Technical presentation on modern earthing

ON MODERN CONCEPTS AND ADVANCED TECHNIQUES OF EARTHING

1

Technical Presentation to PROJECT PRICIPALS

BY PANKAJ CHAKRABORTY

OF

PBC INDUSTRIAL SERVICESBLOCK – 19 , F LAY – 1 1 7 ,

PARNASREE GOVT . HOUS ING ,

KOLKATA – 700 060 , (WEST -BENGAL )

7/2/2013PBC Training Document PBC/ER/005 Rev 0

Objectives

Return path during continuous operation should not be thru earth path unless designed.

Instantaneous fault current should not flow thru the neutral circuit creating live and dangerous neutral.

Dissipate the stray currents, lightning, surges

2

Dissipate the stray currents, lightning, surges keeping system hazard free.

Maintain the whole facility at equipotential.

Provide reference to run the system at specified voltage


TN System

Transformer neutral is earthed

Frames of electrical load are connected to neutral

Fault is cleared by SCPD

3

Fault is cleared by SCPD

TN-C, TN-S, TN-CS

SCPD/ loop impedance matching

Not recommended for premises having electronic and communication system

Not used when cross-section of live cond. < 10 sqmm

PE may get damaged by loads generating 3rd harmonics.


TT system

Transformer neutral is earthed

Frame of electrical load is connected to earth connection

Fault cleared by RCD.

4


IT System

Transformer neutral is not earthed.

Frame of electrical load is earthed.

First fault does not present risk, indication is sufficient

Second fault cleared by SCPD

5

Second fault cleared by SCPD

Ensure continuity of service where human life is at stake or with furnaces

7/2/2013PBC Training Document MEPL/ER/005 Rev 0

7/2/2013PBC Training Document PBC/ER/005 Rev 0 6

Problem for electronic and communication system


Comparing Earthing system8


Parallel association of Earthing system9


BREAK 110

THINKING T IME

PBC Training Document PBC/ER/005 Rev 0 7/2/2013

Soil Resistivity of different Soil Types11


Resistivity Plot

Use Wenner 4 point method. Take at least 4 readings in one direction from 7mt spacing to spacing equal to diagonal of the electrical installation.

Repeat these readings in all 8 directions separated by 450

angle.

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angle. Calculate the resistivity. Make a polar plot. Calculate the area under the polar plot

Draw a circle of equal area Radius of the circle is the resistivity of the point.

Variation in Resistivity

13



Resistivity Plot

15


Wenner’s method16


Schulemberger method17


Essential Practices

Use meter having stray current filter.

where the injected current frequencies can be changed

Injected current can be made high or low depending on current probe resistance.

Apply water and compact the current and potential

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Apply water and compact the current and potential probes to avoid undue high probe resistance.

Lose potential probe can give high value of R and high resistivity.

Avoid measuring along buried or superficial metal conductors

Take readings in multiple directions

7/2/2013

Abstract of IEEE 81 199319


Polar curve method for Uniform Soil Resistivity

• Resistivity taken in min 8 directions

• Angular distance between readings 450

• Cautiously Interpolate the readings to 7.50

• Join the Points to form a polar curve

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PBC Training Document PBC/ER/005 Rev 0

curve• Calculate the area of the polar curve

• Draw equivalent Circular area• Radius of the circle is the average soil resistivity

• This method is particularly beneficial when the resistivity varies significantly in different directions

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Inverse Slope method for 2 layer soil Resistivity

1

1.2

1.4

1.6

1.8

Spacing s / apparent resistivity ρa

Approximate method

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0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35

0.5/5=.11/.1=ρ2=10Ωm

Spacing s / apparent resistivity

Spacing s in m

0.2/5=.041/.04=ρ1=25Ωm

H=13

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Sunde’s Method22


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THINKING T IME


Essentials of Earthing Design

Crossection Area

Current Density

Dangerous Potentials

Resistance

The Crossection of the conductor to be sufficient for carrying

Continuous surface current density

Step potential Horizontal Plane

Touch potential Vertical Plane

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for carrying GRID fault current Instantaneous

