RCD
Residual Current Device
A safety switch that compares the going out on the line conductor with the current coming back on the neutral. If they differ, current is leaking somewhere it should not, possibly through a person, and the RCD cuts the supply in a fraction of a second. Most socket circuits in UK homes need RCD protection, usually rated at 30 mA.
Where to look it up: BS 7671 Regulation 411.3.3.
Where RCD fits in the learning path
The Installation section covers these units:
- Wiring Systems & Enclosures
- Cable Selection & Circuit Design
- Earthing, Bonding & Consumer Units
Curriculum mapping: C&G 2365 Units 203/204/305.
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- RCD TypesInstallation
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A single DIN-rail device combining an RCD and a circuit breaker, so one module gives a circuit both residual-current protection and overload/short-circuit protection. The practical win is selectivity: a fault on one circuit trips that circuit alone, instead of dropping every circuit that shares a bank RCD.
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RCBO — Residual Current Breaker with Overcurrent protection
A single DIN-rail device combining an RCD and a circuit breaker, so one module gives a circuit both residual-current protection and overload/short-circuit protection. The practical win is selectivity: a fault on one circuit trips that circuit alone, instead of dropping every circuit that shares a bank RCD.
Read the RCBO entryADS — Automatic Disconnection of Supply
The standard protective measure in UK installations: earthing, protective bonding and a protective device working together so that a fault to earth causes disconnection within a prescribed time. If any leg fails — poor earth path, missing bond, wrong device — the fault can sit live on touchable metalwork instead of clearing.
Read the ADS entryFault protection
Protection that operates when basic protection has failed and a live conductor has reached touchable metalwork. In UK practice it usually means automatic disconnection of supply: earthing and bonding hold the touch voltage down while the protective device removes the supply within its required time.
Read the Fault protection entryAFDD — Arc Fault Detection Device
A protective device that recognises the electrical signature of dangerous arcing — the sputtering current of a damaged cable or a loose terminal — and disconnects the circuit before the arc starts a fire. Ordinary MCBs and RCDs cannot see these faults, because a series arc neither leaks current to earth nor draws obvious overcurrent.
Read the AFDD entryA definition is a start. Using it in a real question is where it sticks.