Earthing q & A

8/12/2019 Earthing q & A

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What is Earthing

The main reason for doing earthing in electrical network is for the safety. When allmetallic parts in electrical equipments are grounded then if the insulation inside the

equipments fails there are no dangerous voltages present in the equipment case. Ifthe live wire touches the grounded case then the circuit is effectively shorted and

fuse will immediately blow. When the fuse is blown then the dangerous voltages areaway.

Purpose of Earthing:

(1) Safety for Human life/ Building/Equipments:

To save human life from danger of electrical shock or death by blowing a fuse

i.e. To provide an alternative path for the fault current to flow so that it willnot endanger the user

To protect buildings machinery ! appliances under fault conditions.

To ensure that all e"posed conductive parts do not reach a dangerous

potential. To provide safe path to dissipate lightning and short circuit currents.

To provide stable platform for operation of sensitive electronic equipments

i.e. To maintain the voltage at any part of an electrical system at a known

value so as to prevent over current or e"cessive voltage on the appliances orequipment .

(2) Oer oltage prote!tion:

#ightning line surges or unintentional contact with higher voltage lines can

cause dangerously high voltages to the electrical distribution system. Earthingprovides an alternative path around the electrical system to minimi$e

damages in the %ystem.

(") #oltage sta$ili%ation:

There are many sources of electricity. Every transformer can be considered a

separate source. If there were not a common reference point for all these

voltage sources it would be e"tremely difficult to calculate their relationshipsto each other. The earth is the most omnipresent conductive surface and so

it was adopted in the very beginnings of electrical distribution systems as anearly universal standard for all electric systems.

&onventional methods of earthing:

(1) &late type Earthing:

'enerally for plate type earthing normal Practice is to use

&ast iron plate of si$e ()) mm "()) mm "*+ mm. ,-

'alvani$ed iron plate of si$e ()) mm "()) mm "( mm. ,-

&opper plate of si$e ()) mm ()) mm /.*0 mm


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Plate burred at the depth of 1 feet in the vertical position and 'I strip of si$e

0) mm"( mm bolted with the plate is brought up to the ground level.

These types of earth pit are generally filled with alternate layer of charcoal !

salt up to 2 feet from the bottom of the pit.

(2) &ipe type Earthing:

3or Pipe type earthing normal practice is to use

'I pipe 4&5class6 of 70 mm diameter *) feet long welded with 70 mm

diameter 'I flange having ( numbers of holes for the connection of earth

wires and inserted in ground by auger method.

These types of earth pit are generally filled with alternate layer of charcoal !

salt or earth reactivation compound.

8ethod for &onstruction of Earthing Pit 9Indian Electricity oard;:

E"cavation on earth for a normal earth Pit si$e is *.08 < *.08 < /.) 8.

=se 0)) mm < 0)) mm < *) mm 'I Plate or igger %i$e for more &ontact of

Earth and reduce Earth -esistance.

8ake a mi"ture of Wood &oal Powder %alt ! %and all in equal part

Wood &oal Powder use as good conductor of electricity anti corrosive rust

proves for 'I Plate for long life.

The purpose of coal and salt is to keep wet the soil permanently.

The salt percolates and coal absorbs water keeping the soil wet.

&are should always be taken by watering the earth pits in summer so that the

pit soil will be wet.

&oal is made of carbon which is good conductor minimi$ing the earth

resistant.

%alt use as electrolyte to form conductivity between 'I Plate &oal and Earth

with humidity.

%and has used to form porosity to cycle water ! humidity around the mi"ture.

Put 'I Plate 9E>-T? P#>TE; of si$e 0)) mm < 0)) mm < *) mm in the mid of

mi"ture.

=se @ouble 'I %trip si$e /) mm < *) mm to connect 'I Plate to %ystem

Earthling.

It will be better to use 'I Pipe of si$e +.0A diameter with a 3lange on the top

of 'I Pipe to cover 'I %trip from E>-T? P#>TE to Top 3lange.

