11 ms·
Why three prongs?
- iveney 14y agoClears the mystery :)
- xd 14y agoYeah, I dug this up when wondering why it mattered that the sensor for the wireless power usage meter I have could be clamped to either live or neutral but was told by the building manager that it had to go to live; yet I wasn't convinced. Anyway, as you say .. mystery now clear.
- danbmil99 14y agoBeen waiting 40 years for this explanation. Electricians have tried to explain it but their answers always ended up sounding circular and ill-founded.
- tjic 14y agoAn electrician explaining electricity is like a cop explaining the law. They're hired to follow an instruction book. That doesn't mean that they have the least idea of the whys and wherefores of what's in the book. Look at any of 1,000 videos of cops making up laws, falsely citing either the Patriot Act of 9/11, or explaining that parts of the Constitution and Bill of Rights don't apply in certain circumstances. I'd rather listen to a cattle farmer talk about bovine genetics - he's likely to have some clue!
- TazeTSchnitzel 14y agoYep. The correct person to ask about electricity is a Physicist, or in your analogy's case, a lawyer.
- DougBTX 14y agoAs a physicist*, I would suggest speaking to an electrical engineer.
- deleted 14y ago[deleted]
- K2h 14y agoAs an EE, I say ask one specific branch of the Electrical Engineering profession. see [1], here there are 3 specific sub disciplines you can test in, and all are considered "EE" computer electronics power you want to talk to an expert in the power arena, but from what I remember when studying for the exam, the content was mostly things way above residential (which is what this discussion has been about) and more of power gen and conversion. Y, delta configurations and conversions, power angles and factors, etc. the kind of thing you come out really knowlegable in one very small area and still clueless on the NEMA power plug. [1] http://www.pels.ca.gov/applicants/refs.shtml http://www.pels.ca.gov/applicants/refs.shtml
- mturmon 14y agoBut, conversely, be careful about asking a physicist to wire your home. They may not be conversant with the codes, which contain a lot of practical wisdom. I once tried to convince a physicist friend to not use just solder on his ground connections. He thought that by using solder he was going above-and-beyond the code. But actually, ground connections are required to be pressure-fit, not just soldered, to ensure that heating during a current spike does not allow the connection to separate.
- ars 14y agoSorry, but that's not true. For your solder joint to fail it would have to be hot enough to cause a fire. Pressure type connectors are simply cheaper and easier, and were not initially allowed because soldered connections are better. http://www.justanswer.com/electrical/18vcb-not-allowed-solder-connecting.html http://www.justanswer.com/electrical/18vcb-not-allowed-solde... Soldered (sweated) fittings are not allowed on gas lines for this reason though: If there was a fire you don't want the gas lines to leak after the solder melts.
- ams6110 14y agoHow many electricians do you actually know?
- tjic 14y agoThree. One I walk dogs with daily.
- rdl 14y agoFor a random residential cable puller, sure. (there are good electricians who do small scale, but it isn't necessary to do most residential work; it's really more about knowing how to retrofit things in various eras of construction, techniques for opening walls/patching, etc. -- kind of general handyman stuff) Commercial electricians. power engineers, etc. are quite smart. To use your analogy, the residential cable puller is like a local street cop, but commercial electricians and power engineers are FBI, SWAT, or lawyers. EEs are judges and legislators. You wouldn't get a comprehensive view of the law by talking to only one. (I took EE classes in college, although no power classes, and was really glad to know some commercial electricians when doing power systems for datacenters. It was all in unlicensed places, so there were no code/compliance issues, just performance and safety.)
- ajross 14y agoElectricians are the wrong people to ask. It's like asking an IT exchange admin to explain the the SMTP protocol.
- deleted 14y ago[deleted]
- bartwe 14y agoWhy don't Schuko plugs have this orientational bias ?
