Generally in pollution induced flashover of porcelain insulators, if the bottom is more polluted compared to the top, then the movement of arc starts from pin towards cap. Under such nonuniform cases, why with increase in the ratio of ESDD at top to ESDD at bottom does the pollution flashover voltage also increase?
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=6507585&queryText%3Dpollution+perormance+of+string+insulatrors+2013
Generally, Bottom ESDD will be higher than Top surface. Initiation arc need not be from bottom alone. Probability of initiation is more from bottom.. Many times T/B ratios will be specified in certain pollution conditions. This is required especially, in DC pollution tests, as the bottom surface behaviour is critical in DC.
Generally take the performance of Different ESDD on top and bottom of the insulator. This performance will be and must be same if insulator is coated with Average ESDD. Therefore, uniform pollution has been suggested in standards.
Hope this will clarify your doubt.
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