&over Top of 'I pipe with a T Boint to avoid Bamming of pipe with dust ! mud

and also use water time to time through this pipe to bottom of earth plate.

8aintain less than one ,hm -esistance from E>-T? PIT conductor to a

distance of *0 8eters around the E>-T? PIT with another conductor dip onthe Earth at least 0)) mm deep.

&heck Coltage between Earth Pit conductors to Deutral of 8ains %upply ++)C>& 0) ?$ it should be less than +.) Colts.

3actors affecting on Earth resistivity:

(1) Soil 'esistiity:


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It is the resistance of soil to the passage of electric current. The earth

resistance value 9ohmic value; of an earth pit depends on soil resistivity. It is

the resistance of the soil to the passage of electric current.

It varies from soil to soil. It depends on the physical composition of the soil

moisture dissolved salts grain si$e and distribution seasonal variationcurrent magnitude etc.

In depends on the composition of soil 8oisture content @issolved salts grain

si$e and its distribution seasonal variation current magnitude.

(2) Soil ondition:

@ifferent soil conditions give different soil resistivity. 8ost of the soils are very

poor conductors of electricity when they are completely dry. %oil resistivity ismeasured in ohm5meters or ohm5cm.

%oil plays a significant role in determining the performance of Electrode.

%oil with low resistivity is highly corrosive. If soil is dry then soil resistivity

value will be very high.

If soil resistivity is high earth resistance of electrode will also be high.

(") oisture:

8oisture has a great influence on resistivity value of soil. The resistivity of a

soil can be determined by the quantity of water held by the soil and resistivityof the water itself. &onduction of electricity in soil is through water.

The resistance drops quickly to a more or less steady minimum value of about

*0 moisture. >nd further increase of moisture level in soil will have little

effect on soil resistivity. In many locations water table goes down in dryweather conditions. Therefore it is essential to pour water in and around the

earth pit to maintain moisture in dry weather conditions. 8oisture significantly

influences soil resistivity

(*) +issoled salts:

Pure water is poor conductor of electricity.

-esistivity of soil depends on resistivity of water which in turn depends on the

amount and nature of salts dissolved in it.

%mall quantity of salts in water reduces soil resistivity by 1). common salt

is most effective in improving conductivity of soil. ut it corrodes metal and

hence discouraged.

(,) limate ondition:

Increase or decrease of moisture content determines the increase or decrease

of soil resistivity.

Thus in dry whether resistivity will be very high and in monsoon months the

resistivity will be low.

(-) &hysi!al omposition:


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@ifferent soil composition gives different average resistivity. ased on the

type of soil the resistivity of clay soil may be in the range of 2 F *0) ohm5

meter whereas for rocky or gravel soils the same may be well above *)))ohm5meter.

(.) o!ation of Earth &it :

The location also contributes to resistivity to a great e"tent. In a sloping

landscape or in a land with made up of soil or areas which are hilly rocky orsandy water runs off and in dry weather conditions water table goes down

very fast. In such situation ack fill &ompound will not be able to attractmoisture as the soil around the pit would be dry. The earth pits located in

such areas must be watered at frequent intervals particularly during dryweather conditions.

Though back fill compound retains moisture under normal conditions it gives

off moisture during dry weather to the dry soil around the electrode and in

the process loses moisture over a period of time. Therefore choose a site thatis naturally not well drained.

(0) Effe!t of grain si%e and its distri$ution:

'rain si$e its distribution and closeness of packing are also contributory

factors since they control the manner in which the moisture is held in thesoil.

Effect of seasonal variation on soil resistivity: Increase or decrease of

moisture content in soil determines decrease or increase of soil resistivity.

Thus in dry weather resistivity will be very high and during rainy season theresistivity will be low.

() Effe!t of !urrent magnitude:

%oil resistivity in the vicinity of ground electrode may be affected by current

flowing from the electrode into the surrounding soil.

