Electrode resistance. From what to what?

I probably should already know this, but I don't. Please, forgive my ignorance.

When we say we have a grounding electrode resistance of say, 5 ohms, what does that mean, exactly? IOW, we have a resistance of 5 ohms from the electrode to what? 'The earth' is always the answer I get, but I really don't understand what that means. Resistance is a value measured from two points. How can 'the earth' be one of those points?

For example, if I have two individual electrodes that measure 5 ohms by accepted testing procedures and they are a hundred or so feet apart, would there be 10 ohms of resistance between the two rods? I would think not. But maybe I am wrong.

I have read the methods for ground rod testing (AVO has a good book on it) and know how to do it, I just don't really understand it perfectly.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I believe that the concept that you are looking for is 'resistance to distant earth'.

You are quite correct that if you have two '5 ohm' ground electrodes, and measure the resistance between them, the result will be something different than 10 ohms. The exact resistance will depend upon the distance between the electrodes and the material between them. In theory you could make a graph of resistance versus electrode distance.

The resistance of a conductor increases with length and decreases with cross section. In the case of current flowing through the earth, the 'cross section' is a bit difficult to define; the earth is huge but your electrodes are small. What happens is that current has to spread out into the earth from your tiny electrodes.

Roughly, the further apart your electrodes are, the further the current can spread, and the larger the effective cross section of the earth carrying your test current.

The net result is that your graph of electrode-electrode resistance versus spacing will asymptotically approach some fixed value as the spacing gets large. (Assuming that the characteristics of the individual electrodes don't change as you move them.....)

With this in mind, you can _define_ the resistance of a single electrode as the resistance between that electrode and a (fictional) zero ohm electrode infinitely far away.

The resistance between any two electrodes that are sufficiently far apart is then given by the sum of the two resistances as defined above.

This is not reality; you could never have a zero ohm electrode and the earth is only of finite size. But it is a useful fiction to provide a baseline for assigning values to individual components.

In much the same way voltage is _always_ measured between two points, but if you select an appropriate reference zero (even a fictional one) you can assign voltage values to individual points and use those values in your calculations.

-Jon
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The point is that measured over long distances the resistance of "the earth" really is zero.
Between any two points the area through which the charges flow increases faster than the distance separating those two points.
The resistance we assign to each ground electrode takes into account the steadily decreasing resistance of each spherical shell around it.
The result is that if the electrode resistance is measured properly the electrode to electrode resistance will be equal to the sum of the two resistances.
Or, if they are close enough that the significant zones overlap, the resistance will be LOWER.


Tapatalk...
 

templdl

Senior Member
Location
Wisconsin
Doesn't the code say that you should have 25 ohms or less from a ground rod to ground? If not you must drive another rod at least 6' away from the first. Since it's questionable if it is even possible to measure the 25ohms for one rod save the anxiety and just drive the second rod.
 

edlee

Senior Member
Doesn't the code say that you should have 25 ohms or less from a ground rod to ground? If not you must drive another rod at least 6' away from the first. Since it's questionable if it is even possible to measure the 25ohms for one rod save the anxiety and just drive the second rod.
I've always wanted to try this with a ground rod system : take a pigtail light socket, screw in a 5a fuse, wirenut it in series to a 1-pole breaker and the GEC (which has been disconnected from the bussbar and any grounded metal in the panel), turn on the breaker and see if it blows the fuse. Quick and dirty test to see if it makes 25 ohms :)
 

templdl

Senior Member
Location
Wisconsin
I've always wanted to try this with a ground rod system : take a pigtail light socket, screw in a 5a fuse, wirenut it in series to a 1-pole breaker and the GEC (which has been disconnected from the bussbar and any grounded metal in the panel), turn on the breaker and see if it blows the fuse. Quick and dirty test to see if it makes 25 ohms :)
What the heck, why not try it. Just keep safety in mind. I live in Wisconsin and certainly don't want to see any smoke and flash from here and then hear an "oops, I guess that wasn't such a good idea."
 

kwired

Electron manager
Location
NE Nebraska
Better IMHO, would be to put a light bulb in the socket and measure the voltage.


Tapatalk...
Voltage? Current? The resistance of an incandescent lamp is very low until the filament gets heated and then it increases, something with a more fixed resistance would probably work better.

