Breaking Down of Porcelain

The overal height of the insulators may be 3.7 meters but the tracking distance from the top of the insulator to the cloesest earth point at the insulator bas is probaly around 6m as tracking takes place over the surface of the insulator, i.e. down and over the surface of each insulator ring or commonly known as a rain shed. So, if you strectche the insulator out and had a smooth surface, you will be in the region of 6m+.

Dirt collects on the shed tops decreasing the creepage distance, rain pollution etc. Once the top of the shed is polluted the 'length'is reduced considerably so in order to maintain a safe creepage distance and reduce overall height, the sheds are made. You also get a clean surface underneath the rain sheds, which is 'normally' fee of pollution which causes tracking, (creepage). Air does not have a stable dielectric constant, it changes with weather conditions, so a safe length is produced for the voltage being applied. Also the equipment can be installed at sea level or at altitudes so a standard length is assumed to be used in any location at any elevation and for any condition. Also, porcelain is cheap and has voids and if the glazing is poor, or the porcelian is poor quality, internal stress are created, as the porcelain is hollow, so there is an inbuilt safety factor which involves some height. All designed to keep you safe and the power on and stop flash overs.