Grounding Impedance Test
- Thread starter bzartsky
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- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Grounding Impedance Test
Do an Internet search for the phrase ?fall of potential test.? There are a number of web sites that can give you information on this test. Here is a quote from one of them, giving a basic description of the process:
Do an Internet search for the phrase ?fall of potential test.? There are a number of web sites that can give you information on this test. Here is a quote from one of them, giving a basic description of the process:
Re: Grounding Impedance Test
AVO and AEMC are two popular instrument manufactures. AVO/biddle makes the "Meggar" brand 3-point "Fall-of-Potential meters for around $400, and AEMC makes the clamp-on probes for about $4K.
As Charlie point out you can to the web for details on use.
The ironic thing is the Meggar is the most accurate and reliable method. However the
Meggar is much more time consumming and labor intensive. Where as the AEMC is very easy to use, but not extremely accurate.
[ November 22, 2004, 05:14 PM: Message edited by: dereckbc ]
AVO and AEMC are two popular instrument manufactures. AVO/biddle makes the "Meggar" brand 3-point "Fall-of-Potential meters for around $400, and AEMC makes the clamp-on probes for about $4K.
As Charlie point out you can to the web for details on use.
The ironic thing is the Meggar is the most accurate and reliable method. However the
Meggar is much more time consumming and labor intensive. Where as the AEMC is very easy to use, but not extremely accurate.
[ November 22, 2004, 05:14 PM: Message edited by: dereckbc ]
- Location
- Massachusetts
Re: Grounding Impedance Test
About 7,926 miles in diameter.
Re: Grounding Impedance Test
Bzartsky, go to the link below and in the top right window scroll down to "Ground Resistance Testing-Soil Resistivity" and "Measuring Soil Resistivity-4 Point Method"
http://www.leminstruments.com/grounding_tutorial/html/
Roger
That can be a good question, so we should be using the terms "Earth, Earthed and Earthing" to be technically correct and in this case the answer would be big, real big.
Bzartsky, go to the link below and in the top right window scroll down to "Ground Resistance Testing-Soil Resistivity" and "Measuring Soil Resistivity-4 Point Method"
http://www.leminstruments.com/grounding_tutorial/html/
Roger
- Location
- Massachusetts
Re: Grounding Impedance Test
Roger.
Roger.
Re: Grounding Impedance Test
Saadt,
If the required 25 ohms can not be met and measured in this space we would drive a second rod to meet the NEC requirement.
If there were performance levels to meet per specifications, and an acurate "fall of potential" test could not be done, we would simply ask the engineer how he wanted it measured.
Roger
Saadt,
so what exactly are we expecting this electrode to do, and why do we spend the ridiculous prices for the equipment to measure it?
If the required 25 ohms can not be met and measured in this space we would drive a second rod to meet the NEC requirement.
If there were performance levels to meet per specifications, and an acurate "fall of potential" test could not be done, we would simply ask the engineer how he wanted it measured.
Roger
Re: Grounding Impedance Test
Roger,
The initial question was:
What kind of instruments are used to measure ground impedance? ... What is the most common method of a ground impedance test?
For the simple case of two grounding rods, ok.
But the question is a general one. One can not know "a priori".
Roger,
The initial question was:
What kind of instruments are used to measure ground impedance? ... What is the most common method of a ground impedance test?
For the simple case of two grounding rods, ok.
But the question is a general one. One can not know "a priori".
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounding Impedance Test
Is a ground impedance test the same as earth resistance test? I thought that a ground impedance test measured the impedance of a fault return path back to the power source.
Don
Is a ground impedance test the same as earth resistance test? I thought that a ground impedance test measured the impedance of a fault return path back to the power source.
Don
Re: Grounding Impedance Test
Hello bzartsky-
Are you trying to measure the EGC impedance? If so, the correct tool is an impedance meter, which is generally built-in to higher end wiring analyzers. Equipment guidance can be found in IEEE Standard 1100, "Recommended Practice for Powering and Grounding Electronic Equipment".
These units run from $500.00 to $6000.00 and are not available as a clamp-on type tool.
If you are trying to measure the building electrode system, the you need the previously mentioned 3 point fall of potential testers. Clamp-on testers may work for your application, however, we stopped using them as they were frequently way off our fall of potential test results.
