The typical AC appliance is connected to one hot wire and one neutral (ground) right? So how exactly does current flow in this situation? Where exactly is the circuit? Electricity flows from the power plant to the appliance through the hot wire, right? How does it get back to the power plant? Does it take the same path back? How does the ground come into play? What’s so special about the ground in electrical terms, anyway?
Obviously my understanding of DC current does not apply very well to AC.
Your mistake is confusing ground and neutral. Neutral goes back to the power plant, while ground goes to, well, the ground. That’s what the 3rd prong is for in plugs.
Ground has a few meanings. In the home appliance case it tends to mean the voltage of the environment around you which tends to be dominated by the physical ground. Every thing conducts electricity to a greater or lesser extent. So you and your surrounding environment is at some voltage called ground.
Ideally the circuit is back to the transformer that reduces the high voltage to domestic voltages through the wires that the utility connects your house to the power grid. One is the hot wire the other is the neutral wire. The neutral wire should be a the same voltage as ground.
The third prong of the plug is meant to provide a path from the hot voltage to the surrounding area in a way that does not run through people using appliances. The basic idea is to connect the conducting case of the appliance to ground so that in the case of damage to the appliance a circuit from hot to ground happens through the third prong of the plug instead of people touching the appliance.
[QUOTE=Phantom Dennis]
The typical AC appliance is connected to one hot wire and one neutral (ground) right? So how exactly does current flow in this situation? Where exactly is the circuit? Electricity flows from the power plant to the appliance through the hot wire, right? How does it get back to the power plant? Does it take the same path back? How does the ground come into play? What’s so special about the ground in electrical terms, anyway?
Obviously my understanding of DC current does not apply very well to AC.
[/QUOTE]
The ground wire is typically connected to the chassis of whatever is being powered. Let’s use a toaster as an example. If the hot wire come loose inside, or the insulation rubs or melts away and it contacts the metal case, nothing happens, the circuit isn’t complete. BUT the case of the toaster is now hot. If you touch it, you’ll get a shock. If the toaster chassis is grounded, when the wire comes loose and touches the case, the current will flow thorugh the ground wire and back to the breaker. Becuase there’s no load, however, it will trip the breaker and the line will go dead, and you’ll be safe.
As for how the power comes to your house, you have two hots (180 degrees apart) and a neutral. The power flows through the hot wire, into your house and back to the plant through the neutral. I’ll let someone else explain the nuances of the ground/neutral relationship once the power enters your house. It’s late and I’m not thinking clearly enough to explain it.
Okay, so the neutral is not the same as the ground. I always thought they were interchangeable. The neutral is connected to the ground at some point though, right?
Another question: as I understand it, AC current changes polarity 60 times a second. So how is it always flowing from hot to neutral? Shouldn’t it flow from neutral to hot half the time?
Thanks. 
[QUOTE=Phantom Dennis]
Okay, so the neutral is not the same as the ground. I always thought they were interchangeable. The neutral is connected to the ground at some point though, right?
[/quote]
No, the neutral goes back to the power plant to complete the circuit with the hot. The ground can be connected to the neutral, but it doesn’t necessarily have to. The point of the ground is that the hot always has a path back to neutral so that you don’t become that path. For example, if the hot somehow came into contact with the ungrounded metal chassis of a piece of equipment and you touched it, the current would flow through you to ground. On the other hand, if the chassis were grounded and the hot wire touched the chassis it would flow through the chassis to the ground and trip the circuit breaker.
Ground refers to the voltage that the earth is at, i.e. 0 voltage. Neutral refers to the return wire to the voltage source. It too is at essentially 0 voltage, but has current flowing through it, while the ground typically doesn’t.
[QUOTE=Phantom Dennis]
Another question: as I understand it, AC current changes polarity 60 times a second. So how is it always flowing from hot to neutral? Shouldn’t it flow from neutral to hot half the time?
Thanks. 
[/QUOTE]
It does flow both ways.
[QUOTE=treis]
No, the neutral goes back to the power plant to complete the circuit with the hot. The ground can be connected to the neutral, but it doesn’t necessarily have to.
