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We've probably all seen these guys...

3-prong to 2-prong adapter

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?

Adapter Installation

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?

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I can't authoritatively answer this, but it is my impression that the grounding tab comes in contact with the coverplate screw, that is in contact with the metal frame of the receptacle, which is in contact with the metal box, which is hopefully grounded to the metal conduit that carries the wire to the box from the breaker. The conduit of course is grounded to the electrical panel. –  maple_shaft Dec 13 '13 at 12:01
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@maple_shaft In a perfect world, but what about those 2 prong receptacles that are supplied by cable with no grounding means (which may be a large portion of 2 prong receptacles)? Also, if the receptacle is not a self grounding type (which it's not since it's 2 prong), the yoke is not an acceptable grounding means. –  Tester101 Dec 13 '13 at 12:09
    
It can be... I have measured decent continuity between the plate screw and grounded conductor in older homes where I knew the box was metal and it was grounded to the panel. It is certainly more likely to offer ground fault protection than the cheater plug without a ground tab. Why are these things legal anyway? Surely the NEC doesn't put its stamp of approval on this? –  maple_shaft Dec 13 '13 at 12:42
    
@maple_shaft There may be continuity, but it's not designed to carry current, and is not an accepted grounding means. –  Tester101 Dec 13 '13 at 13:23
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Strange that you ask the question and then immediately argue against it. –  Ecnerwal Dec 13 '13 at 16:18

4 Answers 4

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.

Pigtail adapter

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.

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In the days of Metal Conduit, Armored BX and Metal Electrical Boxes, it was a ground for the inserted three wire device. Your conduit system was supposed to attach to a cold water pipe bonded to ground with a ground rod. The center screw in the cover plate attached to the metal bracket the sockets attached to and grounded to the electrical box via the two screws that held the socket there. –  Fiasco Labs Dec 14 '13 at 3:08

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.

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I have come across one that didn't even have a grounding tab. –  Brad Gilbert Dec 13 '13 at 23:14
    
@Brad 1. Research if the manufacturer still exists and if so, if the company is wealthy. 2. Ensure you have purchase receipts, warranty registration card filled out, etc. 3. Operate some machinery using the adapter, perhaps something that doesn't have safe wiring (wink, wink). 4. Sue for damages and profit. –  JYelton Dec 13 '13 at 23:21
    
Based on the design I would say that the latest it would have been produced was in the 1970's. ( It had 1940's 1950's styling. ) I don't remember any identifiable markings on it either. –  Brad Gilbert Dec 14 '13 at 0:26
    
plausible deniability Nice conspiracy theory, works if the house wiring used properly installed and grounded metal boxes and conduit. Which is what this is supposed to be used with. –  Fiasco Labs Dec 14 '13 at 3:11

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 ;)

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Just because I know the answer (sort of), doesn't mean I can't ask the question. –  Tester101 Dec 13 '13 at 16:54
    
One of the great things about SE sites is that you can ask a question (even if you know the answer) because you want to either a) answer it yourself as a reference for others, b) get additional answers to elaborate and expand on something, or c) you just want to get more internet points! –  JYelton Dec 13 '13 at 21:09
    
@Tester101: Fair enough. I hope you didn't read that in an offending tone - I was just curious, that's all. –  BlueRaja - Danny Pflughoeft Dec 13 '13 at 22:21
    
It is ok to answer your own question. I advocate waiting a while though. –  Brad Gilbert Dec 13 '13 at 23:13
up vote 0 down vote accepted

Grounding from plug to Box

Looking at the second picture in the question, makes the grounding path to the box evident.

Grounding path to box

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.

Grounding from box to earth

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.

Potential Hazards

Without an adequate path to ground from the box, this setup could actually be more dangerous.

Short to ground inside the box

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.

Inadequate grounding path

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.

Broken grounding path

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.

tl;dr

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.

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