[mcguyver]

You are exactly right memel. My Fluke will only measure up to 10 Mohms, so I get the O/L (Outside of Limit) indicator. Basically you're just reading any resistance the air offers. But there is still resistance there. My megger will read up to 1 Gigaohm and it will pick it up, as that's what it's designed to do.

This assumes that all is well with the switch and the contacts are nice and dry. I've seen overlubed mechboxes where the switch is juicy and there really is no break in current flow. Not enough when the switch is open to drive the motor, but enough to kill a battery. Resistance would still be high and it would take days or even weeks to kill a battery, all other things discounted.

With low-voltage DC systems, the dielectic breakdown point will be high relative to the circuit voltage. In higher voltage AC systems (600 volts let's say) these things are very destructive as the potential and kinetic energy capacity of the circuit is extremely high.