Why three prongs?
amasci.com
amasci.com
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?
A bit of an interesting side note: it is possible to build a radio set passively powered by radio waves, requiring no external power:
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?
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.
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
[2] http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Comp...
Wikipedia has a nice explanation.
It induces current in all metal near it, which causes sensitive devices to fail.
And it was only economical when you assumed a small number of high current devices. But these days when every outlet has something plugged into it the benefits are nill (since you have to have enough capacity for all the outlets, not just a small number of them).
I have no idea how many sockets are installed in UK houses and how many plugs are fitted to UK equipment but a new plug / socket design would have to be God-Tier to justify the cost of replacement.
Also, I've seen people wiring plugs and it's sometimes scary, so anything that avoids the need for many people to botch a new plug onto their equipment is a good thing. And I guess this is where most improvement has happened - plugs that can be built quickly, cheaply and safely in Chinese factories for use in UK. You can change the fuse, but the plug is molded and you can't take it apart.
The IEC 60906-1 design linked by ricardobeat above, for example, can handle 250V at 16A and is way smaller and more compatible with other european standards. It was designed in the mid 80s, taking advantage of the new injection molding processes popularized in the late 70s.
I was wondering if there were any other designs that took advantage of the newer technologies since: I seem to recall a concept plug designed by an art student that was on HN a few years back that folded up rather neatly: does anyone know what happened to that?
It seems terribly inefficient to require fuses in every plug even when the electrical wiring may not need them.
Now that is inefficient!
This 1994 legislation made it the law that plugs must be fitted to certain consumer devices: http://www.legislation.gov.uk/uksi/1994/1768/contents/made
Before that time, almost everything (certainly everything I purchased) came without a plug and you had to buy a plug and wire it yourself. Probably because these things are expensive, and costs could be reduced by leaving them out.
I'm not certain on the exact motivation, but I think it's a mix of preventing people doing it wrong, and better mechanical properties (especially strain-relief) of moulded plugs.
It's also required now that the live/neutral pins be partially insulated (the 1/3rd closest to the plug or so) to prevent shock hazards to people curling their fingers around the plug when inserting or removing.
I remember us having to wire up kettles, lamps, and irons when I was a kid. And you'd sometimes go to antique (or second hand) shops and come across old appliances that had no cord attached. Anecdotally, we bought an old style telephone that we then had to wire back up.
That being said, I can't corroborate any claim that this was backed by legislation.
No, the earth pin is noticeably longer. This serves two purposes - first, to ground the device being plugged in before it is live; second, to retract the mechanical barriers that are ordinarily across the entrance to the power pins, in order that the plug can be inserted, but stray fingers, paperclips, keys, or other conductive items cannot (without some effort and intent).
As an aside - as a result of the mechanical barrier across the power pins, the plastic socket safety inserts that parenting stores sell are completely redundant, and arguably a scam.
See: http://www.nfpa.org/itemDetail.asp?categoryID=1508&itemI...
The ground pin is longer because it is far safer to ground first before running current, and to continue to ground after disconnecting the current.
You have to insert prongs into both sides of the outlet at the same time in order for it to release.
http://en.wikipedia.org/wiki/NEMA_5-15#Tamper-resistant_rece...
They are labeled with "TR". (There is also WR for water resistant, and of course TR and WR together, sigh. Prices are going up since now stores need to stock 4 times as many types.)
The requirement only applies to dwellings (houses and apartments, not hotels or other commercial buildings), and only in jurisdictions which have adopted the 2008 NEC - the United States, unlike Canada does not have a national building code.
In addition, many jurisdictions have adopted the International Residential Code for One and Two Family Dwellings (aka, "IRC) which contains its own electrical requirements. While their is provision for tamper resistant receptacles in the 2009 IRC, many jurisdictions have remained on older versions of that code because the 2009 edition requires fire sprinklers in single family homes and therefore its adoption creates substantial political tension between building departments and the homebuilders whose fees fund those departments.
To put it another way, in the US, tamper resistant receptacles are not required in any non-residential structure, and because codes are adopted at the local or state level on an ad hoc basis, many dwellings are not required to conform with NEC 2008 or later versions.
Think of new editions of building codes like Windows, lots of people still use XP.
And you don't get hurt or break anything if you step on it :)
(Legally) imported products must conform to the local standards and pass inspection anyway :)
granted, the last time i looked at the paper was 2,5 years ago and there was still a lot of allowances for transition.
And i'm certain the Nokia adapters sold via local phone operators only have two plugs.
And don't even get me started on adapters for international products... Which btw, are the vast majority of brazil consumer gadgets.
When you imagine there are hundreds of millions of these plugs that bit of over-engineering really adds up. Really, when was the last time you plugged in anything above 2KW?
I call bullshit on the whole "earth pin position as a safety measure" debate anyway. Copper wiring of any substance is physically to rigid to move, even if completely free of other mechanical constraints, so isn't going to "drop onto the live pins" no matter what, even if the internal arrangement of a socket facilitated it. (They - don't they're insulated, shrouded, separated etc.)
For example you plug in a drill but it slowly works its way out, you are working with a screw driver against the wall and accidentally drop it, at this point the screw driver can come to rest on the live pins of the drill that is plugged partially into the wall. With the ground pins up the screw driver will pick a side and fall off...
Ah, so awesome.
The longer ground pin at the top provides a mechanical advantage when a downward force is applied to the plug. Since this is the most common accidental force (e.g. something dropped or a cord on the ground dragged) it slightly reduces the risk of disconnect.
In addition, the ground at the top allows for a better view of the pin and receptacle hole.
