The PVC - covered conductors have been developed primarily to give protection to telecommunication lines which are crossed by power lines and to give protection to the public from low voltage lines in case of accidental contact for short periods.
The PVC - covered power lines have also been found useful in corrosive atmospheres and for preventing faults due to birds and trees. 
The insulation thickness of Type 8 is intended for use where the operating voltages does not exceed 650 V r.m.s. between any two conductors or 250 V r.m.s. between any conductor and earth.