[/QUOTE]
Yes, it does. This is an NEC requirement. I’m at work, so I don’t have my copy to quote the relevant section to you.
[QUOTE=treis]
The ground can be connected to the neutral, but it doesn’t necessarily have to. The point of the ground is that the hot always has a path back to neutral so that you don’t become that path. For example, if the hot somehow came into contact with the ungrounded metal chassis of a piece of equipment and you touched it, the current would flow through you to ground. On the other hand, if the chassis were grounded and the hot wire touched the chassis it would flow through the chassis to the ground and trip the circuit breaker.
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While this is true, it does not explain why the AC power system in your house is referenced to earth ground in the first place.
The question you need to start with is this: why is the secondary of the step down transformer (hanging out on the pole next to your house) referenced to earth ground?
(The transformer’s secondary has three wires, BTW: one wire goes to one end of the coil (Hot 1), the other wire goes to the other end of the coil (Hot 2), and a center tap. All three wires go to your house. The center tap is called neutral. And for some strange reason, the center tap is also connected to earth ground.)
When you first think about it, it sounds kinda dumb to connect the center tap (or any of the secondary wires) to earth ground. You can power your house just fine if the secondary were not referenced to earth ground, so why do it? At first glance, it seems like connecting the center tap to ground would actually make the situation *more * dangerous, since you are now guaranteeing there will be a potentially lethal voltage between Hot 1 and earth ground, and Hot 2 to earth ground. By *not * connecting the center tap to ground, there would *not * exist a lethal voltage between Hot 1 and earth ground, nor Hot 2 to earth ground, which means if you’re grounded (which is often the case) you won’t get shocked by touching Hot 1 or Hot 2.
Or so it seems. 
The reason we ground the secondary of the transformer is because, if we didn’t, the common-mode voltage of the secondary winding could float up to the primary voltage. This would not be a good thing, as you might image; while there would still be 120 VAC between each hot and the neutral line, there would be *thousands * of volts between each of the secondary lines and earth ground. :eek: And why would it float up? Because transformers do not exhibit perfect isolation; there’s always a little bit of leakage or coupling impedance between the primary and secondary windings.
So to fix this problem, we connect one of the secondary wires to earth ground. It really doesn’t matter which wire; you could connect Hot 1 to earth ground, for example, and it would solve the problem of having the secondary float up to the primary voltage. In all installations, however, we connect the center tap to earth ground. It’s a bit safer this way… if we connected Hot 1 to earth ground, there would be 240 VAC rms between Hot 2 and earth ground and 120 VAC rms between the center tap and earth ground. On the other hand, if we connect the center tap to earth ground, there would be 120 VAC rms between Hot 2 and earth ground and 120 VAC rms between Hot 1 and earth ground, which is safer.
[QUOTE=Q.E.D.]
Yes, it does. This is an NEC requirement. I’m at work, so I don’t have my copy to quote the relevant section to you.
[/QUOTE]
I think he meant that it doesn’t have to in order for the circuit to work.
But otherwise, you’re right, the NEC does require the neutral to be bonded to the ground at the service disconnect.
The OP might want to read this- “how stuff works” link.
The ‘neutral’ leg would probably more properly be called the ‘return’ leg. It causes less confusion that way. Assuming that “neutral” means “no voltage present” has gotten many people shocked over the years. A cardinal rule when I was going to electrician school in the dark ages was to always open the hot leg first, neutral second, ground last; the reverse can get you killed.
Electrical systems don’t have to be grounded. Hospitals, for example, run isolated power systems that are actually safer than a grounded system, because you can touch either wire and not get shocked (if you touch both, though, you still get zapped). If you are ever in the hospital, look for the red colored outlets (in the US). Those outlets are fed by isolation transformers.
The problem with running a large isolated system is that mother nature tends to ruin your isolation. Hospitals have a heck of a time keeping their isolated systems isolated, which includes a lot of maintenance and yearly testing, and that’s on a system that doesn’t even leave the building. On large scale power systems, if you try to run an isolated systems, what you end up with instead is a randomly grounded system. We’ve found that it is better and safer to have an intentionally grounded system where you know where the grounds are instead of a randomly grounded system where you don’t know where the grounds are.
