What Does Ground Mean Exactly?

  • #1
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I've never been given a clean cut explanation of ground. I've heard some people refer to it as a common return path for current. Another as being a relative point that has a potential of 0V. Sometime people say earth ground which I'm pretty sure actually means the ground of the earth (not exactly sure on this one). Can somebody please give me a clear explanation of what ground means in several different contexts and give examples if you can. Would appreciate it a ton.
 

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  • #2
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It has always been my belief that grounding and the theory of it is the most difficult aspect of practical electrical. If you doubt this, try to trace out noise/ground loops in an audio system and fix the issue. Without getting too much into ground loops, here you go:

Electric potential, aka voltage, aka a difference in energy is point-to-point measurement of the concentration of electrons between two points. Because of this, we need a reference point to compare a point in question to. When electrical power is generated, we are creating a potential difference that allows electrons to move through a conductor (this is what current is) until both points are at the same potential (which would be 0V, they both contain roughly the same concentration of electrons). Once that happens, no more electrical energy is stored in the circuit, and electrons don't flow. Take a digital multimeter with a high resolution (thousandths of a volt) and go around just taking readings on random things (the wall to the floor, between the plastic on your computer monitor and the plastic on your keyboard, between you and a friend). You will see that you get very different readings between different objects. This electrical potential exists everywhere.

Now, when electricity is generated, that power is transmitted over very long distances, and given the above information it is quite easy to see how that can be a problem. I'm pulling these voltages out of my head, and real-life does NOT work this way, but lets assume that it does. Now lets say electricity is being generated at 120V and transmitted to several houses over several miles. At one house, the electricity may be at 180 volts, while another 32 volts, or any potential you can imagine. This is where a ground comes in.

By generating power at 120V relative to the EARTH we can minimize the difference in electrical potential over different areas, so that the electrical energy generated has close to the same potential across a much wider area and is much more stable. This gives us common "ground," pun intended, to compare potential to.

The grounding system in distribution systems serves the same purpose. I get roughly the same voltage reading from a functional, properly installed lighting system that I do from a receptacle installed in a wall outlet. We use the Earth as our reference point for voltage, the neutral as our return path for the circuit and bond the neutral and ground together at the first means of disconnect of a service entrance point to bring them to the same potential. We accomplish this bonding by many different means, the simplest of which is by driving long rods (5/8" by 8' is common) into the ground, then running a wire from that conductive rod up to the distribution panels of the electrical system. Utility lines have ground wires spaced out every so often to keep the electrical potential consistent as well. Lets cover some other topics.

Grounding systems are also used for equipment safety. By bonding the normally non-current-carrying parts of equipment, we can detect short circuits with specific devices (commonly called over-current and/or short circuit protect devices) like fuses and circuit breakers. This way, if the frame/chasis/etc of the device is raised to the line potential, current will flow without resistance causing the OCPD to open up the circuit, effectively de-energizing it.

Now, on to electronics. It is VERY important to be able to distinguish between earth-ground and floating ground (also referred to as "above-earth" ground). In electronics, we often have circuits that are not bonded to the earth. You can take a voltage reading between positive and negative, and may get something like 3, 5, 12, 24, etc volts. The reading would be completely different between either terminal and the earth, however within that electronic system, the negative (also called 0V or GND) is the ground for that system. That is a part of the reason this can get so confusing. Your car is a good example. It sits on rubber tires, so it is insulated from the earth. The frame of the car is actually used as a common reference point for voltage throughout the vehicle's electrical system, so it is effectively the ground for that floating-ground system.

So lets sum it all up. The ground is simply the common reference point for a generated voltage, regardless of how that voltage is generated. It could be chemical (car battery) or mechanical (generating rotating a magnet around a copper wire), it doesn't matter. Ground, in its broadest, loosest sense is the reference point for an electrical system so that voltages are fairly consistent throughout the system. Because of the variety in use, we segment them by referring to them as Earth (which is an earth ground), common ground (which can be any reference point in an electrical system that is ubiquitous) or just plain old GND, which tends to be the voltage reference pin for DC and/or electronic systems.

Quite a long-winded post, but as I mentioned this isn't exactly a simple topic.Remember, Ground is ground the world round!
 
  • #3
Can somebody please give me a clear explanation of what ground means in several different contexts and give examples if you can.
The post above has several errors, especially in how it defines electrical potential.

