Simple Electrical Question

[salman213]

Hey guys I have a few simple questions:

So on a light bulb I have connected to my lamp it says "120 V" and 25 Watts. From the outlet we receive 120 V. If that is true is the light bulb using the maximum E.M.F. the outlet can provide (120V)? and therefore it is using a very low current (p=vi = 25=120i i=25/120 A)

Is that right?

if that is right, do all things made in US for example use 120 V? Or if we plug something in the max they can use is 120 V but the outlet provides anything below that as well.

What is the maximum CURRENT our home outlets can give?... (Is there a maximum)?

LASTLY

There is always a ground or neutral wire, i know it is there to prevent consequences from short circuits but when does the current flow through this wire? What causes the current to start going into the neutral wire? At what point does the current "know" to start flowing through the neutral wire?


If im not being clear please tell me.

[isly ilwott]

Hey guys I have a few simple questions:

So on a light bulb I have connected to my lamp it says "120 V" and 25 Watts. From the outlet we receive 120 V. If that is true is the light bulb using the maximum E.M.F. the outlet can provide (120V)? and therefore it is using a very low current (p=vi = 25=120i i=25/120 A)

Is that right?

if that is right, do all things made in US for example use 120 V? Or if we plug something in the max they can use is 120 V but the outlet provides anything below that as well.

lastly
What is the maximum CURRENT our home outlets can give?...

If im not being clear please tell me.Home convenience outlets in the USA are normally rated for use with devices designed for operation at 115 or 120 volts. The rating on the outlet device may be listed as 125 volts. The voltage applied to the terminals will be 120 volts nominal.

Things designed to be plugged into these outlets usually have a voltage rating of 120 volts. You will find some lamps rated at 130 volts. (Put these in a 120 volt circuit and they will last damn near forever.)

Whatever is plugged into the outlet will "pull" current according to its resistance.

A 100 watt lamp will pull .8333 amps because its resistance is about 144 ohms.

A 200 watt lamp will pull 1.66 amps because its resistance is about 72 ohms.

A 15 watt lamp will pull .125 amps because its resistance is about 960 ohms.

The lower the consumption rating of the driven device, the higher its resistance to current flow is. The higher the resistance is, the lower the current is. Thus a low wattage lamp has a higher resistance to current flow than a high wattage lamp. They are all driven by the same 120 volt supply voltage.

This is applicable to 120v outlets only. We also have incandescent lamps designed for 240 volts...and 277 volts.


You should NOT plug things into the outlet unless they are rated for 115/120 volts, especially if the rating is lower than 115 volts. Applying more voltage than the equipment is designed for will cause overcurrent which leads to overheating which leads to fire which leads to destruction of materials and possibly death.

[mgb_phys]

What is the maximum CURRENT our home outlets can give?... (Is there a maximum)?
It's limited by the fuse/circuit breaker to prevent too much current being drawn which would overheat the wiring.
Usually the US limit is 15Amps, but since this is at the fuse box it is shared by all the appliances on the circuit. Large appliances like ovens, washers, dryers will be on a separate 30A circuit which is often also 240V.
In the UK it is typically 32A at 220V for the circuit with a separate 13A fuse in the plug for each appliance. This is why electric kettles and electric heaters are more common in europe.


There is always a ground or neutral wire, i know it is there to prevent consequences from short circuits but when does the current flow through this wire? What causes the current to start going into the neutral wire? At what point does the current "know" to start flowing through the neutral wire?
You are confusing ground and neutral.
The neutral wire completes the circuit so the light bulb (or whatever) is connected between the incoming live wire and the outgoing neutral - exactly the same current flows in both.
The earth wire is a separate wire which is usually connected to the neutral at the fuse box. It never normally carries any current but is connected to any external metal parts of the appliance. The idea is that if the live wire should touch the external metal (due to some fault) a large current will flow down the earth wire which will blow the fuse and so disconnect the faulty appliance.
A more modern solution is an RCD which measures the current flowing in through the live and back through the neutral - if there is any difference it turns off the circuit. This stops any current flowing to earth through a different path such as your body!

[salman213]

so the wattage on the bulbs are as a result of calculations made on VOLTAGE(120V) and the RESISTANCE.

