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Can one assume that the surface will remain hydrophobic during the entire service life of the insulator? Or, to be safe, should the engineer design the insulator assuming a hydrophilic surface?
It is almost impossible to know years in advance the service conditions of a particular transmission line. Should these conditions become much more severe than originally anticipated, an insulator assumed to always remain hydrophobic may actually prove to be quite vulnerable during the periods when it is hydrophilic. Therefore what is important for a silicon rubber weathershed is not so much the level of hydrophobicity of its surface as how easily that surface can turn hydrophilic and, even more critical, how fast the surface can recover. Usually the surface will recover a sufficient level of hydrophobicity within several hours to a few days. This dynamic behaviour makes the design of silicone rubber composite insulators that much more demanding.
What dictates how well and how long an insulator will perform in service is mainly the evolution of the leakage currents flowing along its surface?
It just so happens that, most of the time, when the surface is hydrophobic, there is not leakage current while when the surface is hydrophilic, leakage current may be present. A measurement of the hydrophobicity level of an insulator is therefore not sufficient to know if leakage currents will be present.

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