Problems understanding Hot, Neutral and Ground in AC circuits

profbuxton

Joined Feb 21, 2014
421
Paulo, regarding your drawing. Please note that there is NO flow of electrons or anything else into or from the ground connection, end of story.
Consider the case of a very low frequency alternator(say 1 cycle per second). Now connect a center zero analogue meter across the output. This alternator is of course not connected to anything else.
The meter will swing from one side to the other at 1 cycle per second as the polarity changes with each cycle.
Note connect a load across the terminals. A current will flow at the same frequency as the voltage.
Now connect one terminal (either one) to a large metal object. The voltage on the meter will not change and current will continue to flow through the load.
Now connect your meter from the metal object to the other terminal. The meter will still show the same swing as before and load current will still flow. You have now a "Neutral" connection using the large metal object. Now you can measure between any point the large metal object and the other terminal and see the meter reading. Replace the large metal object with the "earth" and the above applies also.
One side of power distribution is connected as "neutral" for safety reasons. One side is connected to a solid earth at the supply transformer and each user has a "MEN" link in their switchboard. This connects the incoming "neutral"
conductor to a local earth stake. This ensures that if the"active" contacts any earthed metal it will trip the local protection.
Railway signalling supplies are "floating"(not connected to earth) but they use earth leakage detectors to alarm if one side becomes earthed. If a second earth occurs on the other wire the that could cause a short and rail problems.
 
Before the neutral is connected to ground, it's identical to the hot wire, but of course opposite polarity at any moment. Once it's connected to ground just as we are, we label it neutral because there is still no (or little) EMF to drive current from it through our bodies. If we touch the other wire, we get zapped!
I like that explanation of neutral.
 

wayneh

Joined Sep 9, 2010
17,496
That's really all it is. We call it neutral because we force it to have the same voltage that we do. If we are floating in space and grab one pole of the generator, it becomes the neutral.

Touching the other one then makes us part of a circuit and we call it hot.
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
No, it isn't, if I read your diagram correctly. You are showing a flow of current to ground, and none is required. The EMF that generates power to move current around is between the two poles of the generator, not between one to the other and ground.

Before the neutral is connected to ground, it's identical to the hot wire, but of course opposite polarity at any moment. Once it's connected to ground just as we are, we label it neutral because there is still no (or little) EMF to drive current from it through our bodies. If we touch the other wire, we get zapped!
So is that it ??? What the hell ? So the problem is actually the complete circuit........ So if I actually hold the HOT while not connected to the floor... Will I not get shocked??? Omg............ You have hit me so hard Now Wayneh........... In terms of physics, I think the best way to explain something is to remove the superfulous.......... You have removed the whole earth and have left me alone with the circuit and now I get it !!!

So its the current that is the problem not the voltage!!!! By that I mean that even if there is a ridiculously high voltage between two ends of a wire, If I touch one of them we will be at the same potential in relation to the other wire, but still nothing will happen !?!?!
 
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Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
Paulo, regarding your drawing. Please note that there is NO flow of electrons or anything else into or from the ground connection, end of story.
Consider the case of a very low frequency alternator(say 1 cycle per second). Now connect a center zero analogue meter across the output. This alternator is of course not connected to anything else.
The meter will swing from one side to the other at 1 cycle per second as the polarity changes with each cycle.
Note connect a load across the terminals. A current will flow at the same frequency as the voltage.
Now connect one terminal (either one) to a large metal object. The voltage on the meter will not change and current will continue to flow through the load.
Now connect your meter from the metal object to the other terminal. The meter will still show the same swing as before and load current will still flow. You have now a "Neutral" connection using the large metal object. Now you can measure between any point the large metal object and the other terminal and see the meter reading. Replace the large metal object with the "earth" and the above applies also.
One side of power distribution is connected as "neutral" for safety reasons. One side is connected to a solid earth at the supply transformer and each user has a "MEN" link in their switchboard. This connects the incoming "neutral"
conductor to a local earth stake. This ensures that if the"active" contacts any earthed metal it will trip the local protection.
Railway signalling supplies are "floating"(not connected to earth) but they use earth leakage detectors to alarm if one side becomes earthed. If a second earth occurs on the other wire the that could cause a short and rail problems.


In this analogy then, you could use an earth point 50000 miles away and connect one end of the meter there, and the other end at the battery terminal and have the same reading ? Disregarding the voltage drop of the 50000mile wire....
 

Tonyr1084

Joined Sep 24, 2015
7,853
If I remember the numbers right, 6 mA is all that is needed to stop the heart. So if you take a shock from left hand to right hand (through the chest) 6 mA can very easily stop your heart. Or so I believe.

Now, I was working on a DC-9 and accidentally took 120 VAC (400 cycle) from hand to hand. Knocked me on my butt. That was back in 1979. If I'm typing now then either I'm a Ghost Writer - or I survived. I don't know if it was the cycles per second that made the difference, but like Wayne H said, I never forgot that sensation. And yes, I WAS on my backside after that hit. Dazed for about half a minute too. 35 years later and that much older, I don't know if I would survive that today.

