RCDs not operating as expected

[vwes59]

Hi
Ive looked around and i cant see this particular scenario being discussed.

I think i may have been a numpty, but bear with me.

I have just completed an electrical install into an outbuilding. Its distance meant i went with installing its own earth rod, thus not exporting the pme from the house. Installed a 2 core SWA, earthed back to the main house via its steel sheath. Thus the sub board has no earth connection back to the main house and is essentially TT.

All circuits in the main house including the new SWA are covered by a single RCD type AC. Circuits in new sub main are RCBO Type A.

All dead tests were fine, and earth rod came out at 56ohms. I confirmed there is no earth path from the main house into the sub main, it stops in a plastic junction box outside the consumer unit.

However when i tested the new RCBOs, sometimes they tripped, other times the main RCD in the house tripped. All the RCD tests passed, just it was random weather the main RCD or RCBO would trip.

Given i have separate earths i am surprised selectivity is an issue if that is what it is.

 

[vwes59]

When one is stuck with multiple RCDs for whatever reason, it is possible to test only the downstream RCD without risk of tripping the upstream one, by connecting the earth lead of the tester to the neutral on the line side of the RCD under test. This returns the test current to the circuit so that the upstream device does not see an imbalance.

Thats an interesting thought - thanks for that. That would get my paperwork sorted, but I suppose it is a bit of a fudge, as the underlying problem still exists.

Since my original post, I actually have a dual rcd board that I took out from another job which could be adapted to solve this issue.

 

[Lucien Nunes]

I've just noticed the sloppy use of terminology in my post:

the neutral on the line side of the RCD under test

That should of course be 'neutral on the supply side'. I had used the old meaning of 'line' contrasting with 'load' instead of the new one contrasting with 'neutral.'