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June 3, 2012 11:22 PM   Subscribe

"With AC power, aren't both the wires of the pair interchangable? Why is one wire called 'neutral?' What's all this stuff about 'grounding?' Why are three prongs needed?"
posted by koeselitz (73 comments total) 119 users marked this as a favorite

 
So when my grandfather's old razor trips the GFCI when I turn it on (because the "power screw" is turned too far), I should be thankful that it's not murdering me in my bathroom!
posted by disillusioned at 11:25 PM on June 3, 2012 [1 favorite]


Fun fact: most all appliances in Japan have no ground prong. Just two prongs, like it used to be in the US. Ungrounded!
posted by flapjax at midnite at 11:29 PM on June 3, 2012 [1 favorite]


This guy's essays are great. A few years ago, when I was taking an introductory class on semiconductors, his essay on transistors was an excellent supplement to my textbook.
posted by BigSky at 11:29 PM on June 3, 2012 [1 favorite]


This was fantastic. I've learnt something. I had, indeed, wondered why the two wires in an AC circuit are somehow considered different, when the current is just oscillating in both of them.

Now I'm going to go find that essay on transistors BigSky mentioned...
posted by Jimbob at 11:35 PM on June 3, 2012 [1 favorite]


Thank you so much! I just about went nuts trying to find an explanation of this a few months ago.
posted by starfishprime at 11:36 PM on June 3, 2012


(because the "power screw" is turned too far)

Holy shit. I've also just learnt what that big funky screw in the side of my hair clippers is for as well...
posted by Jimbob at 11:37 PM on June 3, 2012 [5 favorites]


This guy is indeed very good. I can't find the article on transistors, but I am enjoying "Frequently-asked Electricity Questions."
posted by koeselitz at 11:37 PM on June 3, 2012 [3 favorites]


disillusioned: "So when my grandfather's old razor trips the GFCI when I turn it on (because the "power screw" is turned too far), I should be thankful that it's not murdering me in my bathroom!"

Certain loads will cause nuisance trips on GFCI circuits/outlets. It doesn't necessarily mean the equipment is faulty and would murder you if given the chance.
posted by wierdo at 11:48 PM on June 3, 2012 [1 favorite]


There's a more elegant solution to differentiating live and neutral than having one bigger and one smaller prong.
posted by wilful at 12:04 AM on June 4, 2012 [2 favorites]


There's a more elegant solution to differentiating live and neutral than having one bigger and one smaller prong.

Yeah, but then your walls have little scared ghost-faces all over them...
posted by Jimbob at 12:23 AM on June 4, 2012 [6 favorites]


This stuff is really interesting, but now I have a new question: Why can I insert Europlugs and Schuko plugs both ways (which is a quite convenient feature if you ask me)? Is the clear assignment of polarities not that important after all?
posted by wachhundfisch at 12:30 AM on June 4, 2012 [2 favorites]


'How do transistors work?' by Williamm Beatty
posted by BigSky at 12:44 AM on June 4, 2012 [4 favorites]


There's an essay I'm working on that might take me about 10 years to complete. It has a working title of something like: "The Neological Fallacies of Engineers.". And I will certainly be including this part in it:

In electrical circuits, the word "ground" can be very confusing. The word has several different meanings. Your teacher might know which meaning he or she is using in any spoken sentence. But this spreads confusion, because students aren't even aware of the multiple meanings. Or, even when students know that "ground" is a multi-facted term, they may lack skill in connecting up each definition. Perhaps it's best to simply use of the word "ground" in the classroom. That forces everyone to use accurate and less-confusing words.

Thanks for the post.
posted by coolxcool=rad at 12:46 AM on June 4, 2012 [1 favorite]


I tried to read that transistor article, but it was horribly painful. Here's a quick attempt at a better explanation:

Transistors work because they cheat. They don't quite follow the usual rules of potentials and conductors and big chunks of metal that you're used to in normal circuits. The Base in an NPN transistor acts like a valve which can open, close, or just partially open to let current flow through the other leads.

