In order that a cable may be connected, for example into a socket, the insulation needs removing. First the sheath is stripped back to reveal the separate conductors within (and the basic insulation protecting them).
These are then spread to the correct terminals in the accessory. Once this is done a small amount of the basic insulation is removed to expose the inner copper core. This is then connected to the terminals.
All sounds OK so far right?
Well the wiring regulations require that no outer sheath of the cable is stripped back outside of the enclosure to which it is connected. This means that no basic insulation should ever be visible outside of the enclosure.
If the inner “basic insulation” is unsheathed outside of an enclosure, this greatly increases the risk of damage occurring to this. This is the only defence against dangerous live cores within the cable.
Knocks, bangs, movement, rubbing of the cable or excessive bending (which can cause splitting) are all possible ways in which the basic insulation can be damaged.
Expect this to be classed as C2 – POTENTIALLY DANGEROUS if the basic insulation is unsheathed anywhere where it may be accessible to touch. A prime example of this would be in the meter cupboard, an area easily accessible by the end user.
This fault may attract a C3 – IMPROVEMENT RECOMMENDED code if the cable is in an area not accessible to touch.
Regardless of whether accessible to touch or not, unsheathed basic insulation should not be in contact with metallic enclosure/sharp edges, this would be a C2 – POTENTIALLY DANGEROUS issue