Understanding the Role of Grounding in AC Single Phase Systems

[klmnopq]

helllo ALLLLLL

my question
in Ac single phase without earth

only one phase live and one neutral

why we say that live line would kill but neutral won't kill

isn't neutral carry current for return "same live current"

 

[Drakkith]

Since the current oscillates directions at the frequency of your power supply, both wires carry current at all times. I believe the "neutral" wire is simply a distinction to label the wires differently. Someone correct me if I'm wrong.

 

[Ryoko]

The neutral wire is tied to ground.

 

[berkeman]

helllo ALLLLLL

my question
in Ac single phase without earth

only one phase live and one neutral

why we say that live line would kill but neutral won't kill

isn't neutral carry current for return "same live current"

By code (at least in the US), the Neutral wire is Earth grounded at the breaker panel. So there is very little voltage developed between the Neutral wires and grounded metal surfaces. The Hot lead is carring the full AC Mains voltage, and both the Hot and Neutral wires carry the current to the load from the source distribution transformer.

The usual risk from electric shock is from a "ground fault". That is, when you are touching some grounded piece of metal, and you come in contact with the Hot wire. That's bad. But if you are in contact with a grounded metal surface, and you come in contact with an exposed Neutral wire, you will only experience a few volts of AC voltage at most, which you will likely not even feel.

 

[mickybob]

helllo ALLLLLL

my question
in Ac single phase without earth

only one phase live and one neutral

why we say that live line would kill but neutral won't kill

isn't neutral carry current for return "same live current"

It's possible you might be suffering from a couple of common misconceptions. The first is that that the absolute 'voltage' matters. It doesn't. All we care about is the difference in potential (the potential difference or 'voltage') between two wires, points etc.

The second is that wires somehow 'carry' a fixed current. Current flows between two points where there there is a potential difference and sufficiently low resistance. That why we only care about the potential difference, not absolute voltages.

In a single phase AC supply, the potential difference between the 'live' wire and 'neutral' oscillates between (for example) around +340 and -340 V in Europe. So if you connect them together via, say a motor, there is a potential difference and current flows (in this case an alternating current).

The 'neutral' wire is earthed, so is at the same voltage (roughly) as you. So if you touch the neutral wire, nothing happens as there is no potential difference and hence no current flow.

The live wire varies between +340 and -340 V relative to the neutral wire. So there is a large potential difference between you and the wire - if you touch it, bang!

 

[rollingstein]

By code (at least in the US), the Neutral wire is Earth grounded at the breaker panel. So there is very little voltage developed between the Neutral wires and grounded metal surfaces.

If Neutral gets tied to Earth what's the purpose of the third grounding pin / wire that's sometimes found?

 

[Studiot]

Hello mickybob, Berkeman is quite correct, the building neutral is earthed by code in the US.

This is not the case in Europe, where it is illegal to Earth the neutral.

This is because the two distributions system work entirely differently.

Hello rollingstein.

The neutral in both systems carries the return current.

The aditional Earth in both systems should carry zero current in normal operation. It's sole purpose is for safety protection, in which case it carries a fault current, sufficient to blow the fuse or breaker. It also serves to prevent hazardous potentials developing on exposed metalwork.

Edit:
Hello ,klmnopq

mickybob is on the right lines, the actual current flowing in the wires does not present the danger. It is the voltage difference between yourself and something else.
The power company maintains the neutral at somewhere near Earth potential (mine is often about 30 volts) and the phase at line voltage above that. It is this potential difference the delivers the electrical power to your appliances.

You are also at approximately Earth voltage so if you touch the neutral of the supply wire you are probably safe, but don't try it to see.

If, however your supply is from a (portable) generator then you will get a belt off either terminal. Neither terminal of a portable generator is maintained near earth, without further intervention.

Remember safety first, second and last.

 

[mickybob]

Hello mickybob, Berkman is quite correct, the building neutral is earthed by code in the US.

This is not the case in Europe, where it is illegal to Earth the neutral.

This is because the two distributions system work entirely differently.

Hello rollingstein.

The neutral in both systems carry the return current.

The aditional Earth isn both systems should carry zero current in normal operation. It's sole purpose is for safety protection, in which case it carries a fault current, sufficient to blow the fuse or breaker. It also serves to prevent hazardous potentials developing on exposed metalwork.

Yes, but it's earthed the other side of the circuit breaker/fuse box. Which is a crucial difference under fault conditions.