So why doesn't current flow out through the Base? Because the Silicon its made of is engineered to very special in that region. First of all its very thin and very clean, so moving electrons have very little chance of hitting something that could divert them in that direction (this fudges over some imporant things but never mind).

Also the interface between the Base and Emitter and the Collector is really really clean, so there aren't any imperfections there that allow current to be diverted to the Base (usually its only 1 part in 10^6 that comes out the base in a run of the mill BJT).

This is all much easier to understand in FETs, field effect transistors. There, you apply a voltage to the gate electrode, whose electric field "opens or closes" the conducting pathway between the two other terminals.

So why didn't we start with FETs? That was the first thing people did try to build in the solid state, but it turned out to be really hard. So they figured out another clever way to do it. Then after a couple of decades of trying to make FETs they eventually studided the surfaces of the silicon carefully enough that could make FETs.
posted by Chekhovian at 12:59 AM on June 4, 2012 [2 favorites]


I like the British standard. "32 amp main! Put the fuse in the plug! Make the whole thing the size of a child's fist and put prongs in it that you can't bend with a hydraulic press! We need the copper to beat old Jerry!"
posted by Grimgrin at 1:35 AM on June 4, 2012 [12 favorites]


As someone with "electrician" in her job title, I found that pretty painful to read. On the other hand, the guy does know his stuff. I suppose I am more irritated by the writing style than anything else. So cutsey.
posted by mollymayhem at 1:42 AM on June 4, 2012 [2 favorites]


Now that (apparently anonymous) electrical engineers have worked out how to stop us electrocuting ourselves so often the world awaits only a sane solution to those traveling it with electrical devices. It looks like that may be finally here.
posted by rongorongo at 2:15 AM on June 4, 2012


If you can't do serious damage to your feet by stepping on the up turned prongs in the middle of the night, it's not a proper plug.

BS1363 4 LYF.
posted by MattWPBS at 2:22 AM on June 4, 2012 [6 favorites]


Brought to you by my inner MacGyver:

An interesting quality of Ground Fault Interrupters - as explained in the article, while their job is to look for lost current, that's not what they actually do, instead they check to see if the current on one side matches the other.
What that means is that they will also trip if you add power to the system.
What that means is that you can use a ground fault interrupter as a radio-controlled switch.
Plug an appliance into the socket and turn it on - you can switch it off at a distance by hitting the "transmit" button on a walkie talkie. (Proximity of the antenna to house wiring, or to the power cord helps. As does having a powerful walkie-talkie. Basically, the wiring acts as an antenna so your transmitter induces a small current)
posted by -harlequin- at 2:34 AM on June 4, 2012 [8 favorites]


I learned wiring on automobiles first. It always confuses me that black is hot on house wiring. Black is ground and red is hot in cars.
posted by narcoleptic at 4:05 AM on June 4, 2012 [2 favorites]


RED KILLS FRED & BLACK SAVES JACK
posted by Smart Dalek at 4:12 AM on June 4, 2012 [2 favorites]


I grew up around British bikes. Hot positive grounds. Zenier diodes. Oh dear god I can't understand anything electrical now.
posted by 1f2frfbf at 4:15 AM on June 4, 2012 [3 favorites]


blue neutral!
brown live!
green and yellow earth!
posted by scruss at 4:24 AM on June 4, 2012 [5 favorites]


Ok, so can anyone explain why there is so much inconsistency in wiring colours?

Why is brown live, and not, say, red for danger? Why black in the US? That's for negative, isn't it?

I do like striped green and yellow for earth, though. It's pretty.
posted by milkb0at at 4:47 AM on June 4, 2012 [1 favorite]


Fun fact: most all appliances in Japan have no ground prong. Just two prongs, like it used to be in the US. Ungrounded!
Well, according to the article, they are grounded, they're just not double grounded for extra bonus safety

(Which is apparently safety against people connecting the other two wires to the wrong holes, as the odds of someone connecting all three holes wrong would be much lower - connecting the 'hot' and 'neutral' wires would still work, but be dangerous, but I guess you would have to connect the 'live' wire to the 'neutral' prong in order to be dangerous, but if you did that, devices wouldn't work properly if you plugged them in.
posted by delmoi at 4:50 AM on June 4, 2012 [1 favorite]


