The cheap dry chemical extinguishers (class B & C only) have a white powder that is a form of baking SODA.
This is a good example of why I like pipe; you can replace wires without tearing up the wall.
Mike holt pointed out that, according to Ohm's law, a 25 Ohm ground will never draw 20 amps. Add some length to the wire, and it's quite possible to arc-weld with a wire, and not ever trip the breaker! Remember this, the next time someone says "it's only 110v, 20a!"
As for the repair, the bad news is that this is one that is probably beyond the skills/tools of the typical homeowner. It's time to hire a pro. Unless, that is, you like this sort of entertainment.