Grounding reading worst after adding additional ground rods

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thewire

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What is the cause of it? Potential the connections or bonding are loose? Lets say reading was 30ohm, then adding 2 more 10ft rods, the reading is now 45 ohm
 
thewire said:
What is the cause of it? Potential the connections or bonding are loose? Lets say reading was 30ohm, then adding 2 more 10ft rods, the reading is now 45 ohm
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Did you move the test points between testing with one rod and testing with three? It might be that there is a local area of dirt that is not real conductive where you stuck your test probe.
 
thewire said:
Using AEMC 3640. Fall potential Method @ 62ft and 100ft from the rods and probe
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And how far apart are the electrodes? You can get interesting anomalies using the FOP method unless your closest test probe is a large number of diameters away from the total area where the electrodes are located.
 
Wouldn’t you use a larger bonding wire from one ground rod to the next to reduce the ohms? Like using a larger ground rod sometimes, or longer ground rod? Or like how sometimes in a raceway you need a larger ground wire than any of the conductors?
 
Adamjamma said:
Wouldn’t you use a larger bonding wire from one ground rod to the next to reduce the ohms? Like using a larger ground rod sometimes, or longer ground rod? Or like how sometimes in a raceway you need a larger ground wire than any of the conductors?
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It wouldn't really matter. I don't recall what size you need, #6 or #8, but 1,000 feet of either won't even get you to 1 ohm of resistance. I don't see how it can be the problem.
 
Adamjamma said:
Wouldn’t you use a larger bonding wire from one ground rod to the next to reduce the ohms? Like using a larger ground rod sometimes, or longer ground rod? Or like how sometimes in a raceway you need a larger ground wire than any of the conductors?
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He is measuring resistance between rod(s) and earth. Even a 18 AWG bonding wire between the rods (effectively making them one electrode when reading the combined resistance) is likely to be much lower resistance then rod to earth resistance.
 
thewire said:
Can you explain this a little more? thanks!
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The three point Fall Of Potential method fits the observed voltage measurement at the midpoint against a known calculated voltage versus distance curve that assumes that the electrode under test represents a single point or line electrode at a known distance.
When you have instead multiple electrodes separated by many feet the shape of the voltage versus distance curve is distorted. In particular, if the added electrode is closer to the middle test point than the other existing electrodes you can be effectively putting the test electrode at the wrong distance.
The actual voltage versus distance profile for a complex array of electrodes (new and existing) can be too complicated to accurately calculate.
And if there are conductive objects, such as buried pipes, concrete slabs, whether connected to some sort of grounding network or not, along the line joining the three points then the FOP method can be wildly inaccurate.
 
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