# Problems understanding Hot, Neutral and Ground in AC circuits

Discussion in 'General Electronics Chat' started by PauloConstantino, Jun 23, 2016.

1. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
7
Dear all,

I have started studying electronics a few weeks ago and I am confused about the concepts of the Hot, neutral and ground wires.

I will explain what I understand and what I don't understand and I will ask for your help in clarifying what I don't understand please.

Let's say we are generating AC in the following way:

So far so good, as we can see, that wire loop has two ends.

Now consider the following badly drawn circuit:

Let's say the AC generator generates an amplitude of 50V, so it oscillates between -50V and 50V.
Here are my difficulties.

1) If we connect one of the ends of the AC generator to Earth Ground, that is, an actual Rod inside the earth, let's say 5 metres, then what is the voltage on that rod really? Is it 0 relative to the AC generator?

I have marked the diagram with the labels S1 and S2 in order to talk about the direction of the current in each AC half cycle.
2) What is the voltage in the "Neutral" wire? Is it 0? But if it is 0, then what about the voltage coming from the AC generator itself?

I don't even know what it is that I don't understand here. This concept of neutral wire doesn't make sense to me.

I guess the biggest question for me is, once you add a ground to an AC circuit, how do you define the voltages? With respect to what? If the current is flowing in one direction, what point do you use to mark a positive voltage and what point to mark a negative voltage, what about 0 voltage?
Oh dear, if someone could please help me with this, I would be grateful until the day I die!

Best wishes

Paulo

2. ### wayneh Expert

Sep 9, 2010
12,391
3,246
I know there are differences between countries but I can tell you how it works in the U.S. Power from the utility arrives at a center-tapped transformer The neutral line is the center and the two opposing poles each give 120V that are 180° out of phase. So against neutral they give 120V but against each other they give 220V. At the house, the center tap is connected to the house's neutral system and both are tied to earth ground at one spot. No significant current should actually be flowing to ground. Local earth ground just provides the reference voltage for the home system, not a current sink.

All voltages are relative, and in this case they are expressed relative to local earth ground.

Apr 5, 2008
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4. ### crutschow Expert

Mar 14, 2008
13,505
3,376
You confusion appears to stem from the definition of voltage.
Voltage measurements are always relative to some point.
If the generator is connected to ground, then you would normally measure the voltage relative to that.
The neutral wire is also connected to ground so it is at the same ground potential (ignoring any voltage drop due to current through the neutral wire resistance).
Note that the "ground" is just a voltage reference point that is connected to earth. It has no other significance.
So in your example, the voltage of the "Hot" line relative to the ground/neutral connection would be +50V to -50V, the same as if you measured it without the ground connection.

Jul 18, 2013
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6. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
7

You see that's my confusion. You said that the voltage on the neutral wire is 0, but if that is so, what happens to the -50 to 50V on the same wire? because the generator is working and generating voltages on both wires! I'm so lost with this bro

7. ### andre_teprom Member

Jan 17, 2016
31
9
If the wire is connected to ground, and if the ground is connected to earth, this wire will ehxibit only 0V.
The other wire is the one on which you will measure -50V to 50V.

Jul 18, 2013
10,851
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The voltage on a Neutral is Zero WRT Earth.
Imagine a 3 phase 440vac supply, you can make any one of these phases a neutral by connecting it to ground, and call it zero, IOW it is just a reference point,
In fact this is often done in delta supplies.
See the U of Penn link I posted for info on the definition of earth ground.
Max.

9. ### crutschow Expert

Mar 14, 2008
13,505
3,376
Bro, you are still missing the point that voltages are relative, in this case the voltage is relative from one wire to the other.
The generator is not generating separate isolated voltages on each wire, it is generating a voltage between the two wires.
Thus both wires don't have ±50V (relative to what?), they just have ±50V between them.
So if you connect one wire to earth, then the voltage is still ±50V between then, it's just that we use ground as a reference for one of the wires
Keep repeating "All Voltages Are Relative" until it's burned in your brain.

10. ### AnalogKid Distinguished Member

Aug 1, 2013
4,701
1,300
Something to keep in mind is that while physical phenomina such as electricity absolutely exist, all descriptions are arbitrary names, terms, descriptions, and other language constructs. Positive negative, line neutral ground, anode cathode, *whatever* - just names. And like all of language, they make no sense unless they have agreed upon meanings and references. So an AC generator sitting on a table top has a measureable voltage and can pump a current through a load, but neither output is automatically hot, cold, neutral, line, or ground because it is not connected to the US power grid, a structure where those names have defined meanings. So when someone says "the voltage on the neutral wire is 0", that isn't exactly true. The voltage on the neutral wire is *defined* as 0 because all of the math in the universe is easier if we first agree where/what 0 is.

The vast majority of the circuits discussed on this forum are powered by a positive DC voltage, and the power supply of battery negative terminal is connected to GND. BUT, the first transistors were PNP, which means that common-emitter amplifier circuits had the battery positive terminal connected to the circuit GND. All early transistor radios (and early automotive ignition systems) were designed this way - with a "positive ground".

Start
Goto Start

ak

11. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
7
Let me see if I understand this.

Even though one of the wires is connected to the earth, one of them at any point will always be at a lower potential than the earth, except when the voltage is at 0 during the cycle. Suppose current is flowing one way inside the wires, one side is at negative voltage, the other is at positive. If current is now flowing into the negative pole, then because I myself am grounded, if I touch the neutral wire, current will not start flowing from my body into the lower negative pole because my body has more resistance than the Earth connection.

Does that make sense?

12. ### wayneh Expert

Sep 9, 2010
12,391
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Yes, but I still wouldn't hold onto the neutral wire. A failure of its connection to ground could cause it to suddenly become hot.

13. ### crutschow Expert

Mar 14, 2008
13,505
3,376
For an AC supply, the hot wire voltage will go positive and negative with respect to the neutral/earth connection.
Don't know why you say it's at a "lower potential"?

14. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
7
I don't see why, because both the hot and neutral wire come from the same generator, one is connected to the ground and the other isnt. If we connect one of them to the ground, why does the voltage go to 0 if it is still connected to the generator that oscillates between -V and +V ?? This seems so strange and I cant crack it!

15. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
7
maybe then, the generator is only between +V and -V with respect to the stopped generator, or ground. But then why even connect one of the wires to the ground ? Even if you say its for protection, in my mind as soon as you connect one of them to the ground, its voltage is 0, and so it's a useless wire? We might as well just use a hot wire and connect the other end of the load directly to the earth ? Oh boy please help.

16. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
7
Does current even move inside the hot wire when it's not connected to anything? If the wire is floating, does current move back and forth inside it ? Or is it only when we complete the circuit that it moves? In that case, if I connect the hot wire to the Earth, does that complete a circuit ? Does the charge go back to the generator through the earth ?

Jul 18, 2013
10,851
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See the U of Penn paper that shows actually using ground as a conductor.
This has actually been done in rural areas many years ago and was often used for single line telephone communication.
Max.

18. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
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Hello Max. I read the page you sent me. It didn't quite remove my confusion........... I have a degree in Maths but this AC stuff is really bugging me !

19. ### PauloConstantino Thread Starter Member

Jun 23, 2016
265
7

Please have a look at my drawing, does it make some sense? It's for the first half of a full AC cycle.