I would also recommend getting a copy of IEEE Standard 81, "Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System". This standard describes in detail the correct way to take ground resistance or impedance measurements.
Can I ask what your application is to help answer your question better?
Thank-you,
Mark
Hello bzartsky-
Are you trying to measure the EGC impedance? If so, the correct tool is an impedance meter, which is generally built-in to higher end wiring analyzers. Equipment guidance can be found in IEEE Standard 1100, "Recommended Practice for Powering and Grounding Electronic Equipment".
These units run from $500.00 to $6000.00 and are not available as a clamp-on type tool.
If you are trying to measure the building electrode system, the you need the previously mentioned 3 point fall of potential testers. Clamp-on testers may work for your application, however, we stopped using them as they were frequently way off our fall of potential test results.
I would also recommend getting a copy of IEEE Standard 81, "Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System". This standard describes in detail the correct way to take ground resistance or impedance measurements.
Can I ask what your application is to help answer your question better?
Thank-you,
Mark
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Grounding Impedance Test
It took me a long time to catch on to one aspect of ground rods and the NEC?s statements about a 25 ohm resistance. Let me try to explain what I (only recently) figured out. We all know that when you place two resistors in parallel, the net total resistance is smaller than either of the resistors you started with. Place a third or fourth or fifth in parallel, and the total resistance continues to go down.
Now let?s play a mental game. Imagine two rods stuck into dirt some distance from each other. Imagine using a voltage source to force current to flow between the two rods. (At this point, you are imagining part of the Fall of Potential Test.) Question: How does the current go from one to the other? And the answer is:
</font>
The net result of this game is that if you have a chunk of dirt that is deep enough, you will get an effectively infinite number of parallel paths through the dirt. That gives you a net resistance through the dirt of nearly zero ohms. In other words, the resistance of the dirt between two rods 8 feet apart is the same as the resistance of the dirt between two rods 80 feet apart.
A ground impedance test measures the resistance between the ground rod and planet Earth. More specifically, it measures the resistance between the point on the ground rod at which you connect the GEC and the dirt that is far enough from the rod to be representative of the Earth as a whole (actually not the whole Earth, but just the local area).
It took me a long time to catch on to one aspect of ground rods and the NEC?s statements about a 25 ohm resistance. Let me try to explain what I (only recently) figured out. We all know that when you place two resistors in parallel, the net total resistance is smaller than either of the resistors you started with. Place a third or fourth or fifth in parallel, and the total resistance continues to go down.
Now let?s play a mental game. Imagine two rods stuck into dirt some distance from each other. Imagine using a voltage source to force current to flow between the two rods. (At this point, you are imagining part of the Fall of Potential Test.) Question: How does the current go from one to the other? And the answer is:
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">From a point on Rod #1, one inch into the dirt, draw a straight line to Rod #2, again one inch into the dirt. That is the first path for current flow.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">From a point on Rod #1, two inches into the dirt, draw a straight line to Rod #2, again two inches into the dirt. That is the second path for current flow, and it is in parallel with the first.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Now pick any point you like on Rod #1 and any point on Rod #2, and draw a straight line between your two points. That is the third path for current flow, and it is in parallel with the first two. Keep this up until there are no more straight lines to be drawn.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Now start drawing curved lines. Draw curved lines that start at Rod 1, go downwards into the dirt (i.e., deeper than either rod), and curve back up to Rod 2. You get a whole bunch of parallel paths that way too.</font>
The net result of this game is that if you have a chunk of dirt that is deep enough, you will get an effectively infinite number of parallel paths through the dirt. That gives you a net resistance through the dirt of nearly zero ohms. In other words, the resistance of the dirt between two rods 8 feet apart is the same as the resistance of the dirt between two rods 80 feet apart.
Re: Grounding Impedance Test
I going to make the assumption you are trying to measure the resistance to ground of either a ufer type ground or a ground rod/plate.
Only two things out there I know of.
1. Fall of potential meters: These require you set a couple of ground stakes out away from the one you want to measure, hook up the wires and press the button. They generally work okay for the intended purpose if one follows the directions. I have not seen a lab grade type instrument - just grunt grade.