[salman213]

ok so the ground wires are connected to each load ?

[mgb_phys]

so the wattage on the bulbs are as a result of calculations made on VOLTAGE(120V) and the RESISTANCE.
Yes - it's a little tricky because the resistance of the filament changes as it heats up and as it ages - the wattage is only an approximation.

ok so the ground wires are connected to each load ?
The ground wires are all connected together at the fuse box and then usually connected to a copper rod into the ground or a cold water pipe.

[salman213]

I made a quick diagram if u can help me,

attached

can u give me an example of how a ground would be connected

[mgb_phys]

The ground would be a separate wire from the -ve terminal of the supply to the metal case of the equipement.
At the negative terminal it would also be connected to the Earth (in the real world through a buried metal rod).

[Redbelly98]

The load might be enclosed in a metal case, and the ground wire connected to that case. Somewhere else in the house, the ground wire and neutral are connected together (perhaps near the "source" in your diagram) and also (as mgb_phys mentioned in post #6) "usually connected to a copper rod into the ground or a cold water pipe."

Normally, the ground wire and metal case are not part of the circuit. But if the wiring inside the box becomes loose and touches the metal walls, that completes the circuit. Lots of current flows, trips the circuit breaker or fuse, resulting in no voltage being applied to the device.

It's a safety feature. Without that ground circuit, if the wiring in the box gets loose and touches the metal case, there is no complete circuit ... until somebody comes along and touches the box (which is in contact with the live wire) and electrocutes themself.

Edit added: just saw mgb_phys's figure. A picture is worth a thousand words!

[salman213]

so when exactly then will current go into this ground wire?
like if there is too much current? but if there is too much current flowing into the load how does it know to start flowing in the ground wire? Wouldnt the current try to pick the least resistive path and as a result continue to travel through the load?




Thanks to everyone by the u guys are helping a lot

[isly ilwott]

so the wattage on the bulbs are as a result of calculations made on VOLTAGE(120V) and the RESISTANCE.The wattage will determine the current draw at a specific voltage. For purely resistive devices (no phase shift associated with the current), the wattage will be the product of the voltage and the current.

W = E x I

We already know that E = I x R

so, in the W = E x I formula, replace E with I x R and you get

W = I x R x I = I^2 R

It is not so simple for inductive/capacitive devices because the power factor must be considered. When dealing with incandescent lamps, it is negligible.

ok so the ground wires are connected to each load ?

The green ground wire is usually connected to the metal casing of the electric device, or to the mounting yoke on a receptacle. The connection to the casing of the device plugged in is through the rounded hole on the receptacle. The white neutral is connected to the lamp through the screw-shell and usually goes through the shorter of the two vertical slots on the receptacle. The hot wire is connected to the other side of the lamp filament, usually through the button on the bottom of the lamp and is wired through the longer of the two vertical slots on the receptacle.

The green ground wire is not part of the current carrying circuit as are the white neutral and the colored hot wire. The green ground wire and the white neutral are connected together at the main panelboard only.

[Redbelly98]

so when exactly then will current go into this ground wire?

If the wiring becomes loose inside the case. It's not supposed to happen, but it can and does. Screws holding the wires in place can become loose. Soldered joints can crack. Most electric devices warm up when in use, then cool down when turned off. With heating and cooling comes thermal expansion and contraction. This places stress on wiring joints. After many times of the stress pulling on the joint or screw one way (while heating up) and then the opposite way (while cooling down), the joint can fail eventually. When that happens, you have a loose, live electrical wire inside the box.

If the loose, live electrical wire makes contact with the walls of the box, it's a safety hazard. A person who touches the box while standing on the ground (or floor) completes a circuit, and becomes electrocuted.

By making a path from the box to ground (via the ground wire), the circuit breaker is tripped and power is shut off. No more safety hazard.

