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Why three prongs? (amasci.com)
581 points by xd 2056 days ago | hide | past | web | favorite | 146 comments

Even 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?

A 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?

all 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:


Doesn't grounding the networks essentially return the atmospheric charge to the state it would be in if there were no networks?

Ever 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.

British 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

[2] http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Comp...

" There's 3 prongs arranged in a triangle, each of equal length"

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.

Most receptacles [in the US] do not have flaps or any other mechanical part. If they had the sort of contraption described, ungrounded plugs would not work and there would be a significant risk due to failure.

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.

Not anymore. All new receptacles in the US have a protection that will not allow you to insert anything into just one side.

You have to insert prongs into both sides of the outlet at the same time in order for it to release.


The change is in NFPA 70 - 2008, also known as "The National Electrical Code."

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.

I've never seen one of these things, not even at Home Depot.

I live in an older home (1955), but when we bought it in 2010 we had the original owner do electric work that had failed inspection-- as a result (and due to the local laws here), this is the only kind of socket we have.

Look again - I had a hard time finding anything else last time I went.

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.)

They exist, but they sell them alongside the normal ones at an additional cost.

Why would there be a significant risk of failure? Granted it would be more complex, but as the UK is a country of 65 million people I'd expect that if there were a lot failures the design would have changed by now.

Oh, I notice the Earth pin is longer now that you mention it, you're right. Makes sense, considering it retracts the flaps.

It's simultaneous pressure on the two live prongs which allows the plug to enter the receptacle - a two pronged plug still works.

See: http://www.nfpa.org/itemDetail.asp?categoryID=1508&itemI...

We're talking about UK plugs here. It's impossible to insert a plug without the ground pin entering first. I've had a few double-insulated AC adaptor where the dummy plastic ground pin has broken off (in the socket!) rendering the plug unusable in most sockets.

If this happens and you don't want to replace the plug, you can get a pencil and put that in the ground hole to allow you to put the other two pins in.

I used another plug turned around along with some deftness.

After an hour-long detour through Wikipedia's various power plugs articles (shocking how many of them there are), I have to ask this: historical reasons aside, isn't there any kind of consensus on which is the "best" plug/socket system (in purely objective terms)? The comparison table in [2] would have us believe BS1363 has the most features, but surely, there must have been more improvements in plug design since that huge lump was designed in the 1940s?

The IEC 60906[1] standard was designed to replace BS1363 too, but the british requirement for fuses prevents it's adoption.

[1] http://en.wikipedia.org/wiki/IEC_60906-1

Couldn't they just install the fuses in the sockets?

It seems terribly inefficient to require fuses in every plug even when the electrical wiring may not need them.

Their rules require different fuses according to the appliance's rated current and wire diameter, so no. I think you could make a smart outlet that switches fuses according to what's plugged in, but it would cost a fortune.

In the UK, until fairly recently, it was illegal to sell a product with a plug attached. Consumers had to wire their own plugs to their electrical devices before use.

Now that is inefficient!

Citation needed.

I may have had it a little wrong. I can't find older legislation about this subject that confirms my supposition.

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 have a suspicion you have it exactly backwards. It's now a requirement for some|many devices to have non-removable moulded plugs (although they still have replaceable fuses).

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.

It wasn't uncommon for us to have spare cord and plugs either, although the change of law to require moulded plugs obviated the need for any of it.

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.

It's a huge lump, but it handles 240 v at 13 amp and does so without burning down houses or killing people who plug it in.

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.

you misunderstand my intent: I was merely interested in finding out if there were any new ideas in plug design than what was indicated in the wiki article.

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?

Oh! That was posted here very recently. It's going into production with a few small changes.


The sad part is even with all the AC plugs, there are multitudes more DC plugs. I guess USB is kind of a universal standard, but I really wish Apple had licensed their MagSafe to everyone to make a world DC standard wall plug.

It'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.

True, 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.

We'll be laughing when copper hits $20,000 a ton - the ring main is very close to the most economical possible design.

The ring main makes wifi much more difficult, and encircles the building with a strong (and varying) magnetic field causing problems for many devices.

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).

Then we'll switch to copper clad steel/aluminum :)

What is the "ring main" system?

