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Conflict in AC electricity

by klmnopq
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mickybob
#19
Jan29-13, 07:36 AM
P: 34
Quote Quote by russ_watters View Post
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
#20
Jan29-13, 07:45 AM
P: 34
Quote Quote by rollingstein View Post
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
#21
Jan29-13, 07:55 AM
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Quote Quote by klmnopq View Post
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
#22
Jan29-13, 07:57 AM
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Quote Quote by russ_watters View Post
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
#23
Jan29-13, 07:58 AM
P: 5,462
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 apparantly requires UK neutrals to be earthed?

I have previously posted at PF the Wiring Regulations section that prohibits this.
sophiecentaur
#24
Jan29-13, 08:25 AM
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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
#25
Jan29-13, 08:30 AM
P: 34
Quote Quote by Studiot View Post
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...430%202011.pdf

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


Finally would someone post the section of the Wiring Regulations that apparantly 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
#26
Jan29-13, 08:37 AM
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@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
#27
Jan29-13, 09:28 AM
P: 5,462
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
#28
Jan29-13, 09:40 AM
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Single transformer for many customers: the cheapest option in dense housing.
mickybob
#29
Jan29-13, 10:53 AM
P: 34
Quote Quote by Studiot View Post
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
#30
Jan29-13, 11:00 AM
P: 5,462
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
#31
Jan29-13, 02:12 PM
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Quote Quote by Studiot View Post
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
#32
Jan29-13, 02:52 PM
P: 5,462
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
#33
Jan29-13, 02:57 PM
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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
#34
Jan29-13, 03:06 PM
P: 5,462
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
#35
Jan29-13, 03:15 PM
P: 6
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 seperate 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
#36
Jan29-13, 04:48 PM
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Quote Quote by Studiot View Post
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 earths 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.


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