Ground Resistance Test

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tlona1

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I need to perform a ground resistance test on an isolated ground rod. Can this be achieved by simply installing an auxiliary ground rod and using a multimeter (fluke 787) to measure OHMS between the auxiliary rod and the isolated rod or doe I need and ground resistance testing piece of equipment?
 
That won't tell you anything. You will be getting resistance from both of the rods. The only method I have heard of is called the "fall of potential method." Do a google search for that phrase (or look for it on this forum), and you will learn more than you can stand. ;)
 
May I ask why you need to perform the ground resistance test, AND what this information will provide you?

For example, say you perform a fall of potential test and get a measurement of 25.6 ohms. What does that mean to you?
 
The best thing you can do in my opinion is hire a firm that specializes in electrical testing, you get expierence, specialized equipment you would have little use for on a regular basis and an impartial firm with equipment calibrated offering documentation for you engineer to review.

I am assuming this is part of a job spec?
 
brian john said:
The best thing you can do in my opinion is hire a firm that specializes in electrical testing, you get expierence, specialized equipment you would have little use for on a regular basis and an impartial firm with equipment calibrated offering documentation for you engineer to review.

I am assuming this is part of a job spec?
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Couldnt agree more. You can find a certified testing company near you at www.netaworld.org
 
tlona1 said:
I need to perform a ground resistance test on an isolated ground rod. Can this be achieved by simply installing an auxiliary ground rod and using a multimeter (fluke 787) to measure OHMS between the auxiliary rod and the isolated rod or doe I need and ground resistance testing piece of equipment?
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On school projects we had to provide the ground resistance test in the presence of an inspector we use a tester similar to the AEMC 4610 4-Point Ground Resist. Tester. These have 3 or 4 electrode probes installed in ground at cirtain intervals past the ground for the building, like 25?, 50?, & 75??

You can find other types at Test Equipment Depot, or even search online at Google ?ground resistance tester?.
 
tryinghard said:
On school projects we had to provide the ground resistance test in the presence of an inspector we use a tester similar to the AEMC 4610 4-Point Ground Resist. Tester. These have 3 or 4 electrode probes installed in ground at cirtain intervals past the ground for the building, like 25?, 50?, & 75??

You can find other types at Test Equipment Depot, or even search online at Google ?ground resistance tester?.
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Not trying to make light of you test, but there is more to this than just driving rods at set intervals.

A 3-point test is utilized to test installed electrodes, the 4 point test is utilized to determine the soil resistivity typically as part of the design phase.

You need to know the size of the grid, typically you will need to go out 10 times the diagonal of the grid, then take 10 reading spaced out from the grid under test and the furthest probe. Typically we do a minimum of 3 test. Then you need to plot the reading on a graph and see if your reading at 62% was outside the sphere of influence. If it isn't you move you Z probe further out and start again.
 
brian john said:
Not trying to make light of you test, but there is more to this than just driving rods at set intervals.

A 3-point test is utilized to test installed electrodes, the 4 point test is utilized to determine the soil resistivity typically as part of the design phase.

You need to know the size of the grid, typically you will need to go out 10 times the diagonal of the grid, then take 10 reading spaced out from the grid under test and the furthest probe. Typically we do a minimum of 3 test. Then you need to plot the reading on a graph and see if your reading at 62% was outside the sphere of influence. If it isn't you move you Z probe further out and start again.
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Yah I don't know we tested installed electrodes, maybe we did use 3 point?

If it did not prove a 25 ohm or less we had to install additional electrodes.
 
I need some similar testing done for a project I am working on. Can someone recommend a qualified grounding testing company?
 
I did one with a cut lead cord, a 100 watt lamp, plastic light socket, and a digital multimeter. Put the hot to the light, neutral to multimeter, then multimeter to ground rod. Had to do this test for a Plasma cutter for a tinknocker.
 
