earthing sytem

8/13/2019 Earthing Sytem

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Grounding System

Grounding is a connection donethrough a metal link between the body

of any electrical appliance, or neutralpoint, to the deeper ground soil. Themetal link is normally of MS flat, CI

flat, GI wire which should bepenetrated to the ground earth grid.


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Purpose of Grounding

Safety of Personnel

Protection of Installation

Superior performance ofelectrical system


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Design of Grounding System

Touch and step potentialswithin permissible limits

Low value of earthresistance

Objectives:


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Design of Grounding System

Design of grounding systemmainly depends upon:

Soil Resistivity,

Magnitude & duration of fault

current, Area available for earth mat

laying.


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Soil Resistivity

Soil resistivity depends on natureof soil, humidity, temperatureand climatic conditions.

Type of soil Average Resistivity

(-m)

Wet organic soil 10

Moist soil 100

Dry soil 1000

Bedrock 10000


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Soil Resistivity


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Grid Fault Current

The maximum grid current IGwhich is discharged by thegrounding system to the ground,

IG= Df . Sf . If

Df: Decrement factor for entireduration of fault

Sf: Fault current division factorIf : rms symmetrical ground fault

current


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Tolerable Step and Touch

VoltagesEstep= (1000 +6CS )*0.116/tEtouch= (1000+1.5CS)*0.116/tt : Duration of shock current,

S: Resistivity of the top layer of soil,

C : Reduction factor.


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Size of Conductor

depends on :

Magnitude and duration of fault current,

Type of material,

Type of joint and

Corrosion effect.


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Size of Conductor

Cross-sectional area of conductor(in mm2) is determined by,

Amm2= K. If. tc

If : Fault current in kA,

tc: Fault clearing time in sec,

K: Factor dependent on material ofconductor.


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Values of K for Steel, Aluminium andCopper are as follows:

Material Welded joint Bolted joint

Steel 12.15 15.7

Aluminium 8.4 12.0

Copper 4.7 5.8

Size of Conductor


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For steel conductors, corrosionallowance is also considered.

Soil Resistivity(ohm-m)

Increase inconductor size

(%)

Up to 25 30Between 25 to 100 15

>100 0

(Contd)Size of Conductor


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Step and Mesh Voltages

Es=.Ks.Ki.IG/LsEm= .Km.Ki.IG/LM

Factors Km, Ksand Kidepend on

spacing between conductors,depth of earth mat, diameter ofconductor, number ofconductors etc.


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Resistance of Earth Mat

The earth resistance depends on soilresistivity, available area, total

length of mat conductors and depthof mat in soil.

Rg= .( 1/LT + (1/(20.A))x(1+1/(1+ h(20/A)))


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Dif f icul t Condi t ions

Corrosion

High Soil resistivity

Transferred Potential


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Corrosion

When dissimilar metals are in electrical

or physical contact through anelectrolyte, galvanic corrosion can takeplace.

High saline nature of soil


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Soil resistivity and corrosion

Range of Soilresistivity range

(ohm-m)

Class

Less than 25 Severe

25 to 50 Moderate to severe

50 to 100 Mild Above100 Not likely


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Equilibrium Potential- Ref.

Copper electrodeMetal Equilibrium Potential

(volts)

Copper 0

Aluminium -2.0

Magnesium -2.7

Zinc -1.1

Iron -0.7

Lead -0.4


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Methods to reduce the

corrosion effect

Resistive coatingallowances in size selection of

material for corrosion

Cathodic protection.


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Cathodic protection

To reduce corrosion by minimizing thedifference in potential between anode

and cathode. This protection can beaccomplished by two widely usedmethods Sacr i f ic ial anode Method

Impress ing DC Current


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Design sequence

The criterion of selection is based on currentdensity required and soil resistivity. If thesoil resistivity is low (less than 50 ohm-

meters) and the current density requirementis low (less than 1 milli ampere per squarefoot), a galvanic system can be used. How-ever, if the soil resistivity and/or currentdensity requirement exceed the abovevalues, an impressed current system shouldbe used.


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High soil resistivity

The sites of hydroelectric power

plants are located in rockyareas where earth resistivity isvery high and space for layingof earth mat is limited.


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High soil resistivity

Various methods to lower the groundresistance are :

Connection to Penstocks

Laying of conductors in tail racearea

Satellite Earthing Use of Bentonite clay (generallynot recommended)


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Connection to Penstocks

Penstocks which are metallic andburied in soil are connected with the

main earthmat.

Because of large diameter and longlength of penstocks, low overall

ground resistance is easily achieved.


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Laying of conductors in Tail

Race Area

The tailrace area is generally lowresistivity area as earth mass willalways have a fair amount of waterseepage.

A separate earthmat in tailrace area

is laid and it is connected with mainearthmat to reduce the overall earthresistance.


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During fault conditions, high voltageequal to Grid Potential Rise (GPR) isattained by the grounding grid.

This voltage shall not be conveyed toplaces outside the power plant where it

can be dangerous to the person and theequipment.

f


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The L.T. (415 V, 3-phase) powerrequirements of the power plant areashould be met from the station

transformer located within the groundinggrid area of the power plant.

No L.T. line should either be taken out ofthe grounding area or brought fromoutside to inside the power plant area.


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Any telephone line should leave orenter the grounding grid areathrough isolating transformer.

This transformer shall be capable ofwithstanding voltage equal to GPR

between windings and between eachwinding & the ground.


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In pipes and metallic conduitsentering / leaving the grounding gridarea, transferred potentials may be

reduced or stopped at the stationboundary by inserting insulatingsections of sufficient length.

If there is any metallic gate within thearea covered by ground mat, theground mat shall be extended to thefull opening position of the gate.


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CONCLUSION

For a power plant having high soilresistivity, interconnection of dissimilar

material grids, high corrosivity, the safeearthmat design can be achieved fromvarious described methods and at the sametime it is very important to take care of

safety of the men and equipments whichmay come in contact with the area whereearthmat is laid and which may attain a highpotential rise.

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