12
\$\begingroup\$

Why it is not important where we connect our hot and neutral wire when connecting them to our Schuko plug? Is it because current flows in both directions?

enter image description here

It is only important that our ground wire is connected in the middle but I don't understand why the order of our hot and neutral wire is not important while connecting them to Schuko.

\$\endgroup\$
  • 2
    \$\begingroup\$ Inherently stupid design from 1925 that is safe enough when certain limitations are met. It is UTTERLY ESSENTIAL that anything it is used with may be safely used with phase and neutral in either of the two possible arrangements. Wikipedia- read all about it \$\endgroup\$ – Russell McMahon Aug 7 '15 at 12:04
  • \$\begingroup\$ Specifically, there is a requirement on matching device enclosure and connector type. Schuko plugs are required for devices with metal cases, the case needs to be connected to earth ground in several places, so any electrical fault would connect line to earth or neutral to earth. That is why @sweber mentions old installations as unsafe. New installations require a GFCI in the outlet at least. \$\endgroup\$ – Simon Richter Aug 7 '15 at 17:25
  • \$\begingroup\$ Are you sure this is a "shuko" plug? If possible, please post a picture of the front face of the plug. Chances are it is a hybrid plug. \$\endgroup\$ – Magic Smoke Aug 7 '15 at 20:17
4
\$\begingroup\$

Schuko is a standard used primarily in Germany and Austria. (PS: Also in the Netherlands.) Neighbouring countries have different standards which are compatible to some extent (protective earth gets in the way) and mostly asymmetric. The plug in sweber's image is the most common type in Europe because it fits into most of them and so covers most of the European market. (I am assuming plugs that have protective earth. Among those without, euro plugs are even more common because they are even more compatible, e.g. with the Italian and Swiss systems. Due to a euro plug's flexibility, it even works with different pin distances.)

The standard dates back to a time when very often both wires were live at the same time. An electrician in Berlin told me that he once still worked in a place with extremely old wiring where this was still the case. Before changing something, he checked one wire, which was live, and so assumed that the other would be neutral. It was live, too. Before that background, the symmetry makes sense, though it does indicate a certain lack of foresight.

While obviously not ideal, this symmetry is not a big problem so long as all appliances are built in such a way as to be still safe when the two main wires are swapped. As almost everything is being produced for a global market today, in practice this is always the case - or at least manufacturers always claim and usually provide it. It will be like that until the countries without the symmetry provide the overwhelming majority of consumers, at which point the few remaining ones will have to switch as well.

The responsible standards bodies and authorities in Germany and Austria are so little concerned about this that when you buy a switchable power strip here, the switch almost(?) always cuts off only one of the wires, which of course in a symmetric system may be the neutral one. Though this should theoretically be a factor in accidents occasionally, I have never heard of one and nobody seems to be worried about this. Nowadays the old schuko plugs without the French hole are so rare that it would be no problem to introduce new schuko sockets that simply add the French pin to the existing schuko standard, resulting in three connections for protective earth and solving the problem at once. Or, even simpler, to just switch to French sockets. Apparently something similar already happened in Poland, where the old Russian system (like schuko but without the two protective earth connections that gave it its name; consequently, schuko plugs fit into Russian sockets but not vice versa) was replaced by the French system, though reportedly people are careless about the wiring, sometimes switching live and neutral.

The schuko system is rather strongly incompatible with the British one even now that the voltages have been unified all over Europe. The British system is traditionally asymmetric. During the Second(?) World War, as part of a war effort to save metal, people started to use thinner wiring than would otherwise have been necessary, but closed the wiring to a circle. So you have a neutral loop and a live loop, and you can add sockets or light switches to that at various points. This is actually quite dangerous because if the live or neutral circle gets disconnected once, you have no way to know that the wires are now loaded beyond their specification. I believe modern British standards still take this problem into account, and that this is why all British plugs must be individually fused, which could actually be dangerous if you put the fuse on the neutral wire. (In other words, schuko plugs must not be fused.)

