An RCD (residual current device) is a safety device in an electrical installation that monitors the difference in current between the live and neutral conductors. In normal operation these are equal. If someone touches a live wire - or a fault develops that allows current to leak to earth through a person or a faulty appliance - the RCD detects this imbalance and disconnects the supply in under 25 milliseconds (faster than the heart can go into fibrillation). A 30mA RCD can prevent a fatal electric shock.

What is the difference between an RCD, RCCB, and RCBO?

RCD is the general term for any residual current device. An RCCB (residual current circuit breaker) provides RCD protection only - it does not protect against overcurrent, so it is used with separate MCBs. An RCBO (residual current circuit breaker with overcurrent protection) combines both RCD and MCB functions in a single device - it protects against both earth faults and overcurrent on a single circuit. RCBOs are used in high-integrity consumer units where each circuit has its own independent protection.

Why does my RCD keep tripping?

A nuisance-tripping RCD usually indicates a faulty appliance connected to one of the circuits it protects. Unplug all appliances on the affected circuits and reset the RCD - if it holds, plug appliances back in one by one until it trips again to identify the faulty one. Common culprits are old washing machines, tumble dryers, fridges, and electric cookers with deteriorated element insulation. If the RCD trips with no appliances plugged in, there may be a fault in the fixed wiring that requires an electrician to investigate.

How often should I test my RCD?

RCDs should be tested every 3-6 months by pressing the test button (marked T) on the device. The RCD should trip immediately when the button is pressed. If it does not trip, or if it trips but cannot be reset, the device has failed and must be replaced immediately - an RCD that does not trip provides no protection. After testing, ensure the RCD is reset (lever back to ON) and check that all circuits have been restored. BS 7671 recommends this routine testing by the user.

Electrical noun

RCD

/ ɑː siː diː /

Also known as: residual current device, RCCB, RCBO, earth leakage circuit breaker

Definition

An RCD (residual current device) is an electrical safety device that monitors the difference in current flowing in the live and neutral conductors. In a healthy circuit these are equal. If a fault develops - someone touches a live conductor, or current leaks through damaged insulation to earth - the RCD detects the imbalance and disconnects the supply in under 25 milliseconds. A standard 30mA RCD trips fast enough to prevent a fatal electric shock in most circumstances.

In practice

In domestic consumer units, RCD protection is typically provided by one or two RCCBs that protect groups of circuits (split-load board), or by individual RCBOs on each circuit (high-integrity board). The split-load arrangement is more common and lower cost, but has the disadvantage that a single fault on one circuit trips all circuits protected by that RCD. The RCBO approach provides individual circuit protection so a fault on one circuit only affects that circuit.

RCDs have a test button that mechanically simulates a 30mA earth fault. Pressing it should cause an immediate trip. Users should test their RCDs every 3-6 months to confirm they are operating. RCDs can fail mechanically over time - a device that does not trip when the test button is pressed must be replaced. RCDs do not provide protection against all shock scenarios - they are ineffective if both live and neutral contacts are touched simultaneously (as no earth leakage occurs).

Building Regulations

Relevant PartPart P - Electrical safety in dwellings

StandardBS 7671:2018+A2:2022 (18th Edition IET Wiring Regs)

Trip sensitivity30 mA at 25 ms maximum (shock protection)

Mandatory sinceRequired in all new domestic installations since 2008

BS 7671 (IET Wiring Regulations) 17th Edition (2008) first required 30mA RCD protection for all socket circuits in domestic dwellings and for all circuits in bathrooms. The 18th Edition (2018) extended this to require RCD protection for all circuits in new domestic installations, with very limited exceptions. Part P notifiable work requires an Electrical Installation Certificate confirming RCD protection is provided as required by BS 7671.

Full Building Regulations guidance

RCD

Just so you know...