[SOLVED] Can someone explain ac system with grounded neutral?

Thread Starter

Simon Pressure Jet

Joined Oct 28, 2020
13
I’m trying to visualise how current flows from neutral to load in a typical ac system.
My understanding is (and probably wrong?) is that during the negative half cycle electrons flow from the line to load, with equal current flowing back through the neutral.
What I can’t seem to understand is how during the positive half cycle electrons flow from load to line when the neutral is grounded and said to be at 0 volts. How, if the neutral is at earth potential, do electrons move this direction with the same current flow when there is no voltage behind the charges?
I hope someone can see my confusion and clarify for me what I’m missing.
 
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MrChips

Joined Oct 2, 2009
25,938
Current flow requires a potential difference and a path to flow (loop).
AC line current comes from the electric utility and must return to the utility in order to complete the loop.

On the negative cycle, electrons flow from LINE to the load and back to the utility on NEUTRAL.
On the positive cycle, electrons flow from NEUTRAL to load and back to the utility on LINE.

It does not matter that NEUTRAL is at earth potential (which it is always because it is bonded to EARTH at the distribution transformer supplying your house).

What drives the flow of electrons is the potential difference between LINE and NEUTRAL which reverses at LINE frequency.
 

ericgibbs

Joined Jan 29, 2010
15,378
hi Simon,
Welcome to AAC.

When they say, in your example, when Neutral is at 0V, it means the Neutral wire is connected 0V/Ground.
The path of the current from the power gen alternator, travels out along the Line wire thru the Load and the return path is via the Neutral.
The current flows inthe opposite direction on the negative half cycle from the alternator.

At the alternator the Line is above 0v/Gnd and the Neutral is connect to 0V/Gnd

There is always a pair of wires L and N between the Alternator and the Load

Do you follow OK.?
E
Image UK Single Phase supply 50Hz
 

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Chiho9

Joined Oct 18, 2020
21
Firstly, neutral is always at 0volts(Ground).
The ground(0v) simply refers to common ground. that is the part which you can actually touch.
During the positive cycle the electrons flow from the neutral to the line. By electron flow of current the flow of electrons Start's from the negative terminal of the power source. But by conventional flow it starts from the positive terminal. This shows that there is still a PD flowing along the loop between the neutral and line to allow same current to flow given that the neutral is at 0volts.
 

BobTPH

Joined Jun 5, 2013
4,779
I’m not understanding why you see a diffeence between the negative and positive halves of the cycle. You say that on the positive half, the neutral side is at 0V. But that is also true in the negative half cycle.

If you think having the negative point in a circuit connected to ground would stop current flow, try taking a battery powered circuit and connect the negative terminal to ground. Does the current stop flowing?

Also, You have to stop thinking of potetial as an absolute number, it is not. It is an arbitrary choice.

Bob
 

Thread Starter

Simon Pressure Jet

Joined Oct 28, 2020
13
I’m not understanding why you see a diffeence between the negative and positive halves of the cycle. You say that on the positive half, the neutral side is at 0V. But that is also true in the negative half cycle.

If you think having the negative point in a circuit connected to ground would stop current flow, try taking a battery powered circuit and connect the negative terminal to ground. Does the current stop flowing?

Also, You have to stop thinking of potetial as an absolute number, it is not. It is an arbitrary choice.

Bob
Hi Bob. Thanks for taking the time to reply. So based on what you said am I right in thinking that electrons will flow from a region of low potential (more number of electrons) to a region of higher potential (less accumulation of electrons)? This potential difference is what pushes and pulls the flow of charges?
 

MrChips

Joined Oct 2, 2009
25,938
Also someone in the past tried to be "scientifically correct" by changing physics and electrical/electronics curriculum by replacing current flow with electron flow. This has led to a lot of confused students.

Stick with current flow and your life will be a lot simpler. Current flows from higher potential to lower potential.
In most applications of electricity and electronics there is very little need to be thinking in terms of electrons.
 

