EARTHING


EARTHING/GROUNDING SYSTEMS, AN OVERVIEW


Soil resistivity data & earth systemdesign It is advisable to locate in the area of lowest soil resistivity in order to achieve themost economical earthing systeminstallation as itwill performmore effectively and require fewer earthingmaterials.


Knowledge of the soil resistivity at the intended site, and how this varies with parameters such asmoisture content, temperature and depth, provides a valuable insight into how the desired earth resistance can be achieved andmaintained over the life of the installation withminimum cost and effort.Many different factors have a direct effect on the resistivity of the local soil and the following tables will demonstrate this. The resistivity of different types of ground material ranges from 1Ωm to the upwards of over 100,000 Ωm. Soil is also major factor in affecting corrosion rate, as soil resistivity decreases, corrosion increases. As soil moisture content increases, resistivity decreases. Atmospheric conditions can significantly affect soil resistivity as the charts below demonstrate.


°C 20 10 0 0


-5 -15


Temperature °F


68 50


32 (water) 32 (ice) 23 14


Moisture content % by weight


2.5 5


10 15 20 30


Resistivity Ωm Top


soil


25000 16500 5300 1900 1200 640


Resistivity Ωm


72 99


138 300 790


3300


Ground conditions Minimum Ωm


Sea water


Ashes, cinders, brine, waste


Surface soils


Clay, shale, gumbo, loam


Clay


Same, with varying proportions of sand


and gravel Sandy clay


Shale, slates, sandstone, etc.


Peat, loamandmud Sand


Sandy loam


15000 4300 1850 1050 630 420


Clay mixed with sand


1500 430 185 105 63 42


Moist gravel Dry gravel Limestone Sandstone Granites Concrete Ice


0.1 59 10 34 20


102 1000


Gravel, sand, stones with little clay or loam 5900


10 5


200 500


7000 50


200


9000 3000


10000


Maximum Ωm


10


700 500


1630 1000


13500 1500 45800


100 250


3000 7000


12000 10000 20000 11000 5000


100000


10


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