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Old 11-19-2010, 09:08 PM   #1
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ground/shock question


I was thinking, AC changes all the time, so at once instance a live wire might have 120 volts, another instance it might have 50 etc... as it alternates 60 times a second. Now if you touch the neutral or ground, you are no shocked because there is 0 potential between that, and the ground you are standing on or the air around you.

But if the neutral/ground is opposite of hot and and changes 60 times a second (technically becoming at a different potential with the natural ground), how is it that we do not get shocked by touching it?

Or is it the potential between grounded items (ex: a humid floor) and the ground that matters only? So if you stand on something insulated and touch the hot, then you don't get a shock either? (I'm too chicken to try it lol)

Last edited by Red Squirrel; 11-19-2010 at 09:13 PM.
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Old 11-19-2010, 09:23 PM   #2
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Try holding on to a loaded neutral sometime and then say there is no potential difference--right after you pic yourself up off the ground.
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Old 11-19-2010, 10:36 PM   #3
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NEC refers to neutral as the "grounded" conductor. At some point (first point of disconnection in your service) it is attached to ground. Neutral is always (theoretically) at the same potential as ground (it varies a bit, of course, due to voltage drop), but since your question is based on theory, my answer can be too, so we can forget that whole resistance thing.

Now, if, back at the panel, we removed the bonding jumper, and bonded one of the hot conductors instead (again, this is THEORY. DON'T EVEN THINK ABOUT TRYING IT!), it would now be safe to go anywhere in the house and grab ahold of a hot conductor on that leg, but grabbing neutral would hurt.

I have a friend who works on extremely-high-voltage power lines (the ones on big huge towers). They ATTACH themselves to whichever line they are working so as to be at the same potential as it. For them, the "ground" wire is potentially deadly.
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Old 11-19-2010, 10:38 PM   #4
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brric, I'm curious how "loaded" your neutrals are. 10% voltage drop total in a circuit is very high, and that only brings the neutral about 6VAC away from ground...
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Old 11-19-2010, 10:46 PM   #5
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Quote:
Originally Posted by emolatur View Post
NEC refers to neutral as the "grounded" conductor. At some point (first point of disconnection in your service) it is attached to ground. Neutral is always (theoretically) at the same potential as ground (it varies a bit, of course, due to voltage drop), but since your question is based on theory, my answer can be too, so we can forget that whole resistance thing.

Now, if, back at the panel, we removed the bonding jumper, and bonded one of the hot conductors instead (again, this is THEORY. DON'T EVEN THINK ABOUT TRYING IT!), it would now be safe to go anywhere in the house and grab ahold of a hot conductor on that leg, but grabbing neutral would hurt.

I have a friend who works on extremely-high-voltage power lines (the ones on big huge towers). They ATTACH themselves to whichever line they are working so as to be at the same potential as it. For them, the "ground" wire is potentially deadly.

Yeah I understand this part, but say you are standing outside bare foot in a puddle, and touch the neutral, the earth's ground does not alternate like AC does, from my understanding, so how is it that the grounded conductor always remains at the same potential as the ground I'm standing on?

Or is this a non issue, as long as I'm only touching one conductor I don't get a shock? I have seen some of those high voltage guys and yeah they do attach themselves right to the cables. Just don't want to touch anything grounded.
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Old 11-19-2010, 10:47 PM   #6
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Quote:
Originally Posted by brric View Post
Try holding on to a loaded neutral sometime and then say there is no potential difference--right after you pic yourself up off the ground.
Wont do anything unless it's reverse polarity.
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Old 11-19-2010, 10:56 PM   #7
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The neutral, by design, should be held to 0v potential relative to ground. It is called a grounded conductor, because it is grounded. On residential power distribution, it is connected to the center tap of the transformer, which is bonded to ground. The hot wires, coming from the outside taps of the transformer, swing from -120v to 120v, relative to the neutral, such that there is always 240v between them.
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Old 11-19-2010, 10:57 PM   #8
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Here is where your a little off in your thinking...
Quote:
But if the neutral/ground is opposite of hot
In theory neutral or ground is always Zero. It is the Hot side that is changing from +120 to -120.
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Old 11-19-2010, 10:58 PM   #9
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Quote:
Red Squirrel;537289]Yeah I understand this part, but say you are standing outside bare foot in a puddle, and touch the neutral, the earth's ground does not alternate like AC does, from my understanding, so how is it that the grounded conductor always remains at the same potential as the ground I'm standing on?
because that conductor is connected to earth ground so it is always at 0 potentiol, regardless where in the sine wave the voltage is at any given moment.

Quote:
Or is this a non issue, as long as I'm only touching one conductor I don't get a shock? I have seen some of those high voltage guys and yeah they do attach themselves right to the cables. Just don't want to touch anything grounded.
electricity is terribly misunderstood.

A simple answer; as long as you have 0 potential between two points, you can touch those 2 points without getting shocked.

In an ungrounded 3 phase system, you can actually touch any of the 3 hot conductors, one at a time, and ground and not get shocked. at least theoretically. There is often inductive or capacitive coupling that gets in the way of theory though depending on what else you are touching. There is no relationship with ground in such a system so there will be no current flow from any of the individual hot legs to ground.
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Old 11-19-2010, 11:07 PM   #10
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Quote:
Originally Posted by nap View Post

electricity is terribly misunderstood.
I used to have something like this in my signature....
If I'm sitting in a fiberglass bath tub full of warm water and the drain pipe is PVC and the supply line is Pex, am I grounded?
Another thing. You cannot have voltage unless you have current flowing through some resistance.
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Old 11-19-2010, 11:12 PM   #11
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Actually, I think it just clicked. I was not taking into consideration the two hot legs. Those legs are alternating together (one at +120, other at -120, then they switch and so on), the neutral is just in the center of it so it stays at 0 all the time.

Now that I think of it, if I was to draw a sine wave of one hot leg to neutral, would it be a half sine?
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Last edited by Red Squirrel; 11-19-2010 at 11:14 PM.
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Old 11-19-2010, 11:14 PM   #12
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No, it'll be a regular sine wave, which crosses through zero 60 times a second.
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Old 11-19-2010, 11:17 PM   #13
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No, still a sine wave.

H/N, we start at 0/0, go up to 120/0, then back to 0/0, then -120/0, then 0/0

The very same pair of wires, at the same points, if we flip our multimeter probes backwards so we're calling hot "0", it's now
0/0, 0/-120, 0/0, 0/120, 0/0

Notice we can actually have "negative" voltages. They are "negative" relative to whatever we're calling 0. Meaning, their real-world absolute potential (which doesn't exist anyway) is 120 less than whatever the one we're calling "0" is.



Wow doesn't this mess with people just starting in electronics. Show someone an amplifier schematic with "-81V" written on it and brace yourself for the confusion!
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Old 11-19-2010, 11:23 PM   #14
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Think of red as one hot leg, and green as the other, and imagine that the scale reads from 120 to -120, instead of 1 to -1.
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Old 11-19-2010, 11:24 PM   #15
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Actually it makes sense now. Think it just all clicked. One mistake I tend to make is forget that voltage is simply a potential between two points. When you think of it that way everything just falls together.
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