MaxHeadRoom

Joined Jul 18, 2013
25,019
Firstly, neutral is always at 0volts(Ground).
The ground(0v) simply refers to common ground. that is the part which you can actually touch.
Not quite true, in the case of a 3 phase star connected secondary you have single phase from any one phase to the star neutral, which may, or may not be grounded, its still a neutral.! ;)
In the case of N.A. the residential supply neutral is the 0v or centre tap of a 1ph transformer secondary that happens to be earth grounded in practice for safety purposes.
In instances where a 120v/240v is supplied locally from a H.V. to L.V. transformer either termination of the secondary can be connected to earth ground, this then becomes the neutral.
Max.
 

crutschow

Joined Mar 14, 2008
29,528
There is a slight voltage (potential difference) between the neutral side of the load and the neutral ground connection.
It's equal to the load current times the wire resistance between those two points.
You can likely measure that small voltage between the neutral and ground connection at the outlet for a large load (e.g. space heater).
 

MaxHeadRoom

Joined Jul 18, 2013
25,019
I’m trying to visualise how current flows from neutral to load in a typical ac system.
Just ignore the term neutral for a moment and the fact it may be grounded and you have two identical conductors to supply the 120v or 240v AC with the identical current in each.
No difference between either.
Just because one is then selected to be connected to earth ground changes nothing, one conductor is just using earth ground as a reference point which is not normally intended to carry current.
Max.
 
For the moment ignore the ground to neutral bond. let's just concentrate on the secondary of a 240V center tapped transformer 120(L1)-0(N)-120(L2) VAC. L1 and L2 are 180 degrees out of phase relative to each other.

If all of your loads were 240V, there would be no current at the center tap.

If the L1 and L2 loads were balanced, there would be no current through N.

N really carries the unbalanced current of L1 to 0 and L2 to 0.

We can also use a notation like 5A<-90 for L1 and 2A<+90 for L2. The < sign isn't a < sign, pretend the bottom is flat. So, 1A at an angle of -90 degrees. So, in this case the N has 3A at an angle of -90 degrees relative to N.

AC really doesn't have a polarity, but the measuring devices do. They have a phase relationship. So, current transformers have a dot on them marking the primary and secondary phase relationship.

The dot of the primary is in phase with the dot on the secondary. The AC waveform follows each other.

If you had a 6 VAC transformer and a 24 VAC transformer, you can put the secondaries in series and get (24-6) or 24+6 volts. Knowing the relative relationships of the primary and secondary dots, you can get the right voltage you want the first time.

==

Ground is a reference and a place for fault currents to go. Metalic water pipes, stovetops washer/dryers exposed surfaces should all be at the same potential to avoid shock. Cable TV needs ground as a reference too. Telephone has a "protector" to ground which SHOULD be located near the service ground. If that ground "potential" gets raised because of a lightning strike to the telco cables, the entire reference to the house "changed", but no effects to the house metalic water pipes, stovetops washer/dryers exposed surfaces. A shock hazard doesn't exist.

At only one point when the service enters the house is neutral and ground bonded to earth. Any sub-panel within a structure will keep ground and neutral separated. A sub-panel in a detached structure is a little different. Ground and Neutral stay separated, but another ground rod to earth is installed.
 
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MaxHeadRoom

Joined Jul 18, 2013
25,019
The neutral is also bonded to earth ground at the transformer C.T. itself.
In some jurisdictions it used to be the case where the N was connected to earth GND at the transformer but not at the service panel, and not in-between as it is now.
A ground rod or metalic water pipe was used for the GND at the panel.
In this case, if a ground fault occured, the fault current actually traveled through ground back to the source.
A ground resistance test had to be performed to ensure a low enough ground resistance existed.
Max.
 

sparky 1

Joined Nov 3, 2018
706
The definition for the abbreviation AC is alternating current. It takes both the potential difference and the current flow to define a system using Ohms law as a fundimental basis. By comparing AC to DC it becomes more clear when and why a particular problem solving method is used. The unidirctional DC uses the DC Ohms law and bidirectional flow of current uses AC ohm's law.

