In AC System design, When the flow direction changes, why doesnt the neutral wire become the live wire?

Thread Starter

2Pringles

Joined Apr 9, 2022
38
Im having great difficulty understanding why the Live/Neutral wire do not switch positions when the AC voltage direction changes?


If the flow is coming in through the Live wire, When the AC shift occurs, it seems to me to think of the flow now being pushed from the Neutral Wire whereas before it was being pushed through the Live?

Visually, What am I getting wrong here...
 

nsaspook

Joined Aug 27, 2009
13,554
First, the current only moves a tiny distance (fractions of an inch) in the wire between utility frequency power polarity changes so the net distance moved is zero.

The Neutral wire is a current carrying conductor "live". Depending on the power feed configuration like a US split-phase power feed, neutral might connected to Earth/Safety to set a reference point for potentials (HOT) in the circuit.
 

strantor

Joined Oct 3, 2010
6,837
Im having great difficulty understanding why the Live/Neutral wire do not switch positions when the AC voltage direction changes?


If the flow is coming in through the Live wire, When the AC shift occurs, it seems to me to think of the flow now being pushed from the Neutral Wire whereas before it was being pushed through the Live?

Visually, What am I getting wrong here...
I think you mean "live" as in "if you touch it, it will shock you." If that's what you mean, consider this:

You could put (20) 12V batteries in series For a total of 240VDC. If this is just batteries on a bench, the earth is not part of the circuit. You can measure from earth ground to any of the terminals and you will not read any voltage. Neither you or the earth is part of the circuit, nothing is apart from the batteries. It is safe to touch any of the battery terminals. Even if you connect a load across the 240VDC and cause current to flow through the batteries, it is still safe to touch any of the terminals. As long as you only touch one at a time. If you touch two terminals at the same time, you become a load, and current will flow through you.

Now in between batteries 10 and 11, connect a wire, and on the other end of the wire connect it to earth ground. Now earth is part of the circuit. And since you are standing on the earth, you will be too, if you touch any battery terminal. If you measure from earth ground to one end, you will see +120V and from earth ground to the other end, you will see -120V. And if you touch either end, you will get shocked because you will become a load through the ground wire, through the earth, through your body, and to the "hot" wire. But it is safe to touch the ground wire because it is at the same potential as you.

Now change these DC batteries to a transformer, and nothing above changes. In theory (not recommended in practice) since neutral is connected to ground, you can touch neutral because it is at the same potential as you. Current will not flow through you. And if you could levitate in the air so that you are not part of the circuit with earth, it would also be safe to touch either "hot" wire, for the same reason it was safe to touch any battery terminal early in the example. Bird and linemen in helicopters prove this concept all the time.

So neutral is "live" but not in the sense that "if you touch it, it will shock you," because you and neutral are always at the same potential.
 

Sensacell

Joined Jun 19, 2012
3,502
The power transformer is a "floating" voltage source, the Neutral line is connected to earth ground at the panel.
This makes the neutral "safe" relative to ground, with the other two legs being 'hot"
Current does flow in this line (when the two hot legs are unbalanced) but it should never be far from ground potential.
 

MaxHeadRoom

Joined Jul 18, 2013
28,827
Im having great difficulty understanding why the Live/Neutral wire do not switch positions when the AC voltage direction changes?


If the flow is coming in through the Live wire, When the AC shift occurs, it seems to me to think of the flow now being pushed from the Neutral Wire whereas before it was being pushed through the Live?

Visually, What am I getting wrong here...
Before being referenced to Earth GND either conductor of a 2 wire AC supply are both identical in nature, when arbitrarily, one is connected to earth, nothing changes, with the exception, that particular conductor has a second reference point.
In the case where equipment is supplied with a control isolation transformer, one side is permitted to be referenced to earth, creating a neutral conductor.
 

crutschow

Joined Mar 14, 2008
34,842
Neutral just means that designated wire is connected to, and thus has no significant voltage, to earth ground.
Hot means that wire is not connected to earth ground, and thus does have significant voltage to earth ground.

That has no effect on the voltage between the wires, which is what determines the flow through the load connected to both wires, and the direction of the current through the wire.

Current is not "pushed" or "pulled" it flows due the the potential difference (voltage) between the two wires, causing an electric field along the wire. This field is what moves the electrons.
 

ErnieM

Joined Apr 24, 2011
8,382
Here is a HUGE image of an AC source.
AC.jpg
So as time goes on the line voltage will rise to a positive max, decay to zero, then fall to a negative minimum, back to zero, and repeat.