Surface current density

Mesh Potential Mutualresistance

GPR

Transfer Potential

PASS PASS PASSPASS OK7/2/2013

(Clause 11.3 of IEEE 80:2000)

TERMS:I – rms current in KATCAP – Thermal capacity per unit volume in J/(cm3. 0C)

Calculation of required minimum cross sectional area of grid conductor:

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TCAP – Thermal capacity per unit volume in J/(cm . C)tc – Duration of fault current in secαr – Thermal coefficient of resistivity at reference temperature in 1/

0Cρr – Resistivity of the ground conductor at reference temperature in µΩ-cmKo – 1/ α0 in

0CTm – Maximum allowable temperature in

0CTa – Ambient temperature in

0C

Cross Sectional Area of selected grid conductor must be greater than minimum required Cross Sectional Area as required by Asymmetric fault current.


Crossection area

Equation 37 of IEEE 80 2000 The current is Symmetrical fault current

Enthalpy of vaporization decreases with increase in temperature.

For a large grid the fault current gets

See also

26


For a large grid the fault current gets multiple parallel paths hence Tm doesn’t pose a problem.

If the Tm is allowed to rise beyond a limit in smaller grids or pits the water molecules beyond a temperature will instantaneously vaporizes and escape from the soil surrounding the conductor.

Tm also applies to surface layer coating or cover

CompoundAt 1000C

Heat of vaporizatio

n(kJ mol-1)

Heat of vaporizatio

n(kJ kg−1)

Water 40.65 2257

7/2/2013

Calculate Grid Current

(Clause 5.2.8.1 of IEEE 665:1995)

TERMS:

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TERMS:I – rms current in KASf – Split factor (Annex C of IEEE 80:2000)Df – Decremtent factor (Clause 5.2.5.4 of IEEE 665:1995)Cp – Corrective Projection factor

The Remaining design depends on the Current that takes the ground path to return to the source. It is only a portion of 3I0.


Long term surface CurrentsLong term surface Currents Short Term surface currentsShort Term surface currents

Surface current density as per BS7340 clause 15

If surface current densities are not maintained, junction between electrode and soil, will heat up reducing moisture, failing the electrode or grid.

28


Long term Surface current density is 40A/m2

Independent of soil resistivity

Given by the formula 1000*Sqrt (57.7/(ρ*t))

Dependent on soil resistivity and time of clearance of fault

EFFECT OF COROSSION

7/2/2013

Symbol unit Value

Relay setting at 6KA 0.32Sec IG A 6000 Input

Diameter of electrode d m 0.04 Input

Length of Electrode

Soil ls m 37 Input

Water lw m 1 Input

Resistivity

Soil ρs Ωm 360 Input

Water ρw Ωm 2 Input

Resistance

Soil Rs Ω 12.25394 Equation 55 IEEE 80 2000

Water Rw Ω 1.368891 Equation 55 IEEE 80 2000

Combined R Ω 1.231338


Combined Rc Ω 1.231338

Permissible Current Density

Soil σs A/m2 730.9304 Clause 15.2 BS7340

Water σw A/m2 9806.46 Clause 15.2 BS7341

Area

Soil As m2 4.6472 πdls

Water Aw m2 0.1256 πdlw

Current Division Resistance Capacity Design

Soil 3396.779785

Water 1231.691433

TOTAL 4627 A

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Dangerous Potentials

33


Voltage Variation with proximity

∆V

• The charges are distributed to the surrounding soil.

• The Voltage is thus high near the ∆V

∆V

• The Voltage is thus high near the pit and low away from it


Step Potential

Calculation of permissible Step Potential:

(Clause 7.4 and 12.5 of IEEE 80:2000)

(Clause 8.3 of IEEE

35

TERMS:ρ – Soil Resistivity in Ω-mρs – Soil Resistivity of additional surface layer in Ω-mhs – Thickness of additional surface layer in mts – Shock current duration in secCs – Surface layer de-rating factor




Calculation of actual Step Potential:


TERMS:ρ – Soil Resistivity in Ω-m

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ρ – Soil Resistivity in Ω-mKs – Spacing factor for step voltageKi – Correction factor for grid geometryIG – Grid current in KALs – Effective length of (Lc + LR) for step voltage in m

where Lc = Total length of grid conductorLR = Total length of ground rods

Calculated Step Potential must be less than permissible Step Potential.


