The ground in AC current

[Triskadecamus]

Long ago, in El Paso Texas, I worked on sensitive electronic devices. One of the peculiarities of El Paso is that it has very little near surface ground water for most of the year, and a large ranges of its surface soil is clean sand, gravel, and rock. The Army decided, after several difficult problems were traced to ground faults, to create a ground circuit for the entire work area. Bare copper wire, and aluminum cables were set up in every building, and then run between buildings as well, with deep set ground stakes every hundred feet or so. This entire “circuit” was designed so that ground was, in fact, a stable, and equal potential over the entire facility.

So, every plug had four wires on it. Two hot, one common, and one bare copper wire that went all the way to the building ground wire. We had to test between copper ground and common ground as a first step in any procedure. It was zero volts every time I checked it.

So, the entire set up was to assure that the previously mentioned “isolation” did not take place. Anyway, equipment would work without it, but potential differences between widely separated pieces of equipment could occur, given the unusual terrain character. When long distance communication between very sensitive instruments were being tested, a millivolt difference in ground potential actually mattered. When megawatt systems were being monitored, the reliability of the case ground mattered too. An isolated circuit works fine, as long as nothing unisolates it.

Tris

[Chronos]

(All you physics nitpickers who want to talk about point charges, please stay out of this discussion )

Why? We agree with you. There are some common conventions for reference voltage in physics, such that the “potential at a point” is defined relative to that reference, but they’re still only conventions, and it’s still only the difference in potential which is ever relevant.

Phantom Dennis, the trick that you’re probably missing is that you don’t actually need a complete circuit for a current to flow. All you need is a potential difference. If you don’t have a complete circuit, then charge will accumulate at the ends of the paths, until the accumulated charge cancels out the voltage you had. This is what happens in a capacitor. Well, the Earth is also a capacitor, but it takes an awful lot of charge accumulating in the Earth to make much difference in the voltage, and the current that flows before it reaches that point is still quite enough to have unpleasent consequences.

[Crafter_Man]

[QUOTE=Phantom Dennis]
Okay, I think I understand a little more than I did before.

So when I touch a hot wire while being grounded, the reason I get shocked is that neutral is also connected to the ground, so my body is completing the circuit between the hot wire and neutral. Right?.
[/QUOTE]
When you get shocked, the neutral wire usually has nothing to do with it.

To start, forget about “grounding” for a moment. You have three wires coming into your circuit beaker (CB) panel from the transformer hanging out on the pole: Hot 1, Hot 2, and a center tap. Think of the transformer as an AC battery with three terminals. There is 120 VAC between Hot 1 and the center tap. There is 120 VAC between Hot 2 and the center tap. And there is 240 VAC between Hot 1 and Hot 2. Since these are isolated voltages, the voltage between Hot 1 and earth ground is undefined/unknown, the voltage between Hot 2 and earth ground is undefined/unknown, and the voltage between the center tap and earth ground is undefined/unknown.

In my previous post, I explained why it is necessary for one of the wires (Hot 1 or Hot 2 or the center tap) to be tied to earth ground. I also explained why the center tap is the one.

So after we connect the transformer’s center tap to earth ground (via a wire and copper grounding rod), we have the following:

There is 120 VAC between Hot 1 and the center tap (just as before).
There is 120 VAC between Hot 2 and the center tap (just as before).
There is 240 VAC between Hot 1 and Hot 2 (just as before).
There is 120 VAC between Hot 1 and earth ground (new!).
There is 120 VAC between Hot 2 and earth ground (new!).
There is 0 V between the center tap and earth ground (new!).
Time for an example.

