What is the difference between Earth ground and circuit common?

[anorlunda]

<<<Moderator's Note This thread was split form https://www.physicsforums.com/threads/are-the-terminals-of-a-battery-neutral.939947/>> because the grounding posts are interesting but off-topic to that other thread.>>

The entire Earth (ground) might be charged plus or minus one million volts relative to infinity. It makes no difference. That definition using infinity is a useful abstraction to define the magnitude of one volt, but it has no practical application to real world devices like batteries. (Uh oh; now that I said that someone will prove me wrong with an application I never heard of. )

 

[Borek]

How can batteries have a positive and negative potential if the charges of their terminals are neutral?

I don't see how they can be perfectly neutral. Existence of the potential difference means there is an electric field between the terminals, as far as I can tell that requires charge separation. However, the charges involved are very, very small.

 

[rude man]

The entire Earth (ground) might be charged plus or minus one million volts relative to infinity. It makes no difference.

i never heard of that one before. The Earth is considered uncharged and therefore its potential w/r/t/ infinity is zero.

 

[sophiecentaur]

The Earth is considered uncharged

By whom?

 

[jim hardy]

By whom?

Certainly not by me. Earth has a field on order of a hundred volts per meter or so near its surface.

http://www.siencis-par-furlan.net/wp-content/uploads/Bressan.Ground.pdf

 

[anorlunda]

The Earth is considered uncharged

Yes, with emphasis on considered. The consideration is voluntary and a matter of convenience, not physics. It makes many things simpler to just define Earth potential as zero. But as I said in #2, if we defined Earth as some nonzero voltage, it would make no difference in our calculations.

The net charge of our planet must change with time, especially during bombardment by charged particles from solar prominences. The charge at infinity is yet to be measured. My point in #2 is that it doesn't matter.

Don't confuse this with particle physics, such as that of an atom being a net neutral particle.

 

[sophiecentaur]

Earth has a field on order of a hundred volts per meter or so near its surface.

I'm having a problem with that figure of 100V/m and what it actually means to the man in the street. According to the paper, it's not the value that you would get near the surface of a metal sphere in space. If it were, the overall absolute would be something like 3X10¹⁴V
I used the inverse law for potential so
V(1/r1 - 1/r2) = ΔV
where V is the absolute potential and ΔV is the volts per metre. r is about 6000km
How much net charge would this imply? The Capacitance of the Earth, treated as a conducting sphere, is around 700μF and Q=CV so
Q would be 3X10¹⁴X700X10^-6 = 3X10⁸ Coulombs. I think that would be negative (?)
That charge isn't a lot. It would represent, say 100A flowing for about 1000 hours .
Please check my maths.
But the paper is dealing with local field strength and the variation over time takes the value both positive and negative. Plus the change with height doesn't follow 1/r. So where does that lead us in justifying using Earth as 0V?

 

[anorlunda]

But the paper is dealing with local field strength and the variation over time takes the value both positive and negative. Plus the change with height doesn't follow 1/r. So where does that lead us in justifying using Earth as 0V?

Grounding is one of my least favorite subject, partially because it is so complicated and so mysterious.

Just part of the complications, consider the currents induced by those auroras and the constant bombardment of charged particles at the poles. Then think of relatively high resistance of the ground and you get appreciable voltages. @davenn might be able to illuminate us more on those numbers. Sometimes, the gradients are so high, they cause substantial currents in the power grid, and cause trips. A 1989 blackout in Quebec is notorious in that regard. There may also be ground currents induced when thunderclouds pass over, or when the day/night terminator passes by (I don't have any links, just speculating.)

In big buildings with lots of power hungry electronics, grounding science and grounding practices are non-trivial. Perhaps some member can tell us about modern server farm buildings.

There is also the case of Sisters Creek in Marathon, Florida. Beside the creek is a Voice of America transmitter with lots of RF power. Boats anchored 200m away report radio & radar problems and LED lights that flicker even when turned off. I have observed sparks hopping between the links of my anchor chain when anchored out there at night. That leads me to suspect huge transient fluctuations in the ground potential at that location. But how would you go about measuring that?

 

[dlgoff]

I'm having a problem with that figure of 100V/m ...

Well, according to https://en.wikiversity.org/wiki/Natural_electric_field_of_the_Earth

The static fair-weather electric field in the atmosphere is ~150 volts per meter (V/m) near the Earth's surface, but it drops exponentially with height to under 1 V/m at 30 km altitude, as the conductivity of the atmosphere increases.

 

[jim hardy]

The entire Earth (ground) might be charged plus or minus one million volts relative to infinity. It makes no difference. That definition using infinity is a useful abstraction to define the magnitude of one volt, but it has no practical application to real world devices like batteries. (Uh oh;

Indeed. Since we can't get to infinity we'll never know what is Earth's absolute potential...

