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byjuvc
09-13-2006, 10:45 AM
Sir,
Can anyone give me a clear picture regarding Earth resistance measurement. I have been following the following procedure :
My aim is to measure the earth resistance at a 11 kv Distribution Sub station . I disconnect the earth connection wires from the transformer Neutral. The earth resistance measurement kit consists of a meter and four electrodes(for Wenners 4 electrode method) but we are using only two electrodes. Each electrode is 2 feet long. The first electrode is connected to the C1( current terminal) of the Meter and is usually driven into a ground at a distance of 10 meters from the main earth electrode of the station. The second electrode is connected to P1 (Voltage) measurement terminal of the meter and is driven into the ground at a distance of approximately 6 meters from the main earth electrode of the station. Now the other two terminals of the meter namely C2 and P2 are shorted and connected to the Earth wire connected to the main earth electrode of the station. All the three electrodes are in a straight line. I understand that one electrode is used to inject current and the second electrode is for voltage measurement. So we get V/I = R i.e the earth resistance. But I am unable to understand why the voltage measurement is done at a distance of 0.62 d . Is the above method correct or something is wrong because at times we are getting earth resistance as high as 9 ohms? if anyone can tell me any site where this is explained in detail that would also be helpful.

Thanks
Byju

Hatteras
09-13-2006, 12:13 PM
Try this website: www.biddlemegger.com

Hatteras

thrasher
09-13-2006, 01:33 PM
I don't know if your company has a copy floating around somewhere but the best guide I have seen is a manual called "Getting Down to Ground". It was originally published by Biddle who has since been bought out by AVO international. The manual gives you the proper procedures and warns you of some of the pitfalls.Two things to look into, off the top of my head. When measuring large ground grids you often have to double the lengths of your leads to get out of the "zone of influence". For example if testing a deep driven rod the closest probe must be further away then the depth of the rod. The other common problem is a foriegn ground running thru your test set-up and diverting part of the current. For instance a metal waterline running between the C1 and P1 probes. In some soil types a station on initial design can very easily be 9 ohms or higher, because most engineers design based upon "average" soil resistivity not actual measurements. In an extreme case I know of a station that the company had a well driller install a two hundred foot deep six inch diameter hole. They then screwed together 20 10 foot sectionial rods and backfilled the hole with Loresco ground carbon.

byjuvc
09-14-2006, 11:17 AM
Dear All,
My queries have been answered satisfactorily. Well I am on the lookout for the AVO book now. I will find it somehow.

Thanks a lot

Byju V C

mscheuerer
09-14-2006, 02:57 PM
I don't know if your company has a copy floating around somewhere but the best guide I have seen is a manual called "Getting Down to Ground". It was originally published by Biddle who has since been bought out by AVO international. The manual gives you the proper procedures and warns you of some of the pitfalls.Two things to look into, off the top of my head. When measuring large ground grids you often have to double the lengths of your leads to get out of the "zone of influence". For example if testing a deep driven rod the closest probe must be further away then the depth of the rod. The other common problem is a foriegn ground running thru your test set-up and diverting part of the current. For instance a metal waterline running between the C1 and P1 probes. In some soil types a station on initial design can very easily be 9 ohms or higher, because most engineers design based upon "average" soil resistivity not actual measurements. In an extreme case I know of a station that the company had a well driller install a two hundred foot deep six inch diameter hole. They then screwed together 20 10 foot sectionial rods and backfilled the hole with Loresco ground carbon.


Thrasher is correct, it is now called "getting down to earth" and is put out by Megger Corporation http://www.megger.com and has been the bible of the industry relating to Earth Ground Resistance Testing. Goto their website and fill out the questionaire to recieve your free copy via mail or PDF format. Hope this helps.

byjuvc
09-15-2006, 02:02 AM
Dear Mike,
I went to the megger site. Got myself registered. But I am unable to find any link for putting up a request for a free copy via e-mail. Also on searching for "getting down to earth ", it was available but at a total price of $11. Pls guide me how to get the free copy.or if you are having one. pls send it across. My mail id is byjuvc@rediffmail.com.

Thanks

Byju

byjuvc
09-16-2006, 02:41 AM
Well I got the way . I have filled the Questionnaire..Thanks

mscheuerer
09-18-2006, 08:48 AM
I am assuming you found it, however if not you can go here and fill out the requested information and they will mail you your free copy of the book.

*** FREE *** Earth Resistance Testing Guide put out by the Megger Corporation, Reprinted/Updated 2006 Version, 77 Pages.

http://www.megger.com/las/story/Index.php?ID=146