fundamental electricity question

[Carultch]

is two rods much better?

in any case, it is not useless. the ground rod seller made a few pennies from you.

I've wondered that for a long time. Can two identical rods spaced at 6 ft, connected together by a #6 or #4, really do more than double the conductance to the ground? Why else does the standard "two and done" rule work?

 

[Sahib]

The earth itself is a truly excellent conductor, not because it has particularly low resistivity but because there is so much of it. Enormously parallel path network.

The problem, however, is in making the connection to it.
Depending on the nature of the earth, soil chemistry and moisture content a simple driven ground rod could be a low as 5 ohms or less or 100 ohms and more. That is what limits the effectiveness of earth return at utilization voltages for reasonable power equipment. 10 ohms in a 120V circuit will only pass 12A into a short circuit, and the voltage drop will be a problem for any load over .5A.

But if the two end electrodes are identically configured but in locations 2000 miles apart the resistance of that part of the circuit will still be only 10 ohms.

That earth is an excellent conductor depends on its resistivity because the formula for calculation of earth resistance depends on it and not on assumption of enormous number of parallel paths. So the earth resistance may be very high or very low depending on soil resistivity, the contact resistance playing a small role.

 

[Sahib]

I have a gap in understanding some very basic electricity flow and was hoping you guys could help me out.

in the picture below if I have a source and a motor say 500 feet away and I run just one single conductor source to load.

Run a ground cable directly into earth at source and load, will the motor actually run? The current will flow thru actual earth and return to the source? the earth is that good of a conductor?

thanks


View attachment 18538

But do not do it within a city limit. Though it will work, it may cause corrosion to large number of metal pipes buried in there.

 

[GoldDigger]

That earth is an excellent conductor depends on its resistivity because the formula for calculation of earth resistance depends on it and not on assumption of enormous number of parallel paths. So the earth resistance may be very high or very low depending on soil resistivity, the contact resistance playing a small role.

It depends on how you use the term contact resistance. Rather than just the very short range resistance between the surface of the rod and those dirt particles that are actually touching it, I was using the term to include the zone resistance within the sphere of influence of the rod.
That resistance, and the radius to which there is a significant voltage offset from remote earth, does depend strongly on the conductivity of the surrounding earth.
But even if that zone is hundreds of meters wide for low conductivity rock or soil, the difference in overall circuit resistance between two such electrodes a mile apart and the same electrodes a thousand miles apart is negligible regardless of the nature of the surface soil along the path.

Sent from my XT1585 using Tapatalk

 

[Sahib]

But even if that zone is hundreds of meters wide for low conductivity rock or soil, the difference in overall circuit resistance between two such electrodes a mile apart and the same electrodes a thousand miles apart is negligible regardless of the nature of the surface soil along the path.

Sent from my XT1585 using Tapatalk

But the nature of earth soil close to the electrode ie its resistivity determines the resistance to a very large extent i.e the bulk of earth surrounding the earth electrode and not the contact resistance in ordinary circumstances (in extraordinary circumstances where there is a movement of soil, the contact resistance becomes important and it has to be set right).

 

[Sahib]

I've wondered that for a long time. Can two identical rods spaced at 6 ft, connected together by a #6 or #4, really do more than double the conductance to the ground? Why else does the standard "two and done" rule work?

The resistances of two earth electrodes spaced sufficiently apart combine in parallel. The two and done rule purpose is redundancy, IMHO.

 

[Besoeker]

That earth is an excellent conductor depends on its resistivity because the formula for calculation of earth resistance depends on it and not on assumption of enormous number of parallel paths. So the earth resistance may be very high or very low depending on soil resistivity, the contact resistance playing a small role.

But those paths are of ever increasing length.

 

[GoldDigger]

But those paths are of ever increasing length.

The paths are of ever increasing length, but the "number" of paths increases as the square of the distance, so the number of paths still wins.

 

[Besoeker]

The paths are of ever increasing length, but the "number" of paths increases as the square of the distance, so the number of paths still wins.

I'd need to do the geometry and I'm too indolent to do that..........
And the variations in the nature nature of the mediums through which the paths would pass would make such a calculation possibly irrelevant. The earth isn't a homogeneous sphere.

 

[Sahib]

I'd need to do the geometry and I'm too indolent to do that..........
And the variations in the nature nature of the mediums through which the paths would pass would make such a calculation possibly irrelevant. The earth isn't a homogeneous sphere.

Not geometry but calculus that will demonstrate how resistances of successive layers of earth remote from earth electrode, irrespective of their nature, contribute less and less to the overall earth electrode resistance .

 

[Besoeker]

Not geometry but calculus that will demonstrate how resistances of successive layers of earth remote from earth electrode, irrespective of their nature, contribute less and less to the overall earth electrode resistance .

Then, by all means demonstrate away.......

 

[Sahib]

Then, by all means demonstrate away.......

Is it not off topic?

 

[GoldDigger]

I hereby rule it "on topic", subject to differing opinions by other moderators.

 

[Besoeker]

Is it not off topic?

Since the topic is effectively SWER it is certainly relevant to the ER part of that.

 

[Besoeker]

Yesterday evening I posted that I was going to test our soil today. I did so.
FWIW, I got about 4k ohm in damp soil. It has rained a lot here recently.

 

[Sahib]

This web page deals with earth rod resistance calculation also : www.ijme.us/issues/spring2006/p33.htm

 

[Besoeker]

This web page deals with earth rod resistance calculation

It's about electrical conduction through the earth.

 

[mike_kilroy]

Yesterday evening I posted that I was going to test our soil today. I did so.
FWIW, I got about 4k ohm in damp soil. It has rained a lot here recently.

GREAT! I have heard low voltage ohm testing will not read correctly; need higher voltages than 1.5v

But if you would,, I for one would be VERY interested in hearing how that "4k" responds to putting a 12vdc bulb and battery in series with the two you tested? No doubt no light... and then do it with a 120v in series with a bulb? Bet it lights full brilliance! That will verify that AC does indeed change the molecular structure of the earth in contact to reduce its resistivity to near zero and the actual earth resistance between the two rods is 10's of ohms.

 

[winnie]

It's about electrical conduction through the earth.

The link that sahib posted gives the geometry and calculation of resistance through the earth using concentric shells and assuming homogeneous conductivity.

The calculations show that the total resistivity approaches a limiting value as the volume of soil considered increases, and is >90% of the limiting value 4 rod lengths away.

As far as the resistance to 'distant earth' is concerned, the resistance measured at a mile and 1000 miles will be the same.

Real earth is not homogeneous, but this will only make a difference close in.

The paper goes on to do real measurements and show how they align with the theory.

-Jon

 

[Besoeker]

The link that sahib posted gives the geometry and calculation of resistance through the earth using concentric shells and assuming homogeneous conductivity.
-Jon

I agree with that