I have an outside wood furnace. It has three wires in the fan switch. It will shock you when you touch the furnace itself. I can't find any breaks or bare spots in the wires. Any suggestions?

The switch itself may have a short in it, causing the wood stove frame to be energized. Seems there is a problem with grounding this wood stove. Is there a ground wire in the cable feeding the stove, or is the connection poor?

A proerly connected ground wire would cause the breaker to trip, instead of allowing the stove frame to become energized. This can be dangerous.

It sounds as if the furnace is not bonded to ground. A solution would be to make sure a connector (either bare or green) terminates from the furnace frame (usually found in the furnace electrical box) to the switchbox and back to the panel ground.

Even with this you may still get a tingle if barefoot on wet ground.

I'm sure I'm not using the right terms to describe this.. . But sometimes an old motor or transformer can have some current leakage to ground. If this is the case you can either varnish the windings, or replace the motor itself.