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I removed the plates from outlets to paint walls and noticed that some of my electric boxes have ground wires connected to outlets while others do not have a green wire at all. Is this ok?

I know that neutral wire is connected to ground in the main box. I also understand the need to ground metal-encased appliances. What I don't understand is why there have to be two separate wires (white and green) that connect to a common terminal at the main box?

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What year was the building constructed? What region of the world is it in? –  wallyk Aug 28 '13 at 17:34
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@wallyk: NJ, built 2006. Condo. –  user443854 Aug 28 '13 at 17:45
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Seeing your comment, I'm a little shocked that something built in 2006 doesn't have a ground to each outlet. A home built in the 1950's may only have 2 prong receptacles, but anything built within the last 30+ years should have a ground wire. –  BMitch Aug 28 '13 at 21:56
    
My home, buit in '60, had the grounds connected to the back of the metal box on the bracket screw that secured the incomming sheathed wire. I could not see the gound until I looked closer. Are you sure the cable is only 14/2 or 12/2 wire? I also had 2 prong receptacles but easily replaced them. –  Richard Raustad Aug 29 '13 at 0:14
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4 Answers 4

up vote 10 down vote accepted

To answer your first question, no it's not permitted to have a 3 prong receptacle without the ground wire being attached. See some of the other questions explain how you should fix this situation. I believe the short answer is that it should be a 2 prong receptacle (which will be difficult to find and only to be used in grandfathered situations) or you could use a GFCI with a "no equipment ground" label.

If you use your neutral as a ground, then a neutral fault (a break in the neutral connection somewhere in the circuit) will result in the exterior of every device on the far side of that fault being an electrocution risk. You are also at risk of a shock if you ever form a better path to ground than the neutral wire.

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That last sentence was a separate point. Anytime you're a better path you'll get electrocuted. But a neutral fault means you're much more likely to be that better path. –  BMitch Aug 28 '13 at 18:09
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Note that self grounding outlets exist, removing the need for ground wire to the outlet, but only when used in a grounded metal box. –  Pigrew Aug 28 '13 at 23:24
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The grounding conductor in an electrical system provides a safe path for fault currents to travel along. It's there to prevent electrocution.

No Grounding Conductor

Let's say we have a toaster.

Toaster

Inside the toaster are two conductors, a black ungrounded (hot) conductor, and a white grounded (neutral) conductor.

Toaster with wires

To heat the toaster, current flows into the heating element through the ungrounded (hot) conductor, through the heating element, out of the heating element and back to the source through the grounded (neutral) conductor.

Now let's say there is a short between the ungrounded (hot) conductor, and the metal frame of the toaster.

Toaster with short

Since the frame is metal; and conducts electricity, it is now electrified.

Toaster with short electrified

If you then touch the toaster; and you're sufficiently grounded, current will flow through you to the ground. This could lead to a nasty shock, or potentially death.

Death

With Grounding Conductor

Now let's see what happens when we have a toaster with a grounding conductor, that is properly bonded to the non current carrying metal parts of the toaster.

Toaster with bonded ground

There's a short in the toaster again.

Toaster with short and bonded ground

However, since we have a grounding conductor this time, the current flows through the toaster into the grounding conductor and back to the source.

Grounding conductor in use

This creates a very low resistance path back to the source, so the current in the circuit increases rapidly (I=E/R). The increased current causes the circuit's overcurrent protection to kick in, and the breaker trips.

But the neutral is grounded?

The grounded (neutral) conductor is indeed grounded at the service entrance, however, after that point the grounded (neutral) conductor is a current carrying conductor. It's used to carry current back to the source, and so potentially always has current flowing on it. If this conductor was connected to the toaster's non current carrying metal parts, it would allow current to flow on the metal parts of the toaster.

If there was a break in the neutral conductor further down the circuit. Current could flow through the ungrounded (hot) conductor into the toaster, through the heating element, out of the heating element and down the grounded (neutral) conductor, through the toaster's metal parts, through you, and into the ground. Which could lead to a shock, injury, and/or death.


tl;dr

The grounding conductor provides shock hazard protection, and should always be properly connected. If there is no grounding conductor available, a ground-fault circuit interrupting (GFCI) device may be able to be used to provide this protection (see NEC 406.4(D)(2)(b)). This conductor should only carry current during a fault situation.