[mgb_phys]

so when exactly then will current go into this ground wire?
Only if there is a fault in the equiepment and the live wire touches the metal case.
A light bulb isn't a good example since there isn't ussually anything metal to connect the ground wire to. Modern light fittings often have a screw terminal in the plastic base to connect the ground wire to. This is completely pointless but is needed to meet wiring codes - this might be what is confusing you!
If you look inside a metal desk lamp or a toaster you will see a green/yellow (at least in europe) wire connected to the case.

like if there is too much current?
No, the fuse is there to blow if there is to much current. The earth wire is needed to blow the fuse.
As Redbelly said, suppose there was no earth and the live wire came loose and touched the case. There would be no path for the current and so the fuse wouldn't blow - but if you come along and touch the metal case the current can flow through you (bad!). The earth wire is there so that if the live does touch the case it will flow down the earth wire - this short circuit will pull a lot of current and blow the fuse.

Wouldnt the current try to pick the least resistive path and as a result continue to travel through the load?
there is less resitance directly down the earth wire than through the load. The load must have some resistance to do work. The earth wire is designed to have much lower resistance.

edit - don't know why the picture doesn't show up properly.

[isly ilwott]

so when exactly then will current go into this ground wire?
like if there is too much current? but if there is too much current flowing into the load how does it know to start flowing in the ground wire? Wouldnt the current try to pick the least resistive path and as a result continue to travel through the load?




Thanks to everyone by the u guys are helping a lotThe circuit through the device (lamp, motor, lamp ballast, solenoid, whatever) is normally carried through insulated wires. They are insulated from contact with the supporting case either by insulation material or by physical separation. When the insulation breaks down due to age and excess heat, the circuit can conduct current through the metal case. It is at this stage (failure of containment of the current to its intended path) that the green ground wire becomes helpful. It carries the current to the grounding electrode outside the house and keeps it from going through people that may be touching the metal case. When the current through the multi-breaker (or fuse) becomes too high, the breaker will open (or the fuse will melt) causes the current flow to stop completely.

[salman213]

OOOkkkk i never knew that. THANKS! So basically this ground we are talking about protects from US completing the circuit due to some reason as stated in ur postss .... I was always confused about thattttttt thats coooooool...




BUT in an outlet the "third prong," what is that?

[Redbelly98]

BUT in an outlet the "third prong," what is that?

That is for the ground (or "Earth") wire which is connected to the metal case.

[mgb_phys]

So basically this ground we are talking about protects from US completing the circuit due to some reason as stated in ur postss
Exactly!

BUT in an outlet the "third prong," what is that?
That's the ground, on the equipment (plug) side it just goes to the metal case, on the supply (socket) side it is the earth wire going back to the fuse box and then into the Ground.

In most countries the earht prong is sligghtly longer than the other two, to guarrantee that it is connected first - so if the plug is only partly in your are sure the case is earthed BEFORE any power can be supplied.

[salman213]

I SEEEEEEE I THINK lol...

I hope this is right to say... CHECK PIC :)??



I HOPE THAT IS RIGHT: IF IT IS

IS THE BLACK THE LIVE WIRE
GREEN THE NEUTRAL WIRE
THE RED THE GROUND WIRE:s

[Redbelly98]

Yes, that's it. You also got the narrow & wide slots correct!

[salman213]

AWESOME!!!!!!

thanks to everyone who helped me, i was very confused about that for a while now.

THANKS A LOT!!!!!!!

[isly ilwott]

I SEEEEEEE I THINK lol...

I hope this is right to say... CHECK PIC :)??



I HOPE THAT IS RIGHT: IF IT IS

IS THE BLACK THE LIVE WIRE
GREEN THE NEUTRAL WIRE
THE RED THE GROUND WIRE:sIn your outlet box,

Black is the hot wire (will knock you down if you touch it while you are grounded)
White is the current-carrying neutral wire (is grounded at the panel and will not knock you down if touched separately)
Green is the ground wire (is also grounded and will not knock you down if touched)

[salman213]

Thats in real life right? those were actually referring to my diagram, i just picked random colours.


but wait I see if you touch the black wire you complete the circuit since the "white wire is grounded).

But if you touch the neutral wire how does that create any potential difference. Same with green :s...

[mgb_phys]

Thats in real life right? those were actually referring to my diagram, i just picked random colours.
At least in Isly's country.
Brown=live,blue=neutral, green/yellow=earth is standard for europe


but wait I see if you touch the black wire you complete the circuit since the "white wire is grounded).
If you touch the black wire you create the circuit through your feet and the ground - you don't need to touch any other wire.