It's a circuit layout used mostly in the UK.

Wikipedia has a nice explanation.


So THAT's the reason for the damn fuses. Thanks!

The new standard plug in Brazil[1] is both grounded, polarised and has insulated pins, plus it's obligatorily recessed and connects earth first.

And you don't get hurt or break anything if you step on it :)

[1] http://en.wikipedia.org/wiki/IEC_60906-1

And we have our own exclusive design because no other was good enough. Also, so that using imported products its harder :)

It's based on an international standard. The different plug diameter is an annoyance (why shave .5mm?), but it's still backwards-compatible with the spec, and the standard plugs will probably fit anyway.

(Legally) imported products must conform to the local standards and pass inspection anyway :)

I'm also from Poa :)

Hey, hit me up on LinkedIn/twitter. You should add some contact info to your HN profile :)

Last time i checked the middle plug was optional, so no, not polarized.

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.

Not every appliance needs grounding[1], in those that do the ground pin is mandatory and so the plug is polarized.

[1] http://en.wikipedia.org/wiki/Appliance_classes

Remember that the idea,of polarization is,because cost cuttings in products. A product opting to use 2 prongs and grounding the chassis would cause problems. Yes it's forbidden, but may happen and (and products may be repaired by not so perfect techs) and the current system will offer you no precaution.

And don't even get me started on adapters for international products... Which btw, are the vast majority of brazil consumer gadgets.

And the only kind of plug the size of the aircraft carrier :-)

Exactly. Is there another design that shares the same features but in a smaller (more in line with US plugs) size?

The UK plugs are over engineered for their task though. It's occasionally useful that you can use the same outlet that you use for your 4.5W iPhone to use for 6000W welding torch but IMHO the Euro plugs that are a third the size and a third the price are a better use of resources.

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 love the British design; I've always found two pronged cables wobblier in the socket/not to mention buying-forgetting-rebuying many adapters of course

I hate those plugs because I can't find adapters to usethem in Brazil (while european/US ones can go directly into our (now replaced with 3-pin version) outlets: http://www.osetoreletrico.com.br/web/images/stories/Edicao59...)

Cut them up and replace the plug. You wouldn't want an adapter the size of a brick attached to your outlet anyway.

I just buy replacements from DealExtreme, because they're just USB hubs in the end... (but the Apple and Amazon versions have great build quality).

It looks like only the 30 Amp plus NEMA connectors require ground to be on top. It does appear technically "safer".


Nope - the diagram on that page shows every current rating of NEMA plug with an earth pin at the bottom.

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.)

This isn't for in the wall, but outside of it...

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...

You can try it yourself by dropping a bit of pencil lead across the prongs a slightly unplugged plug. You get a flash and it hops away, but I don't know that a piece of metal would break contact. I don't recall this ever even tripping a breaker, but be careful.

If you want a nice flash, take some low resistance half watt or even quarter watt resistors, place in power strip, plug power strip in ...

Ah, so awesome.

[for the US]

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.

The other way around. Plugs always fall out of ground up receptacles to the point that I will rewire them if I have any in my house.

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.

This story of a kid dropping a tape measure down on a partially plugged in device seems somewhat credible: http://www.electriciantalk.com/f12/1st-hand-ground-down-stor...

I once had a metal 18" ruler that fell off the back of a desk, and fell perfectly flush with the wall catching the top two prongs of an outlet. A spark, blackened the ruler and took a small notch out of it.

The upside-down / ground on top installation is a requirement in hospitals because of the danger of a scalpel falling between the plug and wall.

This was in the NEC a few versions ago, but removed. It was very common in hospitals, and then got used elsewhere, added to NEC, and then people complained (due to some plugs, like transformer wall warts, not working as well in this orientation). Of course the NEC isn't itself law, it's just an industry standard which is sometimes incorporated into local code (which is law).

Kind of not law. Most local codes will have a few items that they consider special and worth mentioning, and then it will tell you to refer to the County code (assuming you were at the city level). Then the County code will add some things and refer you to the State code. The State code may add a few more and then ultimately tell you that the NFPA 70 (NEC) is law.

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.

Prior 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).

