[lcargill99]

Never mind that yes, A/C can count on polarity and that you are isolated from the grid by one or more transformers.

The third prong is the "safety" ground, in case neutral fails. Go back to old guitar amps with "the cap of death" or a "ground" switch for one market in which this evolved. It's a fairly simple mod to make an old amp fully safe. Yes, they cover that in the "bad outlet" story, but it's easy to fix if you know how to run a meter and can find a ground.

The lead guitarist from Badfinger died from this.

I'd never heard about DC buildup on the grid - free power, maybe? :)