Let's say we have a 120 Volt AC source

JaredJames

He said both were likely to happen, not that both would. There is a difference.
There is always a potential difference between two points. It's just a matter of how much of a difference.

If the pole is at 1 million volts and the ground is zero (as per your previous post) then there is a potential difference between the ground and the pole.

You may want to go back and learn the basics of electricity.

Evil Bunny

The million volts was between the two poles. 0 volts between pole and ground, remember...

JaredJames

If your pole is at 1 million volts and the ground is at zero, there is a potential difference between them.

Think of an electric line power line. The line is at 400,000V and the ground is your zero reference. The only reason it doesn't arc is because the path of least resistance is to follow the wire.

Now, if I get close enough to the wire, suddenly the path of least resistance is to arc through the air, through me and into the ground. And so it changes direction and follows that route instead - until I move away.

Evil Bunny

The utility company ties one of their two poles to the ground. That is the only reason there is a potential there. We did not do that. That is a very critical point to this thread. Go back and read carefully.

JaredJames

What you have posted is the inversion of one of the standard procedures for servicing of HV lines.

They use a helicopter to allow someone to work on the lines whilst live. As the helicopter is not grounded, there is no potential (or very little) potential between it and the lines - so no arc.

In this case, it won't arc to the ground but to the other pole. EDIT: Assuming path of least resistance is to the other pole.

However, with a significant charge imbalance you will get an arc between the pole and the ground - as with static (lightning).

Evil Bunny

Perfect... Ok so my main reason for posting this is because I am having a hard time understanding why we can develop a large potential (between the two poles of my imaginary hovering generator) and not have the electrons make their way from the earth to one of these poles. I would think that these electrons in the earth would be attracted to one of these poles because of a huge difference in charge, but this is apparently not the case.

Why?

I think the explanation was given by Naty1 in post # 12 of this thread but I'm still having a hard time grasping the concept.

Lightning was also brought into this conversation and rightly so... Obviously there are scenarios where the potential between cloud and ground becomes so great that we ionize the air and lightning occurs. What is the difference? Is it sheer magnitude of voltage? Are we simply not generating enough voltage to "encourage" interaction between the generator pole and ground? Or does voltage (between the two generator poles) have absolutely nothing to do with this "charge" that causes lightning?

JaredJames

It's worth pointing out, if you moved the generator so that the path to earth had less resistance than to the other pole, it would arc to the earth. This is like static electricity from a car.
The car becomes charged due to sunlight on a hot day. It is not grounded to the earth, but when you touch it you ground it and so receive the shock.
This is true for electrical lines as well, regardless of whether or not they are in grounded at some point. The reason it arcs is not because the company ground the cables at some point.

In the case of lightning, it is only when the charge builds up enough to give you a large enough driving voltage to arc to the ground. So yes, it's all about voltage.

rcgldr

In the USA, at most (but not all) transformers, the center tap of the transformer is is grounded, so that at the final step down transformer, and at a household, the neutral ends up +/- 10 volts of earth ground. At my house, it's close, about .3 to .5 volts, but it's not zero. The final step down transformer only has 2 lines going into it, 6900 volts ac, and steps the voltage down to 220 volts, with a non-grounded center tap to provide a neutral. At my house hold, there is a grounded rod used for the third round pin socket on 3 pin plug sockets. The wide pin socket hole is neutral, and the narrow pin socket plug hole is one of the two end taps from the transformer, to provide 110 volts. A 220 volt socket would use both ends of the transformer, plus the grounded line.

There's a huge potential beteen helicopter and the line. The first part of the procedure is to extend a probe connected to the helicopter and the line being worked on, so that both helicopter and the line end up at the same potential, You often see significant arcing during the connection process. The line is then connected to the helicopter while the worker moves onto the line, then the line is disconnected. This processis repeated again when the worker leaves the line. The worker also uses a faraday cage like suit so any current flow or transformer effects of working on a single wire go thorugh the suit and not the worker. Example video:

http://www.youtube.com/watch?v=r_1T2_l43Xo

There are also line inspector robots a bit less than 1 meter long that rely on a transformer effect from the two points on a single line to power them so they can move on high power, high tension lines. (I couldn't find a video of these in action). Just the field from these lines is strong enough to light up flourescent bulbs:

http://www.stopgeek.com/richard-boxs-light-field.html

JaredJames

Initially yes, I meant later with the guy jumping about. The fact the helicopter never gets to the point of creating the path of least resistance means they can get to a zero potential level. So no arc when he's messing about.

