DESIGN PROCEDURE BLOCK DIAGRAM

FIELD DATAA,r

CONDUCTOR SIZEts,d,Io

TOUCH & STEP CRITERIA

Etouch70, Estep70

INITIAL DESIGND,n,LT,h

GRID RESISTANCERg,LT

GRID CURRENTIG

GPR < Etouch

MESH & STEP VOLTAGES

Em,Es,Km,Ks,Ki,Kii

Es<Estep

Em<Etouch

DETAIL DESIGN

MODIFY DESIGND,n,LT,LR

YES

YES

YES

NO

NO

NO

EARTHING CALCULATION

A GENERAL DESIGN DATA

1 Soil Resistivity, : 600 Ohm-M

2 Gravel Resistivity, : 2000 Ohm-M

3 : 50000 A

4 : 0.3 Sec

5 Design Ambient Temperature : 50 ° C

6 Thickness of Crushed Gravel, : 0.3 m

7 : 0.8 m

8 1 m

9 Standard Used IEEE - 80 : 2000

B SIZE OF EARTHING CONDUCTOR :

Eqn.: 40 Page : 43IEEE Std. 80 - 2000

Where

Material Proposed

= Resistivity of Conductor Material 0.0032 Ohm - M

= Thermal co-efficient of resistivity at reference temperature Tr in 1/°C 20.1

Tm = Max. allowable temperature in °C 419 °C

Ta = Ambient temperature in °C 40 °C

Ko 293

Iefs = rms current in Ka 50 KA

= Duration of Current in s 0.3 Sec.

TCAP = thermal capacity per unit volume from Table 1 3.93 J/(cm³°C)

Amm² = Conductor cross section in mm² 402 mm²

SELECTED CONDUCTOR (GI Strip) = 75 mm x 6 mm = 450.0 mm²

= 0.03 m

Symmetrical Short Circuit Current, Iefs

Duration of Earth Fault Current, ts

Depth of Earth Grid, h

Reference depth of the Grid, ho

= 1/a0 or 1/ar - Tr in °C

tc

Diameter of equivalent Grid Conductor ,d

Amm2=I

√(TCAP×10−4

t c α r ρ r) ln(K 0+T m

K0+T a)

ρr

α r

ρ s

ρ

hs

EARTHING CALCULATION

TOUCH & STEP CRITERIA

Eqn. 21, Page 21, IEEE 80 2000

Eqn. 27, Page 23, IEEE 80 2000

= Reflection factor between different material resistivities

= Thickness of the surface material in m

= Surface layer derating factor

= -0.54

= 0.91

Eqn. 30, Page 27, IEEE 80 2000

Where

= Step Voltage for body weight of 70 kg

= 3412.28 Volts

Eqn. 33, Page 27, IEEE 80 2000

Where

= Touch Voltage for body weight of 70 kg

= 1068.05 Volts

= Resistivity of the earth beneath the surface material in W.m

= Surface material resistivity in W.m

K=ρ−ρs

ρ+ ρs

K

ρ

ρ s

K

C s=

0 .09(1− ρρs)

2hs+0 .09

C s

hs

C s

s

ssstept

CE157.0

)61000(70 r=

E step70

E step70

s

sstoucht

CE157.0

)5.11000(70 r=

E touch70

E touch70

EARTHING CALCULATION

C INITIAL DESIGN ASSUMPTIONS

Length BreadthPreliminary Layout of Grid = 450 250

n = Number of parallel conductors = 15

D = Conductor Spacing = 30 m

h = Depth of grid burial = 0.8 m

= Length of the conductor across perimeter = 1800 m

Nr = No. of Ground Rods = 33

Lr = Length of Ground Rods = 3 m

= Total length of Ground Rods = 99 m

= Total length of buried condcutor = 8200 m

= Total length of buried conductors & rods = 8299 m

= Maximum length of conductor in X-Axis = 450

Ly = Maximum length of conductor in Y-Axis = 250

D GRID RESISTANCE

Where

A = Area of the Grid = 112500 m²

= Grid Resistance

= 0.87 W

E MAXIMUM GRID CURRENT

Where

= Maximum grid current in A 50000 A

= Decrement factor for the entire duration of fault, given in s 0.6

= 30000 A

F GROUND POTENTIAL RISE

= 26042.3 V

Lp

LR

LC

LT

Lx

LT=2×Na×L

Rg=ρ[ 1LT

+ 1√20 A (1+ 1

1+h√20/ A )]

Rg

Rg

I g

gfG IDI =

D f

IG

GPR=IG×Rg

GPR

EARTHING CALCULATION

VERIFICATION FOR HUMAN SAFETY

The safety to personnel is specified by IEEE 80, which requires to limit the development of electrical potential to dangerous value during earth fault current.