Now that (apparently anonymous) electrical engineers have worked out how to stop us electrocuting ourselves so often the world awaits only a sane solution to those traveling it with electrical devices. It looks like that may be finally here.
Yeah, it's called USB! The U is for Universal. Just be careful since if you're plugging in a cellphone it may steal all your data
posted by delmoi at 4:54 AM on June 4, 2012 [2 favorites]


Oh man, I've been endlessly confused by this very issue for years. I look forward to clicking through and discovering all the death traps I've wired in that time period!
posted by DU at 4:57 AM on June 4, 2012 [1 favorite]


I've learned something today! Now off to go rewire the new condo...
posted by backseatpilot at 5:14 AM on June 4, 2012


I too found the essay to be overly confusing. The upshot is: both wires do not oscillate. Only the hot wire oscillates. The neutral wire is the drain.

He also fails to mention double insulated devices, which was how things were made safe-ish before polarized plugs.
posted by gjc at 5:57 AM on June 4, 2012 [1 favorite]


Review of NewerTechnology's Power2U USB outlet.
posted by porn in the woods at 6:00 AM on June 4, 2012


When I bought my first house, whoever had wired all the outlets in the house obviously hadn't read this article and pretty much half of them were wired backwards. I spend a weekend day with an outlet tester going through each room and switching the wires on the outlets with the neutral on the live and the live on the neutral.

That was after I'd taken out all of the 30 amp fuses from the 15 amp circuits and replaced them with the correct ones. Amazing that house didn't burn down before I bought it.
posted by octothorpe at 6:04 AM on June 4, 2012 [3 favorites]


Hmm. I also found the transistor article to be overly confusing. Simplification: transistors are leaky, variable relays.

Maybe it's my backwards wired brain, but if I picture a transistor like a nuclear reactor with control rods, it makes them perfectly easy to understand. The further you push the control rods in (the more voltage you apply to the base), the more electricity that passes. Some work the opposite. Just like relays- some are normally open, some are normally closed.
posted by gjc at 6:17 AM on June 4, 2012


Interesting and answers a few things I had wondered about but never bothered to look up, although I too was a little put off by his style. All in all I liked it.

He did leave out the gold standard for shock protection, though: line isolation (even though it would not be cost effective for widespread use; hell, even hospitals don't use it much anymore).
posted by TedW at 6:20 AM on June 4, 2012


This is a wonderful article which answers my long-standing question about why, when you go back to the fuse box, you realize the ground wire is directly connected to the neutral wire.
posted by odinsdream at 6:21 AM on June 4, 2012 [2 favorites]


I had never heard of USB outlets before; they look like a great way to get rid of the wall warts for all my iOS devices! I will definitely be checking them out.
posted by TedW at 6:22 AM on June 4, 2012


For Electrical Outlets Having a Little Face, one really has to go to the Danish plugs. They're so happy!
posted by frimble at 6:42 AM on June 4, 2012 [3 favorites]


Danish plugs. They're so happy!

A really good social welfare system will do that!
posted by flapjax at midnite at 6:50 AM on June 4, 2012 [7 favorites]


But the other wire has developed a new hazard, because whenever the occasional customer comes into contact with it, that foolish customer is usually STANDING ON THE GROUND! By grounding half of your electric network, you've accidentally connected one entire half of your network indirectly to everyone's feet.

Is he really claiming that the reason a person can get shocked from the hot wire is because the electrical circuit is grounded somewhere in the power grid, potentially miles away, and therefore the earth completes a circuit from the person's feet back into the system? Wow, I'm having a little trouble buying this. Is this just a metaphor for explaining the voltage differences between hot and ground in such a system, or does he literally mean that a circuit is created through the earth (does that question even make sense)?
posted by inkfish at 7:25 AM on June 4, 2012


Excellent essay! I love the sentence "Anyone standing on wet ground will feel pain and death if they should grab that metal case."
posted by languagehat at 7:26 AM on June 4, 2012 [1 favorite]


Is this just a metaphor for explaining the voltage differences between hot and ground in such a system, or does he literally mean that a circuit is created through the earth (does that question even make sense)?

