Well let's continue/try again, and see if we can progress without snide comments, accusations of lying and misdirection, dismissal of questions as tedious and irrelevant arguments etc.IMO when we started it was a worthwhile discussion.....IMHO this argument ....
There seemed to be two strands to the discussion; what is a "17th edition board" and to what extent an existing installation has to be brought up to 17th Edition compliance when working on it.
17th Edition CUs
People are never going to agree on this one, for exactly the same reason that they never agreed on what complied with the 16th. The issue of circuit separation is more acute now given the requirement in practice for almost everything to be on an RCD, but the problem about what complies with 314 is essentially the same.
The IET said:Every installation shall be divided into circuits as necessary to:
(i) avoid danger and minimize inconvenience in the event of a fault
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(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
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"Minimize" is an unfortunate word to have used, as it means to reduce to the least possible degree or amount, and the least possible amount of inconvenience in the event of a fault would be a CU with several 10's of RCBOs, with each individual socket, light, FCU and fixed appliance on their own dedicated circuit. And that's never going to happen.
"avoid danger and take account of inconvenience in the event of a fault" would IMO have been much better, for in reality it is the user of the installation who is best placed to judge what inconvenience he is prepared to suffer when circuits are lost because of a fault on a different one.
If you're going to take the literal meaning of "minimize" then not even circuits arranged as per normal with RCBOs for each one complies, so with that meaning you're never going to comply with the 17th.
Once you back off from that position, and inject practical reality into your design I'm not sure that a dual RCD board is necessarily non-compliant, not for a domestic dwelling.
Updating existing installations
There seems to be agreement that the regulations are not retroactive, yet there also seems to be an insufficiently thought-through belief about "taking responsibility" for what's already there, particularly when replacing a CU.
I say "insufficiently thought-through" because ask about EIC'ing an installed circuit and the usual position is "can't do that because I can't see it", so whatever "taking responsibility" means it cannot mean "ensuring that it complies with BS 7671:2008".
Some of the compliance issues can be addressed in some circumstances - when replacing a CU for example it's reasonable to say that you have to put existing circuits on RCDs, but with other work it isn't so reasonable to insist that what's already there has to be changed.
For example, if you were replacing a light or a switch in a bathroom, would you expect to add RCD protection to the circuit?
Unlikely I know, but if the lighting circuit was long and VD was over 3% but < 4%, would you want to rewire it?
If you were changing a fuse or MCB for one of a different rating because the existing one was the wrong size would you expect to add RCD protection?
There are all sorts of other "what ifs" - no point trying to compile an exhaustive list.
I don't believe that 131.8 means that as soon as you've opened your toolbox and isolated a circuit you're not allowed to turn it back on and leave until it complies with BS 7671:2008.
And I would welcome constructive comments and rational, relevant debate on all of this.