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—Wiring and circuits

RCDs and RCBOs, explained

Circuit breakers protect cables from too much current. RCDs and RCBOs protect people from current going where it should not.

A residual current device (RCD) monitors the balance of current flowing out and back on a circuit. If some current leaks to earth — through a person, damp insulation or a faulty appliance — the RCD switches off very quickly. One RCD often protects several circuits, which is why it can switch off a whole area when it trips.

Wiring guides explain how circuits are put together and changed, so a quotation or a recommendation is easier to follow.

An RCBO combines an RCD and a circuit breaker in one device for a single circuit. A fault on one circuit then switches off only that circuit, not a group. Both have a test button that checks the mechanism; pressing it periodically confirms the device can still trip.

01In short

The points that matter most

  • RCDs detect leakage

    They protect against shock from earth faults.

  • RCBOs protect one circuit

    Combining leakage and overcurrent protection.

  • Test buttons matter

    They confirm the mechanism still works.

02Detail

How they behave

  • Shared RCD

    One device protecting several circuits; one fault switches them all off.

  • Per-circuit RCBO

    Limits disruption to the faulty circuit.

  • Nuisance tripping

    Cumulative leakage from many appliances can trip a shared RCD.

  • Different RCD types

    Some appliances, such as EV chargers and certain inverters, need a particular RCD type; the manufacturer’s instructions say which.

03Checklist

Practical points

  • Identify your devices

    Look for test buttons on the board.

  • Test periodically

    Press the test button as advised by the manufacturer.

  • Note what trips

    Which device and which circuits go off.

04Questions

Questions people ask

Why does one trip switch off half the house?
A shared RCD protects several circuits.
Should every circuit have an RCBO?
Per-circuit protection reduces disruption; an electrician can advise on upgrading.
What if the test button does nothing?
The device may be faulty and should be checked.
Can an RCD stop every shock?
It greatly reduces the risk from earth faults, but it does not replace safe practice around electrics and water.

05Related

Related guides

This guide sits under electrical wiring. All topics are on the guides page.

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