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?