# How does AC have a hot and a neutral?



## Dustincoc

If AC is constantly alternating directions, then how does if have a hot and a neutral leg? It seems like it would have two hot/neutral legs.

I've always wondered how it was possible...


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## JD

The power does flow in both directions, but the neutral is held at (or near) ground potential. Thus the hot swings from being positive to negative to positive and so on. It is reversing. The only thing bonding one of the conductors to ground does is provide perspective. From our perspective, one conductor is a lot more dangerous than the other. We call that one the "hot." Imagine a light bulb connected to a battery. Now imagine connecting a clip lead from one terminal of the battery to ground. Now imagine you moved the clip lead to the other terminal. The bulb wouldn't care which one you chose, yet in one case (from our perspective) the power would be flowing away from the "ground" and in the other case it would be flowing toward it. Doesn't much matter. We only know that whatever terminal we did not connect the ground line to would be the "hot" side as it has a voltage potential that is not at ground. 

By the way- As in our battery example, no power flows through the ground path. The ground only provides the perspective. It is the same with AC. Unless there is a short, or something has gone wrong, no power should actually be flowing in the ground conductor. It's only there for perspective. (In theory!) The ground conductor attempts to keep us safe by being bonded to the neutral as well as the earth. Should something go wrong, we want the equipment housing to stay at ground potential, and not become "hot." The ground conductor diverts power that may accidentally have found a path to the housing keeping the housing at our perspective of ground. Kind of a failsafe. 

Ground and Neutral should never be confused or combined. Although they are at the same potential, they serve very different purposes. 

Hope this helps.


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## porkchop

If you want to get really nitty gritty about it here is a link to the notes on AC power from my ECE 2260 class. The learning curve is a bit steep being that this was a topic covered 3/4 of the way through the semester but it got me through it.

http://www.ece.utah.edu/~ecectools/PowerElectronics/ACpwr/PwrAC_Tutor.htm

This professor has a collection of ECE notes that he maintains for students so if you have any other questions I can send you the link to the repository.


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## derekleffew

Another analogy: Think of the hot conductor as a push-pull. It "pushes" the electricity through the neutral, then "pulls" it. This cycle happens sixty times per second, (60 Hz). This is why AC current is a sine wave, going above (positive) and below (negative) the zero line (zero crossing point).


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## gafftaper

The details of electricity are something I haven't mastered either. Why is it that in my old house the Neutral and Ground wires end up at the exact same place in the fuse panel. How are they doing anything different if they end up in the same block at the panel?


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## icewolf08

gafftaper said:


> The details of electricity are something I haven't mastered either. Why is it that in my old house the Neutral and Ground wires end up at the exact same place in the fuse panel. How are they doing anything different if they end up in the same block at the panel?



In a properly designed AC system the neutral and ground should be tied together at one point and one point only.

This is one of the magic things about electricity. You have to think about it like this: all electricity is, is electrons moving back and forth. A generator induces this motion with magnets. Now, think of the Earth. The Earth is composed of an unfathomable number of atoms which have even more electrons, so it makes a great place to pull electrons from or push them to as the variance is nominal given the size of the Earth.

(A simplified scenario)
So, at the generator, one end of the winding goes to ground (literally into the earth) and the other end goes out for distribution. We know that line as hot. At the usage end, the neutral line eventually goes to ground, and the hot comes from the generator. So, the generator pushes and pulls electrons, half the time it is pulling them from the Earth and half the time it puts them back. On the usage end as electrons are pushed by the generator, they travel through the wires to the device being used and back into the Earth. When electrons are pulled by the generator they come from Earth through the device and wires to the generator and back to earth. Kinda like this:
EARTH -> GENERATOR -> WIRING -> DEVICE -> EARTH (1/2 cycle)
then:
EARTH -> DEVICE -> WIRING -> GENERATOR -> EARTH (1/2 cycle)

The line we call ground is there to protect us from faults in the wiring or device, it provides the path of least resistance to ground so that we don't become that path.

I hope this makes sense to more people than just me, I tried to word it so that it does.


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## JD

gafftaper said:


> Why is it that in my old house the Neutral and Ground wires end up at the exact same place in the fuse panel?