It's not a metaphor. Neutral is grounded frequently, usually there is a copper rod stuck into the ground at the power entry to your house. The circuit is completed by the white wire going back to the power plant (simplified, but basically true), but it is tied to the earth regularly, so the earth is referenced to neutral. It's like being in a boat on the waves.
posted by Carmody'sPrize at 7:37 AM on June 4, 2012 [1 favorite]


Why is brown live, and not, say, red for danger? Why black in the US? That's for negative, isn't it?

In the US/Canada it's actually more complicated. Red and Black are both used for hot wires in homes and other single phase services. The black and red have 240V potential to each other and both have 120V potential to ground. Heavy commercial and industrial sites with three phase power will have red, black and blue and some places are also going to have brown and yellow as hot conductors.

Is this just a metaphor for explaining the voltage differences between hot and ground in such a system, or does he literally mean that a circuit is created through the earth

The circuit is actually completed through the earth. But it doesn't have to go through the earth all the way back to the power plant; the centre transformer tap supplying the neutral connection is connected to the earth where your power comes into your house and at the transformer on the pole or in the vault.
posted by Mitheral at 7:40 AM on June 4, 2012


Why black in the US? That's for negative, isn't it?

Black is used to represent negative in DC power systems. In AC there is not a concept of positive and negative connectors, as the current flow alternates. Instead you have hot which is connected to the transformer and neutral which is grounded at the breaker panel.

Wiring an outlet backwards could result in hot being connected to external parts on old devices. Basic outlet testers are cheap (<$10) - if you've been wiring outlets randomly DU, it would be a good thing to check.
posted by bitmage at 8:04 AM on June 4, 2012


Outlet tester for $5. I make sure to check all the outlets in any place that I live after my experience with my first house.
posted by octothorpe at 8:23 AM on June 4, 2012 [2 favorites]


What octothorpe said, folks. I've lived through two mysterious fires (in a single house), that turned out to be started from bad wiring. Not sure whether bad wiring or sleepy smokers account for more fire deaths...

And, while we're at it, eventually you'll hear someone say, "Well, house current can't kill." Really. People say, and believe, that, probably because we've all been shocked before, and lived. However, how much current flows through you is highly dependent on variables (how sweaty you are, if there's a cut, what shoes you're wearing, the flooring type), and voltage doesn't kill; current kills. Walk across a carpet in the winter and touch a doorknob; that jolt is ~10,000 V... and perfectly harmless. Under the right circumstances, a mere 50 V could pass 0.5-1.0 A through a human, and that would kill in a few heartbeats.

I know of an electrician's widow who would have some choice words for those who believe US A/C can't kill.
posted by IAmBroom at 8:34 AM on June 4, 2012


I really like Bill's writing style, actually. When I was younger and starting to learn about electricity his website helped me quite a bit. The explanations were all very practical for the most part, focusing on the concepts behind the operations (the "Babylonian viewpoint" he mentions in the transistor article). His website is definitely one of the best on the web, and has been since I've been on the internet (the weird science sections are also great).

Anyway, thanks for posting this, I always like running across a succinct article that clears up my thinking a bit more. Now I just need to convince myself to get back to work instead of browsing his website for hours.

I learned wiring on automobiles first. It always confuses me that black is hot on house wiring. Black is ground and red is hot in cars.

It's pretty common in DC circuits for black to be ground. This becomes extra confusing when you are wiring solar arrays using normal household wiring (green, white, black), because it's DC, so you want to use the black for ground, but it doesn't make sense for the white to be positive, so you end up using the white for ground and the black for positive.
posted by nTeleKy at 8:59 AM on June 4, 2012


As someone with "electrician" in her job title, I found that pretty painful to read.

I may be going out on a limb here, but I'm almost certain that that piece was not written for people with "electrician" in their job titles.
posted by yoink at 9:19 AM on June 4, 2012 [5 favorites]


Only the hot wire oscillates. The neutral wire is the drain.

