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Engineer54
To "isolate it" from the house. 40A mcb in house, not rcd protected. SWA earthed at house. Earth pole/rod for garage. But then I do a lot of work out in the sticks.
Why would you leave behind a perfectly good earth with around a 0,35ohm Ze, for an earth rod, with 100 to 200 ohm Ra??
Either the OP can mount a small switch fuse, or MCB at the origin and supply the garage from that, providing a typical 2 way garage CU with RCD, or use his existing CU RCD protected MCB, and leave out the RCD at the garage CU. Nothing stopping the OP from also driving a decent sized/length Rod and connecting that in to the EMT, pretty pointless though if it has a high Ra value!!
Amlu
As there would be no discrimination between 2 X 30mA RCD's it would be anyone's guess which RCD device trips first (or both tripping). Ideally you don't want a garage fault to trip out half the house. There is only one way to overcome this, if using 2 X RCD's, that is by the use of a S type (time delayed) RCD upstream....
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