An ideal ground is simply an infinite source or sink of charge. You can pull charge out of it and push charge into it and the electrical potential of the ground will remain constant. The idea of a ground really is this simple.
 
  • #4
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It has always been my belief that grounding and the theory of it is the most difficult aspect of practical electrical. If you doubt this, try to trace out noise/ground loops in an audio system and fix the issue. Without getting too much into ground loops, here you go:

Electric potential, aka voltage, aka a difference in energy is point-to-point measurement of the concentration of electrons between two points. Because of this, we need a reference point to compare a point in question to. When electrical power is generated, we are creating a potential difference that allows electrons to move through a conductor (this is what current is) until both points are at the same potential (which would be 0V, they both contain roughly the same concentration of electrons). Once that happens, no more electrical energy is stored in the circuit, and electrons don't flow. Take a digital multimeter with a high resolution (thousandths of a volt) and go around just taking readings on random things (the wall to the floor, between the plastic on your computer monitor and the plastic on your keyboard, between you and a friend). You will see that you get very different readings between different objects. This electrical potential exists everywhere.

Now, when electricity is generated, that power is transmitted over very long distances, and given the above information it is quite easy to see how that can be a problem. I'm pulling these voltages out of my head, and real-life does NOT work this way, but lets assume that it does. Now lets say electricity is being generated at 120V and transmitted to several houses over several miles. At one house, the electricity may be at 180 volts, while another 32 volts, or any potential you can imagine. This is where a ground comes in.

By generating power at 120V relative to the EARTH we can minimize the difference in electrical potential over different areas, so that the electrical energy generated has close to the same potential across a much wider area and is much more stable. This gives us common "ground," pun intended, to compare potential to.

The grounding system in distribution systems serves the same purpose. I get roughly the same voltage reading from a functional, properly installed lighting system that I do from a receptacle installed in a wall outlet. We use the Earth as our reference point for voltage, the neutral as our return path for the circuit and bond the neutral and ground together at the first means of disconnect of a service entrance point to bring them to the same potential. We accomplish this bonding by many different means, the simplest of which is by driving long rods (5/8" by 8' is common) into the ground, then running a wire from that conductive rod up to the distribution panels of the electrical system. Utility lines have ground wires spaced out every so often to keep the electrical potential consistent as well. Lets cover some other topics.

Grounding systems are also used for equipment safety. By bonding the normally non-current-carrying parts of equipment, we can detect short circuits with specific devices (commonly called over-current and/or short circuit protect devices) like fuses and circuit breakers. This way, if the frame/chasis/etc of the device is raised to the line potential, current will flow without resistance causing the OCPD to open up the circuit, effectively de-energizing it.

Now, on to electronics. It is VERY important to be able to distinguish between earth-ground and floating ground (also referred to as "above-earth" ground). In electronics, we often have circuits that are not bonded to the earth. You can take a voltage reading between positive and negative, and may get something like 3, 5, 12, 24, etc volts. The reading would be completely different between either terminal and the earth, however within that electronic system, the negative (also called 0V or GND) is the ground for that system. That is a part of the reason this can get so confusing. Your car is a good example. It sits on rubber tires, so it is insulated from the earth. The frame of the car is actually used as a common reference point for voltage throughout the vehicle's electrical system, so it is effectively the ground for that floating-ground system.

So lets sum it all up. The ground is simply the common reference point for a generated voltage, regardless of how that voltage is generated. It could be chemical (car battery) or mechanical (generating rotating a magnet around a copper wire), it doesn't matter. Ground, in its broadest, loosest sense is the reference point for an electrical system so that voltages are fairly consistent throughout the system. Because of the variety in use, we segment them by referring to them as Earth (which is an earth ground), common ground (which can be any reference point in an electrical system that is ubiquitous) or just plain old GND, which tends to be the voltage reference pin for DC and/or electronic systems.

Quite a long-winded post, but as I mentioned this isn't exactly a simple topic.Remember, Ground is ground the world round!
Thank you very much for your well explained post, it helped a lot! But just to recap, you're saying that the ground can be anything as long as it can be referenced by everything. In the case of a battery, the negative terminal is ground and the positive terminal is referenced from it. Is the negative terminal of lets say a 12v battery not necessarily 0V then since it's just a reference? It's only 0V since it's taken as the reference, but if you made earth ground as your reference, the potential difference would not necessarily be 12V between the earth and the negative terminal correct? Lastly, when you say a battery has 12V, that just means if I had a coulomb of charge and brought it from the negative terminal to the positve terminal, I would do 12J of work and bring it to a higher potential, right?
 