I'm having a problem with that figure of 100V/m and what it actually means to the man in the street. According to the paper, it's not the value that you would get near the surface of a metal sphere in space. If it were, the overall absolute would be something like 3X1014V

One could speculate that coulombic forces were at the root of Pioneer anomaly... but they've figured out it was something else.

I really should make a brief insights article based on this ancient post... it's on my bucket-list...
https://www.physicsforums.com/threa...s-on-an-empty-wire.903244/page-2#post-5688002
I was trying to answer a fellow's question about "voltage".. exaggeration is sometimes a useful teaching aid.

I think your trouble stems from definitions. I don't think your definition of voltage is correct.

This is close but not quite right
That's it's units allright
but it's actually a simpler concept than that.

And Rive is right, when we lazy engineers get accustomed to using it we broad brush past the details.

Here's how to think about it

Voltage is Potential Difference. Two words, not one. Difference is the easy one.
So what's potential?
Potential is the work required to bring a unit positive charge from infinity to wherever you're measuring potential.
That takes some thought.
Imagine yourself at Alpha Centauri(close enough to infinity for demonstration purposes)
with a one Coulomb sized bucket full of charge,
a force gage, and a ruler.
I grew up with dynes and centimeters and ergs but Newtons and meters and Joules are easier...

Now start walking toward earth, measuring the force in Newtons exerted on your bucket of charges and tabulating it at every meter along the way.
So as you move toward Earth you're going to tabulate the Newton-meters and keep a running sum.
When you've reached the top of your lightbulb you will write there what is that point's potential. That'd be its absolute potential.
Now repeat but this time walk to the bottom of your light bulb and again write its absolute potential.
The difference between those absolute potentials is the voltage across your light bulb.
When i grasped the concept was the day I imagined myself counting dyne-centimeters all the way from Alpha Centauri to my workbench in Miami Central High School's electronics lab, ca 1962 .

Now since we can't get to Alpha Centauri let alone infinity it's completely impractical to do that measurement,
and that's why we never know what is the absolute potential of anyplace.
So we just have to settle for the difference in absolute potentials between two places we can reach.
That's easily measured with a two wire voltmeter provided its leads are long enough to reach our two points of interest.
That difference in absolute potentials is "VOLTAGE" . The voltmeter reads that.

Whatever is the absolute potential at one end of a battery, it's different at the other end by whatever is the voltage of your battery. We can only measure that difference.

That's voltage. Forget about clouds of electrons.

Now, an electric field will cause charges to migrate along the field if they can. Inside a copper wire they migrate easily so a miniscule field will cause quite a bit of current . That's why the voltage between ends of a wire is miniscule, charges move equalizing local charge densities along its length..

This oversimplified layman's explanation should help you make sense of the concept. I don't mean to come across anti-academic; au contraire.
Don't just memorize formulas, understand what's happening and they'll become intuitive.
Looking up definitions is always a good idea - laying the foundation if you will. Then use your imagination to link them to your everyday experience. That's called "Memory Pegs" .

Working inside circuits is different from electrostatics, we have simplifications like no field along a wire and V=IR neglecting magnetic induction.
Poynting Vectors and Magnetic Vector Potential come later, after you've got used to working inside circuits.

Good luck in your studies.

Apologies for being so basic in an academic forum, corrections to any of above are welcome.

old jim himself

Earth Ground is best thought of as just another wire whose absolute potential we do not know,
but since it goes most everywhere it is a mighty convenient spot to hook our voltmeter's black lead.

old jim

 

[sophiecentaur]

Earth Ground is best thought of as just another wire whose absolute potential we do not know,
but since it goes most everywhere it is a mighty convenient spot to hook our voltmeter's black lead.

old jim

Engineers are pragmatists and, when something works, they use it. A useable 'Ground' can be produced with a cross of just two wires, half wavelength long under a VHF monopole, on top of your chimney.
On a larger scale, I guess there must be some equilibrium between Gravitational and Electric forces which will affect the net charge that the Earth can acquire.

 

[Fisherman199]

Grounding is one of the most interesting electrical subjects, I think. The mystery of it all makes it so. I regret not having spent more time with the subject when I was in college and worked around several substation designers.

I just got an old version of an IEEE Green book in the mail. I'm reading through it as we speak (erm... type). The Emerald Book is next.

Despite the subject's difficulty I'm still constantly surprised how many people think current "flows to ground."

 

[jim hardy]

Despite the subject's difficulty I'm still constantly surprised how many people think current "flows to ground."