But if you touch the neutral wire how does that create any potential difference. Same with green :s...
Correct, if the device is correctly wired both the neutral and earth wires are at (almost) 0 volts and are safe.
But since you never know if a house has been wired corectly it's safest not to touch any of them unless the fuse is removed!

[salman213]

Correct, if the device is correctly wired both the neutral and earth wires are at (almost) 0 volts and are safe.
But since you never know if a house has been wired corectly it's safest not to touch any of them unless the fuse is removed!
if the fuse is removed there is no other grounding going on anywhere else that would cause u to be electrocuted if you touched the black wire?

[mgb_phys]

Correct, if the fuse is removed the black (live) wire isn't connected to anything and so is safe

[salman213]

In your outlet box,

Black is the hot wire (will knock you down if you touch it while you are grounded)
White is the current-carrying neutral wire (is grounded at the panel and will not knock you down if touched separately)
Green is the ground wire (is also grounded and will not knock you down if touched)


so u cannot touch any ONE wire at all ..safely?

[salman213]

Correct, if the fuse is removed the black (live) wire isn't connected to anything and so is safe

at each transformer isnt it grounded before coming to your home so isnt it always going to be grounded even if u remove the fuse?

[mgb_phys]

at each transformer isnt it grounded before coming to your home so isnt it always going to be grounded even if u remove the fuse?
The neutral wire is grounded at the transformer. The live wire comes from the power line, through the fuse and to your appliance. If you remove the fuse it's like disconnecting the live wire from the power line.

so u cannot touch any ONE wire at all ..safely?
In theory only the black wire is live, the neutral and ground are both at (or near) 0 volts.

But since you never know if the socket has been wired correctly it's always best to treat all the wires as potentially dangerous, remove the fuse and check them with a meter before touching anything.

[salman213]

so in this diagram

http://physicsforums.com/attachment.php?attachmentid=15119&d=1219265764

in between the source and outlet on the live wire there should be a fuse right?


and the source is acting like the transformer basically.... is that right?

[mgb_phys]

in between the source and outlet on the live wire there should be a fuse right?
Yes

and the source is acting like the transformer basically.... is that right?
Well the power station + powerline + transformer - yes.

[salman213]

Ok Thank You very much to every individual who helped me. I appreciate it greatly. THANK YOU!

If I have anymore questions I wont hesitate to ask :D.

[XPTPCREWX]

One thing I dont get.....

If Ground is connected to the Neutral Bar at the Breaker Box, which it is.
(in residential)......what is stopping the current of that branch circuit on the neutral side from ground out instead of returning to the source?

The Ground is being "spurred" off of the neutral bar.....and providing the path of least resistance....right???

[MATLABdude]

One thing I dont get.....

If Ground is connected to the Neutral Bar at the Breaker Box, which it is.
(in residential)......what is stopping the current of that branch circuit on the neutral side from ground out instead of returning to the source?

The Ground is being "spurred" off of the neutral bar.....and providing the path of least resistance....right???

It'd only return through ground (the wire or physical ground) if the ground were actually in the path of the current (as is the case when you have a wiring fault or a case short). Think of it as being two paths to ground, one lower resistance than the other, but in which only one (the higher resistance "neutral") is usually connected.

The whole ground wire thing is used as primarily as a safety feature, and as a means of having the same "ground" everywhere in your wiring (which might not be the case from the top of your house to the bottom--potentially, a lot of potential between these points). Some companies cheap out and use only two prongs, connecting neutral to case resulting in an extremely dangerous situation if the wiring is reversed at an outlet, and the whole case is live instead of neutral/ground. MORAL OF THE STORY: AC is AC, and reversed hot and neutral may allow what you're using to work, but stay safe and make sure hot and neutral aren't reversed and that ground is actually connected (it's a $10 plug-in tester at any hardware store).

To keep from fear-mongering that all two-prong==bad, two prong live-and-neutral is okay if the case isn't metallic (and the electrical bits well protected) or if there's an isolation transformer being used which keeps the entire unit isolated (no external paths to ground).

