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### Earth Well

09/15/2012 9:21 AM

HI ALL

I am bored!

I am searching about Earth well resistance calculation for 2 weeks and I can't find anything!

when I speak about Earth Well , my mean is one Well about 4 to 6 meters that we put a copper plate in it.one stranded copper wire connects to plate and the other End of wire come out from well entrance.then we fill the well with conductive concrete and soil.

NOW, I need a mathematical formula to calculate the resistance of well,that depends on the depth of well,the size of plate, the resistivity of soil and conductive concrete , ....

can anyone help me? plzzz

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Guru

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#1

### Re: earth well

09/15/2012 9:29 AM

Do you know, "the depth of well,the size of plate, the resistivity of soil and conductive concrete".

Or, should we just guess?

Why not use an EARTH RESISTANCE TESTER, or is this homework?

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#2

### Re: earth well

09/16/2012 2:43 AM

DEAR lyn

I want a formula that depends on the depth of well,the size of plate, the resistivity of soil and conductive concrete , AND this formula gives the resistance of earth well mathematically.

these are typical values:

- the depth of well : 5 meters

- the size of plate : 66 (cm) * 66 (cm) * 5 (mm)

- the resistivity of soil: 100 (ohm.m)

- conductive concrete that covers the plate ; 10 cm above the plate and 10 com below it :

resistivity conductive concrete : 3 (ohm.m)

We have an EARTH RESISTANCE TESTER, but we must calculate the resistance of well mathematically by a formula .

briefly, I just need a formula.plzz help me .

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#3

### Re: Earth Well

09/16/2012 3:13 AM

There's a good reason you're not finding any references to that earth pit design. It isn't used. I've never heard of or seen this design.

That well of yours does not have an intrinsic resistance. Earth resistance is not like mass or volume. It is the measure of resistance relative to something else normally the ground.

What do you want to work out and why?

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#5

### Re: Earth Well

09/16/2012 7:18 AM

what a problem!

you have never seen or heard about this design BUT in my town,all industrial companies, JUST use this design! they tell me, WE need an earth well that its resistance to remote earth was under 2 ohm!

then I ,as in engineer, must have a mathematical formula for this strange design .

ok. see this story:

in standard BS7430:1998 , part 10.1 , we read about plates as earth electrode. so,what is this?! what's the mean of plate as earth electrode.how we can use it in our design? can you tell me some information about this plates?

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#4

### Re: Earth Well

09/16/2012 3:31 AM

You may only measure it against a given resistive known load, so drive a known copper rod into the nearby surface measure that and then compare..Types of earth are simple free flowing water is the best..Copper into the same wet and dry ground gives different readings. You just need to establish a base

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#6

### Re: Earth Well

09/16/2012 7:25 AM

thank you dear

I establish a base and measure the resistance of well and compare it with the resistance of a copper rod.it's true? is this your mean?

in standard BS7430:1998 , part 10.1 , we read about plates as earth electrode. so,what is this?! what's the mean of plate as earth electrode.how we can use it in our design? can you tell me some information about this plates?

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#7

### Re: Earth Well

09/16/2012 8:03 AM

"Plate" refers to the shape of the electrode ie a flat rectangle of metal normally copper.

The rectangle has a large surface area and that is the ruling parameter for determining the theoretical or design resistance to ground.

To calculate the resistance to ground requires the following parameters:

Electrode surface area (or length for nominally linear electrode shapes like rods or cables)

Soil resistivity which you can measure or look up in a table that lists soil types and typical resistivity values. Soil resistivity can be chemically altered if the native soil has high resistivity.

and other considerations that you can google up for yourself (this is your problem after all and the mathematical method is well documented)

Alternatively, if the earth pit exists and you just want to confirm its resistance to ground you can measure it with the correct test instrument and requisite knowledge.

What type of engineer are you? This is routine for electrical and civil engineers.

Where are you? What is the soil type there? (6m deep holes and "conductive concrete"??!!)

What have you googled so far?

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#10

### Re: Earth Well

09/18/2012 3:01 AM

I have soil characteristic such as resistivity. I measured it by wenner method that described in standard IEEE81.

I made a earth well and measured its resistance with earth tester device.

briefly,in a well with 4 meters depth that we put a plate and a vertical copper wire ,that attached it; the overall well resistance is about 6-8 ohm. consider :

resistivity : 100 (ohm.m) ,

size plate : 66(cm)*66(cm)*5(mm)

cross-section of vertical wire : 50 (mm^2)

NOW,how can you explain this with a formula?? this is my huge problem.

I am an electrical engineer but I haven't many experience .

