Here goes.......
Well, that refers to the identification markers, not the conductor insulation.
But if someone splices a 3 core + cpc cable in the middle where there would be no sleeving and a neutral is known to be one of the cores, at least they would know that grey would most likely be the neutral. Thus when no sleeving/tape is present, the core colour becomes the identification marker; Thus why the standard of using grey and not brown or black for the neutral exists.
Non-harmonized colours did/does use black for neutral and there is plenty left. Do you sleeve these brown and blue?
If you want to, but a new cable colour caution label would be better suited IMO.
Not quite - always sleeve neutral with blue.
Have yet to see the blue wire for a radial/RFC socket circuit sleeved blue. Sleeving a blue core with blue sleeving seems a bit ambiguous, what is the blue sleeving meant to indicate what the blue core does not already?
You may not, but others may and thus is good practice to keep the old and new neutral colours septate.
The same can be said for the grey.
And that is why there is the standard of using grey sleeved blue for neutral in 3 core + cpc when a neutral is required and sleeving it brown when used for L2 in 2 way lighting circuits.
A) find hole in wall with red/yellow/blue wires with no sleeving.
(Did anyone ever sleeve the blue with black?)
One should sleeve the blue core with black sleeving to indicate neutral as blue is not a neutral colour in the old non harmonized colour scheme. Whether someone does is a different story.
Do you connect red to L, yellow to SL and blue to N without checking?
If not, why not? After all, Live is red, Neutral is blue and SL must be yellow.
Of course you would check, but you would hope that the Red core has been used for permanent line, Yellow core sleeved Red has been used for switched line, and Blue core sleeved Black has been used for neutral.
B) find hole in wall with brown/black/grey wires with no sleeving.
What's the difference?
One has the new harmonized colours, the other does not. Unless the fan you are wiring is 3 phase, you sleeve the cores their respective single phase colours.
New Black/Old Yellow and New Gray/Old Blue are 3 phase colours, not single phase colours - thus why you sleeve them appropriately. For not using Black or Brown for neutral, see top of this post.
Regards: Elliott