Understanding Electrical Voltage and Current: Exploring 120V Outlets in the US

[salman213]

Using ur analogy the ground wire is just a short (but there is some resistance due to the ground wire).

But the actual EARTH which the path of the ground wire takes the current to, is not a load? The Earth doesn't have any resistance in the circuit?I cannot model a circuit with a ground like this:

 

[mgb_phys]

Yes it's a very confusing terminology for many people - especially in different languages.
The 'earth' or 'ground' can mean.
1, The safety wire that doesn't normally carry any current - as we described.
2, The electrical point in the circuit that is defined as 0volts.
3, The global zero volt reference for comparing any two voltages.
4, The big spherical bit of dirt you are standing on!

Part of the reason is that although the (planet)Earth isn't a very good conductor it does have a large cross sectional area and so the resistance between two points on the (planet) Earth is low and so the they can be assumed to be at the same voltage.
It also uses the fact that the Earth is big enough and contains enough charges that if you put a little bit of current into it the voltage of the Earth doesn't go up - think of it as a very very big capacitor!

I hesitate to confuse people even more, but you can also have local Earth's. Sometimes in an experiment you need to work at very high voltages, to avoid the dangers of having a million volts between the working parts of a piece of equipement and it's case you can insulate everything from the real Earth and have a single point in the equipement that you declare to be ground. All the Earth wires are connected to this single point. You have to be carefull of course never to have anything between this 'relative' ground and the Real Earth!

 

[MATLABdude]

Using ur analogy the ground wire is just a short (but there is some resistance due to the ground wire).

But the actual EARTH which the path of the ground wire takes the current to, is not a load? The Earth doesn't have any resistance in the circuit?I cannot model a circuit with a ground like this:

It does have a resistance (when measured from one point to another), it just doesn't matter, because everything is connected at one point with metal conductors. The ground wire does not travel through earth, it travels TO earth. There might be a high resistance between what you're standing on and the point at which the ground electrode is implanted. This also (sometimes) keeps you from getting killed if you happen to touch the live wire. Or not, if you happen to be near the Earth electrode, or what you're standing on happens to be metal and connected close to where the electrode is implanted, or just has the same potential, (which is why, as another poster has said, you should never touch the bare metal of a live wire--or any wire, just in case something's been miswired).

Your diagram is incorrect in that the ground wire doesn't have a big resistor right before ground (the three-line thing). To model the above, you'd have a stick man figure (representing you) on top of a resistor which connects to the ground point (as mgb_phys says above). If you touch the live wire, you put yourself in parallel with the other two 'resistors' I've mentioned. Unfortunately, this 'resistor' you stand on top of varies, and might very well be low (low enough to put the 40 mA(?) or so through your heart required to cause your heart to start beating funny, or stop beating all together).

 

[salman213]

Ok thank you

can someone quickly take a look at the following attachment, which diagram is right, looking at the ARROWS which represent the current.My questions:

if DIAGRAM 1 is correct what causes so much current to go into the Earth. why does the Earth take so much current rather than the ground wire back to the negative terminal.if DIAGRAM 2 is correct, which I sort of hope is not correct, why is there no current in the ground wire which is going back to the neutral wire (connected to the negative terminal).

 

[mgb_phys]

Using ur analogy the ground wire is just a short (but there is some resistance due to the ground wire).

The Earth doesn't have any resistance in the circuit?

The Earth does have a resistance, the risk is that you don't know how big it is. If you are standing on dry rock there might be enough resistance that the current through you and the (planet)Earth might be low enough to be safe. But it may also be very low if the ground is wet.

You try to design the (wire)earth to be low resistance so that most of the current flows through that instead of through you - if the case of the instrument becomes 'live'.

See the attached picture, what would happen if the red wire, and you, touched the case?


edit - the answer to your question. current only flows through the Earth if there is a fault and the live wire touches the case. Normaly this isn't the case. Both diagrams are correct but in practice the short circuit means no current floaws through the load (because the case is lower resistance) and so a very high current is drawn and the fuse will blow.

 

[salman213]

ok thank you for drawing that out taht makes a lot of sense actually...

one other question lol...

Through the ground wire a huge amount of current goes through it and INTO the actual EARTH. What does the Earth actually do with this current? (electrons)...

 

[MATLABdude]

ok thank you for drawing that out taht makes a lot of sense actually...

one other question lol...

Through the ground wire a huge amount of current goes through it and INTO the actual EARTH. What does the Earth actually do with this current? (electrons)...

There's always one more question, no? ;-) I think that's the sign of a curious mind, and as long as the questions become more sophisticated, and you can start answering some of your own (simpler) questions based on what you learn, that's perfectly fine!

