I assume you're speaking of mains power AC. As in from the wall socket. Probably 230V in Italy.
The distinction between "live" and "neutral" is not an indication that is would be safe to touch the one (neutral) but not the other. They both kill you dead!
It is however relevant to electricians installing the cables. And to the power people how set the electrons flying through them. I think it has to do with the potential in these wire compared with earth. The neutral will pretty much stay neutral, but the live wire will alternate from high to even higher and back. Not sure how high that is compared to earth, but on average it registers as 230 V AC.
I always try to remember that the electrons in an alternating current are not sloshing back and forth through the circuit, but rather they run and crawl, run and then crawl again and again.
How's that for a non scientific (almost artsy) explanation?
I always thought it was "line" not "live". And, isn't there something to do with three-phase supplies that average out to make neutral neutral.
I did some reading about AC a few months ago. My understanding is that in a regular US home, with 120v AC, the neutral (white) wire is supposed to be at 0v, and yes, at the fusebox where the power comes into your house, the neutral line is tied to ground. The line (black) wire alternates between about +170v and -170v relative to the neutral wire. Since it's a sine wave, the average (Root Mean Square) voltage is 120v, but since it sweeps from +170v to -170v, the total peak-to-peak voltage swing is 340v. But as long as everything is working correctly, it shouldn't hurt you to touch the neutral wire, because it is supposed to stay at 0v, the same as ground.
The issue comes when things aren't working correctly. If the ground were to get disconnected, either at the switch box or inside an appliance, it could be possible for the neutral wire to be at a voltage other than 0v relative to ground.
And yeah, the purpose for the grounding wire is so that if a wire were to break inside an appliance, and a line wire were to come into contact with the metal housing of the appliance, if that housing weren't grounded, then there wouldn't be any short circuit or anything, so no breakers would trip, and the metal housing would happily sit and wait at 120v until someone came and touched it while touching the ground -- then suddenly the current would run through their body and into the ground. So the metal case of the appliance is grounded, so that if a line wire comes into contact with the metal housing, it immediately shorts to ground and trips the breaker. As Wikipeda says:
A third wire, called the bond wire, is often connected between non-current-carrying metal enclosures and earth ground. This conductor provides protection from electric shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools. Bonding all non-current-carrying metal parts into one complete system ensures there is always a low impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault. This low impedance path allows the maximum amount of fault current, causing the overcurrent protection device (Breakers, fuses) to trip or burn out as quickly as possible, bringing the electrical system to a safe state. All bond wires are bonded to ground at the main service panel, as is the Neutral/Identified conductor if present.
An interesting little side note I came across while doing all this research is that in some places, especially where cost of power transmission is a factor like when wiring up remote villages etc, they sometimes use single-wire power transmission called Single Wire Earth Return, where they only run a single power wire, the line wire, and they use the actual planet Earth as the return conductor:
The SWER line is a single conductor that may stretch for tens or even hundreds of kilometres, visiting a number of termination points. At each termination point, such as a customer's premises, current flows from the line, through the primary coil of a step-down transformer, to earth through an earth stake. From the earth stake, the current eventually finds its way back to the main step-down transformer at the head of the line, completing the circuit.
That just seemed amazing to me :)