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Simple Electrical Question

by salman213
Tags: electrical, simple
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salman213
#73
Aug22-08, 04:20 PM
P: 303
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
#74
Aug22-08, 05:59 PM
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Quote Quote by salman213 View Post
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
#75
Aug22-08, 06:43 PM
P: 303
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....4&d=1219436573
Attached Thumbnails
qq.jpg  
Averagesupernova
#76
Aug22-08, 07:45 PM
P: 2,509
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
#77
Aug23-08, 02:29 AM
P: 97
what part of the story do I not have straight?
MATLABdude
#78
Aug23-08, 04:48 AM
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Quote Quote by XPTPCREWX View Post
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/S...NQYQ_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
#79
Aug23-08, 04:57 AM
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Quote Quote by salman213 View Post
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....4&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:

Quote Quote by mgb_phys View Post
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
#80
Aug23-08, 05:23 AM
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Quote Quote by XPTPCREWX View Post
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
#81
Aug23-08, 11:01 AM
P: 303
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. :)
Hirams_bro
#82
Aug4-10, 05:57 PM
P: 10
XTP i understand your frustration. They don't have a clue what you're even asking. What a bunch of amateurs! I read this thread and it made me laugh; it reminded me of that movie idiocracy. If these are anything but high schoolers then this nation is doomed! How are we ever going to survive and compete with other nations? We'll be third world in less than 30 years, GUARANTEED!

What I think XTP is asking is quite simply this (I'll use an example to illustrate):

Say you have a 15A branch on a service panel. To avoid any confusion let's also say there are no other branches - "breakers" - in this panel: so just a single branch carrying 15A.
This would mean we have 15A flowing from the panel through the hot or black wire, traveling to the load and then back to the panel via the neutral wire (KCL LAW: same current going into load must come out.)

So the current is now back to the panel and has reached the neutral bus bar and here is the question: Since the neutral bus bar (in the panel) is bonded to the ground rod, why doesn't some of this current flow into the "ground rod and earth system", in effect forming a constant short to the ground rod.

And I think I might have an answer or at least provide a helpful insight, and I hope some qualified person will correct me if I'm wrong.

QUESTION: The current does in fact short to the "ground rod and earth" constantly, but using the current divider rule, this current is negligible b/c the return to the center tap of the transformer offers a much lower resistance so that it carries the brunt of the current.

Is this right?

If so then my next question would be: what is the most important reason for the neutral being bonded to the ground?

My hypothesis:
Because the "ground rod and earth system" by itself offers too high of a resistance (15-20 ohm) to guarantee that the circuit breaker will trip during ground fault, so it is tied to the center tap return in order to guarantee a low resistance path for the ground and thus a high enough current needed to trip the circuit breaker.

Yea...and if not then I just have to assume it's "white man's magick."

P.S. - I think the confusion resulted from people not being clear about which "Ground" was forming a hypothetical short, XTP using ground to mean what I call here "ground rod and earth system" vs. the others taking it to mean the ground wire before it reaches the bus bar in the panel.
Averagesupernova
#83
Aug4-10, 11:42 PM
P: 2,509
^^^^^^^^You have a lot to learn. Oh yeah, this thread is almost 2 years old.
sophiecentaur
#84
Aug5-10, 07:35 AM
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@Hirams_bro
Your post makes a fair amount of sense but what is the excuse for such an abrasive introduction? A lot of the people who post here are very much "amateurs". Why is that a reason to get so shirty with them? The problem with this sort of thread is that the initial questions can all be sorted out using Kirchoff's Laws but people want 'folksy' explanations involving colloquial terms. That's because a lot of them ARE amateurs. Have a bit of patience and bite your tongue before applying the vitriol.
Hirams_bro
#85
Aug7-10, 10:40 PM
P: 10
nova, thank you for your trenchant observation on the age of this thread. It 's good to know if I ever have any questions about dates, timezones, daylight savings, or any other time-related issues, I know just where to turn. Like you I find the whole concept of timestamps extremely atavistic and that's why I had no idea how old the thread was.

Maybe for you the forum is just an online kaffeeklatsch, but for me its about learning and helping others to learn. I think you have a lot to learn since you don't seem to have an intution about things; otherwise you might have intuited that perhaps a link to this thread was offered as an "explanation" by someone else in answer to a similar question in another thread, OR the fact that the thread hasn't been deleted yet, and maybe for good reason e.g. the admins - in their intuition - thought that it would be a good idea to leave the door open for people to contribute later, so that others in the future might benefit by finding an answer to an age-old question. Had you intuited that perhaps you might have realized why it was important for me to follow up, even on a thread as old as this.

^^^^^^^^^Age is just a number.... & I'm just a good-old patriotic American like yourself!
sophiecentaur
#86
Aug8-10, 07:21 AM
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Bearing in mind that we still discuss Darwinism, Relativity and QM (all around 100 years old), a thread of only two years' running seems quite up to date. Has anything changed since the thread was started that might make it invalid or of no interest? It seems to me that it is chock full of conceptions that need straightening out.
Averagesupernova
#87
Aug8-10, 03:14 PM
P: 2,509
Hiram, I'd recommend you at least research to see the last time the user you are addressing was online when you come to a thread this old. XPT last posted June 5th of 2009. It's pretty unlikely XPT will ever see anyting directed his way by you.


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