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1. Make it physically impossible to throw the switch while it is under load. This could, perhaps, be achieved by locating the switch on the underside of the chassis plate.
2. Change the medium by having the switch sealed from air (an SPDT reed relay?)
3. Use a twelve position switch and wire up just the contacts that are furthest apart
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This is obsolete now but I'm going to give a technical answer anyway.
1. is a fine idea.
2. reed relays have a gas inside; the way to do this is with a vacuum relay. They are hard to find and expensive.
3. It's the distance of the contacts that must meet and part to make the connection. Any switch that works will have infinitely close distance at some point in time.
4. The usual solution is to be aware of the arcing (and ionization - there are two mechanisms to worry about) and choose a switch whose contacts are so large that their erosion takes many many cycles. Anyone who remembers changing the points on a distributor is familiar with this!
Side story - way back when we were dating, my wife had a Dodge with a high-compression V-8 and a 6-volt battery, with dual paralleled points. It needed new points about every 750 miles (and a quart of oil every 1500). She used to pull the distributor and do the operation on the kitchen table, replacing the distributor the next morning to go to work.
