130110109050_2150908

7/23/2019 130110109050_2150908

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PREPARED BY:PREPARED BY:

PRATIK PATEL-130110109035

DEEP SANGANI-130110109050

G H PATEL COLLEGE OF ENGINEERING AND

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7/23/2019 130110109050_2150908


Cables are generally laid directly in the grond orin dcts in the ndergrond distribtion syste!"

For this reason# there are little chances o$ $altsin ndergrond cables" Ho%e&er# i$ a $alt does

occr# it is di'clt to locate and re(air the $altbecase condctors are not &isible"

Ne&ertheless# the $ollo%ing are the $alts !ostli)ely to occr in ndergrond cables *

1. O(en+circit $alt2. ,hort+circit $alt

3. Earth $alt"

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-hen there is a brea) in the condctor o$ acable# it is called o(en circit $alt"

The o(en+circit $alt can be chec)ed by a!egger" For this (r(ose# the three condctors

o$ the .+core cable at the $ar end are shortedand earthed" Then resistance bet%een each condctor and

earth is !easred by a !egger" The !egger %ill indicate /ero resistance in the

circit o$ the condctor that is not bro)en" Ho%e&er# i$ the condctor is bro)en# the !egger

%ill indicate in0nite resistance in its circit"

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-hen t%o condctors o$ a !lti+core cable co!ein electrical contact %ith each other de toinslation $ailre# it is called a short+circit $alt"

Again# %e can see) the hel( o$ a !egger to

chec) this $alt" For this (r(ose# the t%oter!inals o$ the !egger are connected to anyt%o condctors"

I$ the !egger gi&es /ero reading# it indicates

short circit $alt bet%een these condctors" The sa!e ste( is re(eated $or other condctors

ta)ing t%o at a ti!e"

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-hen the condctor o$ a cable co!es incontact %ith earth# it is called earth $alt orgrond $alt"

To identi$y this $alt# one ter!inal o$ the!egger is connected to the condctor andthe other ter!inal connected to earth"

I$ the !egger indicates /ero reading# it

!eans the condctor is earthed" The sa!e (rocedre is re(eated $or other

condctors o$ the cable"

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There are se&eral !ethods $or locating the $altsin ndergrond cables"

Ho%e&er# t%o (o(lar !ethods )no%n as loo(tests are *

(I) 1rray loo( test(II) 2arley loo( test

These si!(le tests can be sed to locate the earth$alt or short+circit $alt in ndergrond cables

(ro&ided that a sond cable rns along the $altycable" 3oth these tests e!(loy the (rinci(le o$

-heatstone bridge $or $alt location"

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The 1rray loo( test is the !ost co!!on andaccrate !ethod o$ locating earth $alt or short+circit$alt in ndergrond cables"

(I) EARTH FAULT :

A3 is the sond cable and CD is the $alty cable4 theearth $alt occrring at (oint F"

The $ar end D o$ the $alty cable is 5oined to the $arend 3 o$ the sond cable throgh a lo% resistance

lin)" T%o &ariable resistances P and 6 are 5oined to ends A

and C res(ecti&ely and ser&e as the ratio ar!s o$ the-heatstone bridge"

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The 2arley loo( test is also sed to locateearth $alt or short+circit $alt inndergrond cables"

This test also e!(loys -heatstone bridge(rinci(le" It di7ers $ro! 1rray loo( test inthat here the

ratio ar!s P and 6 are 08ed resistances"3alance is obtained by ad5sting the&ariable resistance , connected to the testend o$ the $alty cable"

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130110109050_2150908

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