\$\endgroup\$
  • \$\begingroup\$ Not a deadly accident, but I was using a device with a switch on the socket strip that cut only one wire. I was doing some testing so no continuous plug/remove plug from the socket, I only used the switch. At a certain point I touched a wire and I felt a light shock, maybe due to the "charging" of the capacitor represented by me and by ground. I was wearing plastic slippers. So yes, it can happen, but unlikely. I have never seen another single-wire switch from that moment on (that strip was very old). So now you heard of one. \$\endgroup\$ – FarO Jul 28 '16 at 13:22
11
\$\begingroup\$

I doubt there is a problem about the word "SCHUKO".

SCHUKO is a German abbreviation for SCHUtzKOntakt (protection contact), which means this type of outlet / connector:

enter image description here

enter image description here

The metal contacts at top and bottom of the outlet are the "Schutzkontakte", i.e. earth.

Because the plug is symmetrical, there is no way to know which line is hot and which is neutral for a device. Only earth is always the same. There is also no rule for the outlets, if the right or left hole should contain live or neutral. So, how should there be a rule which pin of the connector is what?
(That's the answer to the question.)

However, e.g. French outlets are a bit different. They don't have that earth contacts, but a pin in the outlet, instead. The plug in the picture is compatible with both, the German SchuKo system as well as the French system. Though you could in principle distinguish between neutral and live in France, the device does not know if it's in Germany or France. So, usually all devices should be designed to expect live on both lines. (I also doubt that any device worldwide connects it's outer case to what it thinks is neutral)

And most devices don't switch both lines. This indeed means that the entire electronics is on ground or live potential, when switched off. But I don't see a problem there. The is only one point that could be dangerous:

In... ancient installations, you may only find two wires going to an outlet. Directly at the outlet, earth is connected to neutral. If then, the neutral wire is broken by error or switched by a wall switch or similar , the outer case would be on live potential...

For that reason, only the live wire is allowed to be switched by wall switches, and earth goes down to the house connection box, where it is connected to neutral and ground.


EDIT:

As answer to the comments:

It is true that the plug shown in my picture is a hybrid of a German SchuKo plug (earthed via contacts) and a type E plug (earth pin in outlet) and so fits both, German and French outlets. For the German outlets, it's symmetrical, and you don't know which pin will be connected to live.

For the French outlets, it is protected against reverse polarity, and the French standard says: Looking into an outlet with the earth pin in the upper position, left is neutral and right is live.

However, Poland uses the type E connector to, but to my knowledge, has no rules regarding polarity.

So, if you want, follow the French standard, but the device should always expect live on either wire. (And the question mentioned the German SchuKo connector.)

Also, a circuit breaker should always switch the live, never the neutral line. And a simple switch like this:

enter image description here

inside a device just switches one wire.
There is a similar switch with a more square-shaped lever, which may switch both wires. But if it is an illuminated version, it usually switches one wire, only. Yes, electronics may be on live potential, but if earth is correctly wired to the plug, this does not matter.

\$\endgroup\$
  • \$\begingroup\$ I disagree with "And most devices don't switch both lines. This indeed means that the entire electronics is on ground or live potential, when switched off. But I don't see a problem there." - if a circuit breaker trips because there is a short, there MUST NOT BE any harmful voltage past the circuit breaker. Imagine your shower leaks on the lightbulbs, the CB trips, and you stick your fingers in to fix it thinking the circuit is safe. Bad idea. \$\endgroup\$ – Mister Mystère Aug 7 '15 at 15:07
  • \$\begingroup\$ A switch inside a device often switches one line, only, regardless, if neutral or live. A fuse inside a device does so, too. BUT a circuit breaker always has to switch live. May be, this caused the confusion. \$\endgroup\$ – sweber Aug 7 '15 at 17:06
  • \$\begingroup\$ The picture that you included is a CEE-7/7 plug and it's obviously not symmetrical (the hole in the plug is only on one side). It is a hybrid plug combining the symmetrical "shuko" type F (CEE 7/4) and asymmetrical (due to the hole) type E (CEE 7/5) plugs. For type E sockets, if the single pin is on top, neutral is on the left, live is on the right. Given that this plug is supposed to be compatible with type E sockets, it should be wired accordingly. the source of the image \$\endgroup\$ – Magic Smoke Aug 7 '15 at 20:16
  • \$\begingroup\$ Poland indeed has no rules regarding polarity. There are a lot of „symmetrical” double outlets that have each socket wired in the opposite way to the other. \$\endgroup\$ – kinokijuf Aug 8 '15 at 10:42
  • \$\begingroup\$ @MagicSmoke – beware of those left-right advisories, because in Czech Republic and Slovakia, official standards for CEE 7/5 sockets require to have live on the left and neutral on the right (if the ground pin is on the top). Having neutral on the left and live on the right will fail certification of the building. \$\endgroup\$ – miroxlav Oct 29 '18 at 16:06
8
\$\begingroup\$

