The power cu does have it own rccb and so will the cu for the outbuilding. The lighting unit has a disconnector as an rccb will blow every time a halogen bulb goes. It is the same as a split unit but separated into two separate units because there were more than normal circuits.
That's fair enough, but the problem we've all identified (in some cases more gently than BAS) is that,
unless that single upstream RCD is time-delayed, a fault anywhere, even on a circuit protected by another RCD, is at serious risk of taking out the entire installation in your large house (and I know about large houses
). (BTW, whilst dying bulbs/lamps can trip MCBs, it's pretty unlikley that they would cause an RCD to trip)
Why have RCBO's in the posh outhouse . No extraneous metal or water or gas pipes. I do have RCBO's for the shower and the pond for obvious reasons. ... The outhouse cu is protected with a 63amp 30ma RCCB
I can but presume that BAS was thinking of having RCBOs (or an RCD+MCBs) in the outhouse
instead of in the CU in the house supplying it. As you imply, if you have an RCD in that CU in the house, there's no need/point in having an RCD or RCBOs in the outhouse.
The installation except the outhouse was certified when the two main CU's were installed and again by a seperate electrician when there was a major revamp and all circuits were fully tested one NEICC and the other to a different board which I can't remember . Neither had any problems. ... This was a real quality installation done to an extremely high standard and also looked at by my building inspector
How long ago was this done? As has been said, to have the electrical installation of an entire, large, house at the mercy of a single RCD seems very poor design practice, and not compliant with regulations since the current edition came into force in 2008.
Kind Regards, John