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Most other plugs I encountered have three prongs - hot, neutral and ground I assume. For example all BS 1363 plugs have one. Also CEE 7/7 have ground connections and while CEE 7/16 exists it is not used for high-power appliances such as computers.

On the other hand I might have seen once grounded NEMA plug even for computers etc. What reasons are there for those differences (is it due to 120 V vs. 240V)?

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    \$\begingroup\$ Live, neutral and earth. It's likely, I'd guess, that one reason for items like laptops, is that they are made of plastic, so risk of electrocution by a wire touching a case is eliminated. It won't be anything to do with 120/230V as in the UK we have some devices that only use the figure of eight leads that don't carry earth. \$\endgroup\$ – DiBosco Oct 30 '17 at 9:03
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    \$\begingroup\$ If you look on the plug you should find a symbol " a square within a square" this means it is "double insulated" i.e. it has two layers of insulation between you and the conductor and therefore, under a certain power, does not need an earth connection - which is why some computers and phone chargers etc have only two connections. \$\endgroup\$ – Solar Mike Oct 30 '17 at 9:19
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    \$\begingroup\$ two prongs are cheaper than three. there is no other reason. everything is done for maximum profit. if it was up to manufacturers, you would power your toaster by twisting bare wire around binding posts ... lol \$\endgroup\$ – jsotola Oct 31 '17 at 8:21
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    \$\begingroup\$ I believe that "US" is unnecessary. Some appliances have 2, some have 3, US is same as the rest of the world. I believe that UK is exceptional here with their use of 3-pronged plugs even when only 2 are used. \$\endgroup\$ – Agent_L Oct 31 '17 at 8:32
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    \$\begingroup\$ @Agent_L Three pronged plugs are essential in the UK even when the third (ground) prong is not connected to anything, because many sockets have mechanical shutters to prevent kids from poking things into them, etc, which are retracted by the longer third prong to allow the shorter prongs to enter the socket. The third prong may be just a plastic moulding when no electrical connection to it is required. \$\endgroup\$ – alephzero Oct 31 '17 at 18:57
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Many appliances in the USA and elsewhere have two-prong plugs because they are "double insulated." The third prong is for ground fault protection except where outlets have been designed with protective shutters on the current-carrying slots that are opened by the ground prong. Double insulation provides very effective ground fault protection that is less expensive for most products.

The fact that the US voltage for domestic wall-plug use is 120 volts offers a small amount of additional safety vs. 220 volts, but that is generally not taken into consideration in US safety standards.

Double insulated or class 2 electrical appliances are products that have been designed in a way so as not to require a safety connection to electrical earth (These products must NOT have a safety connection to Earth).

These products are required to prevent any failure from resulting in dangerous voltage levels becoming exposed causing a shock etc. This must be done without the aid of an earthed metal casing. Ways of achieving this include double layers of insulating material or reinforced insulation protecting any live parts of the fitting.

There are also strict requirements relating to the maximum insulation resistance and leakage to any functional earth or signal connections of such appliances. Products of this type are required to be labelled "Class 2," "Class II,", "double insulated" or bear the double insulation (square in a square) symbol.

The safety of electrical product designs are certified by an independent testing agency such as Underwriters Laboratories (UL) or the Intertek ETL laboratory (originally Edison Testing Laboratory). In order to apply the UL or ETL label, the manufacturer must have product design evaluated and samples tested in the independent laboratory or have their own testing and review results evaluated by the independent laboratory. In either case, they must maintain a contract with the independent testing laboratory that includes periodic, un-announced inspection of their manufacturing facility and its quality assurance procedures.

See the product marking illustration below.

enter image description here

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    \$\begingroup\$ That's kind of a backwards non answer though. If it needs a ground it gets three pins, true, but it does not answer the question of why it does not have three pins anyway like other nations on the planet. \$\endgroup\$ – Trevor_G Oct 30 '17 at 12:24
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    \$\begingroup\$ I am not familiar with UK outlets. That is a good feature. US outlets that have shutters that are pushed aside by the prongs are starting to appear, but I don't know how effective or durable they are. I have my doubts. Kids poking forks in outlets is one of the biggest problems with outlets. \$\endgroup\$ – Charles Cowie Oct 30 '17 at 12:43
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    \$\begingroup\$ So, with the UK one, they have to open the shutter then they get to poke something in the live compared to other places where it’s just a hole to shove something in... The UK one gives that extra barrier, oh and if you think it’s easy - just try pushing it open... \$\endgroup\$ – Solar Mike Oct 30 '17 at 12:51
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    \$\begingroup\$ @Trevor, you said it's a "backwards non-answer". That sounds to me like saying there's something wrong with it. \$\endgroup\$ – The Photon Oct 30 '17 at 14:58
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    \$\begingroup\$ @Trevor, Okay it sounds like you kind of thing there's something wrong with the answer. \$\endgroup\$ – The Photon Oct 30 '17 at 16:21
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I think your question is based on misconceptions.

For example all BS 1363 plugs have one

BS1363 plugs always have a pin in the earth position to operate the shutter mecahnism and to enforce polarity. However on some non-rewirable plugs and wall warts this pin is made of plastic (reffered to in the BS1363 standard as an "insulated shutter opening device"). Even when the pin is present and made of metal it may not actually be connected to anything.

Also CEE 7/7 have ground connections and while CEE 7/16 exists it is not used for high-power appliances

There is also CEE 7/17 for high power non-earthed appliances, though i'm not sure how common it is.

such as computers.

In my experiance desktop PCs always have an earth connection. I will admit I haven't spend a whole lot of time in america but image searches for desktop PC power cord show a bunch of grounded amerian plugs and no ungrounded ones.

