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Electrical - PowerPoint Presentation

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Electrical - PPT Presentation

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

system earth resistance electrode earth system electrode resistance soil earthing protection resistivity insulation lightning copper conductor current fault shock

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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