Current flow in neutral but no voltage ?

[Rob678]

Having a challenge understanding the concept behind why when you measure across line and neutral (live circuit) using voltage indicators we have voltage reading of 230V single phase. If there is current flow in the neutral why is it termed as 0V ?
On that basis, second question - If I tested across Neutral and Earth, there is current flow down the neutral back to the transformer, why would the tester show 0V ?

 

[pc1966]

Having a challenge understanding the concept behind why when you measure across line and neutral (live circuit) using voltage indicators we have voltage reading of 230V single phase. If there is current flow in the neutral why is it termed as 0V ?

Strictly speaking it is the true Earth (i.e. our planet) that defines zero volts. All voltages are a relative measure (i.e. a potential difference between two electrodes), but once you define a known and world-wide point as 0V you have the means for absolute measurement.

Neutral is defined on the basis that it is referenced to Earth, where as the other line conductors are not. All of L1-L3, N are "live" as they are current-carrying, so if you have an open fault on N then the load-end becomes dangerous indeed.

On that basis, second question - If I tested across Neutral and Earth, there is current flow down the neutral back to the transformer, why would the tester show 0V ?

It depends on three factors:

• How much current is flowing
• The N cable impedance to the N-E link
• The resolution of you meter

If your meter only measures in steps of 1V then you might be seeing 0.4V and it reports zero. If you have a TN-C-S system then the N-E link is at the supply point (DNO cutout) so the impedance of the neutral cable could be small as it is quite short, etc.

 

[pc1966]

This PDF document from the IET has some useful illustrations of typical earthing arrangements:

https://electrical.------.org/media/1698/earthing-your-questions-answered.pdf

https://tiny.cc/6qosvz

 

[Rob678]

Im struggling to grasp the concept in an AC generator at opposite sides of the generator one side becomes positively charged the other side becomes negatively charged. Then we add a cable to both ends and as the core/ shaft spins within a generator this current changes direction back and forth back and forth, and so on to give us AC electricity. But then when the power comes into a domestic property it’s only on the line conductor that I have amps coming into the property ? Using the explanation of how the generator works I’m stuck in my head trying to work out why it isn’t coming in on the phase, then coming in on the neutral and then on the phase and so on. Any help would really be appreciated guys . Thankyou for the responses so far

 

[Rockingit]

@Rob678 - welcome to the world of Yr 1 college theory?? The reason I'm asking is because we'll all absolutely help you, learning and sharing/teaching knowledge is a beautiful thing, it just helps us tailor our replies and point you towards suitable other resources.

 

[Avo Mk8]

But then when the power comes into a domestic property it’s only on the line conductor that I have amps coming into the property ?

That's the only bit of your last post that isn't quite right! Why do you think that?
The (equal) current that 'comes in' on the live, as you nicely put it, 'goes out' on the neutral. And as it's AC, half of the time it's 'coming in' on the neutral!
If you care to put a clamp ammeter on first the L and then the N tail of a consumer unit you will see the same current flowing in each.

I think you might have got confused about voltage vs current from what you said in your first post, maybe because the N is attached to ground as previously explained.
But your narrative in post 4 is basically correct

 

[Rob678]

@Rob678 - welcome to the world of Yr 1 college theory?? The reason I'm asking is because we'll all absolutely help you, learning and sharing/teaching knowledge is a beautiful thing, it just helps us tailor our replies and point you towards suitable other resources.

Hi Rocking it, yes first year at college for me. Thankyou for the help.

 

[Rob678]

That's the only bit of your last post that isn't quite right! Why do you think that?
The (equal) current that 'comes in' on the live, as you nicely put it, 'goes out' on the neutral. And as it's AC, half of the time it's 'coming in' on the neutral!
If you care to put a clamp ammeter on first the L and then the N tail of a consumer unit you will see the same current flowing in each.

I think you might have got confused about voltage vs current from what you said in your first post, maybe because the N is attached to ground as previously explained.
But your narrative in post 4 is basically correct

Thankyou Avo for your response. To be honest from reading some information online and a few early lessons at college, in the most basic form. If I imagine myself as a generator and I hold a rope in each hand around a tree. If I pull with my left the opposite must happen on my right and visa versa. If I stopped and then inspected the rope where it was touching the tree. The abrasive marks / heat produced by rubbing against the tree can be attributed to the current flow with a load attached. This is how I determine AC (in a very basic form) but some of the guys who I’ve spoke to at work only see it from the view power comes in on the live and leaves on the neutral. And I just feel like that’s theoretically incorrect based on the curious research I’ve done so far.

