Earth Resistance Measurements "Right" Procedure

[lume7006]

Hello friends,

I would like to get your oppinions about the procedure to perform earth resistance measurements oriented to determine resistivity.

1) IEEE std 81 says that at least two traverses may be done and it does not set any limit on the distance between electrodes.

2) Some equipment manufacturer says there is a limit of distance of no more than 30 m between each two electrodes and that's the reasson the "default" leads have that lenght.

3) There are some papers from some recognized consultants (some of them participated on IEEE std 80 preparation) that perform those measurements and the distances reported between each pair of electrodes are in some cases much longer (up to 150 m between each two electrodes)

So I am really confused and I do not know what is the correct way to perform them.
If the area to be measured is too large, doing measurements with electrodes appart 30 m from each other at maximum, may require using many traverses and of course, the deepest lawyers will not be properly know (according to IEEE std 81)

Please, give me your comments about it.

 

[Danilo917]

What sort of equipment you want to provide with earthing system? is it a substation?

 

[jghrist]

Are you performing a resistivity test or a resistance test?

If it is a resistivity test, the probe distance determines the depth of the soil that is being measured. You need to have measurements at shallow depth (short probe spacings) and deep depth (long probe spacings). The size of the ground grid determines how deep you need to go. The accuracy of step- and touch-voltage calculations and grid resistance calculations increases with higher probe spacings.

Improving the Reliability of Power Systems With More Accurate Grounding System Resistance Estimates, by R.D.Southey and F.P.Dawalibi investigated the errors with different pin spacings. The answer is complex and depends on soil structure, grid shape, and the amount of grounding provided by the power system (shield wires). Pin spacings 300% of the grid length gave grid resistance errors of -17% to +9% and touch-voltage errors of -8% to +20%.

If equipment manufacturers have a limit of 30 m spacing, it may be because of equipment current limitations.

 

[lume7006]

Hello,

I am talking about resistivity measurements in an area where a substation and some other electrical rooms will be built.

I hope this may help to get more answers!

 

[lume7006]

Is there any more comments?!!

Thank you!

 

[jghrist]

The Southey & Dawaliby paper I cited is dealing with resistivity measurements and the effect on ground resistance and touch-voltage calculation accuracy. This was presented in a CDEGS users group meeting. You might be able to get a copy from SES

http://www.sestech.com/

The longer you make your maximum probe spacing, the more accurate your soil model will be.

 

[FreddyNurk]

jghrist is right, the maximum probe spacing is primarily related to equipment capabilities rather than as stated by standards.

If you can get a system capable of 100m probe spacings (i.e. total of 400m from outer electrodes) then thats great. If not, you need to be aware that you'll either get erroneous results if you attempt to use equipment that cannot measure at such spacings, or you need to stay within manufacturer's guidelines in order to ensure that the results are accurate.