G.V. Nilanka

Content………..  purpose of earthing  Earthing design  Soil resistivity testing  Resistance to earth of a rod or pipe electrode  Resistance to earth of a strip  Resistance to earth of a infill material

purpose of earthing  We provide a direct path to fault current.  What is fault current?  By earthing we can get common reference potential in electrical system.  There are several earthing arrangement  To achieve good earthing,a suitable low resistance to earth is desirable.

Earthing design  Using Rods or pipe  Using Strip  Using Infill material  Using foundation

Soil resistivity testing  This varies with some parameters.  moisture content  temperature  Depth

Soil resistivity testing

Calculate the soil resistivity ρ=2πAd  ρ=soil resistivity  A=Apperant resistance  D=distance between adjacent electrode

Resistance to earth a rod or pipe electrode R= ρ(ln(8l/D)-1)/2πL L =length of electrode D=diameter ρ = resistivity of soil

Cont….  l varies with electrode arrangement

 Three types of copper rods are commonly available. Solid Copper Copper clad steel rod ( copper shrunk onto the core) Copper Bonded steel core (copper is molecularly bonded to nickel plated steel rod)

Resistance to earth of a strip L – length of the strip h - the depth buried w - width of the strip P and Q are coefficient for strip or round conductor R n =FR

Resistance to earth of a infill material Where ρ = resistivity of soil in Ω-m ρ c = resistivity of infill material Ω-m d = Diameter of the electrode – m D = Diameter of infill – m L = Driven Length – m

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