Electrode resistance. From what to what?

Sahib

Senior Member
Location
India
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?
Really the fall of potential method uses the same principle as behind the fuse breaker method, but more accurately and above all, safely.
 

Sahib

Senior Member
Location
India
If you totally isolate you don't get the same results as you are not connected to the utility MGN network so it is not the same test.
Use a sufficiently low voltage secondary isolation transformer for low output current such as used by a meggar.
 
Really the fall of potential method uses the same principle as behind the fuse breaker method, but more accurately and above all, safely.
I disagree.

FOP is testing electrode to earth resistance. The 'fuse breaker' method gives you electrode to POCO neutral resistance. The two may or may not be the same. I'll bet mostly not as the POCO electrode system is now part of the equation and for testing disconnected electrodes we don't want it to be part of the equation.
 
I believe that the concept that you are looking for is 'resistance to distant earth'.

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
"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."

Conflicts with:

"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."

Is FOP testing not an extrapolation of values, as opposed to a single measurement taken as an accurate value?
 
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...
What I am hearing is that the earth has 'negative' resistance. Instead of resistance increasing with distance, it decreases. Likely due to the addition of parallel paths possible vs. the number of series paths?
 

handy10

Senior Member
What I am hearing is that the earth has 'negative' resistance. Instead of resistance increasing with distance, it decreases. Likely due to the addition of parallel paths possible vs. the number of series paths?
The "addition of parallel paths" is essentially correct. One thought experiment would be to think about what would happen in a rather large tub of poorly conducting fluid(salt water) if resistance is measured between two electrodes. As the distance increases, the parallel paths increase causing resistance to decrease. This behavior is different from the effect you would observe if you hand a slender cylinder of the same fluid. As the length of the cylinder increases, the resistance would increase because there is no more parallelism.
 
Here is what I am getting so far.

The material of the earth has resistance.

The total resistance of the material is very near 0 ohms from one point to another directly on the other side of the earth.

The above can't be directly measured, but must be calculated from other data.

The resistance presented by the earth increases as electrodes come closer together.

This 'negative' resistance could possibly be demonstrated using a large ball of semi-conductor and an ohm meter.
 

Sahib

Senior Member
Location
India
I disagree.

FOP is testing electrode to earth resistance. The 'fuse breaker' method gives you electrode to POCO neutral resistance. The two may or may not be the same. I'll bet mostly not as the POCO electrode system is now part of the equation and for testing disconnected electrodes we don't want it to be part of the equation.
FOP is testing electrode to earth resistance. The 'fuse breaker' method gives electrode to POCO neutral resistance. The POCO neutral resistance is so close to zero that its inclusion in the measurement is unlikely to change the result significantly.
 

Sahib

Senior Member
Location
India
I take it that the electrical point we refer to as 'the earth' exists only mathematically. Unless it's a point on the molten nickel in the center.
If we connect any charged object to earth, it loses its charge. By earthing an object, we ensure that it does not give an electrical shock.
Is there a standard electrode somewhere? One to calibrate test equipment by?
Any sufficient large metal object may be used as 'earth' electrode.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
If we connect any charged object to earth, it loses its charge. By earthing an object, we ensure that it does not give an electrical shock. ...
If the object in question is connected to a voltage source, earthing it does nothing more than raise a small section of the earth around the grounding electrode to that of the object. It really does nothing to prevent a shock, unless you are standing on or very close to the grounding electrode.
 

tom baker

First Chief Moderator
Staff member
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 :)
Mike Holt has done just that and its available on a video. He drives a ground rod, checks the resistance with a fall of potential meter, clamp on meter and and measures the current at 120 volts.
Current > ohms law follows the meter readings.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
"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."

Conflicts with:

"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."
I needed to do a bit of reading to give a better answer, so in part I will refer you to this short page on FOP testing: http://electrictestinstruments.com/data/Fall of Potential Ground Test.htm

I have to admit that I forgot about the concept of 'sphere of influence'. The resistance of a ground electrode is the resistance to 'distant earth', however the distance that counts as sufficiently far away to no longer make a difference is generally not to very far.

If you have two electrodes, each measured at 5 ohms 'resistance to earth', and you instead measure the resistance _between_ these two electrodes, then the resistance will be lower than 10 ohms. _However_ (and this is a very important caveat to my statement) the bulk of the resistance is caused by the 'sphere of influence' of each electrode. Only if the electrodes are close enough for their spheres of influence to overlap will the resistance be significantly lower than the sum of the two electrode resistances.

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

Is FOP testing not an extrapolation of values, as opposed to a single measurement taken as an accurate value?
[/QUOTE]

If you look at the paper, proper FOP testing requires an extrapolation be made. You have two 'driving' electrodes (the electrode under test, and the current injection electrode). You have one sense electrode (the potential electrode). You move the sense electrode between the two driving electrodes, and generate a graph of voltage versus position. For a proper FOP test, you must get to a 'flat' region of the curve.

The difference in voltage between the electrode and flat region gives you V. The injected current gives you I. Apply Ohm's law and you get R.

But there is, of necessity, an approximation being made. Assuming a homogeneous conductor (the Earth isn't, but that just adds confusion), there will _never_ be a flat part in this graph. The graph should always be monotonic, always trending in the same direction at a greater or lesser rate. But if your electrodes are far enough apart then there will be a 'flat enough' part of the graph that the result is good enough. The real world has variability (soil temperature, moisture content, etc); at some point the error caused by assuming the middle of the graph is flat is smaller than the natural variability of the resistance being measured.

One other note: there isn't a region of negative resistance. If you are close enough together there is less resistance simply because the circuit is shorter.

-Jon
 

Sahib

Senior Member
Location
India
Mike Holt has done just that and its available on a video. He drives a ground rod, checks the resistance with a fall of potential meter, clamp on meter and and measures the current at 120 volts.
Current > ohms law follows the meter readings.
Nowhere ohms law is violated (save non-linear circuits). Some resistance/impedance might have been missed in Mike's experiment.
 
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kwired

Electron manager
Location
NE Nebraska
Mike Holt has done just that and its available on a video. He drives a ground rod, checks the resistance with a fall of potential meter, clamp on meter and and measures the current at 120 volts.
Current > ohms law follows the meter readings.

IMO a typical ground rod will give you what appears to be inconsistent readings, there is not enough electrode to soil contact as compared to many other electrodes to maintain a stable overall resistance to earth, especially when higher voltages and currents get introduced.
 

Sahib

Senior Member
Location
India
If the object in question is connected to a voltage source, earthing it does nothing more than raise a small section of the earth around the grounding electrode to that of the object. It really does nothing to prevent a shock, unless you are standing on or very close to the grounding electrode.
If the charge on a metal object from a voltage source is steady, earthing the object will free one from receiving a shock on touching near or remote from it, provided the other terminal of the voltage source is not earthed. Metal objects with fluctuating charges are liable to give electrical shocks when there is considerable earth resistance.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
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.):)
And don't ask me what I think of you; I might not give the answer that you want me to. :D
 
If the charge on a metal object from a voltage source is steady, earthing the object will free one from receiving a shock on touching near or remote from it, provided the other terminal of the voltage source is not earthed. Metal objects with fluctuating charges are liable to give electrical shocks when there is considerable earth resistance.
I do not agree with that at all.

Even in ungrounded systems, there will be coupling with the earth and a huge potential difference between the hot conductors and the earth. Dropping one to ground will still establish possibly fatal step potentials.
 
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