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I've seen these stacked-disc structures on high voltage power lines everywhere. I could not find any information regarding this particular shape, though.

Insulator on Railways (From Wikimedia Commons)

From what I've noticed, high voltage ceramic insulators only insulate conductors end-to-end (not inside to outside, like traditional plastic insulators). I can only assume the shape makes it difficult for an electric arc to travel along the ceramic material, as opposed to, say, along a solid cylindrical piece.

Why exactly are ceramic insulators shaped like that? Is it to reduce cost? Thermal concerns (from possible electric arcs)?

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up vote 7 down vote accepted

Electricity can more easily travel across a surface of an insulator. If the surface is made longer it makes the surface path longer and thus is able to withstand higher voltages before breaking down.

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Plus the shape is intended to ensure at least part of the "cup" stays dry in most normal weather; water tends to reduce the surface impedance. –  Brian Drummond Dec 28 '13 at 10:23
    
@BrianDrummond Indeed, imagine if it was a smooth, ceramic cylinder. In heavy rain, a rivulet of water could easily run along its length between the two terminals. –  Kaz Dec 28 '13 at 15:52
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