Author Topic: How is Neutral Wire Neutral in Mains Electricity?  (Read 17962 times)

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Offline help_me_pick_usernameTopic starter

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How is Neutral Wire Neutral in Mains Electricity?
« on: January 14, 2020, 08:22:21 am »
I am kind of confused about the Live and Neutral wires in Mains wiring. I've seen YouTube videos where someone touches the Neutral wire, but it doesn't shock them, but touching the Live wire does.

I'm going to try and explain my understanding of how Mains AC electricity works, please let me know where I am wrong. My understanding is that, if I simplify the AC waveform to simply a square wave instead of Sinosodial, 60Hz AC is where one wire is +240v and the other is ground 30 times a second, and for the other 30 times, this is flipped, so the other wire is +240v, and the other is ground. I know this is wrong, since you wouldn't be able to touch Neutral in this case, but where am I wrong?
 

Online tszaboo

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #1 on: January 14, 2020, 08:41:41 am »
No. Phase has a sine wave on it. Like below. Sometimes positive, sometimes negative.

Neutral is 0V, constantly, normally conducting electricity.

Protetive earth is 0V as well, but it doesnt conduct normally electricity.


 Dont touch any of these wires with bare hands, even if people like Eleoctroboom do this, it is extremely foolish. Buy a electricians glove, it costs like 3 dollars, and switch off the circuit breaker if you work on the wires.
 
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Offline help_me_pick_usernameTopic starter

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #2 on: January 14, 2020, 08:54:50 am »
It was never my intention to touch any of these wires, I was simply using that as an exmaple to explain my question.

I don't really understand how negative voltage works. I will have to do some more research on this.

When the Live wire is in it's negative state, doesn't that mean that Neutral is +240v relative to the Live wire? Sorry if these are stupid questions. I'm having some trouble wrapping my head around this.
 

Online magic

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #3 on: January 14, 2020, 09:14:04 am »
Absolute electric potential is determined by excess or deficit of electric charge. It can be positive or negative. Earth is neither positively nor negatively charged so its potential is zero. Neutral is connected to earth so its potential is zero. You are neutrally charged too so nothing happens when you touch neutral, unless the installation is broken or you actually are statically charged and get a spark.

Live is alternately positive or negative. Neutral may be +240V relative to live when live is -240V, but it's still 0V relative to ground.

Now, if you isolate your floor from ground, connect it to live, stand barefoot on the floor and touch neutral, that's going to hurt :P
 
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Online tszaboo

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #4 on: January 14, 2020, 09:42:14 am »
Voltage is always relative. Meaning, that you always measure it between two points. It is that way by definition.
You take one point as the reference point, for example protective earth, place the black electrode of the multimeter on it, and measure it against it. So yes, if you swap the mutimeter wires, the polarity of the voltage changes. We just use a reference point, because then everyone understands what voltage we talk about.
 

Offline hsn93

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #5 on: January 14, 2020, 09:44:19 am »

When the Live wire is in it's negative state, doesn't that mean that Neutral is +240v relative to the Live wire? Sorry if these are stupid questions. I'm having some trouble wrapping my head around this.

yes,, relative to the live..

so u cant touch both togother..

however, its still ~0v relative to earth
-------------------------------------
 

Offline jeroen79

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #6 on: January 14, 2020, 09:46:11 am »
The distributiontransformer near your house will have one side of secondary winding connected to earth.
This point is the neutral, the other ends of the windings are the different phases.

In a single phase system the neutral will carry the same current as the phase so there will be a voltage across it.
Therefore it will not be at earth potential at the outlet, although very close to it.

The earth in your house can be provided from several points.

 

Offline help_me_pick_usernameTopic starter

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #7 on: January 14, 2020, 10:54:26 am »
I have attatched a diagram. If, instead of 9v batteries, these were 240v power sources that were being switched on and off to make 240v AC, if you touched the reference point in the centre (the neutral wire...?), you would get shocked, right? Since you are touching the high side of the second power supply?
 

Offline sokoloff

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #8 on: January 14, 2020, 11:24:22 am »
To get a shock, current has to flow. For current to flow, there has to be a complete circuit. If you touch only the center point, you won't get a shock, because there's no complete circuit. In fact, you could touch any part of it, provided you don't have any other means for the circuit to be completed.

Watching this might help ground your understanding a bit. (Sorry for the pun.)
 