In house wiring, there is a bit of a funny legacy issue that kind of set the stage for this odd combination. Houses built early last centaury did not have grounded outlets. They only had two conductors, Hot and Neutral. The problem with this is that wires are not perfect conductors. They are actually like big resistors, that drop voltage if the load is heavy enough. What this means is that if you have a really long run of wire and a heavy load, the neutral conductor at the plug end will actually have some voltage on it! Because of this, old "two wire" appliances did not connect the frame to either wire. In fact, on older two blade appliances you will see that the blades are the same size so they can get plugged in either way! As the years went by, appliances started coming out with one blade slightly larger than the other. This "larger blade" side of the outlet was suppose to be connected to the white neutral wire, although I often found this not to be the case when refurbishing older homes. When wiring an older lamp, you were suppose to wire the outer shell of the lamp socket to the wider blade in hopes that the bigger part would be less likely to electrocute people! (Still true on two blade table lamps) The code writers still prohibited any real connection between the "larger blade" side and the case. Finally, the idea came forth to have a separate third prong that would actually be a ground. As this third prong would not be part of the circuit path, it would always be at "true ground" potential, and could be connected to the case of the appliance. As long as the ground wires are run separate from the neutral wires, no voltage drop will occur. If it was combined with the neutral, some drop will occur and it will no longer be at ground potential. The only time the ground wire sees any current is when there is a short in the appliance. In these cases, this current is looped back through the ground and hopefully the overload quickly trips the breaker. Because the ground conductor may have to handle the full circuit current when this happens, it is required to be the full approved gauge for the circuit in question.

When you looked in your circuit breaker box, you saw a buss where the grounds and neutrals came together. This buss also had your service entrance neutral connected to it, and hopefully also had a thick copper wire that was connected to a large metal stake in the ground. It is usually also connected to a water pipe and a gas pipe so that all are bonded together. 

In a funny twist of fait, technology is eliminating the "third prong" defense with the introduction of ground fault detectors. These devices work like a bouncer in a club, who only lets the same amount of people enter as are leaving! If any imbalance occurs, they trip.


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## gafftaper

So does this make sense? 

The neutral is alternating positive and negative electron flow to and from the Earth. The ground is there essentially as a safety bypass. The fact that they both connect to the same place is irrelevant as they do different things with that connection to the Earth.

And yes half my house is knob and tube wiring. When I first moved in most of the house was on one circuit... at Christmas we would blow a breaker if we had the Christmas tree and several lights in the house on when the refrigerator kicked in.


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## porkchop

gafftaper said:


> So does this make sense?
> 
> The neutral is alternating positive and negative electron flow to and from the Earth. The ground is there essentially as a safety bypass. The fact that they both connect to the same place is irrelevant as they do different things with that connection to the Earth.
> 
> And yes half my house is knob and tube wiring. When I first moved in most of the house was on one circuit... at Christmas we would blow a breaker if we had the Christmas tree and several lights in the house on when the refrigerator kicked in.



That's a pretty good way to look at it. The instantaneous polarity will change but the Root Mean Square (RMS which is achieved by squaring the number to get rid of any negative values, then averaging the values out, and then taking the square root of that number) will be 120V or close to it always.


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## tgates

I hope this doesn't confuse you more but it might help visualize, so I'll try to explain it as minimally as possible.

Voltage is relative.

To a wire that is positive, ground looks negative. To a wire that is negative, the ground looks positive. So as long as there is a path back to the circuit, electrons will flow to any differential in voltage.


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## Chris15

To reinforce the comments made by others about voltage being relative to an arbitrary reference, if one has a properly insulated cherry picker, you could hold onto a 330kV line and not die. Because your whole body would be at 300kv and it's the voltage difference that creates problems. This is why birds can sit on power lines without a single problem. But one wing across to another line and that changes...

Now for those not yet thoroughly confused, look into balanced power. Those already confused would be advised not to as this will only deepen the confusion... Actually balanced power would give the results people expect, but it is very much it's own can of worms...


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## JD

Yea, transmission lines are often repaired by use of insulated platforms that are then jumpered to the HV line being worked on. Not much room for error though !