The thermal characteristics and the moisture content of the soil will determine

if a current of a given magnitude and duration will cause significant dryingand thus increase the effect of soil resistivity

(1) 3rea 3aila$le:

%ingle electrode rod or strip or plate will not achieve the desired resistance

alone.

If a number of electrodes could be installed and interconnected the desired

resistance could be achieved. The distance between the electrodes must beequal to the driven depth to avoid overlapping of area of influence. Each

electrode therefore must be outside the resistance area of the other.

(11) O$stru!tions:

The soil may look good on the surface but there may be obstructions below a

few feet like virgin rock. In that event resistivity will be affected. ,bstructions


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like concrete structure near about the pits will affect resistivity. If the earthpits are close by the resistance value will be high.

(12) urrent agnitude:

> current of significant magnitude and duration will cause significant drying

condition in soil and thus increase the soil resistivity.

8easurement of Earth -esistance by use of Earth Tester:

3or measuring soil resistivity Earth Tester is used. It is also called the

G8E''E-H.

It has a voltage source a meter to measure -esistance in ohms switches to

change instrument range Wires to connect terminal to Earth Electrode and

%pikes.

It is measured by using 3our Terminal Earth Tester Instrument. The terminals

are connected by wires as in illustration.

PPotential %pike and &&urrent %pike. The distance between the spikes may

be *8 +8 08 *)8 /08 and 0)8. >ll spikes are equidistant and in straight line to maintain electrical continuity.

Take measurement in different directions.

Soil resistiity 425'6

- Calue of Earth resistance in ohm.

@istance between the spikes in cm.

J /.*2

P Earth resistivity ohm5cm.

Earth resistance value is directly proportional to %oil resistivity value

easurement of Earth 'esistan!e (7hree point method):

In this method earth tester terminal &* ! P* are shorted to each other andconnected to the earth electrode 9pipe; under test.

Terminals P+ ! &+ are connected to the two separate spikes driven in earth.

These two spikes are kept in same line at the distance of +0 meters and 0)

meters due to which there will not be mutual interference in the field ofindividual spikes.

If we rotate generator handle with specific speed we get directly earth

resistance on scale.
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%pike length in the earth should not be more than *K+)th distance between

two spikes.

-esistance must be verified by increasing or decreasing the distance between

the tester electrode and the spikes by 0 meter. Dormally the length of wires

should be *) and *0 8eter or in proportion of (+ of L@M.

%uppose the distance of &urrent %pike from Earth Electrode @ () ft Then

distance of Potential %pike would be (+ of @ ).(+@ i.e. ).(+ " () ft /7 ft.

8our &oint ethod:

In this method 2 spikes are driven in earth in same line at the equal

distance. ,uter two spikes are connected to &* ! &+ terminals of earth

tester. %imilarly inner two spikes are connected to P* ! P+ terminals. Dow ifwe rotate generator handle with specific speed we get earth resistance value

of that place.

In this method error due to polari$ation effect is eliminated and earth tester

can be operated directly on >.&.

'I Earthing Cs &opper Earthing:

>s per I% /)2/ the resistance of Plate electrode to earth(') 4 (r/3) 9

under root(&/3)6

Where r -esistivity of %oil ,hm5meter.

>>rea of Earthing Plate m/.

The resistance of Pipe electrode to earth (') 4 (1r/25) 9 loge (*/d)6

Where # #ength of PipeK-od in cm

d@iameter of PipeK-od in cm.

The resistivity of the soil and the physical dimensions of the electrode play

important role of resistance of -od with earth.

The material resistivity is not considered important role in earth resistivity. >ny material of given dimensions would offer the same resistance to earth.

E"cept the si$ing and number of the earthing conductor or the protectiveconductor.

