string insulators

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electrics said:
as the altitude increases the string insulator dimensions grows bigger or smaller?
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I had studied this regarding to altitude and insulator in a book.

The insulator flashover voltage is reduced as altitude increases. Above 1500 feet, an increase in the number of insulators should be considered. A practical rule is a 3% increase of clearance or insulator strings' length per 1000 ft as the elevation increases.
 
in fact this is what makes me ask this question, the same book i read it from, so why? i think it should decrease since the gas density of the air decreases and so the possibility of arcing is reduced..
 
electrics said:
in fact this is what makes me ask this question, the same book i read it from, so why? i think it should decrease since the gas density of the air decreases and so the possibility of arcing is reduced..
Click to expand...

Notice this point also that air also acts as a insulator. When altitude increases, air decreases that is why we increase the number of insulators.
 
My _guess_ is as follows: The electric field accelerates ions that happen to be present in the surrounding air. If there is sufficient acceleration, then when these ions happen to bump into other molecules, they will have enough energy to ionize those other molecules. As air density decreases, the 'mean free path' (distance that air molecules travel between collisions) increases. This means that any ions will have a longer time to accelerate and will reach higher speeds, and thus be more likely to ionize molecules that they hit. If you reach the point where each ion (on average) produces more than one ion prior to losing its charge, the you get an arc.

-Jon
 
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