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Offline Mechatrommer

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #9 on: January 14, 2020, 11:48:52 am »
I have attatched a diagram. If, instead of 9v batteries, these were 240v power sources that were being switched on and off to make 240v AC, if you touched the reference point in the centre (the neutral wire...?), you would get shocked, right? Since you are touching the high side of the second power supply?
no. your body potential (voltage) only instantly raised to the same level of center point without you noticing, or vice versa. this is called static, static is no electric flow, no flow no shock. you only get a shock when you touch either low or high point with your other hand. in this case, your body has 2 different potential between your hands, electric will flow through that and you will get a shock...

back to neutral wire, if you closely study the above diagram, neutral is connected to earth. your feet also connected to earth, so when you touch neutral wire, your hand is earth, feet is earth, same potential = no flow. no flow, no shock. but when you touch live wire, your feet is earth, but your hand is not, different potential = different voltage... different voltage, electric will flow. there is flow, there is shock.. regardless of direction +ve or -ve.
« Last Edit: January 14, 2020, 11:51:35 am by Mechatrommer »
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Offline help_me_pick_usernameTopic starter

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #10 on: January 14, 2020, 12:15:30 pm »
I have attatched a diagram. If, instead of 9v batteries, these were 240v power sources that were being switched on and off to make 240v AC, if you touched the reference point in the centre (the neutral wire...?), you would get shocked, right? Since you are touching the high side of the second power supply?
no. your body potential (voltage) only instantly raised to the same level of center point without you noticing, or vice versa. this is called static, static is no electric flow, no flow no shock. you only get a shock when you touch either low or high point with your other hand. in this case, your body has 2 different potential between your hands, electric will flow through that and you will get a shock...

back to neutral wire, if you closely study the above diagram, neutral is connected to earth. your feet also connected to earth, so when you touch neutral wire, your hand is earth, feet is earth, same potential = no flow. no flow, no shock. but when you touch live wire, your feet is earth, but your hand is not, different potential = different voltage... different voltage, electric will flow. there is flow, there is shock.. regardless of direction +ve or -ve.

How is the centre connection connected to earth in my diagram? If the two batteries are completely isolated, does that mean that the Neutral is thought of as connected to earth...?
 

Offline sokoloff

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #11 on: January 14, 2020, 12:38:42 pm »
How is the centre connection connected to earth in my diagram?
I think he is talking about jeroene79's diagram, where neutral is literally connected to earth.

If the two batteries are completely isolated, does that mean that the Neutral is thought of as connected to earth...?
Electricity doesn't "think". In residential service, neutral is connected to earth at the service entrance. After that, neutral and ground should not be considered to be the same.

In the case of someone touching the center connection of your two batteries, also touching earth, and no other connections to complete the circuit, that person becomes the (high resistance/impedance) connection between the center point and earth.
 

Offline calzap

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #12 on: January 14, 2020, 03:52:41 pm »
Deliberately touching an energized neutral wire or terminal with bare skin (or bare metal tool) is a really bad idea.  It might not actually be a neutral!  Even if you test it with a meter first, don't touch it with bare skin.  Someone flipping a switch or plugging something in out of your sight might cause the supposed neutral to become hot.

If you are touching a true neutral connection and the end of that neutral is disconnected back at the breaker box ... guess what?  ... you are now the neutral conductor ...  shocking huh?  A true neutral can kill you if you if you insert yourself in its electrical path. If you were to disconnect the neutral for energized equipment and then touch both the wire end and its connection point, you will get shocked ... maybe lethally.

There are other ways a neutral can shock you  ... don't touch 'em without proper precautions.  Main precautions are knowing what you're doing and de-energizing circuits (and verifying they're dead).

Mike in California

 
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Offline paulca

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #13 on: January 14, 2020, 04:24:32 pm »
To get a shock, current has to flow. For current to flow, there has to be a complete circuit. If you touch only the center point, you won't get a shock, because there's no complete circuit. In fact, you could touch any part of it, provided you don't have any other means for the circuit to be completed.

I tested this several times as a kid.  I got mixed results.  Touching the live wire in a light switch did not get me a shock.  However touching the end of the element in an electric heater did get me a mild shock. 

The difference to me is that the switch wire was live, but not part of a running flowing circuit.  The electric fire element however ever was ON.  Now while my body did not complete a circuit to ground, although there might have been some leakage, I did become part of the circuit through which electricity was flowing.  Electricity follows the path of least resistance is the theory, but I wonder how much current flows into dead-ends momentarily, like water in a stream flowing into coves and bays.

Still it's more likely I leaked a few mA into the chair/carpet/floor and eventually to ground.  It was a mild shock, just enough to make you retract your hand rapidly and mutter "ouch!".
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Offline Mechatrommer

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #14 on: January 14, 2020, 04:45:46 pm »
into the chair/carpet/floor and eventually to ground.
there's your problem. chair/carpet add resistance to earth. you may float with the circuit node you are touching you may even add slight static to the carpet. and there is also difference (resistance) between touching with dry hand and wet hand. next time, try with wet bare feet directly to wet ground and wet hand to get maximum fun (WARNING: use only tip of one finger from one hand, the other hand in the pocket, dont ever touch electricity with your palm, even with one hand. violating this may result RIP)
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Offline CatalinaWOW

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #15 on: January 16, 2020, 03:15:24 am »
Shock is 99% right.  But there are two requirements - to complete a circuit and to have a potential difference.