- bmelton 14y agoSchuko plugs expose some material on the sides to ground to their chassis. I personally think they're the better design, but if you look at this picture: http://upload.wikimedia.org/wikipedia/commons/2/23/Schuko_plug_and_socket_annotated.png http://upload.wikimedia.org/wikipedia/commons/2/23/Schuko_pl... Number '2' is grooved out, and there is a receiving prong that sticks out to make contact to the groove-exposed metal, similar to how the contact chargers you're probably used to seeing on handheld phones (not cellular, but cell batteries are often similar) -- The important thing to note is that length-wise, 2 comes in contact to ground before either hot or neutral wires do (length-wise), so it's safer on paper than the traditional design.
- kalleboo 14y agoThat solves a different problem though - the problem of touching the prongs by mistake when fumbling with a plug. I imagine there's no polarity in european plugs because the "use a third lead for grounding" solution was chosen right away instead of trying to patch things with forced polarity first (possibly with the prior knowledge of incidents from the U.S. - kind of like how PAL could avoid the issues inherent in NTSC) edit: looks like my assumptions may be wrong - the Schuko patent predates the grounded NEMA plug patent by 2 years. But who knows how ideas like this spread back then... This is the kind of history I'd love to read a book on.
- excuse-me 14y agoThe european plugs (shuko and UK) were intended for use with 220-240V - the US was always 110V so it was thought that the lower supply wasn't really dangerous enough to need all the extra protection
- hollerith 14y ago>I imagine there's no polarity in european plugs because the "use a third lead for grounding" solution was chosen right away instead of trying to patch things with forced polarity first I remember when polarized plugs started to appear on appliances in the U.S. (probably in the 1970s) and by then the third lead was many years old.
- davidw 14y agoAs an aside, one of the things that's sorely missing from the day-to-day integration of the European Union is a standard for electrical plugs: http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets Here in Italy there are at least three kinds that are fairly common. Go north to Austria, and things change and you have to buy more adapters.
- joelhaasnoot 14y agoI think day to day integration of voltages is worse. Not so much for gadgets since we have transformers for everything down to 5 volts, but anything else is a nightmare.
- kalleboo 14y agoReally? Aside from white goods, it seems most things these days are multi-voltage. And even then, it's still mainly North America that strays more than 10% from the standard http://en.wikipedia.org/wiki/File:Weltkarte_der_Netzspannungen_und_Netzfrequenzen.svg http://en.wikipedia.org/wiki/File:Weltkarte_der_Netzspannung...
- ralfd 14y agoInteresting effect: http://upload.wikimedia.org/wikipedia/commons/f/f5/Weltkarte_der_Netzspannungen_und_Netzfrequenzen.svg http://upload.wikimedia.org/wikipedia/commons/f/f5/Weltkarte... Never cared about this svg file format, but how does it work? If I make a reload on the link the countries are drawn individually as the file is loaded (relatively slow network connection). Like a puzzle.
- kalleboo 14y agoTry view source and you can see how it's built. Simplified, SVG could be described as "like HTML only with shapes".
- joelhaasnoot 14y ago
- gojomo 14y agoEven already knowing the gist of things, I enjoyed the description of the path-dependent incremental evolution here. But, this sentence makes me wonder about something: "Also, the clear-weather sky current can no longer build up a high voltage, if any excess charge immediately leaks into the earth." Have our region- and even continent-wide grounded electrical networks changed the expression of lightning storms, such that non-electrified areas are noticeably different?