Touch Potential

Calculation of permissible Touch Potential:

(Clause 7.4 and 12.5 of IEEE 80:2000)

(Clause 8.3 of IEEE

37

TERMS:ρ – Soil Resistivity in Ω-mρs – Soil Resistivity of additional surface layer in Ω-mhs – Thickness of additional surface layer in mts – Shock current duration in secCs – Surface layer de-rating factor




Calculation of actual Mesh Potential:


TERMS:ρ – Soil Resistivity in Ω-mKm – Spacing factor for mesh voltage

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Km – Spacing factor for mesh voltageKi – Correction factor for grid geometryIG – Grid current in KALM – Effective length of (Lc + LR) for mesh voltage in m

where Lc = Total length of grid conductorLR = Total length of ground rods

Calculated Mesh Potential must be less than permissible Touch Potential.


Tolerable currents by Humans

Frequency based (lethal values)

DC 25Hz 50/60Hz 3-10 KHz

0.5A 0.15A 0.1A high current

39


Threshold current,

tingling feeling

Let – go current.

Can release

Cannot release

energized object

Ventricular fibrillation

1 mA 1-6 mA 9-25 mA 60 - higher

0.5A 0.15A 0.1A high current

Amplitude based

Tolerable currents by Humans (Con.)

Weight Based Time based

40


Step potential (Permissible Value)It is the concentration of charge that creates Tension or Voltage

Voltage drives the Current

Permissible Estep = (1000+6Csρs)0.116/sqrt(t)

Actual E < Permissible E

∆V∆V

Actual Estep < Permissible Estep


Mesh Potential (Permissible Value)

E touch is the Voltage difference between where the person is standing in the grid and GPR.

E mesh is the Max E touch in a grid

Permissible E touch is the voltage limit that can produce dangerous currents

42

dangerous currents

E mesh < Permissible E touch Permissible E touch =(1000+1.5Csρs)0.116/sqrt(t)


Potential Distribution

Permissible Step PotentialActual Step Potential

43


Permissible Step Potential

Dangerous Step Potential

Actual Step Potential

Corrected Step Potential

7/2/2013







Ring Earth

•The concentric conductors reduce the resistance

significantly

•The reactance is reduced by tying the rings to

each other at the corners

•A ramp arrangement is followed burying the out

conductors deeper than the inner conductors to

47


conductors deeper than the inner conductors to

obtain a gradual potential curve

Formulae to Calculate Resistance

for plate earthingR = (ρ/4)* sqrt (π/2A) for pipe earthingR = (ρ/2πL)* [ln (8L/d)-1] for strip earthingR=(ρ/PπL)* [ln (2L2 /(wh))+ Q]

48


R=(ρ/PπL)* [ln (2L2 /(wh))+ Q] for grid earthingR=ρ[(1/LT)+ (1/sqrt (20A) (1+ (1/1+h) sqrt (20A)

Is Material of the grid important for achieving resistance?No. If corrosion factor is taken care of

7/2/2013

Variation of resistance to earth with length at different Soil resistivity

49


Abstract of IEEE 14250


Effect of Resistance due to Artificial Treatment

50

60

70

80

Radius of Artificial

treatment inmm

Remaining percentage of resistance to remote earth

51


0

10

20

30

40

1 10 100 1000 10000

Series1

30 75

60 62

90 54

150 48

300 32

1500 14

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Influence of MV on LV

If Rp>1, the voltage Rp*Ihmtshould be less than

100V in under 500 ms

500V in under 100 ms

53

If this is not so Rp and Rn must be separate

Exp. of PDS world wide


Internationally accepted MV/LV earthing systems



Measurement of Resistance of the grounding system

Fall of point method

61.8% distance rule

Tagg slope method

Tagg intersection curve

Covered in the paper

56


Covered in the paper already with you

7/2/2013

Positioning of the potential probe57


Essential Practices

Must use insulation gear

Must keep a gap of at least 6 meters between potential and current lead

Must insulate any joint appearing in the lead

Use meter having stray current filter.