That doesn't sound A) possible and B) at all related to what he is saying. But I will admit to seeing electricity as basically magic.
posted by yoink at 9:20 AM on June 4, 2012


It doesn't necessarily mean the equipment is faulty and would murder you if given the chance.

I go on the assumption that any tool that once belonged to my grandfather would murder me if given the chance.
From the old but still scary sharp hedge clippers and the electric drill that I suspect is a misplaced tunnel-borer to the padlock I'm pretty sure was carved out of a solid block of steel and has a key you need to register as a lethal weapon.

I'm pretty confident they all get together at night and talk about what pansies modern men are.
posted by madajb at 9:51 AM on June 4, 2012 [4 favorites]


The upshot is: both wires do not oscillate. Only the hot wire oscillates. The neutral wire is the drain.

Really? I’m not sure about that.
posted by bongo_x at 10:22 AM on June 4, 2012


... both wires do not oscillate. Only the hot wire oscillates.

More accurately, voltages are relative to some reference, which may be arbitrary. In an earth grounded system, the reference is usually the earth, but it does not necessarily have to be so.

For example if a little bird lands on the "hot" wire, it is not harmed. To the bird, it seems that the "hot" wire is not oscillating and instead it is the grounded neutral wire that is oscillating. Voltage is relative to the chosen reference. In this case the reference for the bird is the "hot" wire.

In an earth grounded system it is convenient to chose the earth as the reference. Since the neutral is connected to the ground and a person stands on the ground, it appears that the neutral is fixed while the hot wire oscillates. However, if you stand in rubber boots so you are isolated from the earth and touch the "hot" wire, the "hot" wire appears fixed and you are perfectly safe as long as you don't touch the grounded neutral which appears to oscillate relative to your own isolated body.

You can think of it as similar to reference frames for motion. You have to pick a reference frame to determine velocity.
posted by JackFlash at 10:34 AM on June 4, 2012 [2 favorites]


The neutral wire does carry current, basically the difference between the loads on the two circuits coming from the transformer on the pole. Your house (US) is fed 240V on three wires (2 hot, 1 neutral). Normally the breaker panel splits these up with half of the loads on one circuit, half on the other, and 240V appliances using both. If the loads on each side aren't matched, then there will be current flow on the neutral wire back to the transformer.
posted by bitmage at 10:41 AM on June 4, 2012 [1 favorite]


... the electrical circuit is grounded somewhere in the power grid, potentially miles away, and therefore the earth completes a circuit from the person's feet back into the system? Wow, I'm having a little trouble buying this. Is this just a metaphor for explaining the voltage differences between hot and ground in such a system, or does he literally mean that a circuit is created through the earth

For high voltage DC transmission lines, the return circuit often really is the earth. Submarine DC transmission cables are only a single wire, using the earth for the return path. This reduces cost of the cable.

High voltage DC transmission lines above ground on towers typically use two wires -- one hot and the other referenced to earth for the return current. If one of the wires is broken by some accident, they can still transmit power by using the remaining wire as the hot and using the earth as the return path. This increases reliability of the transmission grid by providing redundancy. However, using the earth as a return path is not generally used for long periods because there can be some electro-corrosion side effects due to the imperfect connection to earth.

The earth has much lower conductivity than a copper or aluminum wire, but it has a much bigger cross section. You can think of the earth a one really immense fat wire that is equivalent to the small highly conductive copper wire. The trick is getting a good connection to that fat earth wire. Electrical transformer sub-stations and power plants require something like a 4-foot grid of buried copper cable covering the entire area connected to 8-foot copper rods driven into the earth at regular intervals.
posted by JackFlash at 10:57 AM on June 4, 2012 [3 favorites]


Wiring an outlet backwards could result in hot being connected to external parts on old devices

Also, replacing the light socket or plug on a lamp and not being careful to keep the silver screw connected through to the wide prong and thus neutral will cause the following dangerous situation:

While you unscrew the light bulb, the gradually exposed (and easily brushed by your finger) metal base of the bulb will be "hot" regardless of the position of the socket power switch.
posted by CynicalKnight at 10:58 AM on June 4, 2012 [1 favorite]


The neutral wire does carry current, basically the difference between the loads on the two circuits coming from the transformer on the pole. Your house (US) is fed 240V on three wires (2 hot, 1 neutral). Normally the breaker panel splits these up with half of the loads on one circuit, half on the other, and 240V appliances using both. If the loads on each side aren't matched, then there will be current flow on the neutral wire back to the transformer.

