It may help everyone to think about two separate advancements that have occurred in home wiring. I’m rusty on the full history, but if you’ve done any upgrades you may have felt like an archeologist, and learned about a few things.
1 - The invention of the polarized plug. When this happened some makers, like lamp makers did in fact use this as a ground. So long as the impedance is near zero, no problem.... but if that lifted for any reason like a loose wire, corrosion etc. voltage at the lamp could happen.
2 - The separate ground conductor which evolved over a few iterations, not sure exactly the timing but:
- Originally no ground wire needed when metal conduit was used. This is no longer acceptable. Conduit not considered reliable.
- Originally you could wrap a conductor around any old place to ground outlet boxes. Now must use special fine thread green screws and tabs.
- The addition of the appliance safety.
The point to #2 here is that much has evolved from the early 2 wire system to today’s 3 wire. It didn’t happen until there was a need. Each of these advancements was made because the previous state of affairs cost too many lives and homes.
Edison didn’t come up with these ideas. They were created out of necessity.
Today gear with a metal exterior must be grounded OR be double insulated. This is why my plastic body blender has no ground, but my stainless coffee pot does. I think there may be some exception for gear with outboard, low voltage supplies (i.e. wall warts) too.
