What Does Ground Mean Exactly?

  • #26
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Actually - "on paper " there were no "loops" of iron, but due to mechanical asymmetry, it was enough to cause problems - like centering conductors in the CT for accuracy. This was around 1998 ? -- the control rooms used CRTs and they could tell the current levels by how much everyone's screens were shifted. If you entered the electrical sub it could erase your credit cards in your wallet, and the steel toe shoes would vibrate in some spots. - I could probably write a solid 4 to 5 pages on that job on both the oddities and the technical errors and discoveries.
 
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  • #27
jim hardy
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You make it sound like there is some return path all the way to the power plant:
No, i do not. I said charge must get back to the transformer winding from whence it came.


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 current may be large or it may be small depending on conductivity of the soil, your skin and your shoe soles.
Back to the transformer is exactly where the current goes. Why do you dispute Kirchoff's Current Law ?
You have some un-learning to do, sir. Meantime you shouldn't confuse beginners with such misstatements.
Have you read IEEE 142 ?


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.
JD=ϵ0∂E∂t​
\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.
The neutral connection between house wiring and the transformer winding is not a capacitor it is a solid wire(well more likely stranded ACSR).
So i don't know where in your mental model are the "plates" of your capacitor.
Seems you're proposing they're at opposite ends of the neutral wire . If so then your dielectric is short circuited by the neutral wire and you have not a capacitor but a very low ohm resistor.
 
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  • #28
jim hardy
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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.
Ah but it does. If your shoes are well insulated and dry you can hold 120V 60 hz in your fingers and feel nothing. High impedance of your shoes limits current to a value so small you can't feel it.
Radio Frequency on the other hand can pass enough current into your fingertip and out your body capacitance to space that it will give you a nasty "RF Burn" where it enters you, ask any Ham operator. That is the "radiative" mechanism you mention.
 
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  • #29
jim hardy
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This displacement current is not restricted to capacitors but can be used anywhere there is an electric field.
How much electric field can exist along a wire ? NEC ampacity charts limit it to about 30 millivolts per foot. That won't push much displacement current through free space at power line frequency. ∂E/∂t is too small. You need the neutral wire to complete the circuit .

Thanks, @Baluncore, for putting numbers to it.
 
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  • #30
Averagesupernova
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Ah but it does. If your shoes are well insulated and dry you can hold 120V 60 hz in your fingers and feel nothing. High impedance of your shoes limits current to a value so small you can't feel it.
Yep. Been there and done that. Dangerous situation when you are working with live wires unknowingly. Suddenly I got a pretty good tingle when the back of my hand touched the ground wire. At that point I knew I had missed something.
 
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  • #31
The current may be large or it may be small depending on conductivity of the soil, your skin and your shoe soles.
Back to the transformer is exactly where the current goes. Why do you dispute Kirchoff's Current Law ?
You have some un-learning to do, sir. Meantime you shouldn't confuse beginners with such misstatements.
Have you read IEEE 142 ?
The resistance of all of that is probably in the mega-ohms, yet people still get electrocuted.
The resistivity of soil is normally around ##\rho=100\Omega\text{m}##. Lets say the contact area between your feet and the ground is around ##A=10\text{cm}\times10\text{cm}## and that the pole transformer ground is ##l=100\text{m}## away (this is about how far mine is). The ground resistance is roughly
$$R=\frac{\rho l}{A}=1M\Omega$$
$$I=\frac{120\text{V}}{1M\Omega}=120\mu\text{A}$$
The point I'm trying to make is that if you stand outside on the ground, both your body and the ground have capacitance. The self capacitance of the earth (##710\mu\text{F}##) is large enough to draw a fair amount of current.
$$R=\frac{1}{2\pi(60\text{Hz})(710\mu\text{F})}=3.7\Omega$$
$$I=\frac{120\text{V}}{3.7\Omega}=32\text{A}$$
 
  • #32
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NFuller, from experience I can tell you this isn't exactly the case (as in I have been bitten hooking services back up and by accidentally touching the line on meter cans when pulling the meter). By those equations I should be dead. You aren't exactly taking into account the resistance of the human body either, and that should be factored into your equation.

Also, just as an observation, being as we are talking about service entrances here, the vast majority of the current is going to take the path of least resistance, which is going to be through the ground rod, through the EGC back to the ground-neutral bond at the panel, and through the neutral conductor back to the transformer, making the assumption that you are standing closer to the service disconnect or meter can, which is an almost certainty if you are getting shocked, excluding exceptions such as energized, conductive building facade materials (like siding, or a tin building).

Good conversation so far, I'm having a lot of fun following it.
 
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  • #33
NFuller, from experience I can tell you this isn't exactly the case (as in I have been bitten hooking services back up and by accidentally touching the line on meter cans when pulling the meter). By those equations I should be dead. You aren't exactly taking into account the resistance of the human body either, and that should be factored into your equation.
I'm ignoring the resistance of your shoes and body to directly compare ground resistance in the two cases.
Also, just as an observation, being as we are talking about service entrances here, the vast majority of the current is going to take the path of least resistance, which is going to be through the ground rod, through the EGC back to the ground-neutral bond at the panel, and through the neutral conductor back to the transformer, making the assumption that you are standing closer to the service disconnect or meter can, which is an almost certainty if you are getting shocked, excluding exceptions such as energized, conductive building facade materials (like siding, or a tin building).
Notice that even if you change the length ##l## from ##100\text{m}## to ##1\text{m}## the current would only increase to ##12\text{mA}##. This is still to small to kill someone.
 
