RCD Testing
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Full course section in Plus
A compares what goes out with what comes back and opens when the difference is large enough to matter. A 30 mA device provides additional protection — the last line when basic and have both been defeated, typically by somebody putting a nail through a cable or grabbing a damaged flex.
Testing walks up in steps. At half the rated residual the device must hold; at its rated current it must open inside 300 ms for a general-purpose ; at five times rated current it must open inside 40 ms. Each test is run on both halves of the waveform and the worst time is the one recorded, because a device can be markedly slower on one than the other.
The integral test button is a functional check of the mechanism and nothing more — it proves the trip works, not that it works in time. It is also worth knowing why devices nuisance-trip: standing earth leakage from filters and electronics adds up across a board, and a device sitting near its threshold before anything happens will fall over the first time a kettle joins in.
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 Testing 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 Testing sit in the City & Guilds scheme?
- Testing maps to Unit 304 / 2391. On TradeTraining it lives in the Testing & Inspection section as bite-size lessons with practice questions.
- Do I need RCD Testing for the 18th Edition exam?
- Yes — the C&G 2382-26 exam is set against the BS 7671, and this topic is part of that ground. The exam is open book, so practising fast navigation to the right regulation matters as much as recall.
- How long may a 30 mA RCD take to trip?
- At its rated residual a general-purpose 30 mA must open within 300 ms; at five times rated current it must open within 40 ms. At half rated current it must not open at all.
- Does the test button prove an RCD is working?
- It proves the mechanism releases. It says nothing about how quickly, and nothing about the device's behaviour at half or five times its rated , which is why instrument testing exists.
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 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