The regulation stipulates the following parameters to be within the permissible limit

a) Step Voltage (Foot to Foot Contact)

b) Touch Voltage(Hand to Foot Contact)

CALCULATION FOR ACTUAL DERIVED STEP & MESH VOLTAGE

A Mesh Voltage

Eqn. 80, Page 91, IEEE 80, 2000

= Corrective factor for current irregu- larity

Where

= 9.1 = 1 for square grids = 1

= 1 for square and rectangular grids = 1

= 1 for square, rectangular and L-shaped grids = 1

= 9.11

= 1.99

= Spacing factor for Mesh Voltage Eqn. 68 Page 113 IEEE 80

Eqn. 81, Page 93IEEE 80, 2000

Where

= Corrective wieghting factor that adjusts the effect of inner conductors on the corner mesh

Emesh(Design)=ρ×IG×Km×K i

LC+[1 .55+1 .22( Lr

√Lx2+lLy

2 )]×LR

Ki

Km

Km=12Π

ln [ D2

16 hd+

( D+2h )2

8Dd−

h4d ]+ Kii

Khln [ 8

Π (2n−1 ) ]

Kii

GPR

K ii=1

(2×n )2n

K i=0 .644+0 .148n

n=na×nb×nc×nd

na=2×LT

LP

na

nb

nc

nd

n

Ki

EARTHING CALCULATIONK ii=

1

(2×n )2n

EARTHING CALCULATION

= 0.64

= 1.00 With Rods

= Corrective weighting factor that empasising the grid depth

=

Where

= Reference depth of grid = 1

= Depth of the ground grid conductor = 0.8

= 1.34

= 0.96

= 4104.44 Volts

Calculated Mesh Voltage is Greater than the Tolerable Touch Voltage. MODIFY DESIGN

B Step Voltage

Voltage developed for step as per the earthing system proposed during full Earth fault current

Eqn. 92, Page 94IEEE 80, 2000

Where

= Spacing factor for Step voltage

Eqn. 94, Page 94, IEEE 80, 2000

= 0.220

= 1.992444444

= 1265.62 Volts

= 1266 Volts

Calculated Step Voltage is Lower than the Tolerable Step Voltage.HENCE SAFE

Estep(Design)=[ (Ks×Ki×ρ×IG )]

[0.75×LC+0 .85×LR ]

Ks

Ks= 1Π [ 12h

+ 1D+h

+( 1−0 .5n−2

D )]Ks

Ki

Estep(Design)

Estep(Design)

Kh

√(1+ hho )

ho

h

Kh

Km

Emesh(Design)

Kii

K i=0 .644+0 .148n

Kii

EARTHING CALCULATION

SUMMARY

A EARTH GRID CONDUCTOR

Type of Conductor Zinc-coated steel rod

Size of Conductor 450 mm

Length of Conductor 8200 mtr

Depth of Conductor 0.8 m below GL

B GROUND RODS

Total Length of Ground Rods 99 mtr

Length of Individual Ground Rods 3 mtr

No. of Ground Rods 33 No.

C HUMAN SAFETY

UNIT Designed Value Permissible Value

Step Voltage Volt 3412 1266

Mesh Voltage Volt 1068.05 4104

TABLE 1 - MATERIAL CONSTANTS

Description

1 1 100 1 0.00393 1 234 1 1083 1

2 2 97 2 0.00381 2 242 2 1084 2

3 3 40 3 0.00378 3 245 3 1084 3

4 4 30 4 0.00378 4 245 4 700 4

5 5 20 5 0.00378 5 245 5 1084 5

6 6 61 6 0.00403 6 228 6 657 6

7 7 53.5 7 0.00353 7 263 7 652 7

8 8 52.5 8 0.00347 8 268 8 654 8

9 9 20.3 9 0.0036 9 258 9 657 9

10Steel, 1020

10 10.8 10 0.0016 10 605 10 1510 10

11 11 9.8 11 0.0016 11 605 11 1400 11

12 12 8.6 12 0.0032 12 293 12 419 1213 Stanless steel, 304 13 2.4 13 0.0013 13 749 13 1400 13

Material Conductivity

(%)

ar factor at 20°C

K0 at (0°C)

Fusing Temperature

Tm (°C)

Copper annealed soft - drawn

Copper, commercial hard - drawn

Copper-clad steel wire

Copper-clad steel wire

Copper-clad steel rod

Aluminium EC Grade

Aluminium 5005 alloy

Aluminium 6201 alloy

Aluminium-clad steel wire

Stainless - clad steel rod

Zinc-coated steel rod

TABLE 1 - MATERIAL CONSTANTS

1.72 1 3.42

1.78 2 3.42

4.4 3 3.85

5.86 4 3.85

8.62 5 3.85

2.86 6 2.56

3.22 7 2.6

3.28 8 2.6

8.48 9 3.58

15.9 10 3.28

17.5 11 4.44

20.1 12 3.9372 13 4.03

rr 20°C(mW.

cm)

TCAP Thermal Capacity

[J/(cm³.°C]