I am assuming you man something line http://pimg.tradeindia.com/00018889/b/0/HV-Bushing-.jpg - in the US we refer to this item as a bushing. There are three major electrical factors at work, Creep: the distance along the SURFACE from the conduct to the mount ( flange or ground - but the surface you are going through) - thus the skirts - depends on environment. Next is Strike - this is the THROUGH THE AIR distance from the conductor to mounting flange resulting in the overall size / length of the bushing. Lastly and less known is uniform voltage gradient - this is how the Voltage gradient inside the insulating material is spread between the conductor and the flange, the higher the voltage the more important this becomes.. This ensures the insulation material does not break down over time, and is the main reason for the cone shape.