It comes out the other end and gets "pumped" back into the wire by the generator! Using the water pump analogy, everything just flows in a loop (note that this is a very simplistic view of things, and that technically, electrons are the real charge carriers, and if you want to REALLY get technical about things, no electron travels all the way down the wire, but these are stories for different days, and are addressed in an introductory electromagnetics class, or textbook).

 

[salman213]

wait a minute if the Earth does not do anything with the current. what is the use of putting the Earth there in the first place, why not just make a complete short connecting the metal case directly to the "negative terminal."

:S?if someone can look at this new diagram that would be good as well

I put current through the load since some current still goes through it but is "negligible."

SO is I4=I2??

and then I5 = I ?

If that is true what is the EARTH DOING ..lol?

 

[XPTPCREWX]

I still don't Understand any of this...If Neutral is connected to Ground...THEN IT IS A SHORT CIRCUIT...period...

Why doesn't this trip the breaker when ground is in fact connected to the Neutral Bar?

 

[Redbelly98]

salman123 & XPTPCREWX,

Think about what would happen in a house where Hot and Neutral are erroneously wired backwards. Do you really want that box connected to "Neutral"?

 

[mgb_phys]

I still don't Understand any of this...If Neutral is connected to Ground...THEN IT IS A SHORT CIRCUIT...period...

Why doesn't this trip the breaker when ground is in fact connected to the Neutral Bar?

Because the ground wire is only connected to neutral at one end, at the other end it is connected to the case - which is connected to nothing.
If the case is connected to live (because of a fault) then the ground wire is a short circuit which will blow the fuse.

 

[XPTPCREWX]

Because the ground wire is only connected to neutral at one end, .

Neutral IS HOT.

 

[XPTPCREWX]

Here is a Schematic...Please correct it.

 

[XPTPCREWX]

salman123 & XPTPCREWX,

Think about what would happen in a house where Hot and Neutral are erroneously wired backwards. Do you really want that box connected to "Neutral"?

HENCE...

If Neutral is connected to Ground...THEN IT IS A SHORT CIRCUIT...period...

Why doesn't this trip the breaker when ground is in fact connected to the Neutral Bar?

 

[mgb_phys]

I think you are slightly confused about AC.
AC doesn't mean the voltage goes + and - and the current flows backwards and forwards. The live voltage is always above 0 and the neutral is always near zero volts( except for any slight voltage due to V=IR in the resistance of the wire)

 

[XPTPCREWX]

no, that's not right. you couldn't be more wrong.

ha,

A/C ..."ALTERNATING CURRENT"...

 

[Redbelly98]

HENCE...

If Neutral is connected to Ground...THEN IT IS A SHORT CIRCUIT...period...

Why doesn't this trip the breaker when ground is in fact connected to the Neutral Bar?

Look at your figure from post #48.

The fact that neutral and ground are shorted together does not cause high current to flow through the fuse, because that current must still flow through the load resistance. (Fuse and load are in series.)

However, shorting hot and ground together (when neutral is already shorted to ground) would cause high current to flow through the fuse (or breaker), since then you are bypassing the load resistance in that case.

 

[XPTPCREWX]

Look at your figure from post #48.

The fact that neutral and ground are shorted together does not cause high current to flow through the fuse, because that current must still flow through the load resistance. (Fuse and load are in series.)
.

I am obviously talking to a bunch of amatures,


Neutral carries the HOT back to the DELTA/STAR configutation to complete the circuit.

NEUTRAL IS HOT.

I dare you to grab a neutral when a load is energized and touch the ground wire.

 

[MATLABdude]

HENCE...

If Neutral is connected to Ground...THEN IT IS A SHORT CIRCUIT...period...

You bet! I freaked out the first time I encountered this after tripping a breaker, because right before that, I had just accidentally made contact between the hot wire and the case in a device box (it was on a different circuit altogether, but I was still paranoid and more naive than I am now). I used the multimeter to check continuity and lo and behold, ground and the neutral wire were shorted out. After first having an "Oh S---!" moment, and then wondering how I was going to rip out all the wiring to correct this 'fault', I did some more reading and realized that yes, this was the normal state of affairs.

Why doesn't this trip the breaker when ground is in fact connected to the Neutral Bar?

As you state in a later post, yes, there is current flowing through neutral (at least, downstream of the load). But in normal operation (with a load connected and energized), the majority of the voltage drop will be in the load (hot wire connected to an ideal "source" is a small resistor, and then there's the big resistor of the load, and then there's another resistor representing the neutral). Thus, there is no tripping of the breaker (assuming your load uses less current than your breaker is rated for) because there's the load present, which controls the amount of current that flows through this loop.