- deleted 14y ago[deleted]
- throwaway64 14y agoall manner of things can cause a build up of charge in what is essentially a massive network of thousand KM+ antennas: radio waves, the earth's magnetic field, cosmic rays, wind interacting with dust particles causing static charges, etc. A bit of an interesting side note: it is possible to build a radio set passively powered by radio waves, requiring no external power: http://en.wikipedia.org/wiki/Crystal_radio http://en.wikipedia.org/wiki/Crystal_radio
- stephengillie 14y agohttp://en.wikipedia.org/wiki/Radio-frequency_identification http://en.wikipedia.org/wiki/Radio-frequency_identification
- js2 14y agohttp://news.ycombinator.com/item?id=3561321 http://news.ycombinator.com/item?id=3561321 (My foxhole radio)
- iliis 14y agoA very interesting idea! It's well known that storm create static electricity, but I've never thought of it the other way around. How does electricity shape the weather? Thunderstorms are a highly complex system and I'm not an expert, so I can only speculate: A storm builds up a high electric potential between the clouds and the ground which results in a lightning strike if it gets strong enough. Now, what does the existence of grounded power-grids change? It basically makes the ground more conducting which may result in a more evenly distributed ground-charge. As this doesn't change the potential between ground and higher air masses, I don't think it directly influences a storm. Maybe the weather is influenced by horizontal potential (Air from point A is attracted to differently charged air at point B), but I don't think this effect would be strong enough to notice. While reading through the article, another idea sprang to my mind: Why don't we use all this electricity? Harnessing power from lighning seems rather impractical (extreme amounts of electcity in one unpredictable instant), but how about building a high tower or just laying a few long, ungrounded cabels an then slowly dischargin a thunderstorm?
- lcargill99 14y agoNever mind that yes, A/C can count on polarity and that you are isolated from the grid by one or more transformers. The third prong is the "safety" ground, in case neutral fails. Go back to old guitar amps with "the cap of death" or a "ground" switch for one market in which this evolved. It's a fairly simple mod to make an old amp fully safe. Yes, they cover that in the "bad outlet" story, but it's easy to fix if you know how to run a meter and can find a ground. The lead guitarist from Badfinger died from this. I'd never heard about DC buildup on the grid - free power, maybe? :)
- thought_alarm 14y agoPrior to the Ground Fault Interruptor sockets found in modern bathrooms, an isolating transformer was often used to protect bathroom outlets in the 60s and 70s. These are two-pin unpolarized sockets, usually with "Razors Only" stamped on the front (because if you plug in a 1000 W hair dryer you blow out the transformer).
- TazeTSchnitzel 14y agoI always wondered why it said "Shavers Only". Luckily the worst thing I ever stuck in was an American phone charger.
- texec 14y agoCurrent installations use Residual-Current Devices instead of individual breakers, at least in Europe. This requires three way wiring up to the distribution. In Germany RCDs are required for all outlets in new installations.
- sc00ter 14y agoOr rather both - individual circuits have a current limiting circuit breaker in addition to a master RCD across multiple circuits.
- brudgers 14y agoIn the US, it is relatively common to use circuit breakers in new construction but in no way universal - protection at the receptacle is still very common and can be significantly cheaper since only the first receptacle in a series needs to be GFCI type...of course this creates an issue of finding the tripped device for a person unfamiliar with the wiring scheme.
- lostlogin 14y agoRCDs are required throughout the house in New Zealand now (used to be bathroom only). They work. I tried angle grinding a metal bar, and it's link to the extension cord fell in a puddle I was standing in. Power clicked off thank god, and I became ok with the re-wiring cost. RCDs are the best.
- zdw 14y agoEver wonder why, in some buildings (mainly commercial properties), the plugs are installed so that the ground is at the top of the outlet? I asked an older electrician/installer about this once, and the reason he gave was that if a wire fell in between the gap between the plug and outlet it could short easily. Having the ground on the top makes it so that if something does land in this way, it will be at an angle and more likely to fall off the shorting outlet. Plugs in other countries, specifically european or british plugs aren't susceptible to this because they're designed not to expose hot wires during insertion.
- TazeTSchnitzel 14y agoBritish plugs[1] have a pretty clever design. There's 3 prongs arranged in a triangle, each of equal length, but they are rectangular prisms, not cuboids, so you can only plug it in one way up. The lower 2 "live" holes only open if there's a prong in the top earth hole. Because of this, double-insulated appliances still have a plastic prong on the plug. And yes, we have insulation on both "live" prongs. Plus, it's the only kind of plug that's both Grounded, Polarised, Fused and has Insulated Pins[2] [1] http://en.wikipedia.org/wiki/BS_1363 http://en.wikipedia.org/wiki/BS_1363 [2] http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Comparison_of_sockets http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Comp...