  • #5
Borek
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An ideal ground is simply an infinite source or sink of charge. You can pull charge out of it and push charge into it and the electrical potential of the ground will remain constant. The idea of a ground really is this simple.
That's exactly the definition that confuses people and makes them think if there is a ground in the circuit the current will always flow there as into a black hole.
 
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  • #6
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Ground actually has many meanings. It can mean the ground of the Earth as you said, for example if you walk outside you are standing on the ground. Another definition is the state of a material such as ground beef or coffee grounds. There is the term ground that is used in electronics which had already been explained. Also, if you want to go a little farther, grounds is used to say you have a cause or reason for something like an argument for example. And there are probably more I haven’t thought of yet.

I apologize if I am answering your question wrong. I feel like a dictionary.

EDIT: Just reread your post and realized what you meant, sorry. :doh:
 
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  • #7
That's exactly the definition that confuses people and makes them think if there is a ground in the circuit the current will always flow there as into a black hole.
It's not that confusing. This definition has the advantage of giving a precise meaning to a grounded conductor. The confusion comes from the many other definitions of "ground" or "earth" that are understood to mean different things in different contexts.
 
  • #8
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It is a reference point, that is all... different types of circuits interact with ground in different ways, and some systems will actually have separate, or separated grounding circuits and systems.
 
  • #9
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Thank you very much for your well explained post, it helped a lot! But just to recap, you're saying that the ground can be anything as long as it can be referenced by everything. In the case of a battery, the negative terminal is ground and the positive terminal is referenced from it. Is the negative terminal of lets say a 12v battery not necessarily 0V then since it's just a reference? It's only 0V since it's taken as the reference, but if you made earth ground as your reference, the potential difference would not necessarily be 12V between the earth and the negative terminal correct? Lastly, when you say a battery has 12V, that just means if I had a coulomb of charge and brought it from the negative terminal to the positve terminal, I would do 12J of work and bring it to a higher potential, right?
In most cars the -ve terminal of the battery is connected to the chassis and that is considered ground or earth. However some cars (mostly old ones) have the +ve connected to the chassis. The chassis is still considered to be ground but now the -ve terminal of the battery is at -12V.


The chassis is considered to be a reasonably good ground for equipment in the car, but in both cases the chassis is only connected to the planet earth via rubber tyres so the chassis is a poor earth for something that connects between the car and say the electricity grid. So electric cars need more than one pin in the charging socket.

Sorry if that's confusing but you could write a book on the subject of electrical ground.
 
  • #10
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Lastly, when you say a battery has 12V, that just means if I had a coulomb of charge and brought it from the negative terminal to the positve terminal, I would do 12J of work and bring it to a higher potential, right?
I'm honestly not comfortable enough with with a more scientific explanation getting into the breakdown of units (remember, I'm just now getting my higher level math/physics in) to give you a solid answer on this, but when dealing with electricity and looking at the work that can be done, you must look at the systems electrical power. In a DC system, this is Power = Volts x Amps. This can also be expressed as Power = (joules/coulomb) x (coulombs/second) = (joules/second) when the unit coulombs are cancelled. This is how we measure how much work an electrical system can do. Voltage or Amperage alone won't give electrical power. Anyway, as I have found with any electrical topic, a good amount of time spent contemplating is a beautiful thing. I wish you the best of luck with your studies!
 
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  • #11
CWatters
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Lastly, when you say a battery has 12V, that just means if I had a coulomb of charge and brought it from the negative terminal to the positve terminal, I would do 12J of work and bring it to a higher potential, right?
That's correct. 1 Volt = 1 Joule per Coulomb

Although as an engineer its not a fact that I've needed to know very frequently.
 