I can't remember who to give the credit to, but a member of this forum has in the past mentioned that the earth has a pretty low resistance, but trying to connect to the earth is not easy to do with a low resistance. Once you are able to make a low resistance connection the earth will carry a pretty significant current, but the trick is making that low resistance connection.

So when we say a rod has a 25 ohm resistance what we really mean is there is 25 ohms between that rod and earth. The rod itself is pretty low, and the earth itself is pretty low, they just don't mingle all that well.
 
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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I've always wanted to try this with a ground rod system : take a pigtail light socket, screw in a 5a fuse, wirenut it in series to a 1-pole breaker and the GEC (which has been disconnected from the bussbar and any grounded metal in the panel), turn on the breaker and see if it blows the fuse. Quick and dirty test to see if it makes 25 ohms :)
Isn't this a bad idea? The current is attempting to return to the POCO transformer and will find a path through the earth. What if that path is through the neighbors nearby pool?
 

kwired

Electron manager
Location
NE Nebraska
Isn't this a bad idea? The current is attempting to return to the POCO transformer and will find a path through the earth. What if that path is through the neighbors nearby pool?
Something to consider. Nearest "in ground" pool that I know of from my place is at least 6 or 7 miles away.

Nearest "portable pool" other than maybe a little kids wading pool, which we do have one of those in the dog kennel in the summer months, is probably at least 1.5 miles away

ETA: Hopefully pool has everything bonded like it is supposed to, because POCO likely has currents circulating in the ground in the first place from their MGN network.
 
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I've always wanted to try this with a ground rod system : take a pigtail light socket, screw in a 5a fuse, wirenut it in series to a 1-pole breaker and the GEC (which has been disconnected from the bussbar and any grounded metal in the panel), turn on the breaker and see if it blows the fuse. Quick and dirty test to see if it makes 25 ohms :)
If it was that easy, there would be no need for specialized testers.

Let's say you used a light bulb and measured the current that was actually flowing. As opposed to just guessing using a blown fuse. And let's say you measured two amps. The math says that's 60 ohms. But it's not 60 ohms to the earth, it's 60 ohms back to the POCO neutral.
 

kwired

Electron manager
Location
NE Nebraska
If it was that easy, there would be no need for specialized testers.

Let's say you used a light bulb and measured the current that was actually flowing. As opposed to just guessing using a blown fuse. And let's say you measured two amps. The math says that's 60 ohms. But it's not 60 ohms to the earth, it's 60 ohms back to the POCO neutral.
But the majority of the resistance is in the electrode to earth junction, the POCO has so many more electrodes in their network that that side of things has pretty low resistance.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Engineer
To me it seems like a reasonable test, unless you shock yourself in the process of testing. I'd be curious of the results of comparing the two methods. Clamping 120V onto a rod is easy for anyone to do.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
So why don't we use a light bulb and an amp clamp instead of a fall of potential test?
One reason is that at the time the electrode is being tested there may not be a solid utility MGN available.
Also, although the MGN network produces a low overall resistance to ground, there can easily be voltage offsets in the terminal available to you, caused by current in the local wire connection into that MGN.
I am sure that there are more reasons, but basically you are not allowed to make that kind of assumptions when conducting a test which certifies something. When doing a Fall of Potential test, you control all of the variables and (should) know whether you are doing it right or not.
 
One reason is that at the time the electrode is being tested there may not be a solid utility MGN available.
Also, although the MGN network produces a low overall resistance to ground, there can easily be voltage offsets in the terminal available to you, caused by current in the local wire connection into that MGN.
I am sure that there are more reasons, but basically you are not allowed to make that kind of assumptions when conducting a test which certifies something. When doing a Fall of Potential test, you control all of the variables and (should) know whether you are doing it right or not.
Would it suffice to say using the light bulb (or fuse for that matter) test just doesn't give us the info we are looking for?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Would it suffice to say using the light bulb (or fuse for that matter) test just doesn't give us the info we are looking for?
If you are not interested in the reasons behind it, that is a fair summary, yes. :)
I might tweak it slightly to say that the method does not always give the information we are looking for. (And you can't tell when it is right and when it is not.):)
 

templdl

Senior Member
Location
Wisconsin
If you are not interested in the reasons behind it, that is a fair summary, yes. :)
I might tweak it slightly to say that the method does not always give the information we are looking for. (And you can't tell when it is right and when it is not.):)
Damn, that's a profound statement. Is that a positive maybe?
 
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