A residual current device (RCD) is a protective switching device that disconnects a circuit when residual current reaches a specified value under specified conditions. In everyday terms, it detects current taking an unintended path and switches off the supply to reduce the risk of electric shock. It can reduce that risk, but it does not eliminate it.
What does an RCD do?
An RCD is intended primarily to protect against electric-shock hazards. If current flows somewhere it should not, the device can open the circuit and interrupt the supply. Electrical Safety First describes this as reducing the risk of fatal electric shock by disconnecting when electricity follows an unintended path.
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The formal definition quoted by the IET from BS 7671:2008(2013) is: “A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions.” This wording comes from that historical edition of BS 7671; it should not be taken as a quotation from the current edition.
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RCD is the generic term for residual-current devices. RCCB and RCBO identify particular device types, distinguished by whether they include integral overcurrent protection.
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| Term | Residual-current protection | Integral overcurrent protection |
|---|---|---|
| RCD | Generic category of residual-current devices intended primarily to protect against electric shock, as described by IEC 60755:2017 (IEC 60755). | Depends on the particular device type. |
| RCCB | Yes | No. IEC 61008-1:2024 covers RCCBs without integral overcurrent protection. Its stated scope includes household and similar-use devices up to 440 V AC and 125 A (IEC 61008-1:2024). |
| RCBO | Yes | Yes. IEC 61009-1:2024 covers household and similar-use RCBOs with integral overcurrent protection. Its stated scope includes devices up to 440 V AC, 125 A and rated short-circuit capacity up to 25,000 A (IEC 61009-1:2024). |
The IEC scope figures describe the standards, not a recommendation for what to use in a particular installation. The two catalogue entries are dated 21 November 2024.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does a portable RCD protect the whole installation?
No. Electrical Safety First’s 2025 landlords’ guide for England and Wales says a portable RCD protects equipment connected through its special socket-outlet, including the equipment’s lead. That coverage should not be assumed to extend to the rest of a building’s electrical installation.
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What should you know before acting on this definition?
- An RCD can reduce electric-shock risk; it is not a guarantee against injury.
- RCD is a category name, not a promise that every device has the same configuration or protection features.
- The right device and installation requirements depend on the installation and applicable local rules. The sources here do not establish ratings or configurations for a particular property.
- For installation or testing, follow current local requirements and the device manufacturer’s instructions; consult a qualified electrician where appropriate.
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