Flex said:
I did one with a cut lead cord, a 100 watt lamp, plastic light socket, and a digital multimeter. Put the hot to the light, neutral to multimeter, then multimeter to ground rod. Had to do this test for a Plasma cutter for a tinknocker.
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I don't understand the test set up and what the results would mean.
 
don_resqcapt19 said:
I don't understand the test set up and what the results would mean.
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I needed to prove less than a 750 millivolt reading on the ground rod. It was totally new to me. I guess it need that low measurement for the plasma cutter to operate properly. The black from the cut lead cord tied onto one side of the light socket with the 100 watt lamp, then the multimeter between the other side of the socket and the ground rod. Plugged the lead in and the light bulb lit. The multimeter measured to the ground rod 13 volts. I ended up calling the rep and he said to bond to building steel as well even though the whole point of the seperate ground rod was to isolate the plasma cutter ground from the building ground. His machine his call. Ended up under 200 millivolts for the final reading.
 
Flex said:
I did one with a cut lead cord, a 100 watt lamp, plastic light socket, and a digital multimeter. Put the hot to the light, neutral to multimeter, then multimeter to ground rod. Had to do this test for a Plasma cutter for a tinknocker.
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Seems like this is measuring the neutral-ground voltage. This should be the voltage drop in the neutral.
 
090212-1416 EST

Flex:

If you disconnected the EGC that runs to the main panel from the plasma cutter, and instead connected the plasma cutter cabinet to an isolated ground rod then you violated the NEC requirement that all conductive components of any machine or device must be connected to the EGC.

I could not understand your ground rod resistance measurement to earth. Whenever you make a voltage measurement it has to be between two points. Your exact circuit was not clear.

If you have an isolated ground rod and assuming the power system has a grounding point somewhere else, then connecting a 100 W 120 V tungsten filament lamp between a 120 V hot wire and your isolated ground rod will at most allow about 0.83 A to flow. Next if you assume no voltage drop on the neutral, then this neutral is a basically test lead going back to the main panel. The EGC might have been a better test lead.

Measuring between the ground rod and the neutral you are measuring the voltage drop thru the ground rods and the earth between.

0.75 V at 0.83 A equals 0.9 ohms. Quite low. The 0.75 V measurement is useful if there are no other ground currents or neutral currents. But to have the EGC disconnected from the plasma is still an NEC violation.

.
 
I apologize for my poor explanation of this test. The ground rod was a ground for the frame of the plasma cutter. The actual machine is properly grounded. Its one of those large cutters like 15' wide and 6' deep. The millivolt measurement must be a certain number that verified the proper resistance to the manufacturer.
 
Flex said:
I apologize for my poor explanation of this test. The ground rod was a ground for the frame of the plasma cutter. The actual machine is properly grounded. Its one of those large cutters like 15' wide and 6' deep. The millivolt measurement must be a certain number that verified the proper resistance to the manufacturer.
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I still don't understand where you are reading the voltage. Exactly what are the two meter leads connected to?
 
don_resqcapt19 said:
I still don't understand where you are reading the voltage. Exactly what are the two meter leads connected to?
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The feed is connected to the hot on the lamp. One lead of the meter is connected to the neutral from the liamp while the other lead is connected to the ground rod. At least I think that's it.
 
090212-2010 EST

Flex:

You can not have the frame of the plasma machine isolated from the EGC.

You essentially have two output wires from the plasma torch power supply. These are high current cables and certainly have at least the voltage drop of the arc as a potential difference. Suppose that internally the cable that goes to the work is not connected to the EGC, and the work table is not connected to the EGC. The work table is connected to the isolated ground rod.

The operator of the plasma torch may come into contact with the work or the table. If from somewhere there is a ground current of 100 A flowing in the ground area relative to this rod, then at 1 ohm ground resistance we could have a potential difference of 100 V between this table and the cabinet of the plasma power supply.

All conductive metal objects to which people are exposed must be connected to the EGC system per NEC.

There may be ground current problems created by the plasma system and these may need to be addressed in some fashion. Same goes for welding systems.

I do not believe the safe solution is to connect the table (frame) to an isolated ground rod.

Dennis:

Flex needs to provide an exact description of the test with the light bulb. I see no logical reason in this application to connect the lamp between a circuit neutral and its 120 hot line and then measure the voltage from that neutral to the isolated rod. If one wants to evaluate the ground rod, then one has to inject a current into that rod. That is why I assume the light bulb is connected between 120 hot and the isolated rod. This injects approximately 0.8 A into the rod and the ground path back to the neutral and ground bus at the main panel. If the ground resistance is real high, 100 ohms for example, then the current will be about 0.6 A.

.
 
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