25 ohm from ground to what?

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dpeter

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elevator mechanic / building maintenance
Thanks in advance for indulging me. I have been reading about the 25 ohm or less for one ground rod or add a second and the various ways to test for compliance but have not been able to figure out where take the measurement. I think obviously one point is the ground rod but what would be the other? Do they all involve driving another rod as a reference to verify the first one? And if you do, does it not then render the values mute because now you have two and you're good to go? Just looking for nudge, push, shove or a kick in the right direction. Thanks
 
See 250.56 for details, but if the resistance of the first electrode, rod, pipe or plate, is greater than 25 ohms an additionl electrode of any type specified by 250.52(A)(4) through (A)(8) must be added. Testing the electrode resistance is expensive, so when you drive two rods greater than six feet apart the code is satified no matter the resistance.
 
OK, yes I get the 25 ohms or less is good and if more add a second rod. Please don't interpret this to be rude or smart a.. but I am used to using a meter with two leads and I place them on wires or terminals and get a reading of ohms, amps or volts depending on the application. Is this as simple as placing one probe on the ground rod and sticking the other probe in the dirt? I can easily reference the ground rod but just HOW does one reference the ground (dirt)?
 
The test is from the electrode or electrode system to the earth

I know of two ways, there is a clamp meter available but having used one and from reading about them I would rule them out as being unreliable.

The other way I know is the 'three point fall of potential test.

Here is the set up and notice that the distances are greater then they appear.


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Here is a testing unit.

GP-2.jpg
 
Thanks Brian John. just spent a few moments reading and it helps alot. The shells analogy also helps a great deal. Seems counterintuitive that that rods can have more resistance closer together. Thanks for the link
 
iwire, that looks like a good set. Is that supporting software in the background? Seems almost all do require one or more rods driven to do the test. Wonder how many extra gound rods do you have to save to pay for that?
 
I own 4 or 5- 3 / 4 point test sets, I think the cheapest was around $1,200.00, most expensive around $3,300.00.

Then there are the accessories (hard shipping cases, cables and rods) and yearly calibration.
 
George Stolz said:
Approximately a 100 ground rods for the cheapest meter, providing it tells you less than 25 ohms each time. :)
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brian john said:
I own 4 or 5- 3 / 4 point test sets, I think the cheapest was around $1,200.00, most expensive around $3,300.00.

Then there are the accessories (hard shipping cases, cables and rods) and yearly calibration.
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...and we haven't yet added in the time and labor involved.

IMO, just the time and labor to do the test appropriately would offset the cost of a second ground rod installation... so in this sense the meter and accessories never pay for themselves. Customers will ultimately have to pay for the test equipment.
 
Smart $ said:
...and we haven't yet added in the time and labor involved.

IMO, just the time and labor to do the test appropriately would offset the cost of a second ground rod installation... so in this sense the meter and accessories never pay for themselves. Customers will ultimately have to pay for the test equipment.
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3-4 hours on site, with test report a typical test runs 8 hours billing plus rental fee.
 
Smart $ said:
IMO, just the time and labor to do the test appropriately would offset the cost of a second ground rod installation... so in this sense the meter and accessories never pay for themselves.
Click to expand...

Not for an EC testing a simple system trying to see if it less than 25 ohms. But guys like Brian and myself don't ever get caled to test a system like that. We test complex groud systems, grids, cell towers, and industrial and commercial systems that are required to be less than 1 or 5 ohms.
 
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