I feel every EE curriculum should include a one credit hour course on "The Green Book" (IEEE 142).

 

[essenmein]

Certainly not by me. Earth has a field on order of a hundred volts per meter or so near its surface.

http://www.siencis-par-furlan.net/wp-content/uploads/Bressan.Ground.pdf
View attachment 220749

I guess that depends on what you call "earth". To me the potential of the earth, that is the planet and its atmosphere as a single system within our solar system, I would expect to have the closest approximation of neutral or "0V" you can get. The constant blast of plasma from the sun would rapidly reduce any net charge to zero (ie integral of charge over all Earth = 0). plus E field and its forces are far stronger than gravity so any net charge would also affect orbits etc.

Now within the system of the Earth there are large potential differences, surface to outer atmosphere from memory is 100's of kV and depending on what we are doing with our power generation grounding I'd say there will also be large potential difference between two points on the ground. Then local events like wind storms would generate even larger voltages (lightning for example).

 

[Fisherman199]

Here's an interesting read. I wonder what he thinks about the Green Book? He seems to be saying Earth ground is a source and sink of charge.
http://www.gohz.com/why-current-flows-to-ground-earth-without-closed-path

 

[essenmein]

Here's an interesting read. I wonder what he thinks about the Green Book? He seems to be saying Earth ground is a source and sink of charge.
http://www.gohz.com/why-current-flows-to-ground-earth-without-closed-path

He sounds very confused.

 

[Fisherman199]

He sounds very confused.

Agreed, but I find many articles saying things along these lines. People have forgotten Kirchhoff's work.

 

[sophiecentaur]

He sounds very confused.

He seems not to know about Capacitance - or he is ignoring it.

 

[anorlunda]

Here's an interesting read. I wonder what he thinks about the Green Book? He seems to be saying Earth ground is a source and sink of charge.
http://www.gohz.com/why-current-flows-to-ground-earth-without-closed-path

I agree. gohz sounds confused. He begins with the following analysis which is completely wrong.

Let's pretend we have a 1,000 Volt DC source. Say the negative terminal is not connected to anything but we connect the positive terminal to a copper rod and bury it in the earth. All of a sudden, when we close the switch between the battery and the copper rod, charge will be allowed to diffuse from the positive terminal into the earth. The charge will continue to expand across the surface of the Earth until the Earth and the positive terminal of the source reach the same voltage.

 

[Fisherman199]

I agree. gohz sounds confused. He begins with the following analysis which is completely wrong.

It sounds to me like he needs that credit hour of the IEEE Green Book Jim was talking about.

 

[CWatters]

Is it totally wrong?... He refers to a voltage source, the terminals of which will have some small capacitance to earth. That capacitance maybe charged/discharged when a terminal is connected to Earth depending on its initial state.

 

[Fisherman199]

Is it totally wrong?... He refers to a voltage source, the terminals of which will have some small capacitance to earth. That capacitance maybe charged/discharged when a terminal is connected to Earth depending on its initial state.

I wouldn't say the article is totally wrong, but it makes many statements that are false to my mind. For instance:

In effect for all practical intents and purposes the Earth can absorb or give up an infinite number of electrons and remain electrically neutral

This seems to say Earth is both a source and a sink for charge. It isn't. Depending on how you look at the system it may or may not be electrically neutral. Relative to power systems, Earth ground is not "neutral." Perhaps he means there are "static" charge differences? That's true, but I'm unclear if that's what he means.
He touches on some truth is that differences in ground potential cause currents to flow in ground. It's not apparent to me how, if ground were "absorbing or giving charges," that any current should flow at all because of potential difference. His explanation just doesn't seem compatible.
It's also not apparent to me why one need concern themselves with the self-capacitance of the whole earth. It's interesting to the astrophysicists perhaps but I haven't run across the phrase in the IEEE Green Book as of yet.

 

[essenmein]

Regarding power systems, generally Earth is only used as a local reference and not for returning load current, dirt is not a very good conductor!

3 phase HV lines are generally delta connected and don't need a neutral, HV DC transmission have both pos and neg on the lines, Earth is only a reference for the converter/transformer/end user.

 

[Fisherman199]

Regarding power systems, generally Earth is only used as a local reference and not for returning load current, dirt is not a very good conductor!

3 phase HV lines are generally delta connected and don't need a neutral, HV DC transmission have both pos and neg on the lines, Earth is only a reference for the converter/transformer/end user.

Right. Perhaps I wasn't clear. I'm not saying Earth is conducting to the source. That's the neutral conductor's job, if there is one.

My point is that many electricians and lineman I talk to are convinced that, once something touches Earth reference, it becomes safe. That's a deadly misunderstanding.

