[SOLVED] AC Current Direnction in Earth Wire

AC Current Direction in Earth Wire

We have studied about AC that it changes its direction. That is why we do not use polarity sign in AC.But in some books a + sign is used with AC source.
My first question is if current changes it direction then both wire will work the same. Then why do we use term Live & Neutral wire.

Secondly, if we connect the neutral wire to earth the current will flow.The question arise here, AC changes it direction but how will current change its direction connected to earth because only Phase wire is connected to AC source. I mean the earth serve as sink.

Hello,

https://en.wikipedia.org/wiki/Earthing_system gives information on grounding systems. + and - signs are to indicate the polarity of the source. A current can have a graph like sin(wt), but when you reverse the source, it will become -sin(wt).

If you connect two AC voltage sources (same frequency same phase and same amplitude) in series, you will measure 0 or twice the voltage of a single source. It al depends on the phase relation between the two sources. you can check this with a transformer with two seperate secondary low voltage outputs (so that means you have at least 4 secondary wires coming out of the transformer). Reversing one of the outputs in a series circuit, will change the output voltage of the series circuits due to the phase difference issue.

In an ideal situation, there would be no voltage between N and PE (protective earth). However as PE is only used for safetey purpose, there is low current in the PE wire (hence very low voltage drop across the PE wire). However the N wire (that is finally also connected to mother earth) carries the return current from the users. It may experience a voltage drop due to the resistance of the N wire.

Touching the Live wire can give you a shock if the current through your body can find a way to earth (or the N wire). Touching the N wire will not give you a shock, as the voltage between N and ground (earth) is low.

To reduce the voltage drop across the N wire, same N wire in the cable in the ground is used for three Live (L) wires. You may know that voltage on each L wire is 120 degrees shifted in phase. When all three L wires carry same current, the net current in the N wire will be zero. You can convince yourself by vectorially adding three same length vectors with 120 degrees phase difference (the result will be zero).

You have given the answer I am confused about that neutral wire has low current. It is true for DC because one terminal always has high potential while other terminal has 0 potential. What I understand about AC from books that both terminals of AC go to high potential alternatively i.e first terminal A goes to high potential while terminal B has low potential then direction reverses terminal B goes to high potential while terminal A has low potential. This is what we call a sine wave. The question arise here how can one terminal of AC always be on low potential while AC changes it current direction 50/60 times per second.

The neutral could also be called a 'return wire'.

It is the center tap on the transformer. (I'm speaking about conventional residential house wiring.)

It will not shock you if you hold it and touch the faucet at the same time. In that sense it is neutral, or a ground.

However when a load is attached to either the +120V or the -120V supply, the neutral wire will indeed have electrons going back and forth in normal AC fashion, precisely as our understanding leads us to expect.

tahir4awan:

Voltages are always values compared to some 0-point. In DC sources the same 5V source can give you either -5 V and 0 V or 0V and +5V, that's simple, right? If you consider an AC (giving a signal of A*sin(x*t)) source you can calculate/think of it in different ways:

1) Again one end is considered as 0 V, no matter what the source gives you, as in DC case. Then you can calculate that in the other end is a A*sin(x*t) signal.

2) Or you can choose the 0 V point be in the middle of the source (as in a transformer that has 3 output leads on the secondary winding). In that case both ends of the AC source give the same signal, with a phase difference of 180 degrees. That means that signal on one end is inverted in the other end (that's why there's a "-" sign). Now the signals would be A/2*sin(x*t) and -A/2*sin(x*t). Calculating with actual voltages and RMS-values is a bit different.

3) You can also have a inverted case of the first case, or whatever level to be chosen as 0V. But case 1) gives probably the easiest formulas for calculations and that's why it's often used.

Exactly the same current goes through the AC source from both sides in either case. the current is dependent on the rest of the circuit. Either side can be considered neutral however you please. And you can touch either end (Live or Neutral) if you keep isolated from the other end. You touch the neutral end probably very often (here in Finland every time you touch kitchen sink or whatever). If you were a bird, you would touch the live wire more often, as you would enjoy sitting on a non-isolated power cable.

tahir4awan said:

You have given the answer I am confused about that neutral wire has low current. It is true for DC because one terminal always has high potential while other terminal has 0 potential. What I understand about AC from books that both terminals of AC go to high potential alternatively i.e first terminal A goes to high potential while terminal B has low potential then direction reverses terminal B goes to high potential while terminal A has low potential. This is what we call a sine wave. The question arise here how can one terminal of AC always be on low potential while AC changes it current direction 50/60 times per second.

Click to expand...

In a mains systems with L(ive) and N(eutral), only the L wire has voltage with respect to protective earth (PE).

The voltage BETWEEN the A and B terminal changes direction, hence A goes above B and the other way around every period. However when you connect terminal B to ground, there will be no voltage between B and ground (as there is a wire in between with low resistance, so the voltage drop across that wire is negligible). when B is conected to ground, A goes below and above ground potential every period.

Take a 1.5V battery, connect the case (minus) to ground and measure the voltage between ground and + terminal. It will be 1.5V (black wire of voltmeter to ground, red one the the +terminal of the battery). Now flip the battery upside down, but don't flip the voltmeter wires. The + terminal is now connected to ground and the case of the battery (minus) is connected to the red wire of the voltmeter. You will measure -1.5V.

In a single phase system, the current through the L wire equals the current through the N wire (of course opposite sign). There will be no current in the PE wire (assuming no equipment malfunction).
Only in a three phase system with same load on al phases (L1, L2 and L3), have zero N wire current. You may forget three phase as this may be confusing for you.