Electrical Earthing

Electrical Earthing
D.K.Pathirana

Applications of earthing
• Protect human against lightning and earth fault condition
• Protect the premises against lightning and earth fault condition
• Provide low resistance and safe path for lightning and fault current
• All metallic enclosure and extraneous conductive parts are at equipotential
• LV System Earth

Functions of Earthing
Equipment Earth : Path for fault current, lower touch voltage, protection against electric shock
Lighting Earth : Low resistance path to diverse the current under lightning attack.
Telecom Earth : Signal Earth, reduce noise and interference, stabilize DC supply voltage and prevent electric shock
Computer Earth : reduce interference, maintain supply voltages

Two classes of protection
Class I protection –
use of barrier/insulation and connection of protective conductor to equipment metallic enclosure in order to protect against electric shock
Class II protection –
beside of the basic insulation, addition layer of insulation apply to the enclosure. Therefore no extraneous conductive part. The additional layer is independent to the basic insulation so that under failure of basic insulation, it offers additional protection

Types of Earthing
Supply System – Neutral Earth
System Earth
Electrical Safety Earth
Lightning Earth
Generator Earth
Protection Earth (i.e. surge arrestor)
Telecom / Computer Earth
Shielding Earth
Integrated Earthing System (Advocated)
Electrostatic Earth (Clean Room / Hospital)

Earthing Arrangements
TN System
TNS System
TN C S System
TN C system
TT System
IT System

TN-S System

TN-C System

TN-C-S System

TT System

IT System

Factors affect to the earth impedance
Soil
Weather
Electrode type
Electrode size
Near by utilities
Electrode in parallel
Distance between electrode

Soil Resistivity
The resistivity of earth may vary over extremely wide limits, depending on the composition of the soil and the moisture content.
Factors that affect resistivity
Type of earth (eg, clay, loam, sandstone, granite)
Stratification; layers of different types of soil (eg, loam backfill on a clay base)
Moisture content
Temperature
Chemical composition and concentration of dissolved salt

WENNER method
ρw= 2 π d RS-ES

SCHLUMBERGER method
ρs = ( π (d2 – A2/4) RS-ES ) / A

Types Of Earth Electrodes
• Solid Copper
• Copper clad steel rod ( copper shrunk onto the core)
• Copper Bonded steel core (coper is molecularly bonded to nickel plated steel rod)

Earth Resistance Of An Electrode
soil exhibits a resistance to the flow an electrical current
not an “ideal” conductor
resistance (can never be zero) between the earth electrode and “true Earth”.
The resistance between the earth electrode and “true Earth”

Rods Driven Vertically Into The Ground
Rg = (𝞺/2𝞹L)[ln(8L/d)-1]
where,
ρ - Soil Resistivity in Ωm
L - Buried Length of the electrode in m
d - Diameter of the electrode in m

Combined Resistance Of n No Of Electrodes
In which
Where
R=resistance of one rod Ω
S = distance between adjacent rods m
ρ = resistivity of soil Ω-m
λ =is a factor selected from Table 2 or 3 of BS 7430
‘n is the no of electrodes as given in Tables 2 and3

Resistance Of A Vertical Electrode With Infill Of Bentonite Or Concrete
Where,
𝞺c – resistivity of the infill material
d – diameter of electrode in m
D – diameter of infill
L – driven length of electrode

Where ,
L – length of the strip
‘h - the depth buried
w - width of the strip
P and Q are coefficient for strip or round conductor in Table 5
Approximate Resistance for a Strip or a round conductor

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Electrical Earthing

by Denuka

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