The path the electrons take through your body is somewhat predictable but not foolproof. A hit from left hand to right foot can go through your side, hips and leg or it can go through your chest somehow. A hit from foot to foot is less likely to end your life but I wouldn't be willing to make that bet, nor would I want that sensation flowing through my manhood. A hit from hand to elbow - that's not likely to hurt you, it's pretty predictable where the charge is going to go.

Just learn to have a healthy respect for the power of electricity and avoid unpleasant lessons as much as possible. I've been hit by 36000 volts in the chin (TV Flyback transformer) and I've been hit by 12000 volts from an ignition coil (car). Spark plugs on lawn mowers carry a healthy bite too. And yes, "BITE" is a good description for the sensation in many cases.

Grab a small transformer. Take the low voltage side and grab a D cell battery. Using your fingers (one hand) touch the wires to both ends of the battery. Then pull the wires off. I'm SURE you're not going to like the sensation. And that's just a single hit. And by small transformer I mean something VERY small.

Good day to all. I'm going to the air show (Hill AFB, Utah). See you tonight.
 

wayneh

Joined Sep 9, 2010
17,496
By that I mean that even if there is a ridiculously high voltage between two ends of a wire, If I touch one of them we will be at the same potential in relation to the other wire, but still nothing will happen !?!?!
That's it. Like a bird sitting on a power line. As long as the circuit is incomplete, no current flows and voltage can be anything. "Absolute" voltage has no meaning. Once a circuit is completed, relative voltage matters and is the driving force, EMF, for driving current around.

I'll just add that it doesn't take a lot of current to hurt us. The bird on the wire may get zapped because the surrounding air conducts just enough.
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
That's it. Like a bird sitting on a power line. As long as the circuit is incomplete, no current flows and voltage can be anything. "Absolute" voltage has no meaning. Once a circuit is completed, relative voltage matters and is the driving force, EMF, for driving current around.

I'll just add that it doesn't take a lot of current to hurt us. The bird on the wire may get zapped because the surrounding air conducts just enough.

I can't thank you guys enough. You have really helped me to understand something that was really mysterious to me.

I studied maths at university for 4 years continuously with no problems understanding anything. But trying to understand the basics of AC voltage has drived me nuts the past days!
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
I've just seen a video where the guy measured the voltage between the hot and ground wires and got 220V. Since the earth is only a rod going into the earth and nothing else, is this 220V voltage there because the tension goes through the earth and then through the neutral wire which is also connected to Dirt ?
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
Wow after 111 posts I though it would have sunk in!
The Earth is the same potential as a grounded neutral!
Here is something to study, figure out how a Ground fault breaker/interrupter works.
Max.
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
Wow after 111 posts I though it would have sunk in!
The Earth is the same potential as a grounded neutral!
Here is something to study, figure out how a Ground fault breaker/interrupter works.
Max.
Yes we are almost there Max. But is that because the earth is conductive kind of like a metal ?
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
Seeing you are in the UK I previously explained how it is, or at least was when I practiced there, the service ground was connected only at the remote service transformer neutral, the local residence used an available ground.
water pipe etc, this was the only connection to ground.
In the event of a fault, e.g. 240v touching the frame of grounded electrical equipment, it would find the path from the grounded frame through earth back to the grounded neutral, because the fault is now the same potential as the ground you stand on, you would not receive a shock, of course if the current was sufficient, it would also blow a fuse.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
I am serious, study how a GFI works, RCD in the UK, where many are used now due to the lack of decent means of grounding via metallic supply pipes etc.
It may help you understand the earth ground part.
Max.
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
I think everything has been clarified for me........... This is a miracle!


So as I understand now, the hot and the "neutral" are identical until the "neutral" is grounded to Dirt and actually becomes neutral because it is now at the same potential as Dirt. There is a voltage between the hot and the neutral which is caused by the electrical generator.

If I float in air and touch the hot wire I will not get shocked as long as I do not touch the neutral wire as well and as long as I do not walk with bare feet on a conductive surface on the earth thus completing a circuit to the Neutral.

The ground wire is actually only a Rod attach deep into Dirt, and is attached to the inside of appliance in in case the hot wire goes astray somewhere in inside and poses the danger of someone touching it and thus completing a circuit to the neutral through the so called conductive earth surface.

If I measure the voltage between the Hot and the Ground (The ground which is only a rod attached deep into Dirt and nothing else), then there will be a voltage between them only because the Neutral is also attached to Dirt and thus the Earth itself would complete a path from the neutral to the Dirt Ground Rod and thus give the voltage reading between Dirt Ground and Hot.

Any point on the earth/Dirt can be used as a ground reference VS the hot wire because again, the earth is conductive and completes a path from Ground/Dirt to Neutral/Dirt.


If anyone disagrees to these points please raise your hand!
 

profbuxton

Joined Feb 21, 2014
421
Paulo, please note carefully that at the point where your supply comes from(usually a big transformer on a pole or substation) one side of the supply is connected to very solidly connected to earth. This forms the reference point. Now two wires are run to you house(or domicile).
one wire is called the "neutral" because it is connected to side connected to earth. That makes the other wire "hot" or "live" with respect to the "neutral" or any earth point. Note I refer to our distribution system on Down Under.
Regarding measuring the volts 5000km away I would suspect that there would be so much stray currents from a lot of sources the result would be meaningless unless you could switch off all other supplies and control any self generated currents in the soil.
 
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