[salman213]

Ok ya thats actually somewhat my question now too..

how is the ground less resistive than a load? wouldn't the ground have a HIGHER resistance than any load.

can someone answer my diagram

[MATLABdude]

I think you may be confusing "ground" (source of electrons) with the ground wire. Ground is not less resistive than a load because ground is where the current goes to through the neutral wire in the end (if you were to cut the wire at any point, the resistance to ground of the ground wire will be similar to the resistance of the neutral wire). You're right, there may still be some current that goes through the load, but this will be negligible compared to the current going through the ground wire to ground.

You can see this if you go back to my earlier analogy about having two resistors (one large: the load, and one much smaller: the resistance of the ground wire) side by side. If both have one end connected to ground, normal operation is represented by the connection of the live wire to the higher resistance. A case fault would be represented by having the small resistance connect in parallel to the high resistance. In this case, the vast majority of the current will go through the small resistance (the ground wire) rather than through the load. Hopefully, a breaker or fuse pops at this point and turns off the live wire.

[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 doesnt 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 earths. 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 peice 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 doesnt 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 doesnt 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 dont 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 dont 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, thats 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 wouldnt 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, wouldnt this be an OPEN CIRCUIT. If current flows through an individual where would it go?

I mean isnt that like saying

you have a -1000V--------person--------0V


Current 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 noone 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 doesnt 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 noone 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 wouldnt 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 noone 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.

[salman213]

Is it possible for someone to explain why this individual gets electrocuted...(check image)..

I know MGB explained but im still confused for some reason. I do not understand how this is a complete circuit. Can someone trace the current for me? :S

(Image: Yes that is a short no ground connection).

[MATLABdude]

Is it possible for someone to explain why this individual gets electrocuted...(check image)..

I know MGB explained but im still confused for some reason. I do not understand how this is a complete circuit. Can someone trace the current for me? :S

(Image: Yes that is a short no ground connection).

In the absence of anything else (e.g. the power supply negative terminal being at higher potential than the ground the guy's standing on), the individual does not get electrocuted.

[salman213]

if the negative terminal is at a higher potential i still do not really understand how current would flow through the individual.....

Is it NOT necessary that in AC Circuits the "End" which is the "EARTH" be connected to the negative terminal for the circuit to work..?

[MATLABdude]

if the negative terminal is at a higher potential i still do not really understand how current would flow through the individual.....

Is it NOT necessary that in AC Circuits the "End" which is the "EARTH" be connected to the negative terminal for the circuit to work..?

But wasn't the premise of the question that the circuit was not, in fact, connected to earth? If the negative terminal of the circuit is at a different potential than ground, then this could be represented as another power supply with the hot end connected to the negative end of the first supply, and with its negative end connected to ground. Since this was not in the question, then no, the individual will not be electrocuted.

You *could* have AC circuits not connected to earth ground. If you had a petrol generator, for instance, and you plugged a power drill into it, that power drill wouldn't be connected to earth ground, and would still work (it'd still have the case grounding present, though this would be connected to the local ground--probably the generator casing). Assuming you sat the generator on a big, thick, sheet of something non-conductive, and you stood off of this sheet, you would have no current flow through you (and to ground) if you jammed your screwdriver into any ONE of the generator's outlet prongs. Now, if you were to jam a screw driver into one prong with one hand, and to jam another screwdriver into the other prong with your other hand, you may (literally) be playing with fire.

In most circuit diagrams, you usually see some kind of ground (or even, several types of these mixed together). These do not mean the same thing, as mentioned in another post in this thread regarding the various faces of "ground", and very often do not mean earth ground. Except when they do (but this is usually stated somewhere).

In a large thread like this, you should make use of the QUOTE button, or MULTIQUOTE button so we can know which post you refer to.

[salman213]

ok..umm, thank you.... I know you are trying to explain so thoroughly, but unfortunately im still not really understanding what the answer to my question is. :(

I made a diagram, can someone complete circuit B and does current flow through the guy?


First Diagram: http://physicsforums.com/attachment.php?attachmentid=15144&d=1219436573

[Averagesupernova]

XPTPCREWX: Do a search on this forum and you will see some pix that I have posted of the inside of a breaker box. YOU are the one who is an amateur. You do not have a very good grasp on electricity in general it would seem, or at least residential AC. Incidentally, I can't recall the last time I seen so much arrogance from an individual who didn't have the story straight.

