Residual current device (RCD)
Plain English. Practical detail. UK context.
An RCD, or residual current device, disconnects an electrical supply when it detects an imbalance between outgoing and returning current. That imbalance can indicate current escaping through an unintended route. An RCD reduces some electric shock risks, but cannot make damaged equipment or unsafe working practices acceptable.
Safety: use a registered electrician, for example one registered with a competent person scheme such as NICEIC or NAPIT, for electrical installation work. Do not remove covers or open electrical equipment yourself.
Last reviewed: (editorial review against the public sources listed below).
Where you will find one
RCD protection may be built into a consumer unit, incorporated in a socket or provided by a plug-in adaptor. An RCCB commonly protects several circuits while separate devices provide overcurrent protection. An RCBO combines residual current and overcurrent functions for a circuit. A homeowner describing every device as a fuse can make fault finding harder; a clear photograph of the closed consumer unit and its labels helps an electrician identify the arrangement without exposing live parts.
Why the device type matters
RCDs differ in the fault-current waveforms they can detect, not just their rated current. Modern appliances, inverters and vehicle charging can need particular protective arrangements. The circuit designer must select the device for the equipment and installation under BS 7671. The figure printed in milliamps is not the same thing as the amp rating for carrying load. Do not buy a replacement merely because it fits the spare space in the board.
Common mistakes
Repeated tripping is a reason to investigate, not to bypass the protection or keep resetting indefinitely. Water ingress, a faulty appliance or a wiring fault may be involved. A test button checks an aspect of the device’s operation; it does not certify the entire installation or measure every required performance characteristic. HSE recommends test-button checks for plug-in RCD adaptors before use and says faulty devices should not be used. Follow the installed device’s instructions for routine user checks.
Safer choices and job information
HSE treats an RCD as secondary protection: a shock can still cause serious injury. Battery tools or appropriate reduced-voltage equipment may be better choices for some site tasks. An RCD does not replace secure isolation before electrical work. Tell the electrician which appliances were running, whether the trip followed rain and which parts lost power. Keep observations factual; do not remove the consumer unit cover to collect more evidence.
Frequently asked questions
Does an RCD protect against overload?
An RCD function alone does not. An RCBO combines residual current and overcurrent protection.
Can I replace an RCD myself?
Selection, compatibility, isolation and testing need competent electrical assessment; this is not a like-for-like shopping exercise.
Does pressing the test button prove my wiring is safe?
No. It is a functional user check, not a substitute for electrical inspection and testing.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.