They hold nicely in ground down receptacles though.
The insertion force of the one ground pin is lower than the two live ones. So when force is allied downward the plug pivots out of the receptacle.
In a ground down orientation the plug will pivot into the wall. Since the ground pin moves easier than the other two, it moves into the wall instead of out.
Oh and by the way, did you want to know what the NFPA 70 actually says, since it's you know, law? Up until a few years ago that cost you a pretty penny. In recent times you can read it for free through their special online viewer.
It must be for the same reason that an electric shock from an ungrounded AC circuit doesn't travel through your feet as the article mention.
But I'm having difficulties understanding this concept. My (admittedly naive) understanding of electricity is that when one terminal has more electrons than another you can wire both terminals to a light bulb and it will light up. i.e. electron travel through the light bulb from one terminal to the other.
If my understanding was correct it would mean that one terminal from the input circuit of the isolation transformer (the neutral wire and the ground it is connected to) would always have the same number of electrons than either terminal of the transformer output.
Since the reason we have an isolation transformer is to prevent electrons from traveling between the transformer output and the ground, both outputs and the ground need to have the same number of electron. i.e. zero voltage. Right?
Is there really no voltage at all between the two circuits? I find this confusing.
The important thing here is that as soon as you ground the shaver circuit by touching it, there is a (tiny) static discharge, and then there is no force that restores any potential difference between ground and shaver circuit. The only force is the transformer, creating a potential difference between the two ends of the shaver circuit. But since that circuit is open, and again has a low capacity, barely any electrons need to move in order to create that potential difference.
Essentially electrons must travel around in a circuit. Sort of like a racing car. Electrons can't teleport from one location to another, and the number of electrons in a circuit will never change.
If touching you touch a wire, and you don't make a circuit (a complete loop) then you will receive no current (electrons can't move in a circuit), thus there is no voltage (since V = IR)
Usually when you touch the active wires in a powerpoint a circuit is made:
from the powerplant to the powerpoint, from the powerpoint to you, from you to the floor you are standing on, from the floor back to the powerplant.
A transformer works by having two coils of wires, and when electrons flow through one coil, it excites the electrons in the other coil, and they start to move too. (This is done through magnetism).
A transformer is usually used to change the voltage of an AC source, but in the case of the "Isolation transformer" the voltage on the input side and the output side are the same.
In a transformer, the two coils of wire aren't actually connected to each other. So if you touch one of the output wires, you don't make a circuit, as the electrons need to get back to the other output wire for a circuit to be formed.
The same thing happens with a battery. Unless you are touching both terminals, there will be no current flow.
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? :)
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!
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.
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.
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-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.
"Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder and then be soldered."
So, the joint has to be totally secure, mechanically and electrically, without the solder.
My friend was relying on solder and tape, with an inadequate mechanical connection that relied partly on the solder to hold it together. His installation was therefore not compliant with the NEC, although he did not want to admit it.
If you read the explanatory note related to the code, it gives the rationale I gave. Maybe it's BS, but it's the NEC.
You are not ever supposed to do that. I was assuming he wrapped the wires together securely first. http://en.wikipedia.org/wiki/Western_Union_splice or even http://en.wikipedia.org/wiki/Rat-tail_splice
I was just saying you don't have to use a pressure type connector - you can use a wrap splice, then solder it. (Electrical tape is not sufficient insulation, but you said this was a ground.)
http://www.ecmag.com/?articleID=8721&fa=article
The relevant part is:
"Connection of Grounding and Bonding Equipment. Grounding conductors and bonding jumpers shall be connected by exothermic welding, listed pressure connectors, listed clamps, or other listed means. Connection devices or fittings that depend solely on solder shall not be used."
You really cannot just twist-and-solder for a ground. It is specifically forbidden by the NEC.
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.)
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.
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.
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.
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.
Well, one of them is the Europlug (the "small" 2-pin variant) and that works everywhere in EU except UK.
The problem is that the two grounded versions are peculiar to Italy and not the standard ISO ones. Italy lost its chance to switch to the ISO sockets (compatible with the Europlug) sometimes in the 90s, when the EU forced the adoption of 230V (+-10V) from the previous 220V. The legislator failed to force the use of the ISO grounded plug and now it is not uncommon to see in Italy sockets that are compatible with Europlug, the 2 Italian grounded kinds of plugs, Schuko and type C. It is obvious that these sockets are both clumsy and expensive.
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???
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.
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.
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.
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.
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.
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?
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.
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.
> “Occasionally an electrician will accidentally wire an outlet backwards. This can't be helped, because Perfect Electricians are far more expensive than the normal human variety. And so we cannot intentionally wire appliance cases to the Wide Prong of the plug, since it would cause a lethal hazard if the appliance was plugged into a miswired wall outlet. Miswired outlets look exactly the same as the normal ones.
> “The solution? Why, add a Third Prong! Connect this 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. Give this wire a new color, one which is different from the other two. Give this this third prong a very different shape as well, so even Highly Imperfect Electricians will rarely connect the special prong to the wrong wire. Inside metal-cased appliances, insist that manufacturers connect this third wire to the case.”
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.
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.
You are over exaggerating how useful the ground is - a GFCI is far more effective, the ground pin hardly does anything at all.
I believe this is why they are labeled "test monthly". This is a lot more frequently than I check that neutral is not accidentally wired to hot or that the ground wire actually goes to the ground and not some random piece of plumbing that was removed in a renovation. (I live in a building that was built in the 1860s, and plumbing and wiring are being redone all the time.)
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 This appears to be the company: 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?
Anyway, as you say .. mystery now clear.
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.