Download - Grounding & earth resistance
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What is grounding ?
A conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth
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By MHD WimalaratneElectrical Engineer([email protected])
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Why ground ?To protect people from electrical
shock.To protect plant & equipment from
damage.To limit voltage due to lightning
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Ground Electrodes
Consist of three basic
components :1. Ground Conductor2. The connection of the
conductor to the electrode3. The ground electrode itself
The earth resistance (R) has three basic components :
RA - The resistance of the ground electrode itself and the connections to the electrode
RB - The contact resistance of the surrounding earth to the electrode
RC - The resistance of the surrounding body of earth around the ground electrodeR = RA + RB + RC
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(RA) - The electrode resistance depends on :• Length/ depth of the ground electrode• Electrode material
- Solid Copper, Stainless steel (High conductivity, high corrosion resistance but low strength & high cost)
- Copper clad steel (high strength, high corrosion resistance and low cost)
• The diameter of the rod - Has little effect
- Resistance would only decrease by 10% by double the diameter
(RB) - The contact resistance of the surrounding earth to the electrode is negligible.
(RC)- The resistance of the surrounding body of earth around the ground electrode
- depends on soil conditions such as soil resistivity
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Soil Resistivity depends on
Soil type
Soil Type Resistivity (Ωm)
Marshy ground
2-2.7
Sandy gravel
300-500
Rock 1000 +
Moisture% by
weight
Resistivity (Ωm)
Sandy loam
0 % 10000000
2.5 % 1500
5 % 430
10 % 185
15 % 105
20 % 63
30 % 42
Moisture Content
The earth electrode should be installed deep enough to reach the water table or permanent moisture level
Added salt % by
wt of moisture
Resistivity (Ωm)
0.0 107
0.1 18
1.0 4.6
5.0 1.9
10.0 1.3
20.0 1.0
E.g. Effect of salt on resistivity for sandy loam (15.2 % moisture)
Salt not recommended due to corrosion
Temp
(ºC)Resistivity
(Ωm)
20 72
10 99
0 138
0 300
-5 790
-15 3300
Soil Temperaturee.g. Effect of temperature on resistivity for sandy loam, (15.2% moisture )
Chemical Composition
To increase/ retain moisture content we use Soil Resistivity Reducing Agents such as Bentonite or Marconite 5
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Types of Grounding Systems
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1. Single Rod
e.g.. Lot-1 : Used in resettlement sites,
2. Multiple Rods
e.g.. Lot-2 : Powerhouse ground level at E.L. 682
4. Conductor mesh
e.g. 1). Lot-2 : At each floor of power house there is a conductor mesh/grid 2). Lot-2 : On the Dam surface there is a conductor mesh buried
3. Copper Plates
e.g.. Lot-2 : 18 pieces of copper plates are embedded under 6m of Switchyard surface
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Thank You
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