Simple enough question but....,

[conny]

...I don't know the answer to it.

You take a supply from the main consumer unit to an outbuilding, which has a two-way consumer unit with main switch RCD.
From experience I know that if the RCD trips in the outbuilding, it will often/always trip the one in the main CU.

So, if you don't use, (export?), the earth from the main CU, but instead use an earth rod in the ground at the outbuilding, does this prevent the main CU from tripping.
My brain says yes, as there is no earth connection back to the main CU. But there is something niggling at the back of my brain saying it's not as simple as this.

 

[EFLImpudence]

No. RCDs don't use the earth. They only detect the imbalance between L and N.

That is, if some current leaks to earth, it does not return through the N.

 

[bernardgreen]

So, if you don't use, (export?), the earth from the main CU, but instead use an earth rod in the ground at the outbuilding, does this prevent the main CU from tripping.

No it will NOT prevent the RCD in the main CU from tripping.

The current that flows into the earth rod has passed through the main CU on it'e way to the Earth rod but does not return to the main CU. Hence the RCD sees an unbalance and trips

 

[conny]

Thanks guys. I knew it didn't seem right. Just couldn't get my brain around it.

As I said, simple question.

 

[bsr]

You could presumably use a time delay RCD in the CU for that circuit which would allow the shed RCD to trip first?

 

[EFLImpudence]

You could presumably use a time delay RCD in the CU for that circuit which would allow the shed RCD to trip first?

Not if the cable from CU to shed requires an RCD; if it doesn't then no RCD is required.