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An electric train is typically powered with something like 1.5 kilovolts of direct current of something like 25 kilovolts of alternating current. In my experience electric device manufacturers charge a fortune for even 220-volts (mains voltage in many regions) appliances that can be used outdoors, so atmospheric moisture is a huge problem for electrical devices.

Yet an electric train will move under the heaviest rain with its pantograph contacting a bare overhead wire and no short-circuiting happens.

How is that achieved?

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First, notice that there is nothing near where the train is touching the overhead wire. Distance is insulation. Then if you look closely you will probably see insulators in serveral places in the mechanical gizmo that pushes the contact against the overhead wire.

The same problem applies to high voltage transmission lines. The big ones have metal towers, which of course can't be allowed to have high voltage on them. In fact, they are deliberately grounded. The high voltage cable is usually suspended from a long ceramic insulator. This has a number of ribs or discs to increase the surface distance from one end to the other and keep water from collecting along the whole surface. Smaller but quite visible ceramic insulators will be on the power lines in your neighborhood and at the primary side connections to the transformer for your house or building. Google "high voltage ceramic insulator" and you will see many pictures and descriptions for these devices.

The insulators on the train may be of a different material, but are probably visible if you look closely.

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    \$\begingroup\$ High voltage ceramic insulator Wow! \$\endgroup\$ – Russell McMahon Dec 29 '11 at 15:45
  • \$\begingroup\$ I guess I've wondered about this myself. How would the insulators prevent water running down the side of them from creating a lower resistance pathway? I guess the shape of them is designed to prevent that? \$\endgroup\$ – darron Dec 29 '11 at 15:46
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    \$\begingroup\$ @darron: As I said above, the "ribs" or "discs" in the insulator are there in part to prevent water from collecting or coating the whole surface. \$\endgroup\$ – Olin Lathrop Dec 29 '11 at 16:51
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    \$\begingroup\$ @darron - that's why the segments have a generally cone-shaped design; the undersides tend to stay dry, so there is no continuous wet surface. The distance between the wet edges is good for a few 10's of kV apiece, so they stack up some number of segments to achieve the necessary isolation. \$\endgroup\$ – JustJeff Dec 30 '11 at 22:42
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    \$\begingroup\$ @darron: Also if there were continuous surface water it would be vaporized quickly. As long as the distance isn't enough to sustain a arc thru air this is a minimal short duration event. \$\endgroup\$ – Olin Lathrop Jan 1 '12 at 14:02

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