- kalleboo 14y agoIt's also the only plug that'll wake the neighbors when you step on it going to the loo in the middle of the night.
- lmm 14y agoTrue, but you learn after the first time, and it's pretty non-lethal (I think? It doesn't seem sharp enough to do serious damage, but for something to hurt that much your body must think something's up). The real problem with British electrical standards is the nutty "ring main" system.
- 14y ago
- tjic 14y ago> Another problem springs up. At present, some appliance manufacturers INTENTIONALLY connect the outside of their metal products to one of the power wires. This must be stopped. I disagree with this. If one side is tied to ground (and it is), then tying the chassis of a piece of equipment to that same line is the same as tying it to ground...a ground that you're already standing on. Am I missing something?
- dattaway 14y agoVoltage drop across the neutral side can and does happen. Also, loose connections often happen on the neutral side supplying the plug. This could give you the full voltage across your chassis.
- mturmon 14y ago"Voltage drop across the neutral side can and does happen." This is true. Even though the neutral conductor is grounded at the panel box, loading elsewhere in the circuit can cause the neutral conductor to have a potential above ground. That is, there is a small resistance of the copper wire as it goes from the electrical outlet back to the panel box. If something in the circuit is drawing significant current, this small resistance can cause a voltage to appear in the grounded ("neutral") conductor. If your home uses "shared neutral" wiring (two opposite-phase 120V hot lines sharing a single neutral wire for the return), then even throwing the circuit breaker on a given circuit can leave a potential in the neutral wire. This is because the paired hot conductor can be carrying current, causing the shared neutral wire to have potential. This can be surprising.
- harpastum 14y agoFrom my reading, the reason it was unacceptable was that since the wires were previously identical, the manufacturers attached an arbitrary wire to the case, so 50% of the time it wasn't the neutral one. So, once we switched the polarized plugs, some of the appliances would always have a hot chassis; unless we made sure appliance-makers always attached neutral to the chassis.
- fshaun 14y ago
- HarrietTubgirl 14y agoThis is an awesome article. Here's an example of the usual non-explanation: http://electronics.howstuffworks.com/everyday-tech/question110.htm http://electronics.howstuffworks.com/everyday-tech/question1... This is how it was always explained to me. Yes, neutral and ground are the same thing. Why can't you just connect the case to neutral? Because you connect it to the ground plug. Huhh???
- bpowah 14y agoWonderfully entertaining to read. I was once told by a certified electrician (and physicist) that you don't even need a neutral wire and many rural services only have one "hot wire" and use the earth as the neutral "return" circuit (which really just need to have capacitance. I think the author could have started with a "one slot" age. More info: http://en.wikipedia.org/wiki/Single-wire_earth_return http://en.wikipedia.org/wiki/Single-wire_earth_return
- cynicalkane 14y agoYou still need two slots, unless all of your appliances happen to rest on copper stakes hammered into the ground.
- bpowah 14y agoor have some capacitance of their own.
- bpowah 14y agoNow would somebody please tell me what the little holes are for in the end of the plug prongs!? http://www.worldofstock.com/slides/BEN2097.jpg http://www.worldofstock.com/slides/BEN2097.jpg
- pndmnm 14y agoMany sockets have little bent-wire brackets that pop into those holes when the plug is fully inserted in order to help hold it in place.
- sc00ter 14y agoA tangential use (but not the original design intent) is to put a padlock on the plug to prevent it being plugged for safety reasons. (Think power tools, high temperature devices etc).
- ars 14y agohttp://www.straightdope.com/columns/read/579/why-are-there-holes-in-the-prongs-of-electrical-plugs http://www.straightdope.com/columns/read/579/why-are-there-h...
- jrockway 14y agoDo we need grounded sockets now that we have GFCIs?
