100mA RCD location TT??

So I believe a 100mA time delay RCD is required for a TT system if a split load board but not an RCBO board, why the difference?? Also any regs reference please?

I have been told that I can just swap out the isolator switch in the board for an RCD however I read that the RCD should be upstream in a plastic enclosure?

Any input appriciated Slight smile

Metal enclosure BTW...