At the outset we wish to thank you for the curtsey and excellent response from you and active interest in taking a comprehensive view of modern safe Lightning Protection / Earthing and the underlying scientific principles as we posted in our Linkedin Profile, which is for your ready reference.

1. O N M O D E R N C O N C E P T S A N D A D V A N C E D
T E C H N I Q U E S O F E A R T H I N G
1
Technical Presentation to
PROJECT PRICIPALS
B Y P A N K A J C H A K R A B O R T Y
OF
PBC INDUSTRIAL SERVICES
B L O C K – 1 9 , F L A Y – 1 1 7 ,
P A R N A S R E E G O V T . H O U S I N G ,
K O L K A T A – 7 0 0 0 6 0 , ( W E S T - B E N G A L )
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2. 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
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3. 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.
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4. TT system
Transformer neutral is earthed
Frame of electrical load is connected to earth
connection
Fault cleared by RCD.
4
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5. 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
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7. Problem for electronic and communication system
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8. Comparing Earthing system
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9. Parallel association of Earthing system
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10. BREAK 1
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T H I N K I N G T I M E
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11. Soil Resistivity of different Soil Types
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12. 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.

13. Variation in Resistivity
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15. Resistivity Plot
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16. Wenner’s method
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17. Schulemberger method
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18. 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
18
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
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19. Abstract of IEEE 81 1993
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20. 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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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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21. Inverse Slope method for 2 layer soil Resistivity
1
1.2
1.4
1.6
1.8
Spacings/apparentresistivityρa
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=.1
1/.1=ρ2=10Ω
m
Spacings/apparentresistivity
Spacing s in m
0.2/5=.04
1/.04=ρ1=25Ω
m
H=13
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22. Sunde’s Method
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23. BREAK 2
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T H I N K I N G T I M E
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24. 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 Mutual
resistance
GPR
Transfer
Potential
PASS PASS PASSPASS OK
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25. (Clause 11.3 of IEEE
80:2000)
TERMS:
I – rms current in KA
TCAP – Thermal capacity per unit volume in J/(cm3. 0C)
Calculation of required minimum cross sectional area of
grid conductor:
25
TCAP – Thermal capacity per unit volume in J/(cm . C)
tc – Duration of fault current in sec
αr – Thermal coefficient of resistivity at reference temperature in 1/0C
ρr – Resistivity of the ground conductor at reference temperature in µ -cm
Ko – 1/ α0 in 0C
Tm – Maximum allowable temperature in 0C
Ta – 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.
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26. 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
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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
Compound
At 1000C
Heat of
vaporizatio
n
(kJ mol-1)
Heat of
vaporizatio
n
(kJ kg−1)
Water 40.65 2257
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27. Calculate Grid Current
(Clause 5.2.8.1 of IEEE 665:1995)
TERMS:
27
TERMS:
I – rms current in KA
Sf – 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.
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28. 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.
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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
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29. 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
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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
29

