VK2TVK,
Thanks for the compliment. I do have to say, though, that I disagree on altitude. Air is a pretty good insulator most of the time. Ionized air is a pretty good conductor, though, and air gets ionized when electricity flows through it.
The typical problem with a switch is that when the contacts open, there's always an instant when they're still very close together. An arc will jump that gap, starting ionization, then grow as the contacts spread further apart and the ionization process continues.
Simple switches are spring loaded, so the contacts will separate faster than ionized air can fill the gap and maintain an arc. Some high-voltage switches actually keep the contacts in a vacuum to avoid ionized gas (though the contacts themselves can also create enough ionized particles to sustain an arc in some circumstances).
Other high-voltage switches use a blast of air to physically blow out the arc. These might not work well at altitude, but I don't think they're used for typical aircraft voltages anyway.
Other than that, I'm not sure why arcing would be a bigger problem in unpressurized areas. But there's probably one I haven't thought of.
*** Should have googled before posting. Apparently electrons that bump into too many gas molecules get slowed down and aren't effective at ionizing. So up to a point -- somewhere around 150,000 feet -- less air means more ionization and more arcing.
Last edited by Chu Chu; 5th Jul 2013 at 21:42.