"pickerel" wrote in message
m...

Q What is an 'insulated wall'? Are walls built with light(ish)weight
building blocks considered 'insulating'; or lightweight plasters such
as carlite browing?

No to both of those, IMO. Stud or cavity walls with thermal insulation
materials in them, or "special thermally insulating plasters" which
incorporate polystyrene beads etc. count at insulated. I've never seen a
strict definition in terms of U or k values though. The diagrams in
Appendix 4 give a clue as to what they mean by insulated.

eg 2 x 2.5mm2 FTE cables (on wall under plaster say) touching would
reduce full load rating from 27A to 0.85 x 27A = 23A.

That's correct.

but still over the 20A ring rating - so still OK unless there is some
other adverse factor in the ring?

Agreed.

Please can you clarify what is considered best (or standard) practice
when running ring cable up & down a wall? Separate the cables by the
required gap? Find a completely different routes for the 2 legs? Or
just run them up & down touching, just accepting the grouping
reduction?

Side-by-side (touching so far as Appendix 4 is concerned) or bunched are
usual. They're perfectly OK unless the presence of other cables in the same
chase gives a problem, in which case re-routing the other cables may be the
solution.

streuth, you need to be a lawyer for this. That heading needs reading
very slowly many times: Send for the plain English people...

I think the organisation and layout of the OSG could be much better than it
is. It may be logical (in terms of relationship to BS 7671) but it's not at
all user friendly -- esp. w.r.t. the division of material between the body
and the appendices.

Appendix 4 of BS 7671.

been there, done that, obviously didn't get the t shirt. but will try
again.

Took me some time to get the hang of it too. You have to read it several
times. "Education" -- as one of the profs used to say when I was at
college, many years ago -- "is a process of diminishing deception."

--
Andy