Unbalanced loads on the two legs of single-phase wiring can cause current to flow in the neutral which is why you should never consider the neutral to be an earth ground. It isn't typically much of a problem in small house installations but in factories or skyscrapers it is possible to have significant currents in the neutral due to unbalanced loads. Since the neutral has a finite resistance to earth, that current flow means that the neutral can have a high enough voltage to kill you.
posted by JackFlash at 11:08 AM on June 4, 2012


Wikipedia has a nice page on the US system. I hadn't considered the cost-savings aspect in that smaller conductors can be used.
posted by bitmage at 11:51 AM on June 4, 2012


milkb0at: "Why is brown live, and not, say, red for danger? Why black in the US? That's for negative, isn't it?"

I was taught that brown is easy to remember, because you'll shit yourself if you touch the line.

Here's a full list of the color standards used internationally for both AC and DC systems.

Interestingly, both the US and IEC setups reuse colors between DC and AC systems, which just seems dangerous and unnecessary. Also, quite a few of the systems are ambiguous -- there's actually no specified color for the hot wire in US AC systems, and only a recommendation to use red and black in DC systems. The UK's got quite a bit of old wiring left that doesn't follow the IEC standard; if you see red or yellow leads in a UK circuit, it's time to grab the circuit tester just to be safe.

That said, Britain got things right with the BS1363 plug. Even though the underlying circuitry can be somewhat scary, the plug is rock solid from an electrical standpoint. Even the old BS546 plugs were a vastly superior design to the US setup. (Also, 220V allows extension cords to be a lot less hefty, which was nice when I was doing theatre and touring work over there; I still can't get over how heavy American power cords need to be to support high-current stuff)
posted by schmod at 12:14 PM on June 4, 2012 [1 favorite]


This is a wonderful article which answers my long-standing question about why, when you go back to the fuse box, you realize the ground wire is directly connected to the neutral wire.

Only at the main fuse/breaker box. If it's a sub-panel fed by the main box, the ground and neutral would be (should be) kept separate. That's why he says "Connect this [third] prong to the neutral side of the network, but do it only in one place in the circuit, and run a new third wire out to all of the wall-outlets."
posted by Knappster at 12:24 PM on June 4, 2012


there's actually no specified color for the hot wire in US AC systems

Yep. US AC color coding: white=grounded (neutral), green (or bare)=groundING, any other color=hot. Black and red are most common, but far from the only colors used.
posted by AstroGuy at 1:07 PM on June 4, 2012


Well, I've gone from educated to confused and back again. What a ride! I think part of the confusion is the mix between UK (where I am) and US systems, which doesn't help when you're trying to find a pattern in the mess of wiring standards and options. I'll stick with the UK system and its UK plugs. I haven't electrocuted myself or set my house on fire yet, but I reckon a socket tester is probably a good investment.
posted by milkb0at at 1:36 PM on June 4, 2012


AstroGuy: "Yep. US AC color coding: white=grounded (neutral), green (or bare)=groundING, any other color=hot. Black and red are most common, but far from the only colors used"

The US sure hates the red/green colourblind, huh?
posted by Proofs and Refutations at 1:37 PM on June 4, 2012


That said, Britain got things right with the BS1363 plug.

Only if you ignore the 274,574 people who have been crippled by stepping on it.
posted by srboisvert at 2:36 PM on June 4, 2012 [1 favorite]


Is he really claiming that the reason a person can get shocked from the hot wire is because the electrical circuit is grounded somewhere in the power grid, potentially miles away, and therefore the earth completes a circuit from the person's feet back into the system? Wow, I'm having a little trouble buying this. Is this just a metaphor for explaining the voltage differences between hot and ground in such a system, or does he literally mean that a circuit is created through the earth (does that question even make sense)?