While I completely agree with the cautions about implementing this in practice, in principal the neutral and ground are at the same potential.  So there will generally be no shock from touching neutral while being connected to ground by standing on it or other means.  While you might get away with this far more often than not, there can actually be a shock for many reasons.  The shock can be quite severe so don't mess with it.  Don't make a practice of touching these connections when power is applied.  Use meters to verify that there is no potential even when you think that no power is applied.

The line side has it's potential driven positive and negative relative to the neutral by the power source.  Since the neutral is normally tied to ground that results in a potential between line and ground.  In this case a shock will almost always result.  But just as in the case of the neutral lines there are conditions that can reduce or eliminate the shock.  But these are rare and often not robust, so no matter what your experience in getting a mild or no shock don't depend on that.  Assume that the line is hot relative to ground when energized.  If you must touch the hot line take appropriate steps to de-energize the circuit.  And before you touch it use a meter to verify that you were successful.

The OP's post with two nine volt batteries has confused him because he hasn't fully understood the physical connection that (almost always) occurs between ground and neutral, and possibly also doesn't realize there is a variable but real connection between a human standing on a floor and ground (or earth depending on the terminology used in your area).
« Last Edit: January 16, 2020, 03:16:56 am by CatalinaWOW »
 

Offline tkamiya

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #16 on: January 16, 2020, 03:24:37 am »
Let's simplify this.

A simple, single phase mains feed has two wires.  Live wire, and neutral wire.  Voltage exists as difference between those two wires.  When there is a difference in voltage, when connected, current flows.

At this stage, it does not matter which one swings up and down and which one does not.  It's all relative.  If you base your measurement on neutral wire, live wire swings.  If you base your measurement on live wire, neutral swings.

What makes difference is that at power pole and your main feed, neutral wire is connected to ground.  Therefore, it is at ground potential.  So, it is now simple to think, neutral wire does not swing up or down.  It stays at the ground potential.  This is why it is neutral.  This is why you can grab the neutral wire (white wire in USA) and not be in danger of electrical shock.

I am not going to go into 3 phase stuff.  It is not necessary for this discussion.  In USA, a regular socket has 3 poles but last one is ground.  Just ground.  Remaining two is neutral and live.  If you measure those two lines ASSUMING neutral is at ground potential (which it is), live swings 120 * 1.4 volt 60 times a second.  (peak of AC is higher than what people refer to as "Voltage".)

Since YOU (your body) is coupled weakly to ground, if you touch live, you complete the circuit and feel the shock.  If you touch neutral, since you and the wire are both at ground potential, you won't be shocked.  If you actually put your feet on the actual ground and touch live wire, you might die, since you are strongly coupled to the ground.  Now, if you stand on a perfect insulator and few feet off the ground or away from anything metallic, you can touch live and not feel a thing.  This is because with absence of ground, there is no way for live wire to "swing".

Now, the disclaimer....  above is all true assuming everything is done correctly and no fault exists.  I have seen many times things are screwed up.  Never assume any wires are safe to touch.  I am a licensed electrician but I always measure few times before touching anything.
 

Offline HobGoblyn

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #17 on: January 16, 2020, 09:36:48 am »
I always presumed in the UK, neutral and earth are the same thing. ( I’m a beginner and learning a lot, just want to check my assumption is correct)

I presumed the only reason it was necessary to also have earth pins on our plugs was to make double sure there was always a direct route to ground?

In theory the way UK ring mains work, the neutral should already provide this?

Am I talking rubbish?

I also thought that if you do have to mess around with live mains voltage, you should always do it with your right hand and never your left, the reason being, a shock through your right hand to ground, through your right foot is much less likely to stop your heart ?

Thanks
« Last Edit: January 16, 2020, 09:47:36 am by HobGoblyn »
 

Online Ranayna

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #18 on: January 16, 2020, 10:08:08 am »
For the longest time I had massive problems internalizing what is going on there with live and neutral. My thoughts were: The mains polarity changes 50[60] times per second, so both ends have to be dangerous (for half the time), right?

What helped me to "get it" was the following thought: "The electricity changes from 'pushing' to 'sucking' 50[60] times per second. The live is what does the pushing and sucking, the neutral is pushed to/sucked from". That is an analogy that fits the concept of live and neutral better for me.
Maybe the issue for me was that I grew up with the water analogies in my Kosmos kit  :-// I likely also did not help that german plugs are not polarized, making the distinction largely irrelevant for non electricians.
 

