RCD Types
Installation · C&G 2365 Unit 203 · Full course section in Plus
A compares the flowing out along the line conductor with the current coming back on the neutral. A difference means current is escaping — possibly through a person — and the device disconnects in milliseconds.
Modern loads complicate the picture: electronics can produce residual with components that an old Type device simply cannot detect. That's why device selection now matters — Type A for most modern circuits, Type F and Type B where inverters and variable-speed drives distort the waveform further.
Add (an and protection in one module) and questions about nuisance tripping and per-circuit protection, and this small topic earns its place in every modern exam.
This guide is free to read. There is no public quiz specifically for this topic yet. Explore the key terms below without leaving the page.
Common questions
- Can I try RCD Types free on this page?
- This guide is free to read, but there is no public quiz specifically for this topic on this page. Browse public practice samples for other subjects, or create an account for the learning path. The full course section requires Plus.
- Where does RCD Types sit in the City & Guilds scheme?
- Types maps to Unit 203. On TradeTraining it lives in the Installation section as bite-size lessons with practice questions.
- What is the difference between an RCD and an RCBO?
- An detects residual (leakage) only — it won't trip on a simple overload. An RCBO combines that residual protection with overcurrent protection in a single device, so one faulty circuit trips alone instead of dropping every circuit sharing an RCD.
Explore key terms
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RCDResidual Current Device
A safety switch that compares the current 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.
Read the full RCD glossary entryRCBOResidual 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 full RCBO glossary entryAFDDArc 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 full AFDD glossary entrySPDSurge Protective Device
A device that diverts brief, high-voltage spikes — from lightning activity or heavy switching — safely to earth before they reach sensitive equipment. Usually fitted at or near the consumer unit, SPDs have moved from a luxury to an expected part of new installations as homes fill with vulnerable electronics.
Read the full SPD glossary entryADSAutomatic 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 full ADS glossary 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 full Fault protection glossary entry