Laptops tend to be more variable, years ago they all used the two-prong figure 8 inlet but an increasing number seem to have moved towards the 3-pin cloverleaf inlet.

Small power supplies for phones and tablets generaly don't have an earth connection. Again from what I have seen this is true on both sides of the pond.

Ultimately the desicion whether to have an earth connection or not comes down to whether it is more economical to add the earth connection or to meet the class 2 insulation/leakage standards.

I do get the impression that the US used class 0 appliances (which are not "single fault safe) for longer than other countries did but modern stuff will almost certainly be class 1 (single fault safety provided by earthing) or 2 (single fault safety provided by "double or reinforced insulation").

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    \$\begingroup\$ "In my experiance desktop PCs always have an earth connection. I will admit I haven't spend a whole lot of time in america but image searches for desktop PC power cord show a bunch of grounded amerian plugs and no ungrounded ones." I think you're right. My bad. \$\endgroup\$ – Maciej Piechotka Oct 30 '17 at 15:21
  • \$\begingroup\$ CEE 7/17 is quite common, e.g. for hair dryers or vacuum cleaners \$\endgroup\$ – realtime Oct 30 '17 at 16:52
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    \$\begingroup\$ I've lived in the US my whole life, and from the earliest IBM PC (you know, the one with an 8088 in it), desktop machines have all had grounded, 3-prong plugs. Many laptops don't, but I believe that grounded plugs are becoming more common. Sadly, I don't recall if the Apple ][ had a grounded plug or not, though this supply for a ][gs shows a grounded plug. \$\endgroup\$ – FreeMan Oct 30 '17 at 20:00
  • \$\begingroup\$ PC plugs will always have an earth pin, because the PSU has an earth pin, and these things need to connect the one to the other. \$\endgroup\$ – Kevin Oct 31 '17 at 0:04
  • \$\begingroup\$ @Kevin Outside of the U.S., the wall sockets in many countries don't even have a third pin. All desktops I've encountered in the U.S. have a ground pin, though. \$\endgroup\$ – reirab Oct 31 '17 at 6:43
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Mostly cost... the two pin things are cheap as dirt. Three prongs are harder to make so cost more.

As such, if the appliance does not need a ground, they go with the two pin job.

There is no dictated standard in North America that a plug on the end of the cord MUST have three pins regardless of insulation type.

Further, there are countless legacy outlets that have no hole to plug the ground pin into, even in relatively modern buildings. So adding the third pin can actually increase your product return numbers and reduce your market share.

The North American outlet system is rather old. Plugs never seem to fit right and it is really easy to touch the live pins while inserting and removing the plug from the outlet. They also contain no ground pin activated safety features.

On UK receptacles the ground pin opens a protection shutter on the power pins whether the ground is wired up or not. To reduce cost, some "double insulated" appliances have a dummy plastic ground pin on the plug.

enter image description here

Notice also on the plug that the live and neutral pins have an insulated length on them. The pins do not become live until the metal part is fully inside the receptacle.

enter image description here

Much safer and more reliable than this thing....

enter image description here

Electrically, three prong plugs (two power pins and ground) are only electrically required for devices that need to be earth grounded.

Many if not most small domestic appliances, tools, and other systems, are double insulated, that is, insulated in such a way that two faults must occur in the insulation before the user is exposed to lethal voltages. The fact that the customer can quite easily kill themselves while plugging it in, seems to be covered by an S.E.P. field. ( Somebody Else's Problem. )

The current mechanical nature of the aging North American plug itself, means that, unlike in other countries like the UK, there is NO safety benefit or socket feature that would add value to retaining the ground pin, or a dummy ground pin, when not needed electrically.

Costs therefore dictate that the smaller two pin plug be used when no grounding is required. Costs also dictate that the system is not going to get upgraded any time soon to include better, safer, plugs and outlets.

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    \$\begingroup\$ I have to say, that the mains plugs are one of the few things we do differently here in the UK that is better than the rest of the world. It is by far the most robust and safe system I've come across in my travels around the world. Mainland Europe also has the option of a two pin mains plug; their mains systems gives me the willies, not quite as much as the US one though! \$\endgroup\$ – DiBosco Oct 30 '17 at 12:29
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    \$\begingroup\$ In the USA, UL standards require three-prong plugs for appliances that are not double insulated. The standard is not dictated, but there are hoards of lawyers eager to sue if someone is injured by a faulty appliance. That is a very strong incentive. Three-prong outlets have been required for new construction since about 1960. Local codes require outlets to be converted to three-prong if major renovation is done. The lack of three-prong plugs decreases the value of a property. However, there are still a lot of them around. The prevalence of double insulated products mitigates that somewhat. \$\endgroup\$ – Charles Cowie Oct 30 '17 at 12:32
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    \$\begingroup\$ @DiBosco yes I have to agree with you. When they redid the standards in the UK to switch from the old odd-sized round pin outlets to the uniform square pin jobs they really thought it out well. In addition to the already mentioned features, local fusing on outlet and plug and switched outlets make it a far better system and it is mechanically much more robust. The only down side, unfortunately, is it is not cheap. \$\endgroup\$ – Trevor_G Oct 30 '17 at 12:57
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    \$\begingroup\$ @Trevor - at < £1.50 per outlet and < £0.50 per plug, it's a cost I'm willing to accept. \$\endgroup\$ – Jules Oct 30 '17 at 15:10
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    \$\begingroup\$ @Mark it is not that so much, which I would argue is not that silly, but because plugging your 1A beside lamp into a circuit that is good for 15A can mean the lamp/wiring could seriously overheat in a fault condition without ever blowing the main fuse. \$\endgroup\$ – Trevor_G Oct 30 '17 at 17:35

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