But what is zero?

That is the neutral wire, which is also connected to earth ground. So I hope you can see when the line voltage is "negative" with respect to neutral then (somewhat confusing here) the neutral is positive with respect to the line. It is the change in sign of the line voltage that makes the AC work.

And by connecting neutral to ground we make one more wire safe to accidentally touch.
 

MaxHeadRoom

Joined Jul 18, 2013
28,827
In N.A. residences, the grounded conductor in the panel, completes a path back through GND to the supply transformer where the C.T. of the supply transformer secondary is also connected to Earth GND.
In the UK the Neutral is supplied from a star point of a 3ph transformer. The 'live' conductor is derived from a single phase.
This star point is also connected to earth ground.
 

nsaspook

Joined Aug 27, 2009
13,554
All of the above means the neutral is a 'live' wire. 'Live' in the context that the electrical potential of the circuit is active on that wire. If by accident, failure or miswire, that neutral wire is opened on an active and energized circuit, it's possible that at least some of the line potential will be across that disconnected wire.
 

PeteHL

Joined Dec 17, 2014
478
Current is not "pushed" or "pulled" it flows due the the potential difference (voltage) between the two wires, causing an electric field along the wire. This field is what moves the electrons.
Current produces a magnetic field not electric, isn't that true? Also the magnetic field is the result of current flowing through the wire, not the cause of the current flow.
 

MisterBill2

Joined Jan 23, 2018
19,424
Back to the original question, since I just read the thread so far, the reality has been stated a few times, here comes one more explanation: THE NEUTRAL WILL NEVER BE THE HOT SIDE BECAUSE IT IS SOLIDLY TIED TO GROUND!!!! THAT is how "Neutral is defined! So unless there is a real fault, or major system damage, neutral is always solidly tied to earth ground. There might be local conditions where the connection is not perfect, but it is usually "OK" even at those times.
 

BobTPH

Joined Jun 5, 2013
9,272
I think the confusion of the TS is that he thinks live and neutral have something todo with direction of current flow. They do not.
 

MaxHeadRoom

Joined Jul 18, 2013
28,827
Back to the original question, since I just read the thread so far, the reality has been stated a few times, here comes one more explanation: THE NEUTRAL WILL NEVER BE THE HOT SIDE BECAUSE IT IS SOLIDLY TIED TO GROUND!!!! THAT is how "Neutral is defined!
Neutral is also the term for the star point of a 3ph transformer.
IOW it does not necessarily indicate connected to earth GND. ;)
Also in N.A. the C.T. secondary is termed 'Neutral'. before Earth grounding

1676562524629.png
 
Last edited:

MrChips

Joined Oct 2, 2009
31,083
For all residential, commercial, and industrial power systems, current must flow in a closed loop.
The NEUTRAL wire is the return path to the electrical service panel.

For all intents and purposes, the NEUTRAL wire is "live" and carries current. The NEUTRAL wire reads 0V because it is bonded to EARTH or GROUND.

Do not confuse NEUTRAL with the EARTH connection which must carry no current. This is there for electrical safety.
 

MaxHeadRoom

Joined Jul 18, 2013
28,827
Do not confuse NEUTRAL with the EARTH connection which must carry no current. This is there for electrical safety.
Earth can and does carry current in the case of a ground fault.
At the time of my training in the UK, we had to measure the ground resistance back to the Supply transformer earthed grounded neutral.
IIRC , it was to be below 2.5Ω , in N.A. the NFPA and IEEE recommend a value of <5Ω.
 

MrChips

Joined Oct 2, 2009
31,083
Earth can and does carry current in the case of a ground fault.
As I said, EARTH wire is not necessary (for conduction of electrical power from the source). It is there for safety reasons and is required to comply with electrical code. The EARTH connection is there to detect a fault condition which must then be corrected. After the fault is corrected, no current must flow in the EARTH wire otherwise it is still a fault condition.
 

nsaspook

Joined Aug 27, 2009
13,554
As I said, EARTH wire is not necessary (for conduction of electrical power from the source). It is there for safety reasons and is required to comply with electrical code. The EARTH connection is there to detect a fault condition which must then be corrected. After the fault is corrected, no current must flow in the EARTH wire otherwise it is still a fault condition.
Unless that Earth connection is for conduction of electrical power from the source.
https://curtinmaritime.com/projects/sylmar-ground-return-system-ladwp/
 
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