Earthing DKPathirana 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 ID: 272183
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Slide1
Electrical Earthing
D.K.PathiranaSlide2
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 EarthSlide3
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 voltagesSlide4
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 protectionSlide5
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)Slide6
Earthing Arrangements
TN System
TNS System
TN C S System
TN C system
TT System
IT SystemSlide7
TN-S SystemSlide8
TN-C SystemSlide9
TN-C-S SystemSlide10
TT SystemSlide11
IT SystemSlide12
Factors affect to the earth impedance
Soil
Weather
Electrode type
Electrode size
Near by utilities
Electrode in parallel
Distance between electrodeSlide13
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
af
fect resistivity
Type of earth (
eg
, clay, loam, sandstone, granite)
Stratificatio
n; layers of different types of soil (
eg
, loam backfill on a clay base)
Moisture content
Tempera
ture
Chemical composition and concentration of dissolved salt Slide14
WENNER method
ρ
w
= 2 π d R
S-ESSlide15
SCHLUMBERGER method
ρ
s
= ( π (d2 – A2/4) R
S-ES
) / ASlide16
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) Slide17
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” Slide18
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 Slide19
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 Slide20
Resistance Of A Vertical Electrode With Infill Of Bentonite
Or Concrete
Where,
𝞺
c
– resistivity of the infill material
d – diameter of electrode in m
D – diameter of infill
L – driven length of electrodeSlide21
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 Slide22
Questions
?Slide23
Thank You