 

[russ_watters]

I'm not sure these answers really answer the question. The key, to me, is that the neutral wire comes after the load in the circuit, so the voltage is essentially zero, regardless of if/where it is tied to the ground.

 

[mickybob]

I'm not sure these answers really answer the question. The key, to me, is that the neutral wire comes after the load in the circuit, so the voltage is essentially zero, regardless of if/where it is tied to the ground.

What do you mean by voltage? Voltage is a difference in potential between two points, if you'er saying the voltage is zero, do you mean the potential difference between that point and ground is zero. In which case it's the same as saying it's tied to ground.

The key point is that the p.d. is zero because it is tied to ground. This is irrelevant of whether there is a load or not.

 

[Studiot]

Yes, but it's earthed the other side of the circuit breaker/fuse box. Which is a crucial difference under fault conditions.

No, this is illegal.

 

[Studiot]

If the neutral naturally rests at approximately zero in relation to Earth as Russ suggested, or if it is tied to Earth as mickybob suggested, then how come my neutral is usually about 30 volts under no fault conditions.

Many folks will have seen a neon tester light when touched to a neutral. The striking voltage of a neon is about 70 volts.

Finally why do you get a belt from the 'neutral' terminal of a portable generator if it naturally rests near earth?

I'm not sure these answers really answer the question.

The OP is fully answered in my post#7 edit which was being composed whilst Russ was posting.

 

[mickybob]

If the neutral naturally rests at approximately zero in relation to Earth as Russ suggested, or if it is tied to Earth as mickybob suggested, then how come my neutral is usually about 30 volts under no fault conditions.

You'd expect a small voltage drop, but 30 V seems pretty dangerously highly. Is that under load?

 

[klmnopq]

sorry friends

I got more worried

we difference between

1-single phase 2 wire
not as USA and has only 2 wires for current & return??

IN AC current alternate so, Isn't neutral must carrry current &therefore not safe

2-single phase 3 wire

as in USA there is 3 wires and there is the same problem

 

[klmnopq]

is it mean eath carry current for return ! in case of who say return is grounded

how current return into supply!

 

[russ_watters]

What do you mean by voltage? Voltage is a difference in potential between two points, if you'er saying the voltage is zero, do you mean the potential difference between that point and ground is zero. In which case it's the same as saying it's tied to ground.

The key point is that the p.d. is zero because it is tied to ground. This is irrelevant of whether there is a load or not.

I know it is tied to ground, but it doesn't have to be for this to be true.

Neutral and ground are tied together to ensure exactly zero potential between them, but even if they weren't, there would still be close to zero because the load is in between the neutral and the voltage source.

 

[mickybob]

sorry friends

I got more worried

we difference between

1-single phase 2 wire
not as USA and has only 2 wires for current & return??

IN AC current alternate so, Isn't neutral must carrry current &therefore not safe

2-single phase 3 wire

as in USA there is 3 wires and there is the same problem

The debate has got a bit side-tracked, so let's go back to the 'physics' of your question which I'm sure we'll all agree on. Firstly - please re-read my first post - I think you have some basic misunderstandings.

'Dangerous' means current flowing through you, not through a wire.

You are standing on the ground, then to a reasonable approximation you are grounded, i.e. at what we usually call 0 V. This will be a better approximation if you stand in a pool of water.

The live wire is at, for example, 240 V relative to ground.

The neutral wire is at (give or take) 0 V relative to ground.

If you touch the live wire, there will be a 240 V potential difference between your hand and your feet and hence ground. Your resistance is not that high, so a big current will flow and you'll probably die.

If you touch the neutral wire, there will be (in principle) a 0 V potential difference between your hand and your feet - nothing will happen.

There is a voltage drop of 240 V across the 'load' - this can be your TV or, if nothing is plugged in, air.

If it's your TV, then there is a relatively low resistance, and so there is a current running through the live wire, across the load, through the neutral wire and back into the supply. If the 'load' is air (nothing is plugged in) then the resistance is huge and the current running along this path is essentially zero.

Either way, it doesn't matter - only a current running through you is dangerous and that depends on the voltage drop across you (and your resistance).

Having said all that, in practice things might be slightly more complicated, and I wouldn't suggest you still metal objects in the neutral plug.

 

[rollingstein]

The debate has got a bit side-tracked, so let's go back to the 'physics' of your question which I'm sure we'll all agree on. Firstly - please re-read my first post - I think you have some basic misunderstandings.

'Dangerous' means current flowing through you, not through a wire.

