Fall of Potential Testing in an industrial plant

[rockman7892]

We are getting ready to conduct earth resistance testing in our plant using the fall of potential method. We will most likely contract this work out to a certified testing company.

In all of the research that I've done regarding this procedure it seems that in order to complete this test accurately the ground grid under test must be isolated from any equipment and other ground grids. In our plant we have several of these ground grids surrounding our various electrical rooms containning our 4160/480V unit substations. All of these ground grids are tied together througout the plant as well as tied back to the ground grid installed at the utility transformer.

My question and possbile obstactle to having this testing completed sucessfully is weather or not we must isolate each of these ground grids for testing, or do we treat the whole plant grounding system as one grid? If we treat the plant as one grid then we will need to go out a very far distance with our test rod.

Half of plant is now in service. I know that it is a must to have the systems under test de-energized for these test and I need to plan for this accordingly. I cannot seem to determine however if it is a must to isolate the various ground grids around the electrical rooms as well as isolate the grid from equipment.

 

[cuky2000]

I suggest to investigate the resistance testing procedure using the smart multimeter developed by EPRI.

This meter has the capability of filtering electrical noise such as umbanlnce current and harmonics circulating in the ground.

 

[Elmir]

What is the intent of the test?

Are there records of previous soil resistivity or ground grid impedance tests?

Depending on what's trying to be achieved will determine if the ground grids need to be isolated for testing.

If trying to determine ground grid impedance for the facility, leave the grouding interconnected. The fall-of-potential method does not require the system be de-energized. Testing personnel should be aware of the hazards involved and should wear the appropriate PPE and have a test procedure.

The test should be tested above and below the power system frequency to avoid noise. The 60Hz impedance can be interpolated from the measurements.

Generally isolated ground grids are only tested after contruction to verify the design meets required performance.

Good planning by someone experienced is essential to obtain good test results. Depending on the distances involved and soil resistivity will detemince what type of equipment is required to perform a fall-of-potential test.

Are there any plans to perform ground grid integrity tests?

 

[DanDel]

Depending on the parameters of your facility's ground system, Fall of Potential tests may be conducted using the 'slope' method to get more accurate results. This technique is sometimes used for multiple ground rod locations to isolate and verify that the three-point results are accurate. There is also a clamp-on ground meter which can be used for multiple ground rod systems to determine individual rod resistances.

 

[rockman7892]

Elmir

We are on a site that has an existing plant where we are currenly building a new plant. There have been ground resistance tests done on the existing plant grounding system in the past. The two plant grids tie together at the main substation.

We are not trying to test the ground resistances for the new plant. Based upon your response it seems as if since this is a new system then we would need to isolate the individual ground rings/grids to find the actual resistances of each grid. Should we isolate in this case?

You questioned weather or not we are going to perform ground grid integrity tests? I'm assuming that you are referring to a continuity test between the ground grid and the actual equipment. To do this I'm assuming you measure resistance between ground grid and all equipment connected to the groung grid to verify continuity. Is this what you are referring to?

 

[Kiribanda]

rockman7892,

You should make sure that you are measuring the resistance of the ground grid ONLY. For that you should first remove all other grounding and bonding connections and leave the grid alone.(e.g. grounding and bonding connections to equipment, metal structures, pipelines etc etc)

Since you are going to use Fall of Potential method, take the distance between current spikes (C1C2)as ten times the largest dimension of the grid (mostly the diagonal).

If this distance C1C2 is huge and you donot have enough space to drive the spikes, then you should think of breaking the grid into several convenient grids by removing the bonds between each grid.

Then make sure you get at least five ohm (for Generator station or large substation this should be one ohm) for each grid. Then, when you finally bond all together, the resultant grid will be definitely less than five ohms.

Hope this helps.

 

[Elmir]

Testing the new site isolated after construction only serves to verify the engineers predicted final ground impedance.

Ground potential rise should be evaluated with everything connected as if the plant were in normal operation. Therefore the ground should be tested with everything connected if possible.

Kiribanda made a good point, depending on the available space around the plant for taking out the remote current probe you may want to investigate splitting the ground grids and testing them indiviually. An assement of the ground grid all connected is still required to access GPR. This can be done alternatively through computer modelling using measured results and soil resistivity data.

Buried objects and vicinity of other ground grids will affect measurements so they should be taken into consideration for planning of the test.

For ground grid integrity testing the idea is as you described. Inject and current into the grounding grid and measure the resistance. You can do this in addition to the FOP test to verify each grounding grid is connected.

Do you have an area map you can share to give a better idea of your site?

 

[dpc]

The accuracy of a fall-of-potential test on a site full of underground piping, conduit and other equipment is highly questionable. You will likely get a different result in every direction you go.

http://www.aemc.com/techinfo/appnotes/Ground_Resistance_Testers/APP-Ground-ComparisonofTesters.pdf