Normally the live and neutral are interchangeable due to the AC nature of the voltage. However. If you are using some badly designed equipment in which for example breakers are only present on the live wire (rectifiers can also be made asymmetric), it might matter. Depending on where you live the neutral may be connected to Earth, which means that the live wire is always hundred(s) of volts above your potential. So if the breaker breaks the neutral thinking it's a live wire, the hundred(s) of volts are still there. It really matters when said breaker is a residual current device, and I doubt very much that it would be sold with such design, but since CE marking is no proof that the system is safe (the process relies too much on the manufacturer's knowledge and honesty) one cannot be too cautious.

It's just something to keep in mind when using dodgy equipment (or if you designed such dodgy equipment...) even if it is virtually improbable. Otherwise, what happens? If equipment A has live and neutral wired one way and equipment B has them wired the other way, A and B will be fed the same sine wave with the same amplitude and the same frequency with the same current capability, but these sines will be 180° out of phase. Utterly irrelevant unless it is unacceptable for your application (I can't think of any example).

\$\endgroup\$
  • \$\begingroup\$ Sorry, I don't get the point. Schuko connectors are symmetrical, so there is no way to know which is the live wire. Most devices do only switch one side, and (resiudal) breakers should always break the live wire. (See my answer) \$\endgroup\$ – sweber Aug 7 '15 at 12:31
  • \$\begingroup\$ Plugs and sockets are symmetrical and don't have any markings because the devices are supposed to be designed to be plugged either way. However, in certain countries like France and the UK, the earthing scheme is such that the neutral is earthed. Keeping the same example, correct breakers should break both the neutral and the live at the same time; but if they are dodgy and only break the live, you have a 50/50 chance that the breaker does not cut the high voltage off because it cuts the neutral and not the live wire. \$\endgroup\$ – Mister Mystère Aug 7 '15 at 14:51
4
\$\begingroup\$

There is no rule for the power terminal wiring because the socket is symmetrical and the plug can be inserted either way round. Thus there's no usefule way to distinguish the contacts, they are interchangeable.

The other safety concerns cannot be mitigated by choice of plug wiring, other methods must be used.

\$\endgroup\$
0
\$\begingroup\$

The schuko plug is not unlike the sort of plug used in GB for kettles, they had the side earth contact and were reversible. On AC it does not matter which way the plug could be inserted to operate the appliance, but sometimes reinserting the plug the other way may reduce hum if it's an electronic device. On DC, full operation may only be possible with the plug inserted one way. The side earth is the best part, its safe and easily visible. As for switches, these are meant to break the circuit, so could be in the live or neutral, they are not for isolation. many switches are in the neutral or earth side. Personally I would prefer in the live side. Regarding double pole switches, the idea that having both poles in close proximity is scary, and that double pole switches are not safe is misguided. If a double pole switch is fit for its purpose, e.g. meets electrical and environmental standards, voltage and current ratings not exceeded, it will usually be safer than a single pole switch.

\$\endgroup\$
  • \$\begingroup\$ House outlets only have AC. Also, properly designed switches rated for the voltage and current needed are perfectly safe. \$\endgroup\$ – JRE Mar 17 '18 at 17:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.