Taking an over simplified approach can leave voids in AC Ohms law. This makes it more complex than it needs to be.
By comparing the potential difference of DC to AC we find the necessary methods using AC analog to Ohms law.
By mastering AC Ohms law it becomes one of the most essential tools needed to properly explain an AC system with a grounded neutral.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/acohml.html
 
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crutschow

Joined Mar 14, 2008
29,528
Many years ago, my mom lived in Wisconsin in a house with a faulty connection from the ground rod to the mains ground.
A very close lightning strike generated an arc (I don't remember from where) to a copper water pipe near the water heater with sufficient current to burn a small hole in the pipe, creating a leak.
After that happened they investigated and found the faulty ground connection.
 

MisterBill2

Joined Jan 23, 2018
11,637
The "zero volts" on the neutral line is because that neutral, usually grounded, is the reference for the voltage. With a portable generator set that has not been intentionally grounded, the voltage between the line terminal and ground is close to zero, and likewise the voltage from the generator neutral terminal to ground would also be zero.
The whole thing is a case of measuring from a defined reference point.
 

sparky 1

Joined Nov 3, 2018
706
I put together a low current AC model. The design could be expand into an off grid system model.
( with house wiring a clamp meter could be placed between reference points A and B)
When the switch S1 is open the AC load is balanced. The current thru the neutral is 0.071 uA
The load becomes unbalanced when switch S1 is closed. The AC current measured between the neutral wire and the center tap
The current thru meter U2 jumps to 10 mA and the current thru meter U1 measures 20 mA.

To really understand an AC system having a neutral ground not counting special cases (lightning ect from outside the circuit)
A grasp of AC Ohms Law and other related skill set is needed because of the various types of devices connected to an AC system
are not purely resistive loads. I do agree with zero the whole thing about potential difference is having a reference point.


AC unbalanced load.JPG
 
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Thread Starter

Simon Pressure Jet

Joined Oct 28, 2020
13
For the moment ignore the ground to neutral bond. let's just concentrate on the secondary of a 240V center tapped transformer 120(L1)-0(N)-120(L2) VAC. L1 and L2 are 180 degrees out of phase relative to each other.

If all of your loads were 240V, there would be no current at the center tap.

If the L1 and L2 loads were balanced, there would be no current through N.

N really carries the unbalanced current of L1 to 0 and L2 to 0.

We can also use a notation like 5A<-90 for L1 and 2A<+90 for L2. The < sign isn't a < sign, pretend the bottom is flat. So, 1A at an angle of -90 degrees. So, in this case the N has 3A at an angle of -90 degrees relative to N.

AC really doesn't have a polarity, but the measuring devices do. They have a phase relationship. So, current transformers have a dot on them marking the primary and secondary phase relationship.

The dot of the primary is in phase with the dot on the secondary. The AC waveform follows each other.

If you had a 6 VAC transformer and a 24 VAC transformer, you can put the secondaries in series and get (24-6) or 24+6 volts. Knowing the relative relationships of the primary and secondary dots, you can get the right voltage you want the first time.

==

Ground is a reference and a place for fault currents to go. Metalic water pipes, stovetops washer/dryers exposed surfaces should all be at the same potential to avoid shock. Cable TV needs ground as a reference too. Telephone has a "protector" to ground which SHOULD be located near the service ground. If that ground "potential" gets raised because of a lightning strike to the telco cables, the entire reference to the house "changed", but no effects to the house metalic water pipes, stovetops washer/dryers exposed surfaces. A shock hazard doesn't exist.

At only one point when the service enters the house is neutral and ground bonded to earth. Any sub-panel within a structure will keep ground and neutral separated. A sub-panel in a detached structure is a little different. Ground and Neutral stay separated, but another ground rod to earth is installed.
Hi and thanks for taking the time to reply. Talking about an ungrounded transformer, picture an isolated transforme, 230 VAC on input winding, with the same amount of turns on the secondary winding but without a grounded neutral. Would the voltageon the secondary side still be 230 VAC?
 
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