We’ve also found over the years that it’s safer to run a separate protective ground that carries no current than it is to just use the neutral as a protective ground. In a 3 pronged plug, that third “ground” connection is a safety. It normally carries no current. Your computer case (if it’s metal), the outside of your stove, etc. are all connected to this safety ground. All of the water pipes in your house are required to be connected to this safety ground as well.
[QUOTE=Phantom Dennis]
Okay, so the neutral is not the same as the ground. I always thought they were interchangeable. The neutral is connected to the ground at some point though, right?
Another question: as I understand it, AC current changes polarity 60 times a second. So how is it always flowing from hot to neutral? Shouldn’t it flow from neutral to hot half the time?
Thanks.
[/QUOTE]
As people said, it does, but I don’t think they explained such that you know what is happening. Think of the voltage of the neutral line to be constant line at 0 volts. The hot line is a sine wave that goes both positive and negative (with a frequency of 60 Hz). The current is determined by the difference between the two. Thus when the hot line has a positive voltage the current flows one way, and when it has a negative voltage it flows the other way. The same thing would happen through you if you touched the hot, but is why nothing happens when you touch the neutral.
It is easiest to think of it as a water line. The neutral is at ambient pressure (0 voltage) and therefore never causes any water flow (current) on its own. The hot line cycles from a positive to a negative pressure and therefore either sucks water from the neutral or pushes it into it.
Some old-timer electricians I’ve worked with refer to the ground wire (green or bare) as the “grounding conductor” (because it grounds the system) and the neutral wire as the “grounded conductor” (because it is connected to ground at the panel). It’s technically accurate terminology, but confusing as hell.
Three conductors in a standard ac circuit.
Ungrounded conductor(aka hot) this is powered from the power company.
Grounded conductor(aka neutral) this is a path to ground in the US that path should go back to the service panel before a path to ground is available. Should a service have been set up before current codes it’s possible it may not ground till a transformer or such. In short it goes to ground the power company doesn’t reuse it. The grounded conductor completes the circuit.
A grounding conductor(aka ground) is a safety it is a direct path to ground. It’s purpose is if any problems occur in the circuit stray current has somewhere to go. If a direct short occurs it has an ampacity sufficient cause the over current device(aka circuit breaker) to trip.
[QUOTE=Gus Gusterson]
Some old-timer electricians I’ve worked with refer to the ground wire (green or bare) as the “grounding conductor” (because it grounds the system) and the neutral wire as the “grounded conductor” (because it is connected to ground at the panel). It’s technically accurate terminology, but confusing as hell.
[/QUOTE]
It isn’t just old timers. Those are the terms used in the National Electric Code.
If all of this isn’t bad enough, there are significant changes in store regarding terminology used in the 2008 code. Article 250 promises to be a focus of discussion in the near future.
[QUOTE=flight]
It is easiest to think of it as a water line. The neutral is at ambient pressure (0 voltage) and therefore never causes any water flow (current) on its own. The hot line cycles from a positive to a negative pressure and therefore either sucks water from the neutral or pushes it into it.
[/QUOTE]
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.
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.
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.
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.
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).
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?
I never realized that the earth can conduct electricity in this way.
[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?
I never realized that the earth can conduct electricity in this way.
[/QUOTE]
nope your still missing it.
A path to ground is what completes the circuit. The neutral is a path to ground. If you touch a hot wire while grounded you become a path to ground.
The earth has a balanced number of positive and negative electrons. AC current alternates between a high number of electrons(positive) and a low number of electrons(negative). Negatives travel towards a positive. When the circuit is complete the electrons are traveling back and forth based on the wave of the AC curent.
I apologize for the last paragraph it is the best I can explain things and may be lacking in technical accuracy.
Are you sure?
Oh man. When I’m having this much trouble grasping a concept, experience tells me that one of the fundamental assumptions upon which I’m attempting to build this understanding is flawed. I’m going to look for some reading material on basic electrical theory and start from scratch. Eventually it will all fall into place, and I’ll smack myself on the forehead and wonder why I didn’t figure it out sooner.
Thanks for all of your help though.