Pipe Earthing Cs Plate Earthing:

%uppose &opper Plate having of si$e *.+m " *.+m " /.*0mm thick. soil

resistivity of *)) ohm5m

The resistance of Plate electrode to earth 9-;9 rK>;< under root9JK>;

9*))K+.11;


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#ength of Pipe Electrode and Earthing Pit:

The resistance to earth of a pipe or plate electrode reduces rapidly within the

first few feet from ground 9mostly + to / meter; but after that soil resistivityis mostly uniform.

>fter about 2 meter depth there is no appreciable change in resistance toearth of the electrode. E"cept a number of rods in parallel are to be preferred

to a single long rod.

>mount of %alt and &harcoal 9more than 1Ng; :

To reduce soil resistivity it is necessary to dissolve in the moisture particle in

the %oil.

%ome substance like %altK&harcoal is highly conductive in water solution but

the additive substance would reduce the resistivity of the soil only when it is

dissolved in the moisture in the soil after that additional quantity does notserve the Purpose.

0 moisture in %alt reduces earth resistivity rapidly and further increase insalt content will give a very little decrease in soil resistivity.

The salt content is e"pressed in percent by weight of the moisture content in

the soil. &onsidering *8/ of %oil the moisture content at *) percent will be

about *22 kg. 9*) percent of *22) kg;. The salt content shall be 0 of this9i.e.; 0 of *22kg that is about 7.+kg.

>mount of Water Purring:

8oisture content is one of the controlling factors of earth resistivity.

>bove +) of moisture content the resistivity is very little affected. ut

below +) the resistivity increases rapidly with the decrease in moisture

content. If the moisture content is already above +) there is no point in adding

quantity of water into the earth pit e"cept perhaps wasting an important andscarce national resource like water.

#ength Cs @iameter of Earth Electrode:

>part from considerations of mechanical strength there is little advantage to

be gained from increasing the earth electrode diameter with the obBect inmind of increasing surface area in contact with the soil.

The usual practice is to select a diameter of earth electrode which will have

enough strength to enable it to be driven into the particular soil conditions

without bending or splitting. #arge diameter electrode may be more difficultto drive than smaller diameter electrode.

The depth to which an earth electrode is driven has much more influence on

its electrical resistance characteristics than has its diameter.

8a"imum allowable Earth resistance:

8aBor power station ).0 ,hm.


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8aBor %ub5stations *.) ,hm

8inor %ub5station + ,hm

Deutral ushing. + ,hm

%ervice connection 2 ,hm

8edium Coltage Detwork + ,hm

#.T.#ightening >rrestor 2 ,hm

#.T.Pole 0 ,hm

?.T.Pole *) ,hm

Tower +)5/) ,hm

Treatments to for minimi$ing Earth resistance:

-emove ,"idation on Boints and Boints should be tightened.

Poured sufficient water in earth electrode.

=sed bigger si$e of Earth Electrode.

Electrodes should be connected in parallel.

Earth pit of more depth ! width5 breadth should be made.

S "*"

3$stra!t of S: "*" O+E O8 &'37E 8O' E3'7H=>

>ll medium voltage equipment shall be earthed by two separate and distinct

connections with earth In the case of high.

>nd e"tra high voltage the neutral points shall be earthed by not #ess than

two separate and distinct connections with earth each having its ownelectrode at the generating station or substation and may be earthed at any

other point provided Lno interference is caused by such earthing.

If necessary the neutral may be earthed through suitable impedance.

In cases where direct earthing may prove harmful rather than provide safety9for e"ample high frequency and main. frequency coreless induction

furnaces;. -ela"ation may be obtained from the competent authority.

>s far as possible all earth connections shall be visible for inspection.

Do cut5out link or switch other than a linked switch arranged to operate

simultaneously on the

Earthed or earthed neutral conductor and the live conductors shall be inserted

on any supply %ystem.

This however does not include the case of a switch for use in controlling a

generator or a transformer or a link for test purposes.

'rounding is not likely to reduce the total magnitude of over5voltage

produced by lighting or switching surges. It can however distribute the

voltage between phases and reduce the possibility of e"cessive voltage stresson the phase5to5ground insulation of a particular phase.

