That is single phase. The "hot" (or "live") wire has a nonzero voltage relative to ground, and the neutral wire is usually around the same voltage as ground (do

That is single phase. The "hot" (or "live") wire has a nonzero voltage relative to ground, and the neutral wire is usually around the same voltage as ground (do not count on this). The one-way plug ensures that the "hot" voltage stays in the wall when the device isn't on, so if you shove your wand into a toaster that's off you won't get electrocuted. Devices are always connected to neutral, and the switch connects them to hot.