How to detect the neutral/ground in a power outlet?

October 3, 2005 1:32 PM

Does it make any difference if the ground and the neutral in a power outlet are reversed/swapped? Can this be detected somehow? How?

(And, if possible, why is it possible? Aren't ground and neutral effectively the same thing? Or, am I completely mistaken?)

posted by yeoz to technology (17 comments total)

Ground and neutral are not the same. Yes, it makes a difference. A standard outlet tester available from a home depot or lowes shouldbe able to tell you if they're switched. You can usually tell by examination also: ground is a bare uninsulated wire most of the time, attached to a green terminal. Neutral is usually white, attached to, I think, the silver colored terminal.

The biggest difference is that neutral carries current in normal operation, and ground does not. Ground is an "emergency" line, for use when something goes wrong.
posted by RustyBrooks at 1:36 PM on October 3, 2005

I'm no electrician, but I am comfortable around electricity, and I hate to think of what could happen in this case, I really do. For one, I'm not sure the overload protection (i.e. the fuses or more likely, circuit breakers) would even function properly.
posted by RustyBrooks at 1:38 PM on October 3, 2005

The ground and the neutral are different. The ground is the green wire. It grounds out the current so that if there is a short, the current doesn't pass through the building, it goes into the ground (terre firma) via the ground wire, thus preventing a fire. The neutral is the white wire. The neutral returns the current to the bus.

You might want to call in an electrician to address your concerns.
posted by onhazier at 1:40 PM on October 3, 2005

Out of curiosity, how does an outlet tester determine if it's switched?
posted by yeoz at 1:40 PM on October 3, 2005

One thing I forgot to mention about visual inspection: the outlet may be wired correctly at the outlet, but incorrectly somewhere else (like at the circuit breaker).
posted by RustyBrooks at 1:46 PM on October 3, 2005

The polarized plug thing makes no sense at all. If voltage is a sine wave, i.e. cycling from - to + current, then the electrons should be moving both ways through the wire, making the two plugs identical except having different phases. Wait -- one is hot and the other is neutral? Then let's call a spade a spade: it's DC.

Yeah, I'm being Andy Rooney.. now don't get me started on that cotton in aspirin bottles.
posted by rolypolyman at 2:06 PM on October 3, 2005

I believe Rusty has misread the question, or read the actual meaning of the question which might have been incorrectly phrased. You'd know instantly if the ground and neutral wire in an AC setup were reversed - the breaker would trip (or refuse to latch) and you'd have a rather scary *BZAP* and arc. Somewhere I have a screwdriver missing 1/3 of the shaft where it momentarily touched the hot wire while in contact with the ground.

A far more common situation and what those testers are checking is for the neutral and HOT wire to be reversed. Sometimes it matters and sometimes it doesn't - thus the reason some plugs are polarized, meaning one blade is bigger than the other and cannot be plugged in flipped over.
posted by phearlez at 2:06 PM on October 3, 2005

For one, I'm not sure the overload protection (i.e. the fuses or more likely, circuit breakers) would even function properly.

Of course they would. A fuse or breaker examines one thing only: the amperage draw through itself. If the immediate terminus of the breaker is grounded then the draw is going to be equal to the capacity of that wire or where it's grounded to which will certainly be greater than the breaker/fuse rating.

If you need a visual aid, pick up a pen and imagine it as a barrel fuse where stuff comes in one end and goes out the other, starting at the left and coming out the right. From the right it it's hooked to that hot wire which snakes its way through your house. Where does the neutral wire ever 'reconvene' with it? Nowhere. It's irrelevant to the functioning of the fuse. There's an in and an out, nothing more. That electrical socket in the house can be wired correctly, incorrectly, to a potato, whatever - it doesn't matter, the fuse will still work.

Now how WELL it will work is a different story, and why we have GFCI (ground fault circuit interrupt) sockets for bathrooms and areas where a short could likely cause harm to a human - because as fast as a fuse/breaker is, it's not always fast enough to keep your ass from being killed. or your screwdriver from being 1/3 melted.
posted by phearlez at 2:14 PM on October 3, 2005

Right, but you plain shorted hot to neutral. A plug with nothing in it does not do this. A plug with a device attached to it ALSO does not do this, since it has resistance and does work. I think somethig quite bad could happen but it would not be as bad as plain shorting the two with a screwdriver.

There is a difference between hot and neutral, and yet it's still AC. Take the AC voltage between hot and neutral. In the US, about 110-120. Take the AC voltage between neutral and ground: 0 (or therabouts). The fact that it alternates between -120 and 120 does not mean that hot and neutral are equivalent.