30. Abstract of IEEE 80 2000
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31. Abstract of IEEE 80 2000
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32. Abstract of IEEE 80 2000
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33. Dangerous Potentials
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34. Voltage Variation with proximity
V
• The charges are distributed to
the surrounding soil.
• The Voltage is thus high near theV
V
• The Voltage is thus high near the
pit and low away from it
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35. 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 -m
ρs – Soil Resistivity of additional surface layer in -m
hs – Thickness of additional surface layer in m
ts – Shock current duration in sec
Cs – Surface layer de-rating factor
(Clause 8.3 of IEEE
80:2000)
(Clause 8.3 of IEEE
80:2000)
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36. Calculation of actual Step Potential:
(Clause 16.5 of IEEE
80:2000)
TERMS:
ρ – Soil Resistivity in -m
36
ρ – Soil Resistivity in -m
Ks – Spacing factor for step voltage
Ki – Correction factor for grid geometry
IG – Grid current in KA
Ls – Effective length of (Lc + LR) for step voltage in m
where Lc = Total length of grid conductor
LR = Total length of ground rods
Calculated Step Potential must be less than permissible Step Potential.
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37. 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 -m
ρs – Soil Resistivity of additional surface layer in -m
hs – Thickness of additional surface layer in m
ts – Shock current duration in sec
Cs – Surface layer de-rating factor
(Clause 8.3 of IEEE
80:2000)
(Clause 8.3 of IEEE
80:2000)
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38. Calculation of actual Mesh Potential:
(Clause 16.5 of IEEE
80:2000)
TERMS:
ρ – Soil Resistivity in -m
Km – Spacing factor for mesh voltage
38
Km – Spacing factor for mesh voltage
Ki – Correction factor for grid geometry
IG – Grid current in KA
LM – Effective length of (Lc + LR) for mesh voltage in m
where Lc = Total length of grid conductor
LR = Total length of ground rods
Calculated Mesh Potential must be less than permissible Touch Potential.
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39. Tolerable currents by Humans
Frequency based (lethal values)
DC 25Hz 50/60Hz 3-10 KHz
0.5A 0.15A 0.1A high current
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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

40. Tolerable currents by Humans (Con.)
Weight Based Time based
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41. 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
VV
Actual Estep < Permissible Estep
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42. 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)
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43. Potential Distribution
Permissible Step PotentialActual Step Potential
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Permissible Step Potential
Dangerous Step Potential
Actual Step Potential
Corrected Step Potential
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44. Abstract of IEEE 80 2000
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45. Abstract of IEEE 80 2000
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46. Abstract of IEEE 80 2000
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47. 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
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conductors deeper than the inner conductors to
obtain a gradual potential curve

48. Formulae to Calculate Resistance
for plate earthing
R = (ρ/4)* sqrt (π/2A)
for pipe earthing
R = (ρ/2πL)* [ln (8L/d)-1]
for strip earthing
R=(ρ/PπL)* [ln (2L2 /(wh))+ Q]
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R=(ρ/PπL)* [ln (2L2 /(wh))+ Q]
for grid earthing
R=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
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49. Variation of resistance to earth with length at different Soil
resistivity
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50. Abstract of IEEE 142
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51. Effect of Resistance due to Artificial Treatment
50
60
70
80
Radius of
Artificial
treatment in
mm
Remaining percentage of
resistance to remote earth
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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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52. Abstract of IEEE 80 2000
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53. Influence of
MV on LV
If Rp>1, the voltage Rp*Ihmt
should 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
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54. Internationally accepted MV/LV earthing systems
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56. 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
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Covered in the paper
already with you
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57. Positioning of the potential probe
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58. 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.
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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
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59. Fall of point method
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The average of the flat part of the graph gives the give the ground impedance
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60. 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
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resistance zone of the grid
Similarly the impedance reading increases as the P2
enters the resistance zone of C2
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61. TAGG SLOPE Method
=(R3-R2)/(R2-R1)
0.4 < < 1.6
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62. BREAK 3
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T H I N K I N G T I M E
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63. Earth fault Protection
Earth Fault protection in installations
Selection of device for automatic disconnection
Earth fault protection Devices
Duration 15 min
63
Duration 15 min
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64. 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
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65. Voltage Operated ELCBVoltage Operated ELCB
RCD Current Operated
ELCB
RCD Current Operated
ELCB
Earth Fault Detection
65
Not suitable for protection of human
Trip coil set to operate at 50 V
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66. RCD Connections
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67. IS 3043
67
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68. Earthing of Captive Power Plant
Low voltage generators
High voltage generators
Duration 30 min
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69. • 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
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70. Generators above 10kW Working in parallel
• To avoid flow of
circulating
currents, the
neutral point of
generator is
disconnected in
presence of supply
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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

71. Low voltage standby generators with neutral earthing
transformers
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72. Cable Core Sheath Bonding System
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73. 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
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74. 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 voltagesIEEE 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
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75. S A F E T Y T H R U D E S I G N
Saving life and Assets
75
T H A N K Y O U
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