Three Prongs Do Not Mean Grounded
I was in a Chestnut Hill condo, a unit in a building of more than four, going through the outlets room by room the way I do in every inspection.
Several of the three prong outlets in the bedrooms (some with power blocks as below) and the living room had no ground behind them.
What I had to go on was the testing, not the appearance. Every one of those receptacles has three holes in the face and looks exactly like a modern grounded outlet. The reading is what told me otherwise. I could not see the cable behind them, and in a condo I generally cannot trace it.
Older wiring never contained a ground wire. When a house or a building was originally wired that way, the ungrounded outlets are considered legal non-conforming, meaning they were installed according to the standards of the time. What the ground gives you is a path for electricity to follow if something in the system faults or there is an introduction of new electricity, instead of the path running through a person. A three hole face on a two wire circuit gives you the plug shape without the path.
What I recommended is that the ungrounded three prong outlets be updated to properly wired, grounded circuits, and that an electrician confirm what is actually behind each one rather than working from the face.
My opinion is that these outlets are a confidence problem more than an electrical one. People plug a surge protector and a computer into a three prong outlet and assume they are protected. In older buildings that have been cosmetically updated, swapping the receptacle is a ten minute job and running an actual ground is not, so the ten minute job is often the one that got done.
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