compact unit, which saves space and simplifies installation. Each RCBO protects a specific circuit against both earth faults (RCD function) and overcurrents (MCB function), reducing the complexity of wiring and fault finding.

In certain circumstances, such as circuits supplying socket outlets in kitchens, bathrooms, and outdoors, RCBOs may be required by electrical regulations (e.g., BS 7671) to ensure high levels of safety. Local regulations and installation standards may also specify their use.

When selecting an RCBO for a single-phase circuit, consider the following factors:

• Rated Current: Ensure the RCBO’s rated current matches or exceeds the maximum current expected in the circuit.
• Sensitivity: Typically 30mA sensitivity is used for personnel protection to detect small leakage currents that could pose a risk of electric shock.
• Type: Choose between A-Type (standard) or AC-Type (for circuits with electronic equipment) depending on the application to avoid unwanted tripping.