58


having stray current filter.

where the injected current frequencies can be changed

Injected current can be made high or low depending on current probe resistance.

Apply water and compact the current and potential probes to avoid undue high probe resistance.

Lose potential probe can give high value of R .

Avoid measuring along buried or superficial metal conductors

7/2/2013

Fall of point method59


The average of the flat part of the graph gives the give the ground impedance

7/2/2013

61.8% Distance Rule

The reading levels of at P2 61.8% of the current probe C2

At distances lower than 61.8%, the impedance reading is lower and dips on approaching the resistance zone of the grid

60


resistance zone of the grid

Similarly the impedance reading increases as the P2 enters the resistance zone of C2

7/2/2013

TAGG SLOPE Method

µ=(R3-R2)/(R2-R1)0.4 < µ < 1.6

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THINKING T IME


Earth fault Protection

Earth Fault protection in installations

Selection of device for automatic disconnection

Earth fault protection Devices

Duration 15 min

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Duration 15 min


Earth Fault Protection in Installation

Trip 65V in 10 Sec, and 230 V instantaneously Earth fault protection involves automatic disconnection to prevent dangerous duration and magnitude of touch voltage.

The earth fault loop impedance has to be low enough to trip overcurrent protective devices.

64

trip overcurrent protective devices. Where low earth fault loop impedance cannot be achieved, disconnection may be facilitated by RCCB or Voltage operated ELCB

RCD having minimum operating current greater than 30mA indirect shock risk protection

RCD having minimum operating current less than 30mA direct shock risk protection


Voltage Operated ELCBVoltage Operated ELCBRCD Current Operated ELCBRCD Current Operated ELCB

Earth Fault Detection65

Not suitable for protection of humanTrip coil set to operate at 50 V


RCD Connections66


IS 304367


Earthing of Captive Power Plant

Low voltage generators

High voltage generators

Duration 30 min

68


• It should be connected to installation main earth• The main earth terminal should be connected to earth electrode• Installation should be protected by RCD• RCD cannot protect generator side of the circuit.• For mobile loads the RCD should be 30mA with tripping time of 40mS


Generators above 10kW Working in parallel

• To avoid flow of circulating currents, the neutral point of generator is disconnected in presence of supply

70


presence of supply neutral

• Generators working in parallel, have only 1 common earth point.

• 30mA RCD to be provided for protection of load side of RCD

RCD

Low voltage standby generators with neutral earthing transformers

71


Cable Core Sheath Bonding System72


Statutory Provisions for Earthing

Should follow Indian Electricity Rule 1956

All MV and HV equipments to be earthed by 2 separate earths

Earth electrode should be devised such that testing is possible.

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possible.

The value of earth resistance should be based on degree of shock protection.

No additional source to be added with out verifying the capacity of earthing system


Bibliography IEEE 80 2000 for substation

IEEE 665 1995 for Generating Station

IEEE 142 1991 for Industrial establishment

IEEE 81 1993 for Earthing Measurements

IEEE 1100 for powering and grounding electronic equipments.

IEEE 575 for sheath bonding and induced voltages IEEE 575 for sheath bonding and induced voltages

BS 7430 1998 Code of practice for Earthing

IS 3043-1987 Code of practice for Earthing

IEC 62305 Part 1 to Part 4

NFPA 70 and NFPA 780

API RP 2003 for statics and lightning protection

And many more ref. texts


SAFETY THRU DESIGN

Saving life and Assets75

THANKYOU


Date post:	21-Feb-2017
Category:	Engineering
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Technical presentation on modern earthing

Engineering