I just spent the last 30 minutes trying to get access to the UL 489 specification that outlines this but then I discovered just how inaccessible that stuff is wth
Once upon a time I went to pull an outlet splitter out of the wall, and it broke. It began shorting itself and arcing. Legit one of the most terrifying things I've experienced. I'm lucky nothing exploded and sent shrapnel into my eyes - a possibility I didn't learn about until later.