You are standing on the ground, then to a reasonable approximation you are grounded, i.e. at what we usually call 0 V. This will be a better approximation if you stand in a pool of water.

The live wire is at, for example, 240 V relative to ground.

The neutral wire is at (give or take) 0 V relative to ground.

If you touch the live wire, there will be a 240 V potential difference between your hand and your feet and hence ground. Your resistance is not that high, so a big current will flow and you'll probably die.

If you touch the neutral wire, there will be (in principle) a 0 V potential difference between your hand and your feet - nothing will happen.

There is a voltage drop of 240 V across the 'load' - this can be your TV or, if nothing is plugged in, air.

If it's your TV, then there is a relatively low resistance, and so there is a current running through the live wire, across the load, through the neutral wire and back into the supply. If the 'load' is air (nothing is plugged in) then the resistance is huge and the current running along this path is essentially zero.

Either way, it doesn't matter - only a current running through you is dangerous and that depends on the voltage drop across you (and your resistance).

Having said all that, in practice things might be slightly more complicated, and I wouldn't suggest you still metal objects in the neutral plug.

That's a good explanation. But why is there a separate ground and neutral wire then in some cases?

 

[mickybob]

I know it is tied to ground, but it doesn't have to be for this to be true.

Neutral and ground are tied together to ensure exactly zero potential between them, but even if they weren't, there would still be close to zero because the load is in between the neutral and the voltage source.

0V is an arbitrary concept, its only has meaning if things are tied to ground.

Otherwise I could say the voltage of the live is 10000 V and the voltage of the neutral is 99760 V, giving a p.d. of 240 V. That is possible if it was a floating power supply.

 

[mickybob]

That's a good explanation. But why is there a separate ground and neutral wire then in some cases?

The Earth is often connected to the chassis of the device.

If you did this with the neutral, it would - all else being equal - be okay.

But now suppose there was a break in the neutral wire between the plug and the power supply.

The neutral wire would jump up to 240 V and you've now got a live chassis.

Not good!

There are other reasons as well, to do with the way circuit breakers are set up to detect faults.

 

[sophiecentaur]

is it mean eath carry current for return ! in case of who say return is grounded

how current return into supply!

What are you referring to? A three phase system?

 

[sophiecentaur]

I know it is tied to ground, but it doesn't have to be for this to be true.

Neutral and ground are tied together to ensure exactly zero potential between them, but even if they weren't, there would still be close to zero because the load is in between the neutral and the voltage source.

You refer to the US system, here?

 

[Studiot]

I see you expert folks are carefully ignoring the questions to your theories in my post#13.

In particular what is the potential of the neutral terminal of a portable generator with respect to earth?

Finally would someone post the section of the Wiring Regulations that apparently requires UK neutrals to be earthed?

I have previously posted at PF the Wiring Regulations section that prohibits this.

 

[sophiecentaur]

There's much more 'chat' than actual figures on this when I do a Google search (Dozens of forums for electrical installers etc.- anecdotes, mainly). I did find out that temporary generator installations can get away with 25V N-E but the max one should expect in a permanent installation is only 3 or 4V. This is just due to a realistic value for the neutral cable (all the way from the transformer) and a typical maximum load.
@Studiot
Your measured value of 30V would give me some concern. Where do you measure this - on the input to your consumer distribution unit or on a socket. Also, with what sort of load? If you get that with no domestic load then you should ring your supplier and they should send someone out to see you, I reckon.
(Keep your wellie boots on when you're in the shower!)

Why isn't this thread in the EE section, I wonder?

 

[mickybob]

I see you expert folks are carefully ignoring the questions to your theories in my post#13.

In particular what is the potential of the neutral terminal of a portable generator with respect to earth?

If it's a floating system, it can be anything, see page 20 of:

http://www.ussu.co.uk/ClubsSocieties/societies/stage-crew/Training%20Documens/L2+%20Electrical/REF%20BS7430%202011.pdf

It's entirely irrelevant as everyone else is discussing mains supplies.

Finally would someone post the section of the Wiring Regulations that apparently requires UK neutrals to be earthed?

I have previously posted at PF the Wiring Regulations section that prohibits this.

I believe that, in the UK, the neutral connection is at the transformer, not in people's housing. Since you definitely only want one connection, I would imagine that the wiring regulations would indeed say not to connect them in the house.

In the US I think the connection is in the house, which is why the discussion was getting confused.

But this is all irrelevant to the original post.

The 30 V between your neutral and Earth could be due to a whole host of reasons - the most obvious being impedance of the wires. Seems strangely high to me, but I'm not a sparky.

