Why is there actually zero in the socket, grounding of the housing ...

Could someone explain to me the common sense of the principle of zero operation in the socket? Why also, despite the fact that the device has zero, sometimes it is also connected to a separate ground?

Hello.
As you know, there is 230V AC in the socket.
Through the municipal power grid, it is supplied to your socket from a transformer, which has the neutral wire connected to the ground and to the "N" wire in the socket. The current flow is conventional and runs from the "L" phase conductor to the "N" neutral conductor. On encountering resistance, it turns into heat energy, light ... Now that you have a phase "L" wire connected to the housing, there must be an alternative flow path for the current. The protective conductor (N-PE) is connected to the housing and the current flow in this path is to cause activation of the protections. Anyway, the current flows through the body of the person who touches the housing.
greetings

I'm a bit dark because I didn't get any of it ....

Zero in the socket and grounding (protective conductor) are two things.
You have two holes in the socket and a pin in the middle. in one hole there is phase (L) brown or black cable, in the other zero (N) blue cable. As you know, Prad is always looking for the shortest way to earth that is, let's call it, conventionally, it flows from the L conductor and flows to the N conductor which is in the ground. Between these two holes you plug in a device and the current flows from the conductor L to N through this device and the device then consumes electricity. Grounding is to connect the third wire (PE) yellow green cable, it is connected to the pin in the socket.
Once a bridge was used from the blue pin to the pin, now a separate cable is connected, which you can say is connected to the ground, which gives additional protection.
The metal case of your computer, for example, is connected to this pin. And when a puncture occurs and you touch the casing, the current will look for the shortest path and run into the metal casing connected to the pin (earth) instead of flowing a longer way through your body.
The cable has a lower resistance than the human body, therefore the current will take the shorter, easier path.
I hope that somehow I helped.

ok now I really caught it, well yesterday I was with yesterday ...
that is, the device not connected to the pin can hit the trail? for example, people often turn off the ups from the socket, checking how long they can last?

It is not about the device, but about the human.
A man could have hit if there was a puncture.
Imagine connecting the cable (L) from the wall socket directly to the metal casing of your computer.
It is easy to guess that if you touched such a housing, it could be a painful experience.
If this housing is connected to a pin, the current will flow to that pin and blows the fuses just like you would make a short circuit on the cables in the socket.
The fuses disconnect the power, and you can feel safe touching the housing at that moment.
It is all about human protection.

Normally, the electric (utility) current flows through the L (phase, L-live "live") and N (neutral) conductors that run from your receiver (lamp, TV, iron) to the transformer in a substation reducing the voltage from the transmission value (e.g., 15000V ) to use value (230V).
However, it may happen that the phase wire rubs somewhere and the 230V phase voltage appears on the housing of the device. In this case, touching such a housing may pose a risk of electric shock when, for example, with another limb, we touch something grounded (tap, sink, gas stove, radiator, earthed metal structure of the building - due to water spilled on the floor, in which we are unfortunately standing).

However, if the housing is grounded via PE (Protective Earth) conductor, this current will be led back to the ground. Usually, residual current devices are installed at the input of the installation, which check whether the current "influenced" by the L conductor is equal to the current "flowing" from the installation through the N conductor. ) this disconnects the installation in the shortest possible time (

mass is also marked with GND (Ground)

The mass in electronics with direct current is marked with GND (ground), with alternating current it is the PE conductor and it is marked with such a vertical line and horizontal lines at the bottom.

The earthing of the housing and all metal parts that are not part of the electric circuit is not only intended to protect against a direct short-circuit to the phase conductor housing (230V potential) or current leakage through, for example, damp insulation. Another reason for grounding enclosures and metal parts is to divert any currents inducing on metal parts from transformers, power supplies, chokes, motors, etc. To reduce the risk of electric shock in industrial conditions, equipotential bonding is also used.

In that case, I do not understand, why connect one wire from the transformer to the ground? Why artificially create "phase" and "zero" ?? If we lead two "hanging" wires from the transformer, then the current would flow if and only if it was connected to both of them simultaneously. There would be no 'privileged' phase and zero - just the tension would be just and only between the two conductors. There would be a situation that we have a phase and we close the circuit through the earth, so it would be safer? So I don't understand, what's all this for, grounding one of the power cords, complicating it? And there would be no need for any additional grounding of the device housings, because how would we close the circuit - if it was not for sure?

Luca wrote:

If we lead two "hanging" wires from the transformer, then the current would flow if and only if it was connected to both of them simultaneously. There would be no 'privileged' phase and zero - it would just be tension between the two conductors. There would be no situation that we have a phase and we close the circuit through the earth, so it would be safer? So I don't understand, what's all this for, grounding one of the power cords, complicating it? And there would be no need for any additional grounding of the device housings, because how would we close the circuit - if it was not for sure?

But there are networks and it's called IT. And only the PE conductor is connected to the ground and the conductive parts.
And this network is used, for example, in hospitals, although we also have one in my company.
And in the TN-S network system, thanks to this "zero", a large short-circuit current flows after the insulation damage, capable of disgusting the short-circuit protection, i.e. a fuse or an overcurrent circuit breaker.
http://pl.wikipedia.org/wiki/Uk%C5%82ad_sieciowy