On Sat, 07 Jan 2017 13:52:41 +0000, Graham.
wrote:

When this has been discussed before, it has been stated that a double
layer of insulation is required, ie a sheath within a sheath. How this
is better than a single sheath of twice the thickness escapes me,
especially if they are both the same grade of PVC.

If you take a piece of PVC insulated cable and, with a knife, make a
small cut in the insulation (not all the way through) and then bend
the wire the cut will propagate around the cable leaving the conductor
exposed. If you do the same with double sheathed cable the cut will
spread around the outer insulation but not the inner. Double
sheathing minimises the risk of surface damage to the cable exposing
the conductor.