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Inside a plug there is one grounding wire, while the socket has two grounding pins. I know that the wire splits into two, but why? As the wiring inside the wall is supposed to be the same between different types, I assume it is just a design decision, however does this mean that in such design I could theoretically have one broken pin for grounding and successfully ground a plugged device only by using one pin on the socket?

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  • \$\begingroup\$ Redundancy from oxidation \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 9 '20 at 22:07
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    \$\begingroup\$ BTW it's not correct to call the Schuko socket "EU socket". They are used in most European/EU countries, but by no means in all (see this map: en.wikipedia.org/wiki/…). The only plug that works in all mainland European countries is the non-grounded Euro plug (en.wikipedia.org/wiki/Europlug) \$\endgroup\$ – rob74 Oct 10 '20 at 19:38
  • \$\begingroup\$ @rob74, though looking at just the map gives a somewhat incorrect impression, in that at least up here in the north, pretty much all "Schuko" plugs are those CEE 7/7 hybrids with a much wider compatibility area (red in addition to blue in the map). It's still not the whole of EU or Europe, of course. (Though Brexit will also help there if we look at just the EU...) The text page for Schuko describes it better. \$\endgroup\$ – ilkkachu Oct 10 '20 at 21:52
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enter image description here

Figure 1. Schuko sockets. (Image source: Bran.)

If you are referring to these sockets the dual earth pins give balance pressure on the plug which would otherwise be forced to upwards or downwards making insertion difficult and possibly affecting the contact of the L and N pins in their sockets.

... does this mean that in such design I could theoretically have one broken pin for grounding and successfully ground a plugged device only by using one pin on the socket?

Yes, but it is most likely that the two contacts are stamped out of one sheet of metal and there are no joints so fracture or breakage is unlikely.

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    \$\begingroup\$ @BobbyJ The EU system very much has L and N. When we're at 3 * 230, it's L1, L2, L3 and N. What are you trying to say? \$\endgroup\$ – Mast Oct 10 '20 at 7:04
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    \$\begingroup\$ @Bobby, I live in Europe. Every house in my country has one leg of the 2-wire supply neutralised by connection to earth at the incoming supply box. That makes the other wire live at 230 V with respect to N and to E. The factory where I work has three-phase + N at 400 V between phases and 230 V phase to neutral. \$\endgroup\$ – Transistor Oct 10 '20 at 8:55
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    \$\begingroup\$ @BobbyJ not correct. 380 is only used for industry or maybe a water heater. Certainly not in every socket. \$\endgroup\$ – Esben Skov Pedersen Oct 10 '20 at 10:47
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    \$\begingroup\$ @EsbenSkovPedersen It depends on country. Here 3 phase (400V between phases and 230V from phase to neutral) electricity plus neutral comes to each house so it is not only used in industry. High loads such as water heaters, stoves, sauna, saw to make your own firewood, EV charging etc can use 3 phase electricity, but normal sockets have 1 phase and neutral. \$\endgroup\$ – Justme Oct 10 '20 at 12:17
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    \$\begingroup\$ My main point is that Europe very much have a concept of N \$\endgroup\$ – Esben Skov Pedersen Oct 10 '20 at 16:59
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Two pins is more safe than one pin. And it makes the design symmetric, so it can be connected in any orientation.

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Your claim is not true. EU socket/plug standard is a combination of almost all older standards in the EU. It was design that german DIN standard plugs are somewhat compatible with italian or french ones. Czech plugs are also in compliance with. So...

German standard was rotation symmetric meaning the hot wire is on the left in the wall but in the device it could be one or another. For having the ground wire allways connected to the ground means one have to use two ground connections. Plus the two spring-loaded contacts cancells out the net force.

German wall socket

Italian standard, on the other hand, use anther approach to the symmetry - one central ground connector in between hot and neutral. The outer shape is flat hexagonal and is same as "other" european 2-wire 230 VAC connectors.