- X-Istence 14y agoYes. We still want device manufacturers to ground their cases, and using a third wire that is most likely not mounted the wrong way round is preferable.
- marshray 14y agoI agree. It would be a bad idea to have a stack of metal equipment all plugged into floating outlets. You'd be likely to get all kinds of weird voltages between the metal cases. The electrocution hazard was just one way it was the bad old days. It also sucked for other kinds of signals, hi fi, radio, etc. I would not want to try to have a home office full of computer equipment that weren't sharing a common ground. Did you ever have one of those 17"+ CRT monitors? Recall how they would make the hair on your arm stand on end when you turned them off or on? Without a dedicated ground for those things, guess where all that charge is going to go? Into your computer, via the ground on the signal cable.
- keenerd 14y agoDo you absolutely 100% trust the GFCI breaker? They do require a small imbalance to trip, so something bad could happen in that brief time. Or maybe stopped working one day. But technically yes, ungrounded GFCI will still protect you. Doing it intentionally is a bit like not buckling up because you have 100% faith in your air bags. For the non electrical people: GFCI works by measuring the current in vs the current out. Normally these should be perfectly equal. But if you drop your hairdryer in the bathtub, maybe 10 amps comes in (through the plug), 9.9 amps goes out (through the plug) and 0.1 amps goes out (through the bathtub, killing you). GFCI notices that 10 != 9.9 and shuts down. The trick is detecting 0.01% differences and shutting it down in milliseconds, fast enough to save someone's life. GCFI is way more complicated than a ground wire, and you'd be a fool to not have a cheap, passive and redundant safety system.
- 14y ago
- anologwintermut 14y agoCan anyone confirm the articles assertion that " Before you 'grounded' your system, the AC voltage in general acted pretty safe for your customers. The only way they could get a shock was if they touched both wires at the same time?" It seems that if I stick my finger in the hot socket, there is still a path via me to 0 voltage (the ground). I will still get shocked.
- DanBC2 14y ago> It seems that if I stick my finger in the hot socket, there is still a path via me to 0 voltage (the ground) Over-simplified, but you need a complete circuit. In your example you say that voltage is across you - live at a finger and earth at your feet. But earth is not connected to the circuit, so there's no complete path. That's why the article talks about sticking rods into the Earth to create a ground connection. Now there is a complete circuit. (And this would be across your heart - the current flows through a finger, your arm, your heart, down your leg, and to Earth. This is especially dangerous, and is the reason you see test engineers working with one hand in their pocket. Not having two hands connected to live equipment means it's harder for a voltage across the heart.) There's another thing to mention though: Imagine wearing rubber boots, on a dry wooden floor, and jabbing a metal rod into the socket. That's still risky, because your body will act like a capacitor and the charge can be fatal.
- thought_alarm 14y agoIn an isolated or ungrounded system there is no electric potential between the socket and ground, so the current has nowhere to go but back into the socket. Therefore, you only get shocked when touching both pins.
- no_more_death 14y agoI recall moving a water dispenser that leaked, as a child, stepping on the wet spot barefoot, and plugging in a two-pronged vacuum cleaner. It was a shock.
- colanderman 14y agoI thought the primary reason for a separate ground was because neutral was a current return path and thus could develop a voltage on it due to resistance of wires, whereas a true ground carries no current and thus will not develop a voltage?
- DenisM 14y agoI remember there are two reasons for the third prong, and neither is mentioned in the article. The first reason is the one you have outlined - the larger the current, the bigger the differential between the actual ground via and the neutral wire in the appliance. The second reason is that if you're using a transformer to change the voltage, the output of the transformer is not connected to the input, and thus can have arbitrary large voltage differential with the ground due to charge accumulation, and so it needs to be grounded. Now I'm getting fuzzy here, but I think there is a reason why you can't connect one of the transformers inputs to one of it outputs (the neutral one) to provide the "grounding" this way, so the third wire is used. I've noticed that devices which do not need to provide voltage conversion usually lack the ground prong, e.g. heaters.