I think I read it right. If he's really asking if hot an neutral are reversed, this is a different matter. This is not a holy-hell safety concern, but it does matter. The design of many appliances, light fixtures, etc, assumes that you have hot and neutral right, otherwise you run the risk of zapping yourself by touching an exposed metal part. In the scheme of things it's a relatively minor risk.
posted by RustyBrooks at 2:15 PM on October 3, 2005

(BTW: Hot and Neutral do not mean Plus and Minus)
posted by RustyBrooks at 2:17 PM on October 3, 2005

OK, does this mean that the hot wire is pushing and pulling electrons 60 times per second, and should be pushing and pulling them in/out of the neutral wire? If that's true, then I can understand why the polarized plug is so important. But it still sounds a lot like DC.
posted by rolypolyman at 2:21 PM on October 3, 2005

The Wikipedia entry for AC current talks about the different ways neutral and ground can be configured. To the best of my knowledge the US uses TN-S exclusively.
posted by phearlez at 2:27 PM on October 3, 2005

The polarized plug thing makes no sense at all. If voltage is a sine wave, i.e. cycling from - to + current, then the electrons should be moving both ways through the wire, making the two plugs identical except having different phases.

That's fine if your the two holes in your outlet, and what you're plugging into it are the only things in the entire universe.

As soon as you have something else (let's say, you), then those wires have voltages, not only relative to each other, but to that other thing as well.

If all that exists is those two wires, then yes, all you can say is that at one time, wire X is +156V relative to wire Y, and at another time, wire Y is -156V relative to wire Y. As soon as you have that other thing (you) against which to measure voltage, you can see that "hot" wire fluctuates between (let's say) +256V and -56V relative to you, and the "neutral" wire is at a constant +100V relative to you.

As to the original question, you may want to read Ground and neutral at wikipedia. Ground and neutral are generally at the same voltage, but as others here have already noted, the neutral carries current during normal operation, and the ground only when something's wrong. A ground fault interrupter detects when the ground is carrying current and breaks the circuit.

Because ground and neutral are at the same voltage, a standard outlet tester cannot determine if the ground and neutral are switched. (The picture I linked is the best I cound find; the possible readings are "open ground," "open neutral," "open hot," "hot/ground reversed," "hot/neutral reversed," and "correct." No reading for "neutral/ground reversed," since it can't be detected by using one of these devices.
posted by DevilsAdvocate at 2:31 PM on October 3, 2005

This thread is a bit too chatty for a question about electrical safety.

Unless everything is wired in a crazy fashion, the fuse is in the hot wire, so swapping neutral and ground would produce a functional, fused circuit. It is, however, NOT recommended.

Through appliances, you'd expose neutral to the touch, and it is not guaranteed to be at ground potential. Equipment that is wired properly can have problems with an unstable ground, and this includes your neighbours' things if you share the distribution. If you have a ground fault breaker, it will trip instantly - it's built to prevent these things. You might also have the power company come knocking, asking what the hell you're up to, sending a lot of current through their protection ground wire.
posted by springload at 3:09 PM on October 3, 2005

>Does it make any difference if the ground and the
>neutral in a power outlet are reversed/swapped?

Yes. But the circuit might work anyhow.

>(And, if possible, why is it possible? Aren't ground and
> neutral effectively the same thing? Or, am I
>completely mistaken?)

It's possible because people make mistake. Or you're working with something that predates code. Or it's old and the fabric insulation has all faded so you can't see the color markings and the last guy in there guessed wrong when he put it all back together.

Ground and neutral are cousins, but they are not twins. You are not completely mistaken, you have found a rather interesting point regarding household wiring. Once they get outside the house they might be the same, but inside the house they are not.

A point I'd like to mention is that if you are sending current out the ground instead of the neutral, that current may be exposed somewhere further down the line. You turn on your light here, it looks fine, someone in another room touches the "grounded" cabinet on their stereo, and ZAP!
posted by Ken McE at 7:11 PM on October 3, 2005

There are two obvious and important safety issues with switching ground and neutral.

1. Ground is normally a lighter/thinner gauge of wire. This is a serious fire hazard when used with any heavy load - hair dryer, vacuum, etc. - because the circuit is fused for the heavier gauge of wire.
2. As already mentioned, ground fault interrupters won't function properly. They will probably fail rather than cause a fire, which is nice, but still a problem.

Reversing ground and neutral may also cause problems with EMI shielding and surge suppression... My understanding is that the point at which ground and neutral are connected varies a lot by region. Sometimes right at your home utility panel, other times at a utility pole step down transformer. The exact effects of the reversal would depend greatly on where the connection is made.

I'm not sure about Ken McE's point about grounded devices in other rooms. A functioning neutral can sometimes have a few volts on it, but hardly enough to cause electrocution. On the other hand, if some fool were trying to service an outlet without turning the breaker off, said fool might feel okay breaking the ground circuit and find 120V on one end... Some more explanation of why it could cause electrocution is welcome.

Also, thanks DevilsAdvocate, I assumed that those devices could detect a ground/neutral reversal and I was racking my brain trying to figure out how it could be done. You could probably do some capacitance test to determine if the ground pin and the box behind it are at the same potential, but that wouldn't really be sufficient...
posted by Chuckles at 9:00 AM on October 4, 2005

Actually, the Earthing System wikipedia entry is probably the most direct answer to the original question.

Thanks for mentioning the earthing system terminology phearlez! However, it seems to be a bit broken... According to the Electrical Wiring FAQ - What does an electrical service look like? most/all of North America uses TN-C-S. However, houses in North America also have a direct connection to earth, so maybe the terminology should be something more like T(N+T)-C-S...
posted by Chuckles at 10:06 AM on October 4, 2005