Grounding & earth resistance

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])

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

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

Thank You

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