You have learned why some type of transfer switch is necessary to isolate the systems. I am not familiar with Generac circuitry but you may have cooked the rectifier or electronic voltage control. Have a good whiff around the end plate, if it smells burnt the stator is likely fried.

Life's lessons can be harsh. This generator seems to have only three major electrical parts: Bridge Rectifier, Stator, and Rotor. If it's the rectifier, that would make me very happy since it's the only one of the three under $300. I was hoping for some guidance on how I might check it out.

Thanks for your response - you are of course right about the transfer switch.

And, did you electrocute any electric line repairmen, when you had your jumper wire feeding current into the electric lines? I can just visualize repairmen getting fried, because you energized the lines! MMM-HMMM!

Sorry to disappoint you, but the generator was never running on a circuit open to the outside. It was off when power was fed into it.

But thanks for the help.