The purpose of the insulator or insulation is to insulate the electrically charged part of any equipment or machine from another charged part or uncharged metal part. Creepage Distance is the shortest path between two conductive parts measured along the surface of the insulation as shown in figure below.
When a flashover occurs in the system this distance helps the flashovers not to interfere with the live systems.
The basis for the determination of a Creepage Distance is the long term r.m.s. value of the voltage existing across it. This voltage is the working voltage, the rated insulation voltage or the rated voltage.
The influence of the degrees of pollution in the micro environment on the dimension of Creepage Distances is taken into account in calculation to avoid failure due to tracking. While determining Creepage for an Insulator it should be taken care that for the same equipment, different micro environment conditions can exist.
Mind that a Creepage Distance cannot be less than the associated clearance so that the shortest Creepage possible is equal to the required clearance.