Plate electrodes shall be of the si$e at least () c m < () cm Plates are

generally of cut iron not less than *+ mm thick and preferably ribbed. The

earth connection should be Boined to the plate at not less than two separatepoints. Plate electrodes when made of 'I or steel shall be not less than (/

mm in thickness.

Plate electrodes of &u shall be not less than /.*0 mm in thickness.


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Plate electrodes shall be buried ouch that ii. Top edge is at a depth not less

than *0m from the surface of the ground. ?owever the depth at which plates

are set should be such as to ensure that the surrounding soil is always damp.

Pipes may be of cast iron of not less than *))mm diameter +.0 to / m long

and */ mm thick. %uch pipes cannot be driven satisfactorily and maytherefore be more e"pensive to install than plates for the same effective

>rea. Water pipes shall not be use as consumer earth electrodes.

=nder fault conditions the earth electrode is raised to a potential with respect

to the general mass of the earth that can be calculated from the prospective

fault current and the earth resistance of the electrode. The results in thee"istence of voltage. In the soil around the electrode that may be inBurious to

telephone and pilot cables whose cores are substantially at earth potentialowing to the voltage to which the sheaths of such cables are raised.

The voltage gradient at the surface of the ground may also constitute a

danger to life especially where cattle are concerned. The former risk arises

mainly in connection with large elect rode systems as at power stations andsubstation.

Earth electrodes other than the. =sed for the earthing of the fence itselfshould not be installed

In pro"imity to a metal fence to avoid the possibility of the fence becoming

live and thus. @angerous at points remote from the substation.

The materials used for making connections have to be compatible with the

earth rod and the copper earthing conductor so that galvanic corrosion is

minimi$ed. In all cases the connections have to be mechanically strong.

The cross5sectional area of every protective conductor which does not form

part of the supply cable or cable enclosure shall be in any case not less than

9a; 26, mm2;if mechanical protection is provided and

9b; * mm2;if mechanical protection ii not provided.

Ooints of protective conductors shall be accessible for inspection and testing

e"cept m compound5filled or encapsulated Boints. Do switching device .hall is inserted in the protective conductor but Boints

which can be disconnected for test purposes by use of a tool may beprovided.

>n au"iliary earth electrode shall be provided electricallyM independent of all

other earthed metal for e"ample constructional metalwork pipers or metal5

sheathed cables. This requirement i. considered to be fulfilled if the au"iliary

earth electrode is installed at a specified distance from all other earthed metal9value of distance under consideration;.

The earthing conductor leading to the au"iliary earth electrode shall be

isolated to avoid contact with the protective conductor or any of the partsconnected thereto or e"traneous conductive parts which are or may be in

contact with them.

In TD systems for cables in fi"ed installations having a cross sectional area

not less than *) mm+ for copper and *( mm+ for aluminum a singleconductor may serve both as protective conductor and neutral conductor

provided that the part of the installation concerned is not protected by aresidual current5operated device.

?owever the minimum cross sectional area of a PED conductor may be 2

mm+ provided that the cable is of a concentric type conforming to Indian

%tandards and that duplicate continuity connections e"ist at all Boints andterminations in the run of the concentric conductors.


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When the source of energy is privately owned there should be no metallic

connection with the

'eneral public supply unless there has been consultation with the electricity

authority concerned.

It should be emphasi$ed that an installation together with its source of energy

may not consist entirely of one particular type of system. In such cases each

part of that installation may be required to be treated separately withoutdetriment to other parts of the same installation.

-&@Ms ?aving 8inimum ,perating &urrents 'reater Than /) m> F These

devices are intended to give indirect shock risk protection.

The neutral points of each separate electricity system which has to be earthed

at the power station or substation.

'IK&opper Plate %i$e: 0))mm


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%i$e of * meter diameter and / meter length shall be e"cavated after depth of

/ meter the si$e of e"cavation shall be R))

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Earthing q & A