We've probably all seen these guys...
They come in handy when you need to plug something with three prongs, into something with two prongs. What I've never understood was, what the heck is the "grounding" tab for? It's meant to have the cover plate mounting screw through it. But why?
Is this really designed as a grounding device, or is it there simply as an attachment means to insure the thing doesn't fall out of the receptacle?
Yes, it's designed as a ground attachment. No, it's not simply to keep it from falling out, as evidenced by the style you haven't shown that provides a pigtail green wire, and the fact that the terminal in question is electrically connected to the ground pin of the receptacle side of the device.
While it is true that this will not always result in a ground, and may (I haven't got time to sift through code, nor code on a computer to sift through) not result in a ground approved by code, it is, nevertheless, what it's there for, and in quite a few cases it does, in fact result in a ground - whether or not you like it.
Plausible deniability. I've always regarded these adapters as a 2-to-3-prong converter, rather than a proper grounding mechanism, and here's why:
In every outlet I've encountered without a third (ground) prong, there is no ground wire. Sometimes (my parents' house) there is, but for whatever reason the ground wire is cut off and unused. Thus, the electrical box itself isn't even grounded.
The screw holding the face plate on is electrically connected to the outlet box, and if it's not grounded, then there's not much point in using the screw holding the face plate on as a ground.
The companies who make these grounding adapters probably also know this, but they can't really get away with providing 2-to-3-prong adapter without providing some way to still connect ground. So they provide a way to connect it to something that should be grounded, but often isn't. In this way, it's not their fault if something bad happens because your house wiring is faulty.
If you do happen to have a 2-prong outlet in a properly grounded box, then the little ground tab or wire will actually provide ground continuity, as intended.
So, the answer is that the grounding tab is meant as a way to connect electrically to something that in turn should be grounded (but might not be). It's not meant as a secure attachment mechanism, though it may seem that way because of the use of a faceplate screw.
Many older houses with 2-prong outlets use the sheathing of their BX or AC cables to ground the metal cases. This is not a very effective ground, but it is better than nothing.
These adapters with the grounding tab are a much cheaper and easier alternative to rewiring the entire house, yet still provide (marginally) more safety than using the adapters without a grounding tab.
As the site's expert on electrical matters, I'm sure you knew this already, so I'm curious why you asked this question ;)
Looking at the second picture in the question, makes the grounding path to the box evident.
While the yoke of the device may not be a great (or approved) grounding path (unless it's a self grounding device), it may well serve as a grounding path in an emergency. Which when it comes down to it, is the purpose of the equipment grounding conductor.
In some situations conductors leading to the box may be either in metallic conduit, or cable assemblies sheathed in a metallic covering. In some cases, these metallic pathways can provide and adequate grounding path. In these situations, the grounding ring on the adapter can indeed provide and adequate grounding path.
Unfortunately, there are a lot of situations where nonmetallic cable is used to supply nongrounding type receptacles. In these cases, the grounding ring will not provide a path to ground.
Without an adequate path to ground from the box, this setup could actually be more dangerous.
If for some reason the ungrounded (hot) conductor comes into contact with the metallic box, the box can become energized. If this adapter is connected, the grounding conductor of the plugged in device can also be energized. In this case, if the grounding conductor is attached to a metallic outer covering of the plugged in device. The outer covering can be energized, and becomes a potential shock/electrocution hazard.
If the box is supplied using older armored cable, the cable sheathing may actually be a high resistance path to ground. This could lead to heating of the cable sheath, in some cases to the point of ignition of surrounding building materials. Modern armored cables provide a low resistance path to ground, and are often approved to be used as a grounding means. So if newer cable is used, this may not be a problem.
The box containing the receptacle may be supplied using modern cabling, and a grounding path may be present. However, the modern cabling may be connected to older cabling that does not provide a grounding path. In this case, fault current will follow the grounding path back to the junction between old and new cabling, and lead to potential hazards at that location.
In some cases these devices work as intended, and provide an adequate path to ground. In other cases they do not, and can potentially introduce other hazards. Though we must keep in mind, the equipment grounding conductor is a safety device. Under normal operating conditions, this path will never have current flowing through it. Because of this, the dangers from a device like this are limited.
These are "officially intended" for cases where the sockets are 2-prong, but yet, the receptacle is grounded. The idea is to get proper ground from the receptacle cover plate screw, which would work if the receptacle was grounded.
Obviously, this situation is imaginary. If an electrician went to all the trouble to use the more expensive /2+ground cable in 1964, he certainly would have fit the then-impressive 3-prong receptacle, as a selling point for his effort, and there would be no need for this adapter.
So this is called a cheater for a reason -- it's actually used to plug 3-prong things in locations which are actually not grounded. Which, indeed, is playing with fire. For instance the one in your photo allows polarized devices to be plugged in (taller neutral slot), but its pins are not polarized, breaking polarization!
Now the modern age gives us an alternative here, and that's to fit GFCI protection to the ungrounded circuit (a breaker, deadface or GFCI+receptacle here or upstream), with a "No Equipment Ground" sticker. That will provide all the shock protection of a grounded outlet, but none of the equipment protection (hence the sticker).