Incidentally, yes, the neutral wire gets a little hotter (if that makes sense), but because the neutral wire has such a small resistance compared to the load, this is negligible, and there will not be enough potential on the neutral to drive enough current through you to hurt you were you to grab the bare neutral wire. With the assumption that load resistance is significantly (like order of magnitude) larger than wire resistance. It's still never good practice, on the chance that someone goofed on the wiring, or on the chance that you've got bad neutral wiring which throws out the assumption of the neutral being only a little "hotter" than it was with no load connected.

Now that said, consider what happens when you short live onto the case (ground). Now you've got your tiny resistance going to ground in parallel with your large resistance (the load and small resistance of the neutral wire). You'll have a short between hot and ground, and a huge amount of current through this short (constrained only by the resistance of the wiring and the case). Hopefully, your circuit breaker (on the live wire) trips and stops the current flow through both load and the short.

The exact same thing would happen if you shorted the neutral wire to the live wire. You'd bypass the load, and *really* make the neutral wire hot, and drop all your voltage across the tiny resistance represented by the wiring. And again, hopefully, your breaker trips.

 

[salman213]

Hmm, Without the livewire touching the metal case the ground wire does not create a complete circuit and as a result no current flows through it. As soon as the livewire touches the metal case for any reason, the ground wire will start to carry current.



What i am a little confused about is the purpose have the actual Earth. Why is the connection to Earth? Why not just make a complete short?

Check Image...

 

[mgb_phys]

That is said to be a 'floating' circuit and for a battery powered circuit it wouldn't matter.
But if it is floating then the + and - can be any voltage above 0 there is nothing to make the - terminal the same voltage as the Earth.
So suppose the + was at 1100V and the - was at 1000V, the circuit would work fine. But if you touched the case, even without any fault, the case would be at 1000V above ground and your feet would be connected to 0V so you would provide a path for the current to flow through!

The reason the Earth is used as a reference is that it is also the thing you are msot likely to be touching ! And you can be sure that the Earth in your house is at the same voltage as the Earth at the power station.

 

[salman213]

EDITED: I have a question about your answer now that I think about it

why wouldn't it matter if it was a DC source. ??

and if it was an AC source you said "the case would be at 1000V above ground and your feet would be connected to 0V so you would provide a path for the current to flow through!"

But since it is a "floating circuit" and not connected to the earth, wouldn't this be an OPEN CIRCUIT. If current flows through an individual where would it go?

I mean isn't that like saying

you have a -1000V--------person--------0VCurrent flows? But its not a complete circuit IS IT??

 

[XPTPCREWX]

In Engineering you MUST have a curious mind. If you dont... YOU ARE DOOMED.
The more curious the better. You need to analyze these things for yourself, because no one is going to explain them to you...simply because THEY DON'T KNOW...(as you can see in these posts.) Then you will come to a point where all your questions will look annoying to others..and you might even be hated for it...

Rememebr one thing...in Engineering you must have the "ability" VISUALIZE and expand on you visualizations, perform experiments in your head and draw your own conclusions...this is the splitting point where you leave everyone else in the dust...these abilities are not gained they are in your blood...like a talent.
look at Tesla...an absolute genius. The world hated him but all of his inventions were proven and designed in his head before on paper.

it doesn't matter in DC because DC is Direct Current...meaning that the electrons flow in a ONE WAY diection.

AC flows in both directions, like tug of war...back and forth...the neutral and hot pull electrons back and forth.

 

[XPTPCREWX]

Hmm, Without the livewire touching the metal case the ground wire does not create a complete circuit and as a result no current flows through it. As soon as the livewire touches the metal case for any reason, the ground wire will start to carry current.



What i am a little confused about is the purpose have the actual Earth. Why is the connection to Earth? Why not just make a complete short?

Check Image...

first off you drawings are wrong. and you are basing all your questions on a false basis.

ground will reduce shock/electrocution/arc blast/ and further damage to the equipment and operator, by creating a lower path of resistance and lower potential difference than anything or anyone around it.

what i think everyone need to focus on answering is HOW this is wired. Every drawing i have seen is a short circuit between HOT and NEUTRAL and GROUND.

 

[XPTPCREWX]

I know how it works, what i NEED is someone WHO KNOWS how the Neutral bar is wired...
Can someone who KNOWS please answer this?

CAN NOONE ELSE SEE WHAT WRONG WITH THIS PICTURE?
Look at the drawing and please correct it.

(this is what everyones damn picture is so far for regular operation)

 

[salman213]

This was in my text I just found it..

 

[XPTPCREWX]

Does anyone see this? seriously...

 

[isly ilwott]

I am obviously talking to a bunch of amatures,


Neutral carries the HOT back to the DELTA/STAR configutation to complete the circuit.

NEUTRAL IS HOT.

I dare you to grab a neutral when a load is energized and touch the ground wire.

At the load connection, the voltage above ground on the properly grounded neutral wire will be limited to the voltage drop caused by the current flow in the neutral. It will be a very low voltage compared to the 120volts at the load end of the hot wire.