- gcb 14y agoheh, in brazil they just finished a mandatory replacement of all the plugs. previously we had a plug with a flat face like the US, but the connectors were like "d b" meaning a US flat in the middle with round ones right out. using the same metal connector for both formats in the middle. clever, cheap, worked with everything. (US, Japan, Swiz, ...all except UK and australia as far as I know) now, they have mandatory round plugs only, slightly larger so they can't fit the old ones by one or half milimiter, and a recessed insert like the new Switzerland type J, and an optional ground. It serve the purpose of solving NONE of the mentioned issues in the article, but help lots of people that manufacture the new plugs.
- ricardobeat 14y agoYou could be a little more proud of your country - it's among the first to implement an international standard that is safer, simpler and more reliable. The new plug solves real problems (including all those mentioned in the article) and it does fit the old european-style outlets when groundless - if it has a ground pin or is the 20A variant it should not be plugged into a standard outlet anyway.
- gcb 14y agoRemember that polarized plugs are not to protect against properly designed products. And with the vast amount of international electronics in Brazil (heck, i think i had only a electric shaver that had national plugs, everything else required adapters) the cheap adapters null all the benefits.
- CUR10US 14y agoI love the amasci website (I don't mean the design; I have no idea about what it looks like in a graphical browser; I only mean what there is to read- lots of text! I just extract the text for viewing on 80x25). If only all websites could have this much information density. Long live sites like amasci!
- ajb 14y agoThis is a great article. As an example of innovation in this space, some Chinese company has devices a socket which accepts many international plugs: http://www.bbc.co.uk/news/magazine-18266022 http://www.bbc.co.uk/news/magazine-18266022 This appears to be the company: http://www.europlugs.com/ http://www.europlugs.com/ It's not clear to me that they have managed to fulfil all the safety requirements. I guess it would take some effort to really be sure, but can anyone see any obvious bloopers?
- toddh 14y agoIf there's so much DC energy available can we store it?
- BigTuna 14y agoThat site is an absolute gold mine.
- nroman 14y agoIn college I lived in an old apartment building that had been updated to have 3-prong outlets. One day I was wiring up some audio equipment for a party. As I hooked my computer to a a receiver in another room the audio cable immediately started sparking and the rubber insulation melted off. I yanked it out of my computer. Luckily the only real damage done was the sound card for my computer and the destroyed cable. Later I did some investigation. First I bought one of those standard outlet testers. It seemed to suggest that everything was fine with the outlet. Then I did some more investigation. I got a volt-meter and started measuring the voltage from the different prongs. Finally I measured the voltage compared to a copper pipe in the house. Turns out that both the neutral and ground wires were actually hot (as measured against the grounded pipe), and the "hot" wire was the ground. The outlet in the other room had been wired correctly. The shielding on the cable was grounded so suddenly it sent 120 volts down the audio cable when I plugged it in—destroying it in seconds. The electrician didn't believe me when he came in to fix it, but was finally convinced when he checked against a pipe. Just goes to prove that you can't make any system fully fool-proof.
- losethos 14y agopower is transmitted 3-phase. 3 wires with sin waves 120 degrees apart. The sum of the three is zero (ideally). The earth is used for the neutral (zero) current wire that might be slightly nonzero. 3 phases enter your house and a normal line-to-neutral 117V. Your dryer is line-to-line 240V. current lags voltage due to reactance (inductive loads). Wait. If there are different loads on the three phases, then it will be nonzero current on neutral. For safety, chassey is connected to Earth, so that a circuit breaker DOES blow if there's a problem. Two different outlets may or may not be the same phase. If they are different, a short occurs if you connect the non-neutral wire of two phases together. I guess if you connect line to neutral, too! God says... you_should_be_so_lucky guilty ohh_thank_you I'm_feeling_nice_today rich qed perfect kludge wrath kludge Umm... so EARTH is supposed to be the same as neutral, I think, but you make sure there is no miswiring by connecting ground to earth perhaps in a different spot, I think.