RCD vs RCBO: what is the difference?

Glossary · Consumer units and protection

How an RCD and an RCBO work

An RCD watches the current in the live conductors of a circuit. If the current going out and the current coming back do not match, some of it is leaking to earth. The RCD trips and cuts the supply.^{[1]IET Wiring Matters, Which RCD Type?} That leak could be a person touching a live part, so the device is there to protect people from shock. The Electrical Safety First charity says an RCD can disconnect in as little as 40 milliseconds.^{[5]Electrical Safety First, Why you need an RCD fitted in your home ASAP}

“RCD” is a general term. The IET lists several devices that count as RCDs. Two matter in a consumer unit: the RCCB, which has no protection against overload, and the RCBO, which has it.^{[1]IET Wiring Matters, Which RCD Type?} A circuit breaker (see circuit breaker) deals with overload and short circuit. An RCD deals with leakage to earth. An RCBO does both jobs in one device.

RCD and RCBO side by side

The IET states that, unless it is an RCBO, an RCCB gives no overcurrent protection.^{[2]IET, BS 7671 FAQs: Consumer Units and Protective Devices} On an RCCB there are two ratings. One is the tripping current in milliamps. The other is the current the device can carry. The device is not designed to trip at that second figure, so extra overload protection is still needed.^{[2]IET, BS 7671 FAQs: Consumer Units and Protective Devices}

Where each sits in a home

NICEIC describes the common layouts. In a dual RCD unit, two RCDs share the circuits between them, with a circuit breaker on each circuit. In a high integrity unit, each circuit has its own circuit breaker or its own RCBO, with no circuits grouped under a single RCD. A fault then switches off one circuit only.^{[3]NICEIC, Consumer units and fuse boxes: installation and replacement} There is also a split-load type that mixes the two. See dual RCD consumer unit for how the shared layout works.

When an older consumer unit is replaced, Electrical Safety First recommends that all circuits are protected by Type A RCBOs or by enough Type A RCDs.^{[4]Electrical Safety First, Best Practice Guide 1: Replacing a consumer unit in residential and similar premises} The type letter matters because some equipment produces a direct current leakage that can stop certain RCD types from working. The IET covers this in its RCD guidance.^{[2]IET, BS 7671 FAQs: Consumer Units and Protective Devices}

Replacing a consumer unit is notifiable work under the Building Regulations in England, so a registered electrician must do it or notify building control first.^{[6]Ministry of Housing, Communities and Local Government, Approved Document P: Electrical safety, dwellings} You can read more on the consumer unit upgrades page.

Example

Priya lives in a terraced house in Headingley. Her kitchen sockets and her upstairs lights sit on one RCD in an older unit. A fault in her kettle trips that RCD, and the upstairs lights go out as well.

With an RCBO on each circuit, only the kitchen sockets would have gone off. The upstairs lights would have stayed on. The protection is the same. The difference is how many circuits lose power when one has a fault.

What people get wrong

People often read the amp figure on an RCD as its trip point. It is not. It is the current the device can carry. An RCD does not limit current at all. It limits how long a leakage current can flow to earth.

The second mistake is thinking an RCD makes a circuit breaker unnecessary. A plain RCD does not protect against overload, so a circuit breaker or fuse is still needed alongside it.^{[2]IET, BS 7671 FAQs: Consumer Units and Protective Devices}

The bottom line

An RCD protects people from earth leakage. An RCBO does that and protects the circuit from overload as well, and it does it for one circuit rather than a group. Which layout suits a home is a decision for a registered electrician after looking at the existing wiring. Fitting or changing either device is not a job for the householder.