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explaining the purpose of neutral wire?

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hellmooth

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PostPosted: Sat Oct 10, 2009 5:30 pm Reply with quote

just a general question here,i am an adult trainee in my 2nd year and after asking a few trademen in my workplace this question i couldn't get an answer i understood,most of the answers were 'it just works like that so accept it' icon_biggrin.gif

there seems to be a lot of knowledgable people on here so thought i'd give it a try here icon_biggrin.gif

question being.....can someone explain the purpose (if thats the right word??) of the neutral wire in a circuit,why does it have 0 volts?my understanding was that it was a return for the current flowing from the live wire?if you touched the neutral side of a load under power would you get a shock?if so how can it be 0 volts? icon_biggrin.gif

any answers appreciated thanks icon_biggrin.gif
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PrenticeBoyofDerry

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PostPosted: Sat Oct 10, 2009 5:36 pm Reply with quote

Depends what situation is regarding the zero volts on the neutral,
The neutral in some case is used to carry excess current in an unbalanced circuit.
But in the single phase domestic situation it does carry the current back to the supply and will show evidence of this if you measure it. It will only do this if a load is being used and is a live conductor.
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Adam_151

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PostPosted: Sat Oct 10, 2009 5:43 pm Reply with quote

Before we get onto anything else, you must understand that a circuit needs to be complete to work, so that the electrons have a complete path to flow around (though note that in AC, they just sit there and jiggle back and forth and don't really go anywhere icon_biggrin.gif )

If you had an ungrounded AC source and a lamp, you'd have two wires between them, neither would be live, and neither would be neutral, and to get a shock you'd have to touch both together.... now I won't go into the reasons we use earth referenced systems, but rather just let you know that we do... and to do that in our example... we stick an electrode in the ground, and we connect it to one side of our AC source... and we call that side 'neutral' because it is... *neutral* relative to the ground, and the other side live (cos it is if you touch it, cos you are permantly connected to the other side by standing on the earth...)

Now a neutral will normally be at very few volts (it'll be above ground potential a little bit, due to voltage drop... but not much), but if we break the neutral... the end which doesn't go back to the end of the source connected to ground, that is to say the end connected to live via the lamp, will go upto live voltage (because the lamp is no longer dropping any volatage across it, because it has ceasesed to opperate)

Note that neutrals and lives are both classed as live conductors (they can both bite!), the one that gets called live (and indeed I have done here, is properly termed phase... or line)
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Taylortwocities

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PostPosted: Sat Oct 10, 2009 6:05 pm Reply with quote

I cannot believe that you've got to the second year without understanding a simple circuit.
Probably not your fault but it is the principle of everything you will have been - and will be - taught.

If you dont understand a circuit then you'll never grasp potential difference, or current...

Really, start here: http://www.zephyrus.co.uk/circuits1.html

Its a battery but think of the positive side as live and the negative side as neutral and you'll get the gist of it.........
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hellmooth

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PostPosted: Sat Oct 10, 2009 6:16 pm Reply with quote

i do understand how a circuit works,but this doesn't explain what i was meaning,my main query is how neutral can be termed 0v but still has say 230v running through it?
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chapeau

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PostPosted: Sat Oct 10, 2009 6:51 pm Reply with quote

Quote:
my understanding was that it was a return for the current flowing from the live wire?
Correct, the neutral cable goes back to the substation at the end of your street where it is connected to the ground/earth. The substation gives all the electrons in your cable going to your house (live) a big push, 230 volts worth in fact. They are shepherded through your house wiring in the copper and end up at the substation again where they end up connected to the ground. Earth is where there is no push on them at all, 0 volts, and is where they are all desparate to get to as soon as they can
Quote:
if you touched the neutral side of a load under power would you get a shock?if so how can it be 0 volts?
Lets say you are standing on the ground with wet feet so connected to earth, your body is at zero volts.

In the cable in your house is an electron waiting to go go into a switch to a light, but the switch is turned off. This electron is being pushed hard by the substation at 230 volts and so has lots of energy, so much that it needs to be kept inside a wire surrounded by plastic so it doesn't escape. It wants to get rid of the big push, really wants to get to the ground which is at zero volts, but right now doesn't have a way to get there.

Now turn on the switch and the path to the ground is clear, except first it has to go through a very narrow wire in the light bulb. To get through this wire, the electron has to force its way through, heating up the wire so it glows bright white. And as the wire gets hotter, it gets harder still. As it goes through the light it loses more and more push, and so has less and less volts. By the time it gets to the other end of this narrow wire in the bulb all the push has gone, the electron is absolutely knackered and has lost almost all of its energy, it only has enough to get through the big fat neutral wire back to the substation. As this wire is very thick this doesn't need much push at all and so the electron in the neutral has very little push energy indeed.

