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 :)
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.
so you say that when it is OFFed then the neutral is tied to ground right? That explains why i hear people saying that as long as it is off it is not dangerous! (touching the neutral i mean).
Oh and...in Italy they are exchangeable because you can insert the plug both ways in the socket. Does that make them completely identical?
It just means your appliances are not very choosy about their power inputs. Just like many DC devices do not care about plus/minus. They just correct that internally.
There even exist AC apparati that do care. Like that lamp that switches on/off when you touch it for example.
You still have three wires, don't you? A "hot" wire, a "cold" wire and a wire running to ground?
Dutch regulations have you put safeties in all of those nowadays.