 

[Avo Mk8]

Thankyou Avo for your response. To be honest from reading some information online and a few early lessons at college, in the most basic form. If I imagine myself as a generator and I hold a rope in each hand around a tree. If I pull with my left the opposite must happen on my right and visa versa. If I stopped and then inspected the rope where it was touching the tree. The abrasive marks / heat produced by rubbing against the tree can be attributed to the current flow with a load attached. This is how I determine AC (in a very basic form) but some of the guys who I’ve spoke to at work only see it from the view power comes in on the live and leaves on the neutral. And I just feel like that’s theoretically incorrect based on the curious research I’ve done so far.

Thanks for your thoughts. I am interested to know about your 'curious research'!

Terminology is important. 'Power' in slang is often used to describe the presence of a voltage, but its meaning is more akin to 'energy'. In electrical terms, power is current x voltage = Watts. To be pedantic 'power' does not 'go out' of the neutral. It is given up in the load being fed, and during this process current flows in both live and neutral (even that's a simplification of course, electrons bounce around, rather than 'flowing' en mass, but that's a whole other story!)

Your rope around a tree analogy generating heat works if you think of the distance moved by the rope as current, and tension in the rope as voltage. If you ignore the elasticity of the rope and change in dimensions of the tree (!), then the distance you pull the rope on one side is the same as the movement on the other. But the work you put into the pulling, (force x distance), is absorbed by friction at the tree, and if you were to measure the tension at the loose end of the rope, it will be less due to the tree absorbing some. At the extreme limit where the tree friction absorbs all your pulling, there is no tension on the far end of the rope, equivalent to no voltage on your neutral wire, and the scenario you describe in your first post, asking why the tester shows 0V on the neutral wire. But there is current flowing (movement of the rope). Yeah that's contrived, but go with the flow .
Bearing in mind you are pulling alternately on each rope 50 times a second
Does that work for you?

PS not everything you read online is true (particularly on Quora )

 

[Rob678]

Thanks for your thoughts. I am interested to know about your 'curious research'!

Terminology is important. 'Power' in slang is often used to describe the presence of a voltage, but its meaning is more akin to 'energy'. In electrical terms, power is current x voltage = Watts. To be pedantic 'power' does not 'go out' of the neutral. It is given up in the load being fed, and during this process current flows in both live and neutral (even that's a simplification of course, electrons bounce around, rather than 'flowing' en mass, but that's a whole other story!)

Your rope around a tree analogy generating heat works if you think of the distance moved by the rope as current, and tension in the rope as voltage. If you ignore the elasticity of the rope and change in dimensions of the tree (!), then the distance you pull the rope on one side is the same as the movement on the other. But the work you put into the pulling, (force x distance), is absorbed by friction at the tree, and if you were to measure the tension at the loose end of the rope, it will be less due to the tree absorbing some. At the extreme limit where the tree friction absorbs all your pulling, there is no tension on the far end of the rope, equivalent to no voltage on your neutral wire, and the scenario you describe in your first post, asking why the tester shows 0V on the neutral wire. But there is current flowing (movement of the rope). Yeah that's contrived, but go with the flow .
Bearing in mind you are pulling alternately on each rope 50 times a second
Does that work for you?

PS not everything you read online is true (particularly on Quora )

I think to generate that frequency a lot more physical training would need to be done by myself !! Most of the research I’ve conducted on my own has been through speaking to work colleagues, reading some text books and looking into forums e.g. this one. I don’t know if I’m just really missing a really simple point I’m just struggling to see the points a little bit still. I’ve been having a look at some diagrams and I feel I’m getting there a little bit.
Cables run from the Generator to a step down transformer.
Am I right in saying from the generator to the transformers we don’t need a neutral ? Because as one phase is at a maximum positive voltage, the other two phases would be at half that, but negative voltage ? Therefore it’s all balanced so you don’t need anymore cables.
If that’s correct then this is where I become stuck.
Then from that transformer a single phase comes to a house. We need a complete circuit in the house so we introduce a neutral at the transformer (is this the basis of a star connection) off of this main neutral cable in the road we attach a main neutral that can come into the house ? (not including anything to do with earths or cpcs )