Now lets say you were to touch the wire just before it went into the switch. Remember this electron at 230v has a lot of push behind it, and it finds it far easier to get to earth (which you are standing on) through your body than the thin wire in the light (or if the switch is not yet turned on via the plastic surrounding the copper cable). So through your body it goes, and as it forces all the electrons in your body which are in front of it to move, this energy is let out and you feel it as a nasty shock. If enough electrons get into you and start pushing hard then it could be enough energy to stop your heart or burn your skin.

Now lets say instead you touch the neutral cable just after the electron has come thorough the light bulb, remember the electron is knackered after having pushed through that narrow hot wire. It is no longer at 230 volts push, they all got used up in the light buld wire, it is pretty close to zero. It still wants to get back to earth, but is going to say it's a lot easier to get there via the big fat copper cable than it is going through your flesh and bone, so it will go back via the neutral. Now some will be bloody minded and decide to go to earth through you, but not many and they aren't pushing hard at all, and so little energy gets into your body that you won't even feel it and definitely not enough to do any damage.
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otherparty (29 Dec 2012), hellmooth (12 Oct 2009)
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Spark123

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PostPosted: Sat Oct 10, 2009 7:18 pm Reply with quote

hellmooth wrote:
i do understand how a circuit works,but this doesn't explain what i was meaning,my main query is how neutral can be termed 0v but still has say 230v running through it?


Neutral doesn't have 230v running through it icon_wink.gif
Neutral is at roughly 0v.
Phase is at 230v.
The potential difference between phase and neutral is nominally 230v.
In order to get current to flow you need a potential difference. Plug say a lamp in and you will have a potential difference of 230v across it allowing current to flow illuminating the lamp. The current in the phase wire in this case is the same as the current in the neutral.
Neutral is at about zero volts but as there is current flowing through it and it has a small resistance in reality it will be ever so slightly above zero volts.

Neither of the live wires to cause a potential difference need to be grounded for it to work, a floating system (electrically separate) doesn't have an earth but there is still 230v across the two conductors.
A bathroom shaver socket is an example of electrically separate - it has a small isolation transformer in it.
The main purpose of grounding one side is to tie the system to earth on large systems is for safety.
In a 3 phase system, if it was left floating and one of the phases came into contact with earth there would then be 400v between the other 2 phases and earth. Should someone touch one of the phases they would recieve a 400v shock instead of 230v. We therefore ground the centre "star point" at the transformer to ensure that the max voltage to earth is 230v.
There are other reasons for not using floating systems for larger systems such as all pole MCBs, RCDs don't work and non-noticable single fault conditions. I'm sure there are more...
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flameport

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PostPosted: Sat Oct 10, 2009 8:16 pm Reply with quote

In the picture:
230V power source at the left.
Note that the 0V or neutral wire is connected to the ground (earth).

The voltage between points 1 and 3 is 230V, the box between them represents the load, such as an electric heating element.
With the load (heating element) in place, one end is at 230V (1) and the other end is at 0V (3). A current flows through the element making it heat up.
If you measured between one end of the heating element and halfway along it (2), the voltage would be half. So the voltage between 1 & 2 is 115V. The voltage between 2 & 3 is also 115V.
If the load is removed, no current flows, but the voltage between 1 & 3 is still 230V.

If you stand on the ground at point 4 and touch point 1, you get an electric shock, as there will be 230 volts across your body and some current will flow through you.

If you stand on the ground at point 4 and touch the neutral wire at point 3, the potential difference or voltage between those points is zero, or 0V. As there is no voltage, no current can flow and you don't get an electric shock.

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Goldberg

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PostPosted: Sat Oct 10, 2009 8:31 pm Reply with quote

hellmooth wrote:
just a general question here,i am an adult trainee in my 2nd year and after asking a few trademen in my workplace this question i couldn't get an answer i understood

The fact that your more experienced colleagues can't help you understand probably explains why you think this:

hellmooth wrote:
i do understand how a circuit works,but this doesn't explain what i was meaning,my main query is how neutral can be termed 0v but still has say 230v running through it?

A voltage does not flow through the circuit. Current flows, not voltage.


If it helps, compare electricity in a circuit to water in a hosepipe.

Think of voltage as the pressure, and current as the flow.

