I've a Beko condensing tumble dryer that trips the socket circuit RCD
(labelled 80A 30mA) only under certain circumstances - always at what seems to
be the end of a cycle (in the sense that the clothes are always dry), and only
after it hasn't been used in a while (like at the moment, when most drying is
done on the line).

Resetting the RCD is fine.

Any idea of the cause/solution, and safety issues?

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Cheers, Rob