RCBO Installation Guide — Types, Costs, and BS 7671

An RCBO (Residual Current Breaker with Overcurrent) is a single device that combines the protection functions of both an RCD (Residual Current Device) and an MCB (Miniature Circuit Breaker). It provides overcurrent protection against both overloads and short circuits, and it also provides residual current protection against earth leakage faults.

In a domestic consumer unit, an RCBO is fitted in place of a standard MCB. Because each RCBO is self-contained, a fault on one circuit — such as a faulty appliance causing an earth leakage — will trip only that circuit's RCBO, leaving all other circuits unaffected. This is one of the principal advantages over a split-load consumer unit, where a single RCD protects a group of circuits.

The traditional approach in domestic consumer units was a split-load arrangement: a main switch, one or two RCDs each protecting a group of MCBs. The alternative — now widely adopted for new installations and replacements — is an RCBO board, where every circuit has its own RCBO and a main switch only (no shared RCDs). See our consumer unit types guide for how RCBO boards compare with split-load and high-integrity boards.

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RCBOs (and RCDs) are classified by the type of residual current they can detect. BS 7671 Regulation 531.3.3 defines four device types according to their behaviour in the presence of DC components and frequencies. Selecting the wrong type for the load being protected can result in either nuisance tripping or, worse, failure to trip under a genuine fault condition.

For most domestic circuits, Type A RCBOs are the correct choice. Specify Type F for circuits supplying air source heat pumps, variable speed drives, or any load where high-frequency leakage is a concern. Always check the equipment manufacturer's requirements before selecting the RCBO type.

One of the most common call-backs electricians receive after a consumer unit replacement or LED lighting installation is nuisance tripping on lighting circuits. This is caused by the cumulative leakage current from LED drivers (the electronic power supplies inside LED fittings and modules).

Each LED driver contains electromagnetic interference (EMI) filtering capacitors. These capacitors create a small residual current path to earth. A single LED fitting might leak as little as 0.5 to 1mA. But connect 20, 30, or 40 downlights to a single circuit and the cumulative leakage can approach or exceed the 30mA trip threshold of the RCBO.

Installing an RCBO requires safe isolation of the consumer unit and competence in working on distribution equipment. This is not a DIY task — it must be carried out by a qualified electrician and certified with the appropriate certificate.

One of the most important — and most frequently overlooked — rules in consumer unit work is that protective devices must be compatible with the consumer unit in which they are installed. Consumer unit manufacturers design their busbars and enclosures around their own device ranges and do not generally warrant third-party devices fitted in their units.

RCBO costs vary by brand, type, and current rating. The figures below are indicative market guidance for the UK trade only — not a quote. Always price from current supplier rates and your own labour:

When quoting consumer unit replacement work, ensure you include the cost of the Building Regulations compliance certificate (issued through your competent person scheme) and the Electrical Installation Certificate (EIC) in your price. Elec-Mate lets you generate EICs and Minor Works Certificates directly on your phone at the job, with instant PDF and cloud storage.

The selection and installation of RCDs and RCBOs is governed by Section 531 of BS 7671:2018+A4:2026 (the 18th Edition Wiring Regulations, as amended). The key regulations relevant to RCBO selection and installation are set out below.

For a full analysis of your installation's RCD and RCBO requirements, an EICR will identify any deficiencies in the existing RCD protection arrangement and recommend appropriate remediation.