It is.If, as implied by stillp's way of thinking, an applied voltage could be withstood satisfactorily for 5 minutes but would result in breakdown of the insulation if it continued to be applied for a further 10 minutes, I would be rather surprised - but maybe that's how it is?!
"PVC Type 6" to BS 7655-4.2
Yes, that's called up by BS6004.
All I can see at the moment is that the tensile strength of Type 6 or Type 10, the other allowable sheath material is about half that of the Types TI 1 and TI4 that are used for the insulation.
There can be large differences, depending on the additives used.It would seem that there is relatively little variation in either dielectric strength and resistivity across the whole range of PVCs - it's really just the mechanical/environmental properties which vary appreciably.