Let’s say you standing barefoot in front of a receptacle outside your house, and the receptacle is connected to Hot 1. (It is also connected to neutral and earth ground, but don’t worry about those. Hot 1 is the only one that matters in this example.) If you insert an ice pick into the receptacle’s hot terminal, here is the current path. Recalling that there is 120 VAC between the transformer’s Hot 1 and center tap connections, charges will flow…

1. Out of the transformer’s Hot 1 connection.
2. Through the Hot 1 wire to your house.
3. To your CB panel.
4. Through your CB panel.
5. Through the Hot 1 Romex wire (running through the walls in your house) between the CB panel and the receptacle you’re standing in front of.
6. To the screw on the receptacle, and through the receptacle.
7. Through the ice pick.
8. Into your finger.
9. Out your feet.
10. Into the earth.
11. Through the earth.
12. Out of the earth and into the wire that connects the transformer’s center tap to earth ground.
13. To the transformer’s center tap connection.

(Of course, this only occurs during half a cycle. During the other half, the charges flow in the opposite direction. At least they usually do. Never mind.)

Does this help any?

[flight]

[QUOTE=engineer_comp_geek]
I hate water pipe analogies. They are useful for teaching the concepts of voltage and current to a newbie, but other than that, they just end up leading to confusion and misconceptions.
[/quote]

Based on the OP, that was what I was going for. Further posts after yours suggest that he is relatively new to some of the concepts as I suspected.

Voltage at a point is meaningless. Voltage is a difference between two points (All you physics nitpickers who want to talk about point charges, please stay out of this discussion ). Whenever you talk about voltage in an electrical system, you generally pick one point and arbitrarily call it your “zero”. It’s important to note that this designation is completely arbitrary. We tend to pick earth ground and call it zero because it makes the math work out a lot easier. We could just as easily pick the “hot” wire and arbitrarily call it our 40 Billion and 3 reference. The neutral would then end up being at 40 billion and 3 minus 120 volts, with the same 120 volts difference between the hot and neutral that everyone is used to. Subtracting zero is lot easier than subtracting 40 billion and 3, so we use zero instead, and we use earth ground as our reference since most things in the world tend to be sitting on it, and it makes a convenient reference point.

Though there are reasons to pick apart the pipe flow analogy, this is not one of them. Instead of ground we have “ambient pressure”. Just as with current and voltage difference, flow is dependent on pressure difference. If you connect two pipes each at a 40 billion and 3 Pascals you will not get any flow (other than outward when they explode). You need to have a pressure gradient, like opening one of those pipes to the ambient pressure (ground).

Now just because we’ve picked one wire and arbitrarily called it zero doesn’t somehow transform this wire into something magical. It’s still the potential difference between the hot and neutral wires that causes current to flow. One wire isn’t “sucking and blowing” while the other one does nothing. The difference between the two causes current to flow.

Though this is, of course, true, it is just as true for the pipe analogy. But, since we are using a grounded neutral with a constant voltage level it is easier to think of that way. It is how people think of pipe flow (and vacuum cleaners) even though it is no more accurate there.

Consider an isolation transformer. You don’t have a grounded neutral. Both wires are essentially “hot”, and the whole thing floats relative to earth ground. If you arbitrarily call one wire the 0 wire, then the voltage on the other wire goes positive and negative in the typical AC sine wave fashion. If you arbitrarily pick the other wire as the zero reference, then it’s the first wire that changes. Which wire you arbitrarily call zero doesn’t cause the other wire to suck and blow against it. Both wires are essentially the same. They both suck and blow with respect to each other. Taking one wire and arbitrarily tacking it into the dirt doesn’t change any of that.

Just like pipes.

Nothing happens when you touch the neutral because you’re standing on the earth and the neutral is electrically tied to earth. If the ground connection degrades (which has been known to happen), the neutral voltage with respect to earth ground can become large enough to be deadly (see Crafter Man’s post above).

Just like pipes. If you have one open to air at the other side (and no height change, that would kill the analogy unless you can do some backbending to relate it to running the wires through a changing magntic field or something) and you open your end to take a drink you will get nothing. If the other end is cycling pressure all over the place you will have a water cannon pointed at your head.