RCD Testing
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Premium section
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 current 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.
Common questions
- Is there a free RCD Testing quiz?
- Testing sits in the Testing & Inspection section, which is part of TradeTraining Premium — but the app is free to start, sample questions are open to everyone, and the free sections cover the fundamentals first.
- Where does RCD Testing sit in the City & Guilds scheme?
- Testing maps to C&G 2365 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 current 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 current 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 current, which is why instrument testing exists.