Italian wall socket

French standard was hot wire plug on the left, neutral plug on the right and ground pin above them. Meaning no rotation symmetry.

enter image description here

The standards were changed to gain the compatibility. The sockets in Czech Republic, which followed the french standard, are round (no notches) with the groun pin; the plugs are notched body (to fit in german socket) wit both hole for ground pin and notches for the german ground connectors. The universal EU socket would look like this:

enter image description here

You can see there are no notches, so all round, flat or nothed plugs will fit in. Also it accept two different hot-neutral distances (I think old italy was wider than old german/french). There is also the Italian central ground plug and german top-bottom connectors. This socket can accept any old and new european socket. Except for UK, because,... You know, UK. And old french/czech plugs will lose the ground connection because they do not have the notch. A design failure, I may say...

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    \$\begingroup\$ Going by the AC power plugs and sockets page on Wikipedia, that universal socket also doesn't look like it would accept the Danish or Swiss plugs. Because, standards, it's good to have so many to choose from... \$\endgroup\$ – ilkkachu Oct 10 '20 at 22:07
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    \$\begingroup\$ @ilkkachu For the Danes - one cannot simply satisfy everybody, the combo supports the biggest EU economies (Germany, France) plus others, that accidentaly fits in (former Czechoslovakia). Switzerland has never been part of the EU. You know, xkcd.com/927 \$\endgroup\$ – Crowley Oct 10 '20 at 22:23
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Not all European AC plugs/sockets are the Schuko's you show.

Many have a single pin for earth, or indeed none at all.

I imagine that it's a design feature to ensure reliable earth connection without using a huge pin as in the UK !

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    \$\begingroup\$ In this case it is to cancel out the lateral forces in the first place. Italy, Switzerland and France are usint one single pin slightly thicker than hot/neutral pins. \$\endgroup\$ – Crowley Oct 10 '20 at 21:55
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    \$\begingroup\$ @Crowley Belgium has the same system as well. This is why truly universal europlugs both have the metal pieces at each side of the plug as well as a hole corresponding to this pin. \$\endgroup\$ – htmlcoderexe Oct 11 '20 at 0:10
  • \$\begingroup\$ If you think UK's contemporary pins are large, then.. you've got to see the "legacy" ones: en.wikipedia.org/wiki/… \$\endgroup\$ – quetzalcoatl Oct 11 '20 at 18:50
  • \$\begingroup\$ Ah yes. The 'old' UK 15A plug to BS546, Also used in India fwiw. Do you also have the smaller 5 and 2 Amp versions ? They're cute and great for table lamps etc. upload.wikimedia.org/wikipedia/commons/thumb/4/4a/… \$\endgroup\$ – Graham Stevenson Oct 11 '20 at 19:00
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In the UK we have a single earth pin. The 240V single phase is between live and neutral the latter bonded to earth at the supply. Properties typically run off one of the three phases of the 440v supply in blocks of 6 or so. Utterly different from the us model.shuco plugs are an abomination! The 13A plug with its shrouded sockets and live snd neutral pins making contact after earth (and with insulated shanks) are remarkably safe.

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    \$\begingroup\$ Schukos aren't a US design ! They're German. Did you mean the USA Edison plug ?Pedantically, I'll also point out that, courtesy of Boris, the UK is no longer a member of the EU either. \$\endgroup\$ – Graham Stevenson Oct 10 '20 at 21:55
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    \$\begingroup\$ pfft, if you're going to describe something as an "abomination", you might do well to at least give some reason as to why you'd say that. (Otherwise, in a context like this, you run the risk of just sounding prejudiced toward anything foreign...) \$\endgroup\$ – ilkkachu Oct 10 '20 at 21:59
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    \$\begingroup\$ @Stub, we use BS 13A plugs in Ireland but I notice that the recessed Schuko (not "shuco") sockets probably block finger access to the pins by the time they're inserted far enough. The older ones didn't have shutters on the sockets but I've seen that newer ones do in some places. \$\endgroup\$ – Transistor Oct 10 '20 at 22:07
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    \$\begingroup\$ @Transistor, they do indeed block finger access. I've tried. \$\endgroup\$ – ilkkachu Oct 10 '20 at 22:15

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