anything that it will eat..
if it's poisonous to it you will soon know when it starts vomiting everywhere...
as for the OP, NO you don't need to RCD protect the cables in the surface conduit specifically, however if you have TT earthing then your house should already have and overall RCD..
If it's a seperate garage and you are relying on TT earthing for it then ALL of the circuits inside it will need to be RCD protected..
this is usually done by installing a "garage CU" with an RCD main switch, or a standard main switch and RCBO's..
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