OK, first we need to realise that cable standards are in a state of flux at the moment, due to the Construction Products Directive, there is further cable harmonisation across the EEA.
The "cable" we are discussing here is 6181Y to BS6004.
A typical reliable quality suppliers link here:
Draka 6181Y PVC Insulated and Sheathed Wiring Cable
Now if you look at this page, it confirms the cable complies with BS6004, & that there is another kind of “similar” sheathed cable.
6242Y, commonly known as “twin & earth”.
Now there is a clue to the lack of “double insulation” in the wire code.
e.g. 6181Y, NOT, 6181YY.
The single Y denoting a single layer of PVC insulation.
Now if you look here, the cable is described in the British Standard as “PVC Insulated & PVC sheathed”.
BS 6004:2012 - Electric cables. PVC insulated and PVC sheathed cables for voltages up to and including 300/500 V, for electric power and lighting
As has already been suggested BS7671 does draw a distinction between double insulation & insulation & sheathing.
Whilst the sheathing is PVC, and may well be made of the same material as the insulation, and it may offer insulating properties, as far as BS6004 which is the product standard remember, does not call for the sheathing to have any dielectric properties what so ever.
The sheath does not have to undergo any, electrical testing for the cable to meet the requirements of the standard, ergo, it cannot be relied upon as insulation.
The next thing is, looking at the flat twin pvc insulated & sheathed cable with bare cpc.
Now if we stick this in trunking then we can use the bare cpc as a current carrying conductor, because it is insulated by the pvc outer sheathing, as that is the same standard of material ised to sheath the 6181Y.
No we can’t because it is not insulated.
If you look at the description on the Draka site for the twin & cpc, it even states that the cpc is uninsulated, if the sheath was insulation then it would not state that.
I obviously can’t publish the whole of BS6004 as its copyright, and, it also refers to dozens of other standards for testing and material properties.
However, just a quick couple of crops from BS6004 for you:
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1 Scope
This British Standard specifies requirements and test methods for the construction and performance of cables that:
a) have a polyvinyl chloride (PVC) insulation of rated voltage 300/500 V;
b) are intended for electric power and lighting.
The types of cable included in this British Standard are:
• PVC insulated, PVC sheathed cable 300/500 V, single-core 6181Y, flat-twin 6192Y and 3-core 6193Y (see Table 3);
• PVC insulated, PVC sheathed cable with circuit protective conductor, 300/500 V, single-core 6241Y, flat-twin 6242Y and 3-core 6243Y (see Table 4);
• PVC insulated, PVC sheathed cable with or without circuit protective conductor, 300/500 V, single-core 6192Y, 6241Y and flat-twin 6242Y (alternative conductor versions) (see Table 5);
• ordinary duty low temperature PVC insulated, PVC sheathed flexible cable, 300/500 V, flat-twin 3192A, circular-twin 3182A, 3-core 3183A, 4-core 3184A and 5-core 3185A (see Table 6).
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7 Insulation
7.1 Type of insulation
The insulation shall be one of the following types, in accordance with Table 3 to Table 6:
• TI 1 as specified in BS EN 50363-3;
• TI 4 as specified in BS EN 50363-3.
7.2 Application
The insulation shall be applied by an extrusion process to form a compact and homogeneous layer.
NOTE 1 The insulation can be applied in a single layer, or in a number of cohesive layers.
Where more than one layer is used, all testing specified in this British Standard shall be carried out on the complete insulation as though it were a single layer of the declared insulation type (see 7.1).
NOTE 2 Insulation applied in more than one layer does not conform to the
definition of ‘‘double insulation’’ given, for instance, in BS 7671.
When the application is tested by removing the insulation from the conductor, there shall be no damage to the insulation itself or the conductor.
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10 Sheath
10.1 Type of sheath
The sheath shall be an extruded layer of PVC of one of the following types, in accordance with Table 3 to Table 6:
• Type 6 as specified in BS 7655-4.2.
• Type 10 as specified in BS 7655-4.2.
10.2 Application
The sheath shall be applied by an extrusion process.
NOTE The sheath can be applied in a single layer or in a number of cohesive layers.
Where more than one layer is used, all testing specified in this British Standard shall be carried out on the complete sheath as though it were a single layer of the declared sheath type (see 10.1).
When the sheath is removed, there shall be no damage to the core insulation when visually checked.
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The title of BS 7655-4.2 is:
BS 7655-4.2:2000 Specification for insulating and sheathing materials for cables. PVC sheathing compounds. General application. General application
So you can see again that the product standard for the sheathing makes a distinction, specifically that it is sheathing compound, not insulation.
