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I think he means an additional one at the socket
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It doesn't matter where the RCD is, provided it is of the normal type used to protect against electric shock, and it works. All new domestic ring main circuits are required to have them in the distribution board, many existing installations have them too. Before relying on an older RCD in the distribution board, check that it states IΔN 30mA, (read I-Delta-N) meaning the sensitivity is such that if 30mA leaks from the circuit then the breaker will trip. Test it regularly by pressing the test button which should cause it to trip immediately.
If you are connecting your set to a socket outlet that isn't RCD protected at the distribution board, then a plug-in type such as recommended for garden equipment is equally suitable. Use both, if you like, there's no harm in doing so.
An RCD is not a substitute for earthing, however it achieves one of the same aims (to lower the risk from electric shock) by a different method. It will protect a transformerless (AC/DC universal) set just as well as an AC-only set, which earthing cannot do.
Lucien