 

[essenmein]

My point is that many electricians and lineman I talk to are convinced that, once something touches Earth reference, it becomes safe. That's a deadly misunderstanding.

I'm curious why this is a deadly misunderstanding? Aside from the working voltages (eg 120/230 etc) which will get you even if the ground is correctly connected, generally speaking if the thing you are worried about getting a shock from is grounded and you are also grounded then there is no potential difference between the thing and you to give you a shock?

 

[Babadag]

As usual, I'm late. However, I spent a lot of time looking for a 'true grounding theory', so I am quite intoxicated with usual information from NEC ,IEEE 142,IEEE 80, BS 7430, IEC 60364, IEC EN 50522 and other.
The Earth absolute potential it is not so important but only the voltage drop and the potential gradient since E[Electric Field Intensity]=-grad(V).
If an object is connected between two points of different potential the current passing through the resistance between these two points could be deadly.
The resistance between these 2 points depends on grounding resistance of the grounding electrode in each point [or grounding grid]- which is just a contact resistance between the electrode and the ground. The ground resistance between electrodes is considered 0 [somewhere I found this equation rE=π^2.freq/10^4 Ω/km].More important is the “depth” of this ground considered a conductor buried at this depth [according to Carson] in order to calculate the reactance from a wire to ground.
If we measure the ground resistivity the second spike could be anywhere in vicinity-10 fits. is good enough, for instance. So the infinite [where potential=0] it could be 10 ft. from the measuring point[!].

 

[jim hardy]

If we measure the ground resistivity the second spike could be anywhere in vicinity-10 fits. is good enough, for instance. So the infinite [where potential=0] it could be 10 ft. from the measuring point[!].

That becomes your reference point and you're measuring potential difference with respect to it instead of with respect to infinity. .

If an object is connected between two points of different potential the current passing through the resistance between these two points could be deadly.

Hence the concept of "step potential".

http://www.esgroundingsolutions.com/what-is-step-and-touch-potential/

They design a switchyard grounding grid (network in that image) to hold voltage drop along 'ground' during a fault to a small enough number of volts per meter that it won't electrocute that unlucky fellow standing underneath the line. I don't know about transmission line towers..

3 phase HV lines are generally delta connected and don't need a neutral,

That's a misleading statement that i found posted on Stackexchange...
The lines themselves are just wires. It's the transformers at their ends that are connected wye or delta.
At my plant the main generator stepup transformer had a 24KV delta primary fed by the generator, and a 240 kv wye secondary feeding the switchyard.
So our HV lines were fed at their source end by a wye connected transformer.
Using a delta connected transformer at their load end would force neutral current to ~zero allowing neutral to serve as just a lightning rod under normal conditions. That was important in South Florida.. It also gives a return path via wire for fault current.

any help ?

old jim

 

[Svein]

So our HV lines were fed at their source end by a wye connected transformer.
Using a delta connected transformer at their load end would force neutral current to ~zero allowing neutral to serve as just a lightning rod under normal conditions. That was important in South Florida.. It also gives a return path via wire for fault current.

The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...

 

[Babadag]

Thank you for your remarks Jim and I agree with you.
About the essenmein post: usually HV Systems are Y connected as it is indicated in Stackexchange post.
However, a phase-to-ground short-circuit at HV appears as phase-to-phase short-circuit at 24 kV Delta. No neutral so no neutral current at generator side.
Usually, if the transmission line it is an overhead line then the overhead grounding wire it is also static wire protecting the live conductors and insulators against lightning strokes.
If lightning strikes an overhead ground wire, a traveling current wave will be set up which will
induce a traveling voltage wave. This voltage wave will generally increase in magnitude as it
travels down the wire, until it reaches a structure where the reflection of the traveling wave from
the ground prevents the voltage from further increasing. (The overhead ground wire is grounded
at every structure). If the traveling voltage wave at the structure is sufficiently high, a "back flashover" across the insulation from the structure ground wire or from the overhead ground wire
to the phase conductor will occur. The factors that determine if a back flashover will occur are:
the amount of insulation, the footing resistance (the higher the footing resistance, the higher the
voltage rise at the structure) and the span length.

 

[jim hardy]

The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...

Since the neutral need only carry the unbalance, which is small(or zero if the line feeds a delta transfomer load), it is often a much smaller wire that's run atop the towers to intercept lightning before it reaches the phase conductors. It'll be connected to the structure and to Earth at every tower. Since it doesn't need insulators it's pretty cheap to install. Power company can put a fiber optic cable inside it and earn revenue from a communication company...

Another consideration is that a Delta connected transformer winding nearly eliminates third harmonic distortion.

There's a long discussion at Stackexchange .