As for the "earth" thing, the earth itself does not really play a role. (Witness grounded outlets on the space shuttle.) In fact, if every ground lead everywhere were cut right now, everything would keep on working fine. The ground is there for two big reasons. One, to guide your friendly neighborhood lightning bolt to the earth. (As compared to having it jump from your TV to your sink.) Two, to drain off stray voltage. These stray voltages would occur due to the fact that the electric grid is not perfect and is continually leaking somewhere or another. As high voltage lines leak to ground, their voltage value would drop. This voltage difference would then appear on what used to be the safe neutral. So, instead of having a 13kv hot and a 0 volt neutral, they would "balance" out and both have a 6.5kv potential!

Not sure how long things would keep running before stray voltages or lightning took out a transformer somewhere. 


Tech note: Yea, I know, actually 13.2kv distribution is hot-to-hot, with the neutral to hot of 7,620, but I was making a point


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## TimMiller

The ground is also used as a 0V reference source for digital circuits. Thats why things like audio consoles and especially moving lights, lighting consoles, and computers must have a ground. +5V is on, 0V is off, things would get real confusing if it was seeing 2V as ground.


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## JD

Ahh! Which brings to light that there can be more than one "Ground" !!

On movers, you have an "earth" ground, which is in the power cord and connects to the case. There is also a second "Circuit" ground, which is what is fed out on pin #1 of the DMX. It is the digital ground. In this special case, these two ground must never meet! Thus, the "never connect pin #1 to the shell" rule. Doing so would cause the "Circuit" ground to be connected to the "Earth" ground which is a no-no. The primary reason for this is that if there were an electrical short, power could travel through the data cable causing a lot of damage, and possibly a fire, as the "shield" line in DMX cable is no where near a valid power gauge. Also, electrical noise in the ground system (as sound guys know) would end up mixed in with data.

EDIT: A word or two about DMX receivers and noise;
In the real world, DMX receivers compare the inputs on pin 2 & 3 of the dmx connector and output a digital signal (0v = 0, 5v = 1)
Problem is, they only work if the inputs are within the voltage rails feeding the decoder chip. This is why we need pin #1, the DMX ground. 
The following two scenarios will explain the problem:

#1 Pin 1 is at 0v, pin 2 is at 5v, pin 3 is at 0v. The decoder sees pin 2 as "high" and pin 3 as "low." VALID

#2 A negative noise spike of -5 volts is on pin 1, pin 2 is at 5v, pin 3 is at 0v. As the decoder chip is seeing +10 on one input and +5 on the other, it will read both as high. This is due to the fact that the chip can not compare voltages that are outside of its own power rails. Resulting data will be INVALID 

It takes a lot of "noise" to do this, but it can happen. Most chips use 2.5v as the divide between 0 and 1. The DMX chip is a little more flexible. If pin 2 was at 2.52 volts and pin 3 was at 2.53 volts, it would still read pin 2 as "low" and pin 3 as "high" Pretty good! Once you go above 5 volts or below 0 volts, all bets are off.


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## TimMiller

commonly the logic boards do ground to the frame of the fixture (this is especially true in x-spots, they use the board grounding to different points to decipher what their job is). Usually grounding the logic board depends upon how the power supply is designed. If the power supply provides a true 0V ref ground, then commonly the logics are not grounded to the frame, but if it does not have a 0V ref ground then it commonly does ground to the frame. When i have had logic boards on the bench, i commonly use the fixture frame as the ground reference.


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## JD

Interesting. I have never seen that done. (Other than a 100k bleeder) 

It presents a real and present danger. Example, if the earth ground were interrupted (Faulty plug, etc.) and the fixture developed a hot to frame short, pin #1 would go to line potential destroying not only the logic board on that fixture, but the DMX decoders on all of the system. To say nothing about the concept that the "hot" pin #1 would then feed full circuit amperage (less cable drop) through the data cable to the frame grounded pin #1 of the next fixture.


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## TimMiller

In these fixtures they have a fuse right at the beginning of the circuit, and pin 1 of the xlr never touches the frame ground. Things always get tricky when you need a constant grounded digital buss, and then you have your powersupply ground all in one metal frame.

Upon wondering, i just connected the ground of the plug to one side of my fluke bench meter, and the other to pin one of this trackspot. I got .4 ohms of resistance, which means pin 1 of the xlr is connected to the ground pin of the plug. Which means that the digital ground and the power ground are all the same ground.


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