The grounded (neutral) conductor is a current carrying conductor, used to carry current back to the source. This conductor carries current during normal operation.

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There's great educational value in your answer. But I have to point out one inaccuracy. White being connected to metal lamp is not the same as black being connected. Let's say you are grounded, and offered a choice to touch black or white, which would you rather touch? Hence, the difference. The current through black and white is the same, but the potential difference with ground is not, and that potential difference will determine the current that will flow through you when you touch a wire. –  user443854 Aug 28 '13 at 18:10
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Not exactly, but pretty close. Even if you are standing in a bath tub full of water, your resistance is much greater than that of a wire. You have two parallel resistors, r (wire) and R (you), where r << R. The current through each will be V/R and V/r. So yes, you will get shocked. Compare that to near certain death if you touch a black wire instead. –  user443854 Aug 28 '13 at 19:11
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@Tester101 yeah that's basically correct. The neutral wire, even when carrying current, stays at a nearly constant voltage to ground (almost 0). If you are grounded, the voltage between you and neutral should be almost 0, so little/no current will flow. It could be diagrammed like this: goo.gl/TtRkE2 (and now compare that to the case where you connect to the other end of the device instead). –  BlueRaja - Danny Pflughoeft Aug 28 '13 at 23:28
    
Good explanation, but the example is a little weak because most lamps do not have a ground wire even when they are plugged into a properly grounded outlet. –  bib Aug 29 '13 at 0:19
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@bib Is a toaster better? –  Tester101 Aug 29 '13 at 13:36
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The accepted answer states "it's not permitted to have a 3 prong receptacle without the ground wire being attached." This is incorrect.

406.4(d)2(b)
A non-grounding type receptacle(s) shall be permitted to be replaced with a ground fault circuit interrupter type of receptacle(s). These receptacles shall be marked "No Equipment Ground". An Equipment grounding conductor shall not be connected from the ground fault circuit interrupter type receptacle to any outlet supplied from the ground fault circuit interrupter receptacle

406.4(D)(2)(C)
A non-grounding type receptacle(s) shall be permitted to be replaced with a grounding type receptacle(s) where supplied through a ground-fault circuit interrupter. Grounding-type receptacles supplied through the ground fault circuit interrupter shall be marked "GFCI Protected" and "No Equipment Ground". An equipment grounding conductor shall not be connected between the grounding type receptacles.

Thus, having a three-prong recepticle with no ground is allowed, as long as it's GFCI-protected (either by being a GFCI, or being on the load-side of a GFCI) and has a "No Equipment Ground" sticker.

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+1 Thanks for the clarification. –  BMitch Aug 28 '13 at 22:29
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Also note (probably won't apply to you because of how new your home is): there are these things known as 'self-grounding outlets'. Self-grounding outlets are three-prong outlets that automatically ground to the outlet metal box they are attached to via the mounting screws on the outlet assembly, or via a green pigtail wire from the outlet assembly that is screwed to the metal outlet box. These obtain their ground via a conduit (a metal outer shielding that the wires were run inside) that runs to the main electrical box, which is then grounded to earth-ground somewhere.

This does not work if you have romex wiring with three wires (black,white,green).

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This is what my house appears to have. –  Brian Knoblauch Aug 28 '13 at 23:11
    
@J.Polfer: So maybe that is the case with me. How can I test this? I have a multimeter. Is it sufficient to check that there is electric contact between the metal box, the green bolt on the receptacle (nothing attached to it right now), and the white wire? –  user443854 Aug 29 '13 at 13:30
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@user443854 - There's a shortcut to this... I would recommend getting a receptacle tester (en.wikipedia.org/wiki/Receptacle_tester). Read the entire wikipedia page. I would recommend plugging in the receptacle tester on the suspect outlets you have to see if they test OK, and then run through the Safety multimeter check on their page. That should indicate whether or not it checks out okay. Note: I am not an electrician. If you are not comfortable with this or are unsure, consult with an electrician. –  J. Polfer Aug 29 '13 at 16:48
    
@J.Polfer: What they suggest makes sense: there should be small measurable, non-zero resistance between ground and neutral, measured at the outlet. I'm plenty comfortable checking this with a multimeter. –  user443854 Aug 29 '13 at 18:37
    
@user443854 - In short, yes, if you want to ensure you don't have a bootleg ground. –  J. Polfer Aug 29 '13 at 19:02
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