You are not talking to "amatures". You simply misunderstand what the supposed amateurs are saying.

I'll get out my paint prushes and send you a picture that you may be able to understand.

 

[salman213]

can you post the picture u are drawing...thanks... :)

 

[XPTPCREWX]

Regardless of the voltage drop, however miniscule the voltage in neutral may be in reference to ground...the current in neutral is the same as the HOT leg supplied to the load.

Thus: neutral is HOT.

If no one understands this they are Amatures.

 

[XPTPCREWX]

With that said...

If Neutral carries the same current at the Load end of the Hot wire.

What is stopping Neutral from "equalizing" (shorting) its voltage with Ground and transferring its current into Ground?

Ground has a lower potential difference, and offers the path of least resistance.

 

[isly ilwott]

With that said...

If Neutral carries the same current at the Load end of the Hot wire.

What is stopping Neutral from "equalizing" (shorting) its voltage with Ground and transferring its current into Ground?

Ground has a lower potential difference, and offers the path of least resistance.

The neutral is grounded at the main panelboard. Any potential at the load in of the neutral wire will be due to voltage drop from one end of the neutral to the other, caused by the resistance of the neutral conductor. It will be in the range of 5 volts or less, depending on the length of the circuit. It will not be 120 volts...or anywhere near that high.

 

[mgb_phys]

why wouldn't it matter if it was a DC source. ??

I meant if it was just a small battery.
although the laws of voltage and current are the same there is an important safety difference between AC and DC. For DC to hurt you there has to be enough current flowing to actually cause burns. which since your skin is normally fairly resistive requires high voltages - like those needed for a subway train.
AC is much more dangerous because only a very small current (20-50mA) flowing through your heart is enough to kill you. Basically your heart tries to beat at the rate of the AC electricity 50/60Hz which causes a heart attack. With household AC electric at 110/220V it's easy to get such a small current through your body.

and if it was an AC source you said "the case would be at 1000V above ground and your feet would be connected to 0V so you would provide a path for the current to flow through!"

Sorry I wasn't very clear - I assumed that it was referenced to ground back at the power station.

 

[XPTPCREWX]

You are not talking to "amatures". You simply misunderstand what the supposed amateurs are saying.

I'll get out my paint prushes and send you a picture that you may be able to understand.

I think you are the one who simply misundertands.

The neutral is grounded at the main panelboard. Any potential at the load in of the neutral wire will be due to voltage drop from one end of the neutral to the other, caused by the resistance of the neutral conductor. It will be in the range of 5 volts or less, depending on the length of the circuit. It will not be 120 volts...or anywhere near that high.

who is arguing otherwise?

I am asking you...

What is stopping Neutral from "equalizing" (shorting) its voltage with Ground and transferring its current into Ground?
Ground has a lower potential difference, and offers the path of least resistance.

 

[MATLABdude]

Regardless of the voltage drop, however miniscule the voltage in neutral may be in reference to ground...the current in neutral is the same as the HOT leg supplied to the load.

Thus: neutral is HOT.

If no one understands this they are Amatures.

Ask anybody in the field (power engineers and/or electricians) and the HOT wire (slang for the LIVE wire) is the wire which carries the high potential relative to ground (amplitude-wise, as it's AC). It has nothing to do with the current it's carrying. As the voltage is what drives current through something, you're usually safe grabbing neutral wires (assuming the path from you to ground is sufficiently high resistance that you don't start looking like a convenient path to go through to get to ground).

With that said...

If Neutral carries the same current at the Load end of the Hot wire.

What is stopping Neutral from "equalizing" (shorting) its voltage with Ground and transferring its current into Ground?

Ground has a lower potential difference, and offers the path of least resistance.

Nothing, this is what happens. If you were to crack open your breaker box, you'd see that the neutral bus bar is connected to ground (at only one point, usually via a bolt--this point is code in most locales!) Since you sound like a show-me kinda guy, KILL THE MAIN BREAKER BEFORE STARTING TO REMOVE THE FRONT PANEL--NO SHORTCUTS! Or, take a look about half-way down on this page (breaker panel anatomy):
http://www.hammerzone.com/archives/elect/panel/breaker/install.htm

first off you drawings are wrong. and you are basing all your questions on a false basis.

ground will reduce shock/electrocution/arc blast/ and further damage to the equipment and operator, by creating a lower path of resistance and lower potential difference than anything or anyone around it.

what i think everyone need to focus on answering is HOW this is wired. Every drawing i have seen is a short circuit between HOT and NEUTRAL and GROUND.

Yes, there's a short between neutral and ground. I have never seen a short (i.e. non-load) between hot and these, however.

As for me, all I know is that I know nothing.