  • #12
jim hardy
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@Alex Hughes (@ not working again)

"Ground" is probably the most widely misunderstood word in all engineering jargon.
Three old threads you might peruse:

https://www.physicsforums.com/threads/physically-grounded.675484/#post-4291620

https://www.physicsforums.com/threads/electrical-utility-neutral-return-wire.792299/#post-4977758

https://www.physicsforums.com/threads/grounding-in-electrical-circuits.526008/


Term "Ground" is so used, abused and misused. that you have to evaluate what was in the mind of whoever wrote or said whatever it is you're reading or hearing at that moment.

The three threads above will help you understand its meaning in several contexts.

Really it's not safe to assume it means any more than where the black wire of somebody's voltmeter is connected.
In an airplane that's likely the aluminum fuselage.
In a car it's likely the metal chassis.
In a TV set there are two "grounds" - one on the line side of the SMPS and another on the signal side .
In house wiring it means the green wire that's connected to a copper clad rod driven eight feet into the earth near your electric meter.

a couple more threads
https://www.physicsforums.com/threads/ground-wire-sizing.812849/#post-5103427

https://www.physicsforums.com/threa...t-live-and-neutral-wires.892309/#post-5613568

Once the concept "Clicks" you won't be able to remember when it wasn'n intuitive.
IEEE "Green Book", standard 142 is a great practical introduction to the subject. I think it should be a one credit hour course required in every EE curriculum..

old jim
 
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  • #13
jim hardy
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That's exactly the definition that confuses people and makes them think if there is a ground in the circuit the current will always flow there as into a black hole.
Precisely ! Bravo @Borek

We get that idea as kids watching lightning. I remember being much impressed as a toddler - first memory of it is the September 1948 hurricane . I was 2 years old.
Then we watch other things fall to the ground and mistakenly assume that Mother Earth attracts electricity just as she attracts mass. That's the source of the mistaken idea that "ground" is an infinite reservoir of charge.

Coulombic attraction is not gravity . Electric charge follows Kirchoff's laws not Newton's.

Kirchoff's Current Law tells us that charge in motion(that's what current is) must get back to wherever it started from. Usually it's stated more formally, "..sum of currents entering a node equals zero..." or "..current flows only in closed loops..." .
A current that originated not in Mother Earth, let's say it started instead from in a flashlight battery, has no incentive to flow into earth. It 'wants ' only to get back to that same flashlight battery it started from .
Unless it can find a way back to the other end of its battery by taking a shortcut through Mother Earth, it won' t go there..
.....................................................................................
In most electrical devices there's a power supply that sends currents to various parts of the machine.
After doing their work there those currents come back together, joining up for return to the power supply.
That place where they rejoin is properly called "Circuit Common" but all too often is mis-named "Ground" . It may or may not be connected to Mother Earth.
But whatever you name it it's a convenient place to hook you voltmeter's black wire .


So whenever you hear the term "ground" your ears should perk up and you should figure out what was meant - Mother Earth or Circuit Common ?
A lot of oscilloscopes have got fried by people unaware of that distinction.
A savvy technician always checks for voltage between Circuit Common and Mother Earth.

Train your brain on Kirchoff's laws early . It'll save a lot of unlearning later on.

old jim
 
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  • #14
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HAHA -- to point out how different this is..... we were working on a large chlorine generation plant - this had two large electrolysis processes Sodium and Potassium salts - so one side was D-Y and the other was D-D... a 12 pulse rectifier.. But much to the German engineer's frustration (Sent here to troubleshoot) the PE that laid out the grounding scheme did not calculate the impact of induced current ( ummmm ... >100KA on each rectifier / reactor) and the harmonics....

When we finally dug up a corner of the steel structure - and measured the current flowing from the grounded structure to the 4 x 4/0 grounding cable.... we had over 600A of 720 Hz current --- just circulating around the installed grounding grid.... -- After that I never failed to look for ground loops!

Electrically - on a schematic, diagram - etc. it looks like the "same" ground point. and many if not most would not consider the physical layout as being important. Every point in that plant had a proper connection to ground, but assumptions about what that meant cost thousands. ( The plant was losing $60K an hour being off line... that is another story....)

Ground is just a reference point.
 
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  • #15
jim hardy
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Ground is just a reference point.
Bravo !
You can think of "Earth Ground" as just another wire but one that goes most everywhere.

In the power plant i once put a clamp-on ammeter around the handrail above our isophase bus . Read sixty amps.
 
  • #16
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Thanks for all the great insights guys! I want to hear more details about the ground loop you found and what caused it. I can't quite understand it from your explanation, as far as how the current was induced, at 720 Hz no less.