I think he is taking some liberties here, but the upshot is that electricity will take the easiest path back to home (the generator). If you had a system that wasn't grounded, the easiest path back to the generator would be through the other wire, rather than through the ground. Because there would basically be no circuit to complete.

I guess the thought experiment would be to take a portable gasoline generator, isolate it completely from the earth, and then fire it up and measure the voltage between its hot wire and the earth. It should (conceptually?) be zero.
posted by gjc at 3:00 PM on June 4, 2012


Only if you ignore the 274,574 people who have been crippled by stepping on it.
Oh come on! Think of all the benefits the plugs brings...not the least of which is a ready supply of caltrops should we be invaded.
posted by Jehan at 3:11 PM on June 4, 2012 [5 favorites]


Interestingly, both the US and IEC setups reuse colors between DC and AC systems, which just seems dangerous and unnecessary.

There are a limited number of colours that are distigishable from each other; especially once things get dirty, worn, and faded (Oh the joys of trying to tell whether a wire is white or yellow or faded brown). Also the more colours you intorduce the more expensive stocking becomes. And whether something is A/C or D/C is something that is going to come up very rarely, generally it should be obvious.

Here's a full list of the color standards used internationally for both AC and DC systems.

I don't know about US/IEC but his chart is not complete for Canada. Brown and Yellow are both valid hot colours in Canada and grey is permitted to be used for neutral.
posted by Mitheral at 3:19 PM on June 4, 2012 [1 favorite]


Clarifying: yes, the circuit does go through the earth. If you look at the picture here, the high voltage power goes in the top. The other side of that circuit is the outside case of the transformer, which is tied to the earth. That power is transformed down to (in the US) 120V, with the two legs being 180 degrees out of phase with each other. The center tap is the neutral, which is also connected to the case (ground). (Which, being the earth, is by definition 0V)

The three wires come out of the transformer and attach to your house. The neutral is again tied to the ground, and distributed to your outlets.

A neutral cable *can* have AC on it, but it only gets that power from something downstream that is running. It does not have any power or signal on it coming from the power company, unlike the hot.

This is different from a balanced AC system, where a 120V line would be two 60V hots running 180 degrees out of phase from each other. In that case, neither is a neutral and both have an AC signal on them.
posted by gjc at 3:25 PM on June 4, 2012 [1 favorite]


If you can't do serious damage to your feet by stepping on the up turned prongs in the middle of the night, it's not a proper plug.

And also, because whoever was in charge of electrical standards in Britain immediately following WW2, decided that copper would remain expensive, the standards specify houses wired with a ring main, rather than branches. Among other things, this requires chunky plugs with built-in fuses, and has led to the regulation prohibiting power sockets in bathrooms. (There is an exemption for current-limited sockets using a different plug standard, used by electric razors; these sockets don't provide enough current to run an electric hair dryer.) Which is why you don't see power sockets in British bathrooms.
posted by acb at 3:43 PM on June 4, 2012


> Walk across a carpet in the winter and touch a doorknob; that jolt is ~10,000 V...
> and perfectly harmless.

Harmless it may be but if you touch a cat on the nose it will fear and hate you forever.
posted by jfuller at 4:05 PM on June 4, 2012 [3 favorites]


A neutral cable *can* have AC on it, but it only gets that power from something downstream that is running. It does not have any power or signal on it coming from the power company, unlike the hot.

This is simply incorrect. The neutral is absolutely necessary and does carry power. It is the current return path for both legs of the split phases. On a branch circuit from the panel to an outlet, all of the return current always goes through the neutral. If it didn't, your GFCIs would trip.

From the panel to the transformer, if you have only one 120V circuit turned on, all of the AC current will return on the neutral to the transformer. If you have lots of circuits on both legs turned on, then, since they are 180 degrees out of phase, some of the return currents from the panel to transformer will cancel, but this is not the same as saying the neutral doesn't carry current or power. The neutral wire must be sized in the worst case to carry all of the AC current from one leg of the split phases back to the transformer.

