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Comment by zdw | Hacker News Reader

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.

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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.

http://en.wikipedia.org/wiki/NEMA_5-15#Tamper-resistant_rece...

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.

(http://news.ycombinator.com/item?id=4033328)

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.

(http://en.wikipedia.org/wiki/Ring_circuit)

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".

http://en.wikipedia.org/wiki/NEMA_connector

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.

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