[XPTPCREWX]

what part of the story do I not have straight?

[MATLABdude]

what part of the story do I not have straight?

[TONE = Expository, not angry] Where you go against every convention I'm aware of, and call the neutral, hot? Or disregard the effect of potential relative to ground for current (where you dare us to grab the neutral on the return leg of an energized load). But these are just matters of terminology and convention, but when you're arguing armed with these, people will not understand what you mean, nor you they. Based on this and some of your other posts, you sound like you're probably a first or second year undergraduate in EE (or are higher, but non-power concentration), with some exposure to "practical" AC, but only at a high and abstracted level. So you've got a good start, but definitely haven't finished.

Take a look at this document from APC--makers of UPSs and power correction/cleaning equipment, a company that probably knows their power (Neutral Wire Facts and Myths):
http://www.apcmedia.com/salestools/SADE-5TNQYQ_R0_EN.pdf

It supports your view--neutral as a power carrier, and yet supports everybody else's view on this thread as well: neutral is hooked up to ground (as opposed to hot being hooked up to ground), and this saves you from being killed when you grab the neutral wire because you are ALSO grounded (or for better or worse, have a high resistance to ground, but are not in contact with a high current source which is at higher potential than ground).

So yes, neutral and live can be reversed and things will work. Should you grab the bare live wire? Never! Have I grabbed the bare live wire before? Yes, and I was damned lucky that I was standing on a stack of dry planks with a poor path to ground. When you are at ground potential, can you grab the bare neutral wire? Probably. Should you make a habit of it? No, because you never know when the neutral wire or load or your breaker may be broken, and your neutral has all of a sudden become an extension of hot!

...And to elaborate on my previous posting in this thread regarding neutral as it relates to ground (of the protective earth variety), a summary of various earthing techniques (Canada and US usually have TN-C-S, and the green going to your house is PE-N, using the article terminology):
http://en.wikipedia.org/wiki/Earthing_system

[MATLABdude]

ok..umm, thank you.... I know you are trying to explain so thoroughly, but unfortunately im still not really understanding what the answer to my question is. :(

I made a diagram, can someone complete circuit B and does current flow through the guy?


First Diagram: http://physicsforums.com/attachment.php?attachmentid=15144&d=1219436573

Assuming the 1000V is relative to ground (and the bottom generator of circuit B is what provides the 1000V to the top generator), the poor individual will be in parallel with the bottom generator (and implicitly, his feet are connected to the bottom terminal of the bottom generator via ground). How much current will flow through him? Depends on how much resistance he has. Hopefully, he's got on really thick rubber boots...

EDIT: I believe this is the quote you refer to:

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.

I hope this makes sense, but sometimes, when things have been fuzzy, I've accepted them and moved on. Only later (with more experience, or something that forced me to make use of them), did they finally make sense.

[MATLABdude]

One thing I dont get.....

If Ground is connected to the Neutral Bar at the Breaker Box, which it is.
(in residential)......what is stopping the current of that branch circuit on the neutral side from ground out instead of returning to the source?

The Ground is being "spurred" off of the neutral bar.....and providing the path of least resistance....right???

Maybe if we went back to the original post that started all of this confusion (in between the back and forth with salman213)... The situation you describe is a bit backwards, as the neutral bar is the one being spurred off the ground. Ground out is okay, from ConEd, or Ontario Hydro's points of view (or whoever it is that generates your power for you), since they get their current returned to them. It might not go through the neutral wire, but current will return to source. Might not be so okay for whatever you arc-welded to create this turn of events, but for them, that's your business (as long as you don't black out your neighborhood).

Does everything return to source? It better, otherwise you'd charge up your plot of land. Might be a good security feature against the thieves that cross from public "ground" onto your land, though, if you could somehow electrically disconnect your patch from everything around you. :-)

Possibly of interest: single-wire power transmission (but even here, a loop is formed back to source--just *literally* through the ground):
http://en.wikipedia.org/wiki/Single_wire_earth_return

[salman213]

Ok. Thank you, I guess that makes sense that the individual would become parallel with the "potential difference" and so current would flow through alright. THANKSSSSSSSSSSSSS this is a great thread for me. :)