Getting my head round earthing!

[YTSOxford]

I know that its prefered to place an outbuilding on its own earth rod to prevent faults traveling back to the house, so the way I usually do it is run 2 core swa to the second board, and terminate earth at the supply end.
I use metal boards because the smaller plastic ones are a right pain to get on with the amoured cable, but surely the metal enclosure which is earthed would connect to the earth in the armour... connecting the earths again????

 

[mrloy99]

I know that its prefered to place an outbuilding on its own earth rod to prevent faults traveling back to the house, so the way I usually do it is run 2 core swa to the second board, and terminate earth at the supply end.
I use metal boards because the smaller plastic ones are a right pain to get on with the amoured cable, but surely the metal enclosure which is earthed would connect to the earth in the armour... connecting the earths again????

If you wanted toTT it AND use a metal CU you would need to provide an insulator between your gland and the board eg plastic conduit adaptors or fit a one gang plastic box c/w blank plate.You would also have to ensure that the brass gland is fully shrouded so as the two earthing systems cannot be simultaneously touched.
I'm also not sure about your origional statement about earth faults travelling back to the house.Its often done if there is exposed metalwork or pipework to save having to run 10mm bonds back to the main house.