If the ring main live terminal in the consumer unit will take the extra wire go right ahead. The only time I ever had a problem was with a fuse carrier that had two rings, ie four wires, in it.
Sod's law is quite clear on this. No matter where you want your socket/light/bell transformer, the existing cables will be running off in the opposite direction. Been there, tried that, put a spur in.
indeed he could though its far more likely he meant mcb simply because rcbos are rare on domestic
and i think there aren't many people who would reffer to a rcbo as a rcd (even though it technically is one) unless they were completely ignorant about what mcbs and rcds where.
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