Either 50mm, or not at all.
Not at all only applies if the run is in a specially prescribed zone, which
includes at the end of a wall, at the top of a wall (not bottom), or
vertically from a visible electrical fitting. You can also run horizontally
from one, but this is usually discouraged.
If you install at any other location, you need 50mm depth or mechanical
If you look more closely at the regs this rule only applies if the cable is
concealed in the wall and not being terminated in an outlet. ie. if there is
no way of knowing that the cable is there. With an outlet present you know
there is a cable running vertically or horizontally.(Or, you should be able
to assume so). Again, if it runs diagonally it has to be 50mm or
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