Thank you dear

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#11

### Re: Earth Well

09/18/2012 3:13 AM

Please re-read post #9. Unless you have the said two points, you can not derive a formula. Period.

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#14

### Re: Earth Well

09/18/2012 4:01 AM

Sounds like a lot of work for a very ordinary result. You must be pressed for space.

100Ωm is a typical value for concrete and dry inland soil but can also be dry clay so your measurement seems valid. Unless you have sand there in which case measure it again.

Calculations.......

ASNZ 1768 page 151 has the method for calculation. This link is to a draft copy which will save you having to spend some money for now. I choose this for convenience only...text books and dissertations abound on the subject (I would encourage you to convince your management to purchase a subscription to an online standards library if you need to reference these frequently)

You have 2 elements in parallel. The plate and the vertical conductor.

Work out the resistance of each using equation C3(1) and C3(4) there is a note there regarding square plates V circular plates

The total resistance is the parallel resistance of these.

Easy math, you have all the parameters to do it.

Let us know the results.

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#8

### Re: Earth Well

09/16/2012 11:55 AM

A copper plate with a wire attached; buried will give you a ground that you can measure for resistance. Why do you need "conductive concrete"? A formula is easy, but you can't use a formula unless you have numbers to put into the formula, like the resistivity of the soil which will vary by depth and location. I am not an electrical engineer, so not an expert. This reply is based on my limited knowledge of things electrical. Oh by-the-way, is the wire bare or insulated? Could be the conductive concrete maintains a constant resistance when in contact with a bare copper wire. If in soil alone, the resistance would vary by depth. Any formula would add the resistance of the plate-to-soil and the resistance of bare wire-to-soil per meter (or foot).

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#12

### Re: Earth Well

09/18/2012 3:31 AM

conductive concrete is low resistivity material and protect cooper electrode against corrosion.

if you have a formula that fitted for this design,well,write it.

I have numbers:

briefly,in a well with 4 meters depth that we put a plate and a vertical copper wire ,that attached it; the overall well resistance is about 6-8 ohm. consider :

resistivity : 100 (ohm.m) ,

size plate : 66(cm)*66(cm)*5(mm)

cross-section of vertical bare wire : 50 (mm^2)

NOW,how can you explain this with a formula?? this is my huge problem.

Guru

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#15

### Re: Earth Well

09/18/2012 4:24 AM

Not concrete but native soil with a ground enhancing compound mixed with it right?

Like GEM ( ground enhancing material from Erico) or GRIP (ground resistance improving powder from LPI) or something similar....

Does the same job as salt and doesn't leach away as quickly (forms a gel) or corrode the crap out of your buried elements.

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#19

### Re: Earth Well

09/19/2012 10:12 AM

oh right ,ok ok

my mean was conductive powder. like GEM.

without soil additives OR with it, I can't calculate right :(

due to there isn't any formula for earth well in standard,I guess ,we have a rod that be parallel with a plate.

rod resistance and plate resistance exist in BS7430:1998 lonely.

But with this assume of well, my answer is bigger that true answer.

in below,you can see an example of my calculation :

indeed,the measured resistance is about 6 ohms .

my assume is wrong. but What is true one? what should I do?

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#20

### Re: Earth Well

09/19/2012 12:18 PM

due to there isn't any formula for earth well in standard,I guess ,we have a rod that be parallel with a plate.

That's right. There are many physical configurations that could be construed. They are all combinations of individual elements.

Your Rplate value seems too high.

There is no consideration for the plate's depth in your Rplate calculation. (your equation?)

Recalculate this using formula C3(4) in the link I provided.

What do you get now?

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#16

### Re: Earth Well

09/18/2012 5:07 AM
Guru

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#17

### Re: Earth Well

09/18/2012 5:56 AM

from that interesting academic dissertation ....

A new method for the ground resistance calculation using

finite element method has been presented in this paper.

and

The method is mainly indicated for the calculation of

ground resistance in asymmetric grids and/or in stratified

soils with multiple layers.

I can't see how that helps here.

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#18

### Re: Earth Well

09/18/2012 6:23 AM

You are right. I drew his attention to it as a guidance, so that if he has the capacity, he can plough on and who knows, he may even present us his dissertation!

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#9

### Re: Earth Well

09/17/2012 6:12 AM

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#13

### Re: Earth Well

09/18/2012 3:38 AM

dear Joshi

when we want measure the resistance of a grounding system,we must use such special method such as fall-of-potential OR 62% method , that described in related standards.

in these methods we have at least 3 point.

we usually measure our system resistance by 62% method.you can search in google and find out this route.

after it,can you help me to find or make one formula?