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When an RCD has to be replacedThe test button does nothing. Now what?

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If the test button does not work, replace the device

The device's own test button creates a small imbalance between line and neutral. If pressing it does not trip the RCD, you do not carry on with the RCD tests — the device gets replaced at the inspection stage.

Domestic RCDs are 30 mA. You would not change to 100 mA in a house — 30 mA is what the guidance asks for and more than enough.
Nuisance tripping is a different problem. That may mean you need a different type of RCD rather than a different rating — and types have changed as technology has.
Cost is a real factor in the advice you give. An AFDD is around £120 a device. BS 7671 sets out where they are required; outside that it is a judgement call for the client, not an automatic recommendation.

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