  • #34
Baluncore
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The resistance of all of that is probably in the mega-ohms, yet people still get electrocuted.
The resistivity of soil is normally around ρ=100Ωmρ=100Ωm\rho=100\Omega\text{m}. Lets say the contact area between your feet and the ground is around A=10cm×10cmA=10cm×10cmA=10\text{cm}\times10\text{cm} and that the pole transformer ground is l=100ml=100ml=100\text{m} away (this is about how far mine is). The ground resistance is roughly
R=ρlA=1MΩR=ρlA=1MΩ​
R=\frac{\rho l}{A}=1M\Omega
I=120V1MΩ=120μAI=120V1MΩ=120μA​
I=\frac{120\text{V}}{1M\Omega}=120\mu\text{A}
The point I'm trying to make is that if you stand outside on the ground, both your body and the ground have capacitance. The self capacitance of the earth (710μF710μF710\mu\text{F}) is large enough to draw a fair amount of current.
R=12π(60Hz)(710μF)=3.7ΩR=12π(60Hz)(710μF)=3.7Ω​
R=\frac{1}{2\pi(60\text{Hz})(710\mu\text{F})}=3.7\Omega
I=120V3.7Ω=32AI=120V3.7Ω=32A​
I=\frac{120\text{V}}{3.7\Omega}=32\text{A}
I can assure you that the resistivity of soil is never typical.

Your ground resistance calculations are out by 5 or 6 orders of magnitude. You ignore the fact that the parallel conductive path is very wide and is shortened by metal pipes in the ground. In my experience the Earth is highly conductive because you are usually close to the water table with all that dissolved salt. I have done geophysical resistivity surveys and notice that the resistance drops rapidly once current begins to flow at depth.

Also, the fault current only needs to get to the earth stake at the meter box, then it can cross the link to the neutral and travel along the neutral wire to the transformer. It certainly does not need to go all the way to the transformer through a shallow narrow strip of dry topsoil.

Nor do you have to charge the spherical Earth capacitor. To do that you would need to be standing on the moon with an electron beam gun. Or be the Sun, delivering a solar wind to the ionosphere. You are inside the Earth capacitance, until you are struck by lightning it is irrelevant to the discussion.

Your poor understanding and bad models of the electrical distribution system and the real world environment will get you into trouble, maybe even kill you. Until you realise your lack of experience and understanding you will be dangerous to yourself and others.
 
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  • #35
jim hardy
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well just lost an hour of typing a response. Something logged me out, draft disappeared when i clicked 'login' . @berkeman - anything fishy looking ? do my PF tracks show signs of a virus? old jim.
 
  • #36
jim hardy
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hmm got logged out again on that one. But the draft didn't disappear.

NFuller: a measly one amp will charge earth's 710 uf at the rate of 1408 volts per second , assuming you can distribute the charge equally over its whole surface. That's 11.7 volts every half a line cycle.
Since you can't distribute charge that fast it'll raise local earth voltage more at the point of current injection reducing potential difference across your body.

That's why it's current flow around a closed loop that electrocutes people .

Another thread you might like
https://www.physicsforums.com/threads/trying-to-understand-electric-shocks.883550/
 
  • #37
berkeman
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well just lost an hour of typing a response. Something logged me out, draft disappeared when i clicked 'login' . @berkeman - anything fishy looking ? do my PF tracks show signs of a virus? old jim.
I don't see anything obvious. Nice featured thread about PV, btw. :smile:
 
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  • #38
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The current may be large or it may be small depending on conductivity of the soil, your skin and your shoe soles.
The resistance of all of that is probably in the mega-ohms, yet people still get electrocuted.
I remember being surprised how much I didn't know about grounding when it came time to learn how to use a "ground Megger" and interpret the results. R=pl/A is but a starting point, but as @jim hardy and @Baluncore point out, coming close to an understanding what's actually going on is a wilder and woolier ride. Biddle has a good write-up of the basics, and is worth reading by anyone interested in what constitutes earth ground.

www.weschler.com/_upload/sitepdfs/techref/gettingdowntoearth.pdf

Resistance varies in the manner of concentric shells around a grounding point, and is the reason behind step potential, and using a "bunny hop" or "shuffle step" technique to escape the vicinity of a downed power line (images were copped from a BC Hydro slide show and the document above).

ground_sphere of influence.jpg


ground_BC-Hydro_step potential.jpg
 

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  • #39
anorlunda
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Wow. I first looked at this thread 36 hours ago. It was simple. But now I see it has become a mess. Ground is indeed a term with many definitions and many nuances.

The OP said his questions have been answered. Most of the following posts seem to go in directions other than the OP's question, so I think it's time to close this thread. If you want to discuss specific grounding practices, feel free to start a new thread.
 

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