Does that mean the output of the transformer have no difference of potential relative to the ground? Why is that?

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.

You must understand the concept of capacity: Capacity describes the amount of charge (number of electrons) needed to create a potential difference (voltage) across a gap. The capacity between ground and shaver circuit is tiny, so only a really tiny number of electrons travels through you to put the shaver circuit on the same potential as ground.

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.

I always wondered why it said "Shavers Only". Luckily the worst thing I ever stuck in was an American phone charger.

Current 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.

In 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.

RCDs 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.

Or rather both - individual circuits have a current limiting circuit breaker in addition to a master RCD across multiple circuits.

Now 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

Many 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.

A 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).

Never 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? :)

Been waiting 40 years for this explanation. Electricians have tried to explain it but their answers always ended up sounding circular and ill-founded.

An 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!

For 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.)

How many electricians do you actually know?

Three. One I walk dogs with daily.

Yep. The correct person to ask about electricity is a Physicist, or in your analogy's case, a lawyer.

As a physicist*, I would suggest speaking to an electrical engineer.

As 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"




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

But, 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.

Sorry, 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.


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.

What I said is correct according to the NEC. In fact, the relevant part of your link says:

"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.

> relied partly on the solder to hold it together

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.)

The NEC is even more restrictive with equipment grounding conductors, actually. You are not allowed to use solder-only connections on grounds. I don't want to go farther down the road, since we're only talking to each other, but here is a link FYI:


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.

Electricians are the wrong people to ask. It's like asking an IT exchange admin to explain the the SMTP protocol.

In 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.

I 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?

I 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.

As 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:


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.

> Here in Italy there are at least three kinds that are fairly common.

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.

As somewhat of a neutral observer, I think the 3 pin Italian sockets are actually quite nice: it's easy to put a lot of them in a power strip, and they are a lot less big and clunky than the Schuko ones.

I 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.

Really? 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_Netzspannung...

Ok, so it has gotten a little better, but having lived in close proximity to Americans in Africa and so buying their appliances second hand, etc, there were all to many cases of people accidentally switching plugs. PC Power supplies used to also be manually switcheable - no autodetect (atleast the ones we had access to in Africa), guessing that's changed. The dangerous examples were cords that didn't have the right end or had changed ends so that you could hook a 110V appliance to a 220 outlet.

Interesting effect: 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.

Try view source and you can see how it's built. Simplified, SVG could be described as "like HTML only with shapes".

This is an awesome article. Here's an example of the usual non-explanation:


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???

Can 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.

> 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.

In 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.

Wonderfully 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

You still need two slots, unless all of your appliances happen to rest on copper stakes hammered into the ground.

or have some capacitance of their own.

Why don't Schuko plugs have this orientational bias ?

Schuko 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_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.

That 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.

>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.

The 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

There is also the Europlug.


> 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?

Keep reading. At that point outlets aren’t polarized and the case might end up connected either to ground or to the hot wire depending on how you plug it in. Even when outlets are polarized…

> “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.”

And if there is any current flowing in the circuit and you have a finite resistance in the neutral wire there will be a (small) potential difference between the case and ground.

Voltage 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.

"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.

From 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.

If everything is wired and functioning correctly I believe you are correct; the article mentions the case of electricians mixing up hot and ground wires. Presumably an appliance manufacturer could also have a mistake, but assembly lines are more consistent than humans -- if UL or CE tests a device it's likely all devices of that model are OK.

Do we need grounded sockets now that we have GFCIs?

Do 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.

Actually, it's totally legal. If you have an outlet without a ground wire, you can use a three prong GFCI and put a sticker on it that says "No Equipment Ground" and it's totally legal and to code.

You are over exaggerating how useful the ground is - a GFCI is far more effective, the ground pin hardly does anything at all.

While I never said it was not legal (though I may have implied as such considering seat belt laws), code does require properly grounded outlets in bathrooms and other high risk areas.

Or maybe stopped working one day.

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.)

Yes. 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.

I 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.

This 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 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?

I 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!

I 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.

If there's so much DC energy available can we store it?

That site is an absolute gold mine.

Clears the mystery :)

Yeah, 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.

heh, 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.

You 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.

Remember 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.

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