Difference between Line and Neutral in AC

I want to find out what is the difference between the AC (220 VAC) lines, (phase and Neutral).

• As I know that the Alternating Current (AC) has no polarity, so why do we have a "phase" line and a "Neutral" line ?!

• Why one of them (the phase line) would make danger on human if touched, while the other doesn't ?!

• I also want to know, what's meant by "Analog Ground"? .. is it the Neutral line ? or what ?!

The voltage is a difference between the electric potentials of two conductors. Hence, to change voltage, only one of the potentials has to change (although both can). In AC power only one of the wires (live/phase) changes it's potential, while the potential of the other one (neutral) remains constant.

In the picture above, the orange horizontal line represents the potential of the neutral line (marked as zero for convenience), while the blue curve shows the constantly changing (in relation to zero) potential of the phase line.

Since in properly constructed power network the neutral wire is maintained at a potential level close to ground potential, there is nearly no voltage between the neutral and the ground. Hence, touching neutral will not cause current to flow through human body into ground.

Live line, however has a potential that rapidly changes from highly positive relative to ground potential, to highly negative. This difference in potentials (voltage) of the conductor you're touching with your hand and the one you're standing on causes a current flowing through you and at typical outlet voltages can be very deadly.

Analog Ground is a reference to a constant potential wire, that all other signals (voltages) relate to. Is is what you name your "0" when measuring other signals. For example, in most battery-powered devices it is the wire connected directly to the negative terminal of the battery. Naming "ground" or "0" is a matter of convenience, however in outlet-powered applications the designations are often separate, since "ground" is electically connected to actual ground.

Your phase is just one of three in a 3-phase network.

In the diagram the neutral is at the center, and each arrow represents a phase vector. In this case the mains voltage is 120 V, and when you look at only one phase you could invert the arrow and still have a 120 V sine. But the phases aren't just related to the neutral, they're also used with respect to the other phases. If you would measure the voltage between the A phase and the B phase you'd find that it's $\sqrt{3}$ larger than 120 V, or 210 V. These voltages are often used in industry where 3 $\times$ 210 V will give you more power at lower currents than just 120 V. Under a balanced 3-phase load there won't flow any current through the neutral, while there will be current for each phase.

Analog ground in a circuit is a reference voltage. Voltage is relative and is only meaningful when you say what other voltage you compare it to. So in a circuit we choose a reference level to measure all the other level against, and we call that "ground". It will often be the lowest voltage in the circuit, so that all measured voltages are positive, though you may also have a dual voltage supply of for instance + and - 15 V, symmetrical about ground. "Analog ground" is the reference for the analog part of your circuit, but should be the same as the digital ground. The difference is used during PCB layout to keep noise from the digital part away from the analog part.

The Alternating Current do not have polarity, but Live wire is dangerous and neutral wire can safely be touched. The reason is that, the neutral is the point at which all electrically balance voltage which is 120 degree phase shift from each other is connected together. You can resolve the vector sum of the voltage of three phase given in the picture, assuming that all three phases are balance. You will get Neutral voltage zero. I was not able to post image. You may email me if you want.

• "can safely be touched" That depends on the system you are connected to: TN, TT or IT. On a TN system, the N is grounded "everywhere", while on a TT system, it is grounded only on the generator or transformer. This means that yu can potentially have dangerous voltages at the N. – glglgl Nov 26 '14 at 9:10