Also, you are right about checking for potential to ground. More than once I have found voltage between neutral to ground. No wonder that weight indicator was randomly freezing up or shutting itself off.
 
  • #17
jim hardy
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induced current ( ummmm ... >100KA on each rectifier / reactor) and the harmonics....
Wow !

With that kind of current you have to be careful to not encircle it with iron.
around a loop h dot dl = current enclosed , so you have 100K amp-turns pushing flux around your iron loop. Supply and return wires need to run together. If that's not possible then any steel structure surrounding significant current must not be continuous - break it up with an insulating spacer.
 
  • #19
Kirchoff's Current Law tells us that charge in motion(that's what current is) must get back to wherever it started from. Usually it's stated more formally, "..sum of currents entering a node equals zero..." or "..current flows only in closed loops..." .
A current that originated not in Mother Earth, let's say it started instead from in a flashlight battery, has no incentive to flow into earth. It 'wants ' only to get back to that same flashlight battery it started from .
Unless it can find a way back to the other end of its battery by taking a shortcut through Mother Earth, it won' t go there..
This is another misconception about grounds that I here quite often. An AC ground, like the one in your house, is not a return path to anything. Like I stated earlier, the ground acts as a source and sink of charge.

The earth or any other conductor has charges which will move some distance in an applied electric field. The movement of these charges is a current. A simple example is a dipole antenna where the charges move back and forth in the antenna. Even though the antenna is not a closed circuit, AC current still flows between the transmitter and antenna.

For DC, one must have a return path. This is because a constant applied electric field will move the charges in the conductor until the applied electric field is cancelled by the electric field created by the deficit of charge in the conductor.
 
  • #20
Baluncore
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The different terms Ground, Earth and Chassis are all used to define the same concept.

Electrical circuits are made from conductors with insulation. The conductors must be sufficiently substantial to carry the circuit currents. The insulation must be thick enough to insulate the circuit conductors without breaking down at circuit voltages.

That is all well and good so long as external high static voltages are not present. A very high voltage, low current, can break down the circuit insulation. That may result in a conductive path for a high circuit current short circuit.

By connecting the neutral conductor to earth throughout an AC electrical distribution system, the voltage between the distribution grid and the environment can be held below the breakdown voltage of the insulators.

The chassis of a motor vehicle is usually connected to one side of the battery. That has two advantages. It removes the need for many return circuit wires and prevents breakdown of insulation due to static voltages that may be built up between the chassis and wiring.

In the early days of longwave radio, the signal was taken from between the antenna and ground. The local Earth and AC power system ground were connected to the metal chassis of the radio. The internal DC supply was also connected to the chassis. The common ground guaranteed that voltages between those various systems would remain within the capabilities of the insulation.

So earth, ground and chassis connections are all designed to protect the circuit insulation by preventing the build-up of charge creating excessively high voltages on otherwise isolated subsystems.
 
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  • #21
jim hardy
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An AC ground, like the one in your house, is not a return path to anything. Like I stated earlier, the ground acts as a source and sink of charge.
My ears just perked up. What do you mean by "AC GROUND" ????

Like I stated earlier, the ground acts as a source and sink of charge.
At power line frequency that's just plain wrong.

Charge gets back to the transformer winding from which it originated via the neutral wire.
That neutral wire is connected to Mother Earth near the meter for safety's sake , to limit potential difference between earth and house wiring.

In absence of an electrical fault little or no current enters or exits Mother Earth at that connection adjacent meter. .
However -
Since the centertap of the "pole pig" transformer winding is also earthed via the copper wire stapled to the side of the power pole,
polepigground.jpg


what you have is two parallel paths from the house back to the pole - one through earth and one through the neutral wire.
Current will divide between those paths per Ohm's law with the vast majority of it taking the lower resistance neutral wire.
What little bit of charge enters earth at the service entrance exits earth at the pole, there's no storage.
Capacitance to earth of wiring inside the house will allow a trickle of current maybe a milliamp. But it too goes back to the transformer winding from whence it came.

So in house wiring "Ground" not a spongy source and sink for charge it's a conduction path back to the transformer winding. Like i said , ground can be thought of as just another wire that goes most everywhere.