It's what I was trying to clarify with my last post regarding moving closer to the earth.

Evil Bunny

No it wouldn't. It's all about where these electrons want to go. They don't want to go to the earth, they want to go to the other pole. We talked about it in this very thread. If you put a straight wire from pole A to earth, virtually no current would flow (the same is true between pole B and the earth). If you put a straight wire from pole A to pole B the wire would quickly melt from all the current.

No, it's not actually... And this is what I'm trying to find out. What is different about it? This "static" electricity that shocks you when you touch your door knob or your car or allows you to stick a balloon to the wall is the same as lightning.

But it's not the same as having a voltage potential between these two wires of the "source" (generator) and their relationship to the earth.

This is the whole point of the thread. I'm looking for an explanation of what is different about it.

I guess I'm just not explaining it very well. I'm probably not asking it the right way or something.

Evil Bunny

The primary of the transformer has one side attached to HV (6900VAC in your example) and the other side attached to the "return" which is the wire located underneath the HV line parallel with it on your utility pole). The "return" side of the primary is also attached to ground by a large conductor and a rod driven into the earth... The secondary has two "hots" and one center tapped neutral coming out of it. The center tapped neutral absolutely is grounded. It is grounded with the same conductor that is grounding the primary of the transformer. The two "hots" give you 240 Volts between them. Either of the "hots" give you 120V when referenced to the center-tapped neutral.

The bare copper wire (third round pin) is used for "bonding" metal parts in your structure together and they are also tied into the "neutral" as well as the "ground rod" at your service entrance.

JaredJames

I'm curious how you can know all this without knowing the reasons (physics) behind it - or at least some knowledge.

JaredJames

Right, you need to check a few things here. I suggest you read up what charge is and what voltage is before we continue.

As rcgdlr said, you get an arc between the helicopter and the power lines - note that the helicopter isn't grounded.

Initially, the PD between the two is so high, you get the arc - much like static shocks.

Evil Bunny

The reason for grounding the distribution system in the United States is put forth in the NEC. They mention things like stabilizing voltages and protection against line surges and lightning strikes and unintended contact with high voltage sources... the article is 250.4 (A)(1) I think but I don't have it in front of me.

I definitely have some knowledge, but I'm looking for more. I don't understand how we can have a potential voltage on a pair of wires (like our generator scenario) and not have a potential between either one of those wires and the earth. It just seems like there would be some "charge" on the wire that would want to interact with the electrons in the earth.

My brain hurts... I need to take a break from this and regroup.

rcgldr

Then why do I see .3 to .5 volts between ground and neutral at my house? I've asked a few of my friends to do measure the difference at their houses, and the voltage is a bit under 1 volt, but it's never zero. I've seen the final transformer (it's less than 100 feet from my house) when it was being serviced. Two well insulated lines going in, 3 well insulated lines going out (the two hots and neutral), none of them are grounded.

Studiot

Since my meaning was still in dispute:

Friction causes some clouds to become charged with static electricity. Some become positive, some become negative so act like battery poles.

If you watch the heavens you can see some lightning flashes between one cloud and another.

You can also see lightning flashes between some clouds and ground.

Both types of flash act to discharge the clouds and we all know just how energetic and destructive a lightning flash can be.

Then I (respectfully) suggest you prick up those long bunny ears and do more listening and less expounding.

Evil Bunny

The reason for voltage between neutral and ground is probably voltage drop. They are tied together in your panel and then attached to ground just outside of where your panel is located. They are tied together at no other point in the system, so as you get further away from the box the voltage might tend to "drift".

As to why they aren't grounded at your utility pole, I don't have an answer. Every one that I've seen, including the one at my house, has a large conductor connected to a ground rod at the base of the utility pole. At any rate, the neutral is most certainly grounded at your panel.