 

[sophiecentaur]

@Studiot
You're in for a major re-wire of your house, I expect. No time for more postings on PF. You'll be out earning overtime to pay for it all.

Where ignorance is bliss, 'tis folly to be wise.

 

[Studiot]

I have the Board's certificate of satisfactory test for my own wiring, thank you for your concern.

Any competent electrician would know that the 30 volts would not be maintained if I tried to draw significant current to earth.

I am far more concerned with the hazardous practices suggested by others here, or by the suggestion that portable generators are somehow different or are not covered by the wiring regulations. Of course they are. Much of the world's supplies are from such generators. Portable includes multi megawatt devices towed behind an HGV tractor.

Reading th OP's posts I suspect he is from 'the subcontinent' where the wiring system is based on older British practice - that of providing both earthed and non earthed AC supplies.

I repeat my comment to him that the Earth should not carry current in normal operation - it is a safety conductor.

No one has offered a reason for earthing the neutral. So here is why the americans do and we do not.

Here is the reason for american practice

The consumer neutral is the centre point of a split phase transformer. By itself, it is floating. It is earthed to provide a stable reference for both phases whose differential return currents would otherwise unbalance it.
In the US the consumer's neutral does not carry any other consumer's return current.

Excellent sound practice.

But wholly inapplicable to British practice since the British consumers neutral does not stem from a consumers transformer, but from a local substation where the combined return current from the entire phase arrives. This phase services many consumers.

 

[sophiecentaur]

Single transformer for many customers: the cheapest option in dense housing.

 

[mickybob]

I have the Board's certificate of satisfactory test for my own wiring, thank you for your concern.

Any competent electrician would know that the 30 volts would not be maintained if I tried to draw significant current to earth.

I am far more concerned with the hazardous practices suggested by others here, or by the suggestion that portable generators are somehow different or are not covered by the wiring regulations. Of course they are. Much of the world's supplies are from such generators. Portable includes multi megawatt devices towed behind an HGV tractor.

Reading th OP's posts I suspect he is from 'the subcontinent' where the wiring system is based on older British practice - that of providing both earthed and non earthed AC supplies.

I repeat my comment to him that the Earth should not carry current in normal operation - it is a safety conductor.

No one has offered a reason for earthing the neutral. So here is why the americans do and we do not.

Here is the reason for american practice

The consumer neutral is the centre point of a split phase transformer. By itself, it is floating. It is earthed to provide a stable reference for both phases whose differential return currents would otherwise unbalance it.
In the US the consumer's neutral does not carry any other consumer's return current.

Excellent sound practice.

But wholly inapplicable to British practice since the British consumers neutral does not stem from a consumers transformer, but from a local substation where the combined return current from the entire phase arrives. This phase services many consumers.

So what? The neutral is earthed at the substation instead in the UK. For the OPs question this makes no difference, the point is that neutral ~ OV.

 

[Studiot]

So what? The neutral is earthed at the substation instead in the UK. For the OPs question this makes no difference, the point is that neutral ~ OV.

Do you really not understand the significance of the difference?

 

[russ_watters]

Do you really not understand the significance of the difference?

I don't. Please explain.

If the neutral and ground are tied together at the substation, the neutral will have approximately zero potential art the ground, everywhere. As it pertains to the OP's question, I don't see why it matters where they are connected, only that they are.

 

[Studiot]

I don't. Please explain.

Good evening Russ.

First who said the neutral and ground are tied together at the substation?
I didn't.

In the US, a pair of transmission wires, single phase no Earth (the company feed) are brought to the company side of the consumer supply transformer (actually owned by the company I believe but I'm not sure about that). That is the primary.

The consumer is supplied from a centre tapped secondary on this transformer. The centre tap is earthed locally. The two halves of the 'split phase' are used to supply the consumer 120 outlets and the whole secondary used to supply any 240 heavy demand installations.
As far as possible the split phases are balanced but in any event their current draw does not affect the current through the primary circuit terminals, only its magnitude.

The other point of this is that the consumer is electrically isolated from other consumers and cannot unbalance their supplies, since the consumer is in the secondary and the feed supply in the primary.

In British practice, on the other hand, imagine the effect of multiple grounding one of the two main conductors of a phase. All the phase current would be routed directly to local ground, leaving serious unbalance in the (neutral) return conductor.

The British consumer is not electrically isolated from his neighbour and what he does affects the neighbours supply.