Antenna effects start coming into play around 1/10 wavelength which at 60 hz is something like three hundred miles.

old jim
 

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  • #22
jim hardy
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Don't confuse Charge Carriers with Charge.
Electrons drift at mm/minute. Coulombs move at a goodly fraction of c.
 
  • #23
At power line frequency that's just plain wrong.

Charge gets back to the transformer winding from which it originated via the neutral wire.
That neutral wire is connected to Mother Earth near the meter for safety's sake , to limit potential difference between earth and house wiring.
You make it sound like there is some return path all the way to the power plant: there isn't. If you touch a live wire, you will conduct a large amount of current into the ground. This current does not flow back to the transformer. The resistance of dirt is way too high. The grounding wire in your picture is also connected to one side of the primary with the other side at HV.

Furthermore, if you are standing on the floor in your house, which is not conductive, you can still get a severe shock from a 60Hz line. Again, this is because AC does not require a return path.
Antenna effects start coming into play around 1/10 wavelength which at 60 hz is something like three hundred miles.
If you don't like the antenna analogy, then think of a parallel plate capacitor. AC current can flow through the capacitor even though the plates are separated by an insulator. This is because there is a changing electric field in between the plates of the capacitor which leads to a displacement current.
$$\mathbf{J}_{D}=\epsilon_{0}\frac{\partial \mathbf{E}}{\partial t}$$
This displacement current is not restricted to capacitors but can be used anywhere there is an electric field. So one way to think about AC circuits is that the return path is not necessarily a flow of charge, but can also be a radiated time varying electric field.
Don't confuse Charge Carriers with Charge.
Electrons drift at mm/minute. Coulombs move at a goodly fraction of c.
I think you are confusing charge with the electric field. Electrons carry the charge so if the electrons move slowly, then the charge moves slowly. The electric field propagates at or near the speed of light in vacuum.
 
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  • #25
Baluncore
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You make it sound like there is some return path all the way to the power plant: there isn't. If you touch a live wire, you will conduct a large amount of current into the ground. This current does not flow back to the transformer. The resistance of dirt is way too high.
The current cannot flow back to the power plant because there are transformers isolating that path. The current must flow back to the local transformer because that is where the active circuit started. The neutral bar and earth bar are only linked in the meter box. The earth bar is connected to a buried earth stake that was tested to make sure it would carry the full fault current. When you touch the active line the current flowing through you gets to ground and then through the earth stake and the earth link in the meter box back to the neutral and so the transformer. Some also gets to nearby power poles with the MEN. The resistance of the dirt is not high enough to save you. Don't forget that the dirt is deep and has water with dissolved salt close to the surface. There are even metal pipes buried in it.

Furthermore, if you are standing on the floor in your house, which is not conductive, you can still get a severe shock from a 60Hz line. Again, this is because AC does not require a return path.
If you complete a circuit, AC does have a return path. That path can be AC coupled. But the capacitance of an insulated human body is low, (< 1nF ), and the power line frequency is low, 60Hz, so the capacitive displacement current is also low.
–Zc = 1 / (2 ∙ Pi ∙ 60Hz ∙ 1nF ) = 2.65 Megohm. At 230V, displacement I = 85 uA.

This displacement current is not restricted to capacitors but can be used anywhere there is an electric field. So one way to think about AC circuits is that the return path is not necessarily a flow of charge, but can also be a radiated time varying electric field.
That is a really novel theory. On the face of the Earth, if there is an electric field then there is capacitance, you cannot have one without the other.
The return circuit carries an electric current, 1 amp = 1 coulomb per second, so there must be a flow of charge. AC circuits can have series capacitors. The AC current is limited by the capacitance, voltage and frequency.

I think you are confusing charge with the electric field. Electrons carry the charge so if the electrons move slowly, then the charge moves slowly. The electric field propagates at or near the speed of light in vacuum.
And there I was thinking that electrons were charge.
The electrons that move on the surface of conductors are only there because a magnetic field is being guided by the conductor. That magnetic field accompanies the electric field between the wires. The velocity of the external EM field propagation will be a little less than the speed of light, Poynting towards the load. But the speed of the electric wave and magnetic field in the conductor will be only about 100 m/sec, which is the diffusion velocity of electrons in a good conductor.

Let's face it. Earth, ground and the chassis are there to protect the insulation from puncture by electrostatic discharge.
 
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