AC and neutral

Thread Starter

ilium007

Joined Aug 24, 2013
132
I have wondered about this for years and decided today to research further. In a mains AC installation neutral is tied to ground (here in Australian anyway). Everything I have read states that neutral provides a return path for current flow. I am confused as to what happens during the negative half of the AC sine wave? How can the neutral line 'source' current? Does current flow from neutral to active during this period? When current is generated in the power station how is the negative half of the sine wave generated?
 
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nsaspook

Joined Aug 27, 2009
8,689
Current (a rate) is the flow of charge, it's not electrical energy. All of the charge (electrons) in the wring (including the neutral) just moves slightly back and forward as AC current during power flow as 'real' electrical energy moves only one way, from the source to load. The power station delivers (sources) electrical power using charge separation (inside the generator) for a time&polarity changing electric potential (AC voltage) that results in a circuit EMF using the existing charge in wiring as a system to move electrical energy using electromagnetic fields.

8:00 for an important point.
 
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dendad

Joined Feb 20, 2016
3,914
Look at it like the current is first "pushed" from the power station down one wire, Active, and the return is the second wire, Neutral.
Then, on the second half of the AC cycle, the current is then "pushed" from the power station down the Neutral, and the return is the Active.
The load does not do the "pushing".
Does that help?
 

MaxHeadRoom

Joined Jul 18, 2013
23,652
I have wondered about this for years and decided today to research further. In a mains AC installation neutral is tied to ground (here in Australian anyway). Everything I have read states that neutral provides a return path to ground for current.
Your system is the same as UK, the neutral does not provide a 'return path to ground' the return path is normally through the neutral conductor, which in essence is no different from the live conductor, it just happens to be taken to ground at the service station in order to provide a return in case of a fault, otherwise there is no return current through earth ground.
Max.
 

Thread Starter

ilium007

Joined Aug 24, 2013
132
Your system is the same as UK, the neutral does not provide a 'return path to ground' the return path is normally through the neutral conductor, which in essence is no different from the live conductor, it just happens to be taken to ground at the service station in order to provide a return in case of a fault, otherwise there is no return current through earth ground.
Max.
Sorry, I typed the original post in a hurry. I meant the neutral provides a return path for current flow, the dedicated ground wire provides a fast path to ground in the event of a fault.
 

Thread Starter

ilium007

Joined Aug 24, 2013
132
Look at it like the current is first "pushed" from the power station down one wire, Active, and the return is the second wire, Neutral.
Then, on the second half of the AC cycle, the current is then "pushed" from the power station down the Neutral, and the return is the Active.
The load does not do the "pushing".
Does that help?
But how is the current “pushed” on the negative cycle of the sine wave? It’s like it is “pulled” by the generating station.
 

dendad

Joined Feb 20, 2016
3,914
But how is the current “pushed” on the negative cycle of the sine wave? It’s like it is “pulled” by the generating station.
It is just the same, but reversed polarity.
Think of a battery connected to a lamp. It lights.
Then, swap the battery around, and the lamp still lights.

I s'pose you could say it is "pushed" and "pulled" if you are just looking at it all referenced to the same single point for both cases.
Pushed down one wire, and pulled back on the other. Then they swap over for the next half cycle.
 

boostbuck

Joined Oct 5, 2017
59
If you think of the generator spinning in the power station, it has two wires coming out of it, and these swap polarity, plus to minus, at 50 or 60 Hz. There is no Active and Neutral. Connecting these wires to a load will deliver power. Current flows one way, and then the other.

If one wire is connected to the ground, nothing changes, except there is a big lump of stuff connected to one wire.

It is convenient for you to regard the wire connected to the ground you are standing on as Neutral, and hence the wire not connected to you as Active, since it will feel active, and the one connected to ground quite neutral.

But the flow of electrons hasn't changed. Delivery of power to the load is still the same even though you are now connected to one wire.
 

nsaspook

Joined Aug 27, 2009
8,689
But how is the current “pushed” on the negative cycle of the sine wave? It’s like it is “pulled” by the generating station.
You need to adjust your mental picture of electrical energy for things to make sense. No electrons are transferred from the source to the load with AC power. If they did there would be a huge charge imbalance on a massive scale.

A DC version of energy transfer. Energy travels down both sides of the circuit wires.
DJN0c.png

In a AC circuit the 'battery' polarity reverses but energy flux still travels down both sides of the circuit wires. Neutral is just another conduction wire.

The generating station produces a changing electrical potential (positive/negative electric field) across points that repulses or attracts (push/pull) the free charges (negative charged electrons) in the wire in reference to the applied potential. A EM field travels down between and around the conductors of the transmission wires at near light speed causing the wires free charges to rock slightly with the changing polarity and potential of the moving energy. This is the AC current that gives rise to at changing magnetic field that also moves at near light speed while the electrons, averaged over time, remain stationary while AC power (energy flux) always moves from source to load at nearly light speed.
 
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crutschow

Joined Mar 14, 2008
27,765
Neutral is just a name for one of the wires with the other often labeled hot.
But both wires serve a similar function, that is, to provide a complete path for the electrical current from the source to the load.
Since there is no significant current normally flowing in the neutral connection to ground, then the earth connection has no effect on that, the current through the neutral operates under the same rules as the hot.
The only difference is the relative voltage between hot and ground, and neutral and ground.

Also whether on wire is positive or negative with respect to the makes no difference.
You seem to be trying to make some kind of distinction between them as regarding the flow of the current, but there is none.
 

nsaspook

Joined Aug 27, 2009
8,689
The video makes it all so much clearer now. I’ve learnt something new here today!
Glad it helped, it's tricky to find media that clearly informs about simple circuits in a physics consistent matter. As the video explains current (electrons) is very slow moving in good conductors under normal power transmission conditions. Normally we want the physical charge carriers of current to move as slow as possible to optimize the transfer of electrical energy from point to point. Larger good conductors have more free charge carriers so they can move slower (reducing energy consumption) at the same current levels. When current moves 'fast' it consumes electrical energy by converting it into mechanical kinetic energy as the mass of the charge carrier accelerates. This mechanical 'resistance' energy is normally dissipated as heat in the circuit.
 

andrewmm

Joined Feb 25, 2011
1,757
There was a post on this a few months ago , but I can't find it.

Take basic case first.

If you think of the generator, then that has a two outputs , A and B across which is generated a sine wave. If you put a multi meter across A and B , then you would se a sine wave going +ve then -ve.

That gets fed to the load ( house )
at the house, the two wires come in, and present a sine wave just the same. Put a meter across the two wires, and you see a sine wave going +ve and then -ve.

Now if at the generator, the B side is connected into the ground, at the house, the signal will just be the same,
if we now connect B to ground at the house as well, then the signal will still look the same
A sine wave across A B , going +ve then negative.

Its only if you measure from "ground" to just a or B then you will see different.
B will be basically "zero"
A will be going above and below ground.

And as "ground" is an infinite current sink /source,
then if the generator and the load are both connected to ground on one side, no second cable is in theory needed.
 
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