Earthing Resistance Calculations

IEEE Standard 81-1983
IEEE Standard 81 is the IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System. The most accurate method for measuring soil resistance is what they call the Wenner four-point method. This method is discussed in detail in section 7, page 12 of Standard 81. The four point method consists of driving four short ground rods into the soil, placed in a straight line, separated by a distance (a). The rods are only driven into the ground to a depth (b) = a /10. The test instrument induces a current (I) into the soil between the two outer rods and then measures the voltage (V) on the two inner rods. The resistance is equal to the voltage divided by the current (V/I). The instrument does the calculation. The instrument reading then has to be multiplied by 2pa to get the average resistance.

The resistance we measure with a rod separation of 2 meters is the average resistivity of the soil to the depth of 2 meters. The resistance we measure with a rod separation of 4 meters is the average resistivity of the soil to the depth of 4 meters. If the 2-meter resistance and the 4-meter resistance are different, then the soil type, moisture content, and/or temperature of the top two meters of soil is different from the bottom two meters of soil. One instrument, which can be used to measure the soil resistance

Keep me informed

can u send the IEEE standard 81-1983 it will helpful to me in order to design the switchyard earthing

Standards need to be purchased from the publishers. Doing other than that may breach copyright laws and the implications for that are:

one should not send such copyright doc.

Dear sir,

Thanks,but i want to know how to decide earthpits GI/Copper plate/pipe size or size of conductor?

Hi All,

Further to the questions above. I have few doubts about earth testings as well. Our refinery has about 10 substations for each area and the earth grid is one for the whole refinery. We do Low Current Injection testing for the whole site every 5 years But what sort of Preventative maintenance strategies do we implement annualy just to make sure the bondings of the earth conductor. Every sub has 4 earth bars and 2 test wells. Should we check the continuity form these earth bars or do we check them from the test well near each sub. Can we also check the continuity between the main earth bar and random structures like tanks, pipes and motors to make sure that the resistance is less than 0.5 ohms. Replies will be appreciated. Thanks

BIDDLE has a booklet called "Getting Down to Earth". It's basic, but a very good place to start.

from where i can get it?

http://www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

(BTW I don't know why it is on free when the cover show a price ? )

As people here have implied, there is no way to theoretically calculate your earthing resistance. This is a measured in situ. There are local standards that specify the technique suitable to your location. Typically the local standard refers to one of the international standards, NFPA70 or the IEEE standard mentiond are just two of them. These standards are intellectual property and must be purchased. More importantly, the interpretation of these standards must be by somebody qualified and probably licensed.

how can i get these standards? where? & at what cost?

Two common sites are www.ihs.com and www.techstreet.com

or to the sites www.nfpa.org and www.ieee.org

all these standards are available online