Also, it is incorrect to say the the neutral has no AC signal on it. As I explained above, voltages are relative. From the point of view of the hot wire, the neutral is an oscillating AC signal.
posted by JackFlash at 4:41 PM on June 4, 2012


DU: If you're in the States, the black (hot) wire should be connected to the brass-colored screws on your outlets, the white (neutral) wire should be connected to the silver-colored connectors and the bare or green wire should be connected to the green-colored screw. If an outlet doesn't have these colors (i.e. missing ground screw and prong), throw it away and buy a new one (they're dirt cheap.)

Of course, turn off the circuit first and test for voltage with a test light first. Then test with one of those plug-in testers after you're done to make sure your polarity is correct.

Also, there's no shame in hiring a electrician. Paying to get your wiring right is better than electrocuting yourself or burning your house down!
posted by double block and bleed at 4:45 PM on June 4, 2012


This is simply incorrect. The neutral is absolutely necessary and does carry power. It is the current return path for both legs of the split phases. On a branch circuit from the panel to an outlet, all of the return current always goes through the neutral. If it didn't, your GFCIs would trip.

From the panel to the transformer, if you have only one 120V circuit turned on, all of the AC current will return on the neutral to the transformer. If you have lots of circuits on both legs turned on, then, since they are 180 degrees out of phase, some of the return currents from the panel to transformer will cancel, but this is not the same as saying the neutral doesn't carry current or power. The neutral wire must be sized in the worst case to carry all of the AC current from one leg of the split phases back to the transformer.

Also, it is incorrect to say the the neutral has no AC signal on it. As I explained above, voltages are relative. From the point of view of the hot wire, the neutral is an oscillating AC signal.


1- I don't think I said that the neutral was unnecessary.

2- If you stick a voltmeter into the socket, the neutral has no voltage compared to ground. If you plug something in and turn it on, and you measure the voltage on the neutral prong against a ground, it is zero.

3- You are getting tripped up between the flow of current in a circuit, and what voltage/signal is on the line. Thought experiment: plug in a lightbulb at the end of an extension cord. Strip the conductors of the extension cord. Measure the voltage between black and white: you get voltage. Measure the voltage between the black and the green. You get voltage. Measure between green and white. Nothing.

Now, cut the neutral. The light goes off because the circuit is broken. The two ends of your cut neutral are no longer neutrals, however. The outlet end is neutral, the lightbulb end is hot. And if you grabbed both ends you'd get shocked. But not because there is voltage "on" the neutral, but because you are completing a circuit between neutral and your newly created hot end.

The only reason you need a neutral conductor between your house and the transformer is because the ground is not a perfect conductor. If you could have a perfect grounding rod at your house, and a perfect grounding rod at the transformer, you wouldn't need the cable.
posted by gjc at 7:10 AM on June 5, 2012


schmod: “That said, Britain got things right with the BS1363 plug.”

Wait – you're seriously arguing that plugs should have fuses in them? Like, the wire part of the plug should have a fuse, so that you have to take the thing apart and replace it or even just throw the cable away if it blows? That seems like a very odd design to me, and probably a bad one. Resetting switches seem to work much better. Also, ring circuits have some very odd side effects, to say the least.
posted by koeselitz at 12:03 AM on June 6, 2012


(Seriously, BS1363 has always seemed to me to be broken by design.)
posted by koeselitz at 12:03 AM on June 6, 2012


And also, because whoever was in charge of electrical standards in Britain immediately following WW2, decided that copper would remain expensive, the standards specify houses wired with a ring main, rather than branches.

The above mentioned link suggests that the person ultimately responsible for this was Lord Reith. It seems somebody suggested "The chap did a good job sorting out the telly; lets see if he can do the same for the wiring". In fact both the BBC and the BS1363 plug share the same characteristics design ethos "good for you whether you like it or not".
posted by rongorongo at 4:43 AM on June 6, 2012 [1 favorite]


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