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 )

 

[OnlQQker]

1:35 of the video below kind of visualizes your rope theory!

 

[pc1966]

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.

As @Avo Mk8 says power = current x voltage and as I & V both reverse each cycle of AC then the power is flowing in the same direction. Also while the L & N have equal but opposite I, they also have opposite V (depending on where you reference it) so power flows in the same direction.

Now you might argue if it is 230V & 0V then all power is in the L, which is reasonable enough, but if centre-tapped (as the typical 110V yellow supplies on building sites do for safety, so 55V-0V-55V) then you have half power on one, half on the other. But however you chose to reference 0V you get the same total power for L+N, so the definition of N as 0V (give or take a volt or so drop along the cable) taking power as in the L is adequate..

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.

Yes, typically in the UK the HV feed to a substation has only 3 line conductors, L1, L2, and L3 from a 3-phase supply. The primary windings are normally delta-connected to these 3 line feeds. Dropping N saves a 4th conductor which is a significant cost and as you say, is not actually needed here!

At the sending end (generator, or maybe a higher-voltage substation) then there is a neutral point for the 3-phase that is earthed so all lines are at known potentials w.r.t. the true Earth. A complication in the HV world is often there is an added impedance in the N earthing designed to limit the magnitude of any fault current to a value that is easier to handle, while the protection relay ponders if the fault is temporary or needs disconnection.

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 )

Yes, at the secondary (LV side) in the UK it is almost always a star-connection with the centre point the N which is then earthed via some arrangement of earth electrodes.

To save on cable, it is now common to have a combined N & E in the main cable (protective earth neutral = PEN), and multiple earth electrodes attached along its length (sometimes the lead sheath of old cables that have been superseded but remain buried, etc) to help a little with reliability & safety (protective multiple earthing = PME). This is known as "TN-C" as N & E are common (the 'C') part. For each house the N+E is taken along with one of the L and fed to the house, at that point (usually DNO cutout) the N and E for the installation are separated out. As N and E are now separated, it becomes TN-C-S

In the past there would be separate N & E from the substation, with the E often the cable steel armour or lead anti-corrosion sheath, but for cost saving that is not usually done in public networks (where different rules apply and TN-C is permitted) but it the usual way with private substations where the usual wiring regs apply that prohibit TN-C for safety reasons.

And yes, the safety aspect of an open PEN is a real issue today and has been forced back in to debate with electric cars being attached to that network and folks likely washing them, etc!

 

[Rob678]

As @Avo Mk8 says power = current x voltage and as I & V both reverse each cycle of AC then the power is flowing in the same direction. Also while the L & N have equal but opposite I, they also have opposite V (depending on where you reference it) so power flows in the same direction.

Now you might argue if it is 230V & 0V then all power is in the L, which is reasonable enough, but if centre-tapped (as the typical 110V yellow supplies on building sites do for safety, so 55V-0V-55V) then you have half power on one, half on the other. But however you chose to reference 0V you get the same total power for L+N, so the definition of N as 0V (give or take a volt or so drop along the cable) taking power as in the L is adequate..


Yes, typically in the UK the HV feed to a substation has only 3 line conductors, L1, L2, and L3 from a 3-phase supply. The primary windings are normally delta-connected to these 3 line feeds. Dropping N saves a 4th conductor which is a significant cost and as you say, is not actually needed here!

At the sending end (generator, or maybe a higher-voltage substation) then there is a neutral point for the 3-phase that is earthed so all lines are at known potentials w.r.t. the true Earth. A complication in the HV world is often there is an added impedance in the N earthing designed to limit the magnitude of any fault current to a value that is easier to handle, while the protection relay ponders if the fault is temporary or needs disconnection.


Yes, at the secondary (LV side) in the UK it is almost always a star-connection with the centre point the N which is then earthed via some arrangement of earth electrodes.

To save on cable, it is now common to have a combined N & E in the main cable (protective earth neutral = PEN), and multiple earth electrodes attached along its length (sometimes the lead sheath of old cables that have been superseded but remain buried, etc) to help a little with reliability & safety (protective multiple earthing = PME). This is known as "TN-C" as N & E are common (the 'C') part. For each house the N+E is taken along with one of the L and fed to the house, at that point (usually DNO cutout) the N and E for the installation are separated out. As N and E are now separated, it becomes TN-C-S

In the past there would be separate N & E from the substation, with the E often the cable steel armour or lead anti-corrosion sheath, but for cost saving that is not usually done in public networks (where different rules apply and TN-C is permitted) but it the usual way with private substations where the usual wiring regs apply that prohibit TN-C for safety reasons.

And yes, the safety aspect of an open PEN is a real issue today and has been forced back in to debate with electric cars being attached to that network and folks likely washing them, etc!

Okay I’m feeling like I’ve got abit more knowledge than yesterday now, but I have another question now ?
So during the negative portion of the sine wave, voltage is moving from the neutral through the load to the line ? If this is the case would you get an electric shock from touching a neutral ? For example if you turn the MCB off to do maintenance to a socket. The socket is safely isolated but if it’s still in the neutral bar and the current is alternating into the bar wouldn’t there be a risk of electric shock ?

 

[ElectroChem]

Okay I’m feeling like I’ve got abit more knowledge than yesterday now, but I have another question now ?
So during the negative portion of the sine wave, voltage is moving from the neutral through the load to the line ? If this is the case would you get an electric shock from touching a neutral ? For example if you turn the MCB off to do maintenance to a socket. The socket is safely isolated but if it’s still in the neutral bar and the current is alternating into the bar wouldn’t there be a risk of electric shock ?

Almost there but still not quite.

Rather than thinking of the negative half of the sine wave as 'putting voltage on the neutral', what actually happens is that voltage is still on the active wire, but now it's negative to earth reference rather than positive. The neutral stays at 0V as it's bonded to earth reference.

 

[Rob678]

Almost there but still not quite.

Rather than thinking of the negative half of the sine wave as 'putting voltage on the neutral', what actually happens is that voltage is still on the active wire, but now it's negative to earth reference rather than positive. The neutral stays at 0V as it's bonded to earth reference.

Thankyou for your response Electrochem, is there any other way to explain this as it’s gone over my head abit,
Thankyou