domestic electric AC wiring - Live and Neutral correct or not? On electric circuits, with $120$ or $230$ AC mains, there are the wires Live and Neutral.
The wiring isn't symmetrical : following the circuit, you cannot switch $L$ and $N$.
However, in Italy, sockets and plugs don't have a specific orientation : it is a matter of how the user push the plug in the socket. 
You cannot watch the socket or the plug and guess the N and L, there aren't signs.
Also internal building to sockets wiring isn't specific.
So, here it is an Italian socket - you are sure only about the middle one :
|*|   ?
|*|   Earth Gnd
|*|   ?

If the appliance design expects a specific N and L wiring, it is a random 50 % correct / not correct run.
If I recollect well, most of radio appliance show sign N and L wires on their power supply electrical schema.
I  spoke with some electrical engineers, they cannot explain me how to match the circuit requirements (and hazard regulations) with the real common practices.
Is the electric schema sufficient to pass the local regulations / certification, even if going in production the appliance is build with a non polarized plug ?
 A: In domestic circuits, the point is that the supply is alternating current, your outlet cuts this alternating current and when an appliance is inserted to the plug, the circuit closes and current flows through the appliance. 
The  two lines are the live line, bringing in the current, and the return line, leading the current back to the generators of electricity in the city factories. The return line in the plug is neutral, it has maybe a small potential difference to the ground and is not dangerous to life. The live line carries the power.
The ground connection is an extra security, in principle the neutral should be at ground potential, but because of the great distance to the generators  a small potential difference with neutral and  the earth could create problems;  also in case of an accident the current can flow to the ground. The ground is not necessary for the appliances to work.
Domestic appliances are built so that the direction of the alternating current within them does not matter. The only difference in reversing  the live and return  lines in your plot is the direction of the current, and AC appliances do not care. The direction of the current is important in Direct Current modules, like radios and TVs but all that is internal to the units, they have transformers etc that turn the current into the DC they need, and transformers do not care which way the current is flowing.
It is important  for the electrician who set up the domestic circuit to be careful that the live line, i.e. the line bringing in the current, is in a protected spot on the main board that controls the house electricity, not easily touchable with screw drivers and fingers by naive people, because the live current  will go through a person to the ground and electrocute him/her. That is standard in the plugs and sockets, ( there are protecting covers for children) , all are internal and not easily accessible. 
A: It's only the difference in voltage between two points in a circuit that really matters. As long as a circuit is only connected to two prongs (Live and Neutral), there's no way to determine which is which from the circuit's perspective.  All you see is that they oscillate relative to each other.
The reason it matters to electricians is that the live wire has an oscillating voltage with respect to the local ground/electrician's body, and the "neutral" likely does not. This means that if the circuit comes into electrical contact with anything connected to the ground, it will be the live wire that is going to cause problems.
A: You always can recognise the phase line by using a "phase-meter" or "live-meter" screw driver. You can buy that some 50 cents. Push it into one of the holes while your thumb is attached to the other end of the screw driver. If that hole is live the "neon lamp" in the screw driver upper body turns on in orange color. Otherwise, it remains off. Neon lamp has electrical resistance to reduce the live voltage to a safe vale at the other end that you have attached your thumb. Your thumb works as a null for the lamp. In Britain, socket holes do not open to allow inserting that phase-meter to check if a socket is functioning at all or not. They have something like a British plug with a lamp on its outer body that by insertion turns on. (in Ialy) if you are a bit mythbuster person, push that screw driver in the live (phase) hole and attach the back of your palm -I repeat the back of the palm, not the palm- to a radiator pipe or a metalic tap cautiousely. By pressing further you get to a point that you feel the shock. Then make your hand separate. (You are using the back of your palm in case the electricity clinch your fingers then your hand becomes automatically separated from the point of contact.) Now you understand why for the electrical appliance there is no difference between Phase and null. The poor appliance is firmly attach in one side to the null (say to the tap). Null is safe as far as there is no phase on the other connector. Experienced electricians who have to work with live electricity, recognise the live with phase meter, red tape the naked lives and wear safety shoes to insulate them from the ground. For high voltage overhead lines they have to go into Farady cages to be separate from Zero- potential. Then like birds who safely sit on power-lines they can work with 400000 volts live lines. Perhaps you saw that Syvester Stallone could escape from prison by jumping and sliding on one of these lines (I wonder on the other side how could he come to the ground!) There are situations that you cannot work symmetrical with phase and null. For example you cannot put your switch on the way of a null line. It becomes dangerous, or in some cases like a door bell you frequently receive spontaneous ringing (phase always might find a null around and turns on the appliance). In Italy if you plug the fridge or the kettle in the other way, then by touching (with the back of your hand) the body of the appliance you feel a very faint feeling of the shock. Then immediately correct the plugging into the reverse of the previous connection. VDE now has become the backbone of EU electrical regulations that makes a new safer standard for southern European countries compulsary. When working with live electricity you must have a peer buddy around. You also should put on safety goggles and safety shoes and gloves. You also might like to read chapter nine of Feynman's Lectures on Electricity and Magnetism or Resnick-Halliday chapters on this subject.
A: The main disadvantage of this is that the power switch in the circuit must switch both lines since you don't know which one is live.
Even this is just a safety feature, the item works just as well whichever side the switch is on, but a switch on the neutral side would leave live voltages inside an 'off' piece of equipment. It's also difficult to fit a fuse in the equipment.