 

[russ_watters]

That's all fine, but what does that have to do with the OP's question? To be more precise: if you stand in a bathtub full of water and grab a neutral wire in London, should you expect a shock?

 

[Studiot]

That's all fine, but what does that have to do with the OP's question? To be more precise: if you stand in a bathtub full of water and grab a neutral wire in London, should you expect a shock?

I already answered that in my first post

mickybob is on the right lines, the actual current flowing in the wires does not present the danger. It is the voltage difference between yourself and something else.
The power company maintains the neutral at somewhere near Earth potential (mine is often about 30 volts) and the phase at line voltage above that. It is this potential difference the delivers the electrical power to your appliances.

You are also at approximately Earth voltage so if you touch the neutral of the supply wire you are probably safe, but don't try it to see.

 

[Martinaston]

In a polyphase system if all three phases are not equally balanced the voltage on the Neutral can increase.

If you have three houses in a row where each house is on a separate phase from the substation, two houses are empty and the third is consuming power. The return voltage of the Neutral will rise.
This will not rise in just the occupied house, it will also rise in the other two houses because they all share the Neutral conductor back to the substation.

This voltage should be fairly low and can be felt if you stick your hand between the Neutral and Earth conductors to give you a "tinkle"(so it only passes through your hand, NOT from one hand to the other)
It only takes 50mA to interfere with your heartbeat.

Here in the UK we now use RCD's on all new installations that trip at 30mA, not sure how the US does it ?

This Neutral current can be very dangerous in commercial installations if you have machinery with very high start up currents that unbalance the three phase system while your working on the Neutral condutor.

If you call the power company concerning the 30v on your Neutral they would probably thank you because it means they are earning a fortune.

If the power company had a way of continuously,instantaneously balancing all three phases from the power station they could then get money for old rope.(like they don't already)

To whoever made the comment about the Neutral being connected at the houses in the US, please explain that to me.

As for the generator part, ALL generators should be Earthed.
If its not its giving you a potential difference of 110v or 230v between Live and Neutral but the peak voltage on the Live conductor measured to Earth could be absolutely anywhere and dependant on things like the amount of moisture in the air that day.
If it gets high enough your camper van could look like David Blain was visiting.

 

[sophiecentaur]

The British consumer is not electrically isolated from his neighbour and what he does affects the neighbours supply.

His neighbours three doors either side of him, usually (WYE transformer). But his neutral is shared with everyone. If the neutrals and Earth's were joined at both ends, there could be some really hefty Earth currents flowing constantly from induction. It's just like a Hifi system, where you should connect all grounds to one point.

 

[Studiot]

Hello martinaston, you have pretty well confirmed what I was saying about British practice.

What do you not understand about US practice?

It is indeed a US code requirement that the consumer 'neutral' should be earthed to a local Earth at the entry point to the premises. US power companies do not provide Earth terminals, as a rule.

The transformer is often in a service cupboard these days. Formerly it was on poles.

 

[jbriggs444]

Here in the UK we now use RCD's on all new installations that trip at 30mA, not sure how the US does it ?

In the U.S. individual wall outlets are sometimes protected with "ground fault interruptors" (GFI's). You will generally find such outlets in the wash room or kitchen. There are typically two buttons on such a device. One to reset it in the event of a an accidental trip and one to test it, thereby causing it to trip.

Downstream outlets on a circuit are sometimes protected by the interruptor. An outlet will stop working and you walk into the wash room to reset the GFI.

There is a picture of a U.S. device and some additional details in the North America section of http://en.wikipedia.org/wiki/Residual-current_device.

[I am not an electrician]

 

[rollingstein]

If you call the power company concerning the 30v on your Neutral they would probably thank you because it means they are earning a fortune.

I didn't get that remark. Can someone elaborate?

 

[rollingstein]

This voltage should be fairly low and can be felt if you stick your hand between the Neutral and Earth conductors to give you a "tinkle"

Sounds like a dangerous thing to try!

 

[Studiot]

For example, a crane that was grounded to the system neutral and that contacted an energized line would expose any person in contact with the crane or its uninsulated load line to a touch potential nearly equal to the full fault voltage.

Here is an excerpt from dlgoff's link in this thread.

Note the effect of grounding the neutral.

https://www.physicsforums.com/showthread.php?p=4249959&posted=1#post4249959

 

[rollingstein]

Note the effect of grounding the neutral.

For something mobile like a crane what does "grounding the neutral" mean?

"grounded to the system neutral " to be precise.