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
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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
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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
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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 R
S-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
Approximate Resistance for a Strip or a
round conductor
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
Questions
?
Thank You