Offline paulca

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #19 on: January 16, 2020, 01:14:30 pm »
I always presumed in the UK, neutral and earth are the same thing. ( I’m a beginner and learning a lot, just want to check my assumption is correct)

I presumed the only reason it was necessary to also have earth pins on our plugs was to make double sure there was always a direct route to ground?

The Earth is there to provide a low resistance and deliberate failure path for fault current.  So if LIVE touches the metal case of a toaster, then having that metal connected to earth causes a massive amount of current to flow, which causes fuses/breakers to blow preventing the fault from continuing.  It also has a tendency to try and keep the toaster case close to Earth potential and reduce risk of current flowing into you to ground when it has a lovely low resistance path to take.

Earth and Neutral and separate, someone previously posted the different types of grounds, but I believe in the UK it is often taken from the metal conduit where the mains power enters the house.  In some instances this is not available, is plastic or is not good enough a ground rod is installed.  Neutral remains neutral all the way back to the substation where it joins up into the 3 phase 11kV lines.  It is probably earth referenced there.  However due to voltage drop over the potentially miles of cable there can be a few volts potential between neutral and local ground. 

An RCD will often trip if you short neutral to ground.  It actually makes it next to impossible to cut a cable without tripping the RCD.  Even if you turn off the master power breaker/switch the RCD will still pop when you cut the cable as you will short neutral and earth.

Check your own house, measure the voltage between earth and neutral.  It is unlikely to be exactly zero, but it might be close.
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Offline sokoloff

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #20 on: January 16, 2020, 01:37:41 pm »
I always presumed in the UK, neutral and earth are the same thing. ( I’m a beginner and learning a lot, just want to check my assumption is correct)

I presumed the only reason it was necessary to also have earth pins on our plugs was to make double sure there was always a direct route to ground?
...I believe in the UK it is often taken from the metal conduit where the mains power enters the house.  In some instances this is not available, is plastic or is not good enough a ground rod is installed.  Neutral remains neutral all the way back to the substation where it joins up into the 3 phase 11kV lines.  It is probably earth referenced there.  However due to voltage drop over the potentially miles of cable there can be a few volts potential between neutral and local ground.
In US residential service, the neutral and ground are bonded at the first service disconnect on a property.

In the UK are they really kept separate all the way back to the medium voltage transformer at the substation? That seems unlikely. There's good reason for bonding on the property and bonding to other metal in-ground systems and metal in-building piping.
 

Offline paulca

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #21 on: January 16, 2020, 03:49:02 pm »
In the UK are they really kept separate all the way back to the medium voltage transformer at the substation? That seems unlikely. There's good reason for bonding on the property and bonding to other metal in-ground systems and metal in-building piping.

I believe there are different standards used depending on how long ago the installation was done.

In the UK a set of streets will have an 11kV 3 phase substation usually taking a drop line off an over high cable.  The actual single phase residential cabling is usually underground buried in an armored plastic insulated cable (I believe in a ring).  "Usually" as I think it really depends on if the area was cabled while being built or was cabled afterwards and putting them underground was tricky, so there are flying mains cables off poles to the house in some areas.  My house has a rather thick cable arriving from a pole, but I'm fairly sure it's the telephone line.

The substations are usually hidden away in grey boxes in "dead areas" where there wasn't enough room to build a house, fenced off with warning signs, they hum and buzz.

I think in my locale these join up at distribution/switching yards and combine into 65kV or 110kV distribution lines.
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Offline HobGoblyn

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #22 on: January 16, 2020, 04:08:51 pm »
In the UK a set of streets will have an 11kV 3 phase substation usually taking a drop line off an over high cable.

We had a rare power cut a few months ago.  What was odd was my house had no power, my neighbours house had no power, then next few houses had power, then the next few didn't. Half the small row of shops didn't etc.

It almost looked like a wizard had waved his wand at a selection of random houses and said "no power"

I presume they use single phase to supply various houses, hence the houses without electricity were connected to one (faulty) phase, while the others were connected to one of the other two?
 

Offline paulca

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #23 on: January 16, 2020, 04:15:44 pm »
I'm not sure, but I think the final end distribution cable is a mess of branches and complications as bits will be added on at later times.  It's probably a mess in most residential areas that weren't all built at the same time.  Chances are there are multiple substations in your area and one of them tripped.

Taking out a single phase and leaving the other two in use, while plausible would create a imbalance in the phases and generate more current on the return neutral which in balanced 3 phase carries virtually no current.  It would seem logical that if one phase trips they would trip all 3... <shrug>  No idea.  I'm sure the grey beards will be along soon to correct me.

Apparently it's not 110kV either...
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Offline Monkeh

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #24 on: January 16, 2020, 04:23:09 pm »
Losing one or two phases out of three on a street (or a few streets) isn't uncommon - generally modern installations handle it with no ill effect. It's happened here a few times - some of the underground joints have gone bad over the last decade or so.
 


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