2. Clamp-on measuring devices have gotten popular the last few years cause you clamp it around the GEC to the rod, press the button, and digital readout gives one an "exact" reading. Unfortunately they really don't read the resistance to ground of the rod. Rather they read the loop resistance. Say I have two ground rods set 20 feet apart with a #4 between them. Either at one of the rods, or tapped to the #4 is another #4 that goes to the panel. No other grounding connections to the system.
Okay, clamp the tester around the #4 between the two rods and push the button, and the meter reads 48 ohms. You just measured the series resistnce between the two rods. you don't know if one is 1 ohm and the other is 47 ohms or anywhere in between.
Now clamp on the GEC from the rods to the panel - what do you read? I think it will read infinite - no return path.
The only condition I know of where a clamp-on type will give a true reading is where there is great grounding system in place and one wants to measure the resistance of one rod. Clamp on to the GEC to that rod, and the reading is indicative of the resistnce to ground of that rod, cause the resistance to ground of the rest of the system is near zero.
My opinion on the usefullness of each:
The few times I needed to measure ground resistance, the spec called for less than 10 ohms for the system (rods, ring, ufer in the foundation). A clamp-on won't work, had to use fall of potential, with the aux measuring grounds set waaaay out. Yes it was less than 1 ohm
The only time I saw a clamp-on used was when an inspector wanted to see the individual resistance on the rods driven around the perimeter of a large installation. Likely the whole system is less than 10 ohms, probably less than 1 ohm. Yes, he could measure each rod fine. It looked like useless data to me, I asked the purpose, but never got any answer that made sense.
I did call one of the mfgs of a clamp-on type and wanted to discuss how to interpret the readings cause there was very little in the instruction book. I couldn't get to an engineer, but the sales sort I talked to assured me he was very well versed in the operation. I went through what we were seeing for readings, measuring the loop resistance, and the need for a reference to read to. His response was to get upset and suggest we hire an engineer. sigh
Just in case you haven't guessed, my opinion of clamp-ons is low.
carl
I going to make the assumption you are trying to measure the resistance to ground of either a ufer type ground or a ground rod/plate.
Only two things out there I know of.
1. Fall of potential meters: These require you set a couple of ground stakes out away from the one you want to measure, hook up the wires and press the button. They generally work okay for the intended purpose if one follows the directions. I have not seen a lab grade type instrument - just grunt grade.
2. Clamp-on measuring devices have gotten popular the last few years cause you clamp it around the GEC to the rod, press the button, and digital readout gives one an "exact" reading. Unfortunately they really don't read the resistance to ground of the rod. Rather they read the loop resistance. Say I have two ground rods set 20 feet apart with a #4 between them. Either at one of the rods, or tapped to the #4 is another #4 that goes to the panel. No other grounding connections to the system.
Okay, clamp the tester around the #4 between the two rods and push the button, and the meter reads 48 ohms. You just measured the series resistnce between the two rods. you don't know if one is 1 ohm and the other is 47 ohms or anywhere in between.
Now clamp on the GEC from the rods to the panel - what do you read? I think it will read infinite - no return path.
The only condition I know of where a clamp-on type will give a true reading is where there is great grounding system in place and one wants to measure the resistance of one rod. Clamp on to the GEC to that rod, and the reading is indicative of the resistnce to ground of that rod, cause the resistance to ground of the rest of the system is near zero.
My opinion on the usefullness of each:
The few times I needed to measure ground resistance, the spec called for less than 10 ohms for the system (rods, ring, ufer in the foundation). A clamp-on won't work, had to use fall of potential, with the aux measuring grounds set waaaay out. Yes it was less than 1 ohm
The only time I saw a clamp-on used was when an inspector wanted to see the individual resistance on the rods driven around the perimeter of a large installation. Likely the whole system is less than 10 ohms, probably less than 1 ohm. Yes, he could measure each rod fine. It looked like useless data to me, I asked the purpose, but never got any answer that made sense.
I did call one of the mfgs of a clamp-on type and wanted to discuss how to interpret the readings cause there was very little in the instruction book. I couldn't get to an engineer, but the sales sort I talked to assured me he was very well versed in the operation. I went through what we were seeing for readings, measuring the loop resistance, and the need for a reference to read to. His response was to get upset and suggest we hire an engineer. sigh
Just in case you haven't guessed, my opinion of clamp-ons is low.
carl
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