Extending the analogy, resistance is conceptually the same - a thin wire/hose is more resistant to current/water flow than a wide one.
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properleckie

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PostPosted: Sat Oct 10, 2009 10:01 pm Reply with quote

Goldbergs analogy is good and often used, I find it helps people if you imagine a water tank connected to a long pipe via a pump, the pipe goes in a long loop and back to the tank, like a central heating circuit.
turn pump on (light sw ), if you could put a pressure gauge at tappings around the circuit you would find full pressure (volts) at pump, dropping around circuit until at open return at tank virtually no pressure (0Volts) but however much water flowed into pipe same volume will come out other end (Amps). If only one radiator most of pressure will be lost over coming its resistance to the flow (Volt drop) some will be lost in pipes before and after so return not quite at 0 Volts (Neutral),
For the purpose of your question the fact that N is connected to E at supply point some where merely gives us a reference from which to measure,
hope thats not confused the issue for you,

regards
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securespark

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PostPosted: Sat Oct 10, 2009 10:15 pm Reply with quote

Adam_151 wrote:
they just sit there and jiggle back and forth and don't really go anywhere


What, like RF before his early morning coffee? icon_lol.gif
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ban-all-sheds

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PostPosted: Sun Oct 11, 2009 12:36 am Reply with quote

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matt1e

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PostPosted: Sun Oct 11, 2009 1:41 am Reply with quote

hellmooth wrote:
i do understand how a circuit works,but this doesn't explain what i was meaning,my main query is how neutral can be termed 0v but still has say 230v running through it?

hi hellmouth
neutral can be termed 0v or 230v depending on what you are referencing it to
it is at 0v (more or less)in respect to earth because they are connected together upstream so at the same potential
it is also at 230 in respect to line

Quote:
If you had an ungrounded AC source and a lamp, you'd have two wires between them, neither would be live, and neither would be neutral, and to get a shock you'd have to touch both together

as adam says,in this situation you would have to touch both cables together to get a shock,I agree that neither are neutral but disagree that neither were live,they both are but I can see where he is coming from in trying to explain to you
It might be easier to understand if you think of a simple single phase generator,generating alternating current at 230v
now connect a voltmeter at the two points of output and measure the voltage
=230v
now reverse the leads and measure
=230v
they are both equally live in respect to each other
now take a twin and earth cable and connect brown to one point (doesnt matter which) and connect the blue and the earth to the other point
now go to the other end of the cable and take voltage readings
you will get 230v between brown and blue
you will also get 230v between brown and earth
you will get 0v between blue and earth because they are neutral to each other
matt
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matt1e

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PostPosted: Sun Oct 11, 2009 7:01 am Reply with quote

sorry havn't answered the full question above regarding the purpose of the neutral but adam and spark123 etc have already answered it pretty well just to try and help you a bit further
taking taylortwocity's analogy with the battery a bit further:
there exists a potential difference between positive and negative so when you connect it to a load say a lamp then current will flow and the lamp will light. now if you spin the battery round and connect it the other way round then the light will still work but the current will be flowing in the other direction-keep doing this and you have ac
both of these connections are classed as live because they are connected to the source generating the potential difference and for the light to work both must be connected,and it is the same with our ac mains both connections to the source must also be present and they are both live conductors but have different names
neutral = the live that is connected to the star point (common winding connection)of the sub stations transformer and for safety reasons also connected to earth
line or phase = the live that is connected to the other side of the winding

chapeau wrote:

Now lets say instead you touch the neutral cable just after the electron has come thorough the light bulb, remember the electron is knackered after having pushed through that narrow hot wire. It is no longer at 230 volts push, they all got used up in the light buld wire, it is pretty close to zero. It still wants to get back to earth, but is going to say it's a lot easier to get there via the big fat copper cable than it is going through your flesh and bone, so it will go back via the neutral. Now some will be bloody minded and decide to go to earth through you, but not many and they aren't pushing hard at all, and so little energy gets into your body that you won't even feel it and definitely not enough to do any damage.

the reason you don't feel it is that you are more or less on the same potential as the neutral as already explained not because the the electron is "knackered", try taking the neutral connection off the bulb and put your finger there and see how knackered your electron is!!
for those that do not know better do not do what is underlined in the previous sentence as you will receive an electric shock
I can see what you are trying to say chapeau but as Goldburg has already said
Quote:
A voltage does not flow through the circuit. Current flows, not voltage

put a volt meter across a lit lamp and you will still find nearly full voltage take a look at flameports drawing
matt
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DavidTransformer

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PostPosted: Sun Oct 11, 2009 7:19 am Reply with quote

The fact that Robert and I have the same surname might lead many to the correct conclusion that we are related, and you would be correct, Robert was my youngest Brother.

I know from conversations we had and from his e-mail folders that he had people on this forum who had known him for some time, and because of those who looked on Robert as a friend I thought that they would like to know that Robert is no longer with us. Unfortunately Robert was killed on Monday evening at work in a freak induction accident.

I know that some here may not be too concerned to know of this loss, I knew Robert for 200 Years and I know how difficult he could be, with all his talk of Tesla's work being more important than Faraday's, and his unerring belief that it was possible to derive 240,000 Volts from a 12V supply with only one winding, and his repeated fights with Jacques Daguerre about the properties of synthetic organic materials for camera bodies, but I am also aware of the other side of him, the side that some here saw.

I would like to thank those here who had become Robert's friends, I know he appreciated it and had the utmost respect for many members of this team, including Mickey, Minnie, and Felix.

He will be sadly missed.
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