DESIGN PROCEDURE BLOCK DIAGRAM
FIELD DATA A,r
CONDUCTOR SIZE ts,d,Io
TOUCH & STEP CRITERIA Etouch70, Estep70
INITIAL DESIGN D,n,LT,h
GRID RESISTANCE Rg,LT MODIFY DESIGN D,n,LT,LR
GRID CURRENT IG
YES GPR < Etouch
NO MESH & STEP VOLTAGES Em,Es,Km,Ks,Ki,Kii
NO Em
YES
NO Es
YES
DETAIL DESIGN
EARTHING CALCULATION
A
GENERAL DESIGN DATA
1
Soil Resistivity,
2
r
:
600 Ohm-M
Gravel Resistivity, r s
:
2000 Ohm-M
3
Symmetrical Short Circuit Current, Iefs
:
4
Duration of Earth Fault Current, ts
:
0.3 Sec
5
Design Ambient Temperature
:
50 ° C
6
Thickness of Crushed Gravel, hs
:
0.3 m
7
Depth of Earth Grid, h
:
0.8 m
8
Reference depth of the Grid, h o
9
Standard Used
B
SIZE OF EARTHING CONDUCTOR :
Amm 2 =
50000 A
1 m IEEE - 80 : 2000
I TCAP 10 t ca r r r
4
K 0 Tm ln K T a 0
Eqn.: 40 Page : 43 IEEE Std. 80 - 2000
Where Material Proposed
ar
= Resistivity of Conductor Material
rr
= 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
Ko
= 1/a0 or 1/ar - Tr in °C
Iefs
= rms current in Ka
50 KA
= Duration of Current in s
0.3 Sec.
tc
0.0032 Ohm - M
40 °C 293
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) Diameter of equivalent Grid Conductor ,d
= 75 mm x 6 mm = =
450.0 mm² 0.03 m
EARTHING CALCULATION
TOUCH & STEP CRITERIA
K =
r rs r rs
Eqn. 21, Page 21, IEEE 80 2000
r 0 .09 1 r s Cs = 2 hs 0 .09
Eqn. 27, Page 23, IEEE 80 2000
K
= Reflection factor between different material resistivities
r
= Resistivity of the earth beneath the surface material in W.m
rs
= Surface material resistivity in W.m
hs
= Thickness of the surface material in m
Cs
= Surface layer derating factor
Estep70 = (1000 6Cs rs )
0.157 ts
K
=
-0.54
Cs
=
0.91
Eqn. 30, Page 27, IEEE 80 2000
Where
Estep 70
= Step Voltage for body weight of 70 kg
Estep 70 Etouch70 = (10001.5Cs rs ) Where
Etouch70
=
3412.28 Volts
Eqn. 33, Page 27, IEEE 80 2000
0.157 ts
= Touch Voltage for body weight of 70 kg
Etouch70
=
1068.05 Volts
EARTHING CALCULATION
C
INITIAL DESIGN ASSUMPTIONS Length 450
Breadth 250
Preliminary Layout of Grid
=
n
= Number of parallel conductors
=
15
D
= Conductor Spacing
=
30 m
h
= Depth of grid burial
=
0.8 m
LT = 2 Na L Lp
= Length of the conductor across perimeter
=
1800 m
Nr
= No. of Ground Rods
=
33
Lr
= Length of Ground Rods
=
3 m
LR
= Total length of Ground Rods
=
99 m
LC
= Total length of buried condcutor
=
8200 m
LT
= Total length of buried conductors & rods
=
8299 m
Lx
= Maximum length of conductor in X-Axis
=
450
Ly
= Maximum length of conductor in Y-Axis
=
250
D
GRID RESISTANCE
=
112500 m²
1 1 1 Rg = r 1 20A 1 h 20/ A LT Where A
= Area of the Grid
Rg
= Grid Resistance =
Rg
E
0.87 W
MAXIMUM GRID CURRENT
IG = D f I g Where Ig Df
= Maximum grid current in A
50000 A
= Decrement factor for the entire duration of fault, given in s
IG
F
0.6
=
30000 A
GROUND POTENTIAL RISE
GPR = I G Rg GPR
=
26042.3 V
EARTHING CALCULATION GPR 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
Emesh ( Design ) =
Ki
r IG K m K i Lr L C 1 . 55 1 . 22 L2 l 2 x Ly
L R
Eqn. 80, Page 91, IEEE 80, 2000
= Corrective factor for current irregularity
K
i
= 0 . 644
0 . 148
n
Where
n = n
n
a
=
a
n
b
n
c
n
d
2 LT LP na
=
9.1
nb
= 1 for square grids
=
1
nc
= 1 for square and rectangular grids
=
1
nd
= 1 for square, rectangular and L-shaped grids
=
1
n Ki Km
=
9.11
=
1.99
= Spacing factor for Mesh Voltage Km =
Eqn. 68 Page 113 IEEE 80
D2 D 2 h 2 h Kii ln 1 8 ln 2 8 Dd 4 d Kh 2 n 1 16 hd
Where
Kii
= Corrective wieghting factor that adjusts the effect of inner conductors on the corner mesh
K ii =
1 2
(2 n) n
Eqn. 81, Page 93 IEEE 80, 2000
EARTHING CALCULATION
Kh
Kii
=
0.64
Kii
=
1.00 With Rods
= Corrective weighting factor that empasising the grid depth
1
=
h ho
Where
ho
= Reference depth of grid
=
1
h
= Depth of the ground grid conductor
=
0.8
Kh
=
1.34
Km
=
0.96
Emesh (Design)
=
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
Estep ( Design ) =
Ks
Ki r I G 0 . 75 L C 0 . 85 L R
Eqn. 92, Page 94 IEEE 80, 2000
Where
Ks
= Spacing factor for Step voltage Ks
1
=
1 1 0 .5 1 D h D 2 h
n 2
Ks K
i
= 0 . 644
0 . 148
=
0.220
=
1.992444444
n
Ki
Estep (Design )
Estep (Design )
Eqn. 94, Page 94, IEEE 80, 2000
=
=
Calculated Step Voltage is Lower than the Tolerable Step Voltage.HENCE SAFE
1265.62 Volts
1266 Volts
EARTHING CALCULATION SUMMARY
A
EARTH GRID CONDUCTOR Type of Conductor
B
C
Zinc-coated steel rod
Size of Conductor
450
mm
Length of Conductor
8200
mtr
Depth of Conductor
0.8
m below GL
Total Length of Ground Rods
99
mtr
Length of Individual Ground Rods
3
mtr
No. of Ground Rods
33
No.
GROUND RODS
HUMAN SAFETY
Step Voltage Mesh Voltage
UNIT
Designed Value
Permissible Value
Volt
3412
1266
Volt
1068.05
4104
TABLE 1 - MATERIAL CONSTANTS Material Conductivity (%)
Description
Copper annealed 1 soft - drawn
2 3 4 5 6 7 8 9 10
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 Steel, 1020
ar factor
K 0 at
at 20°C
(0°C)
Fusing Temperature Tm (°C)
1
100
1
0.00393
1
234
1
1083
1
2
97
2
0.00381
2
242
2
1084
2
3
40
3
0.00378
3
245
3
1084
3
4
30
4
0.00378
4
245
4
700
4
5
20
5
0.00378
5
245
5
1084
5
6
61
6
0.00403
6
228
6
657
6
7
53.5
7
0.00353
7
263
7
652
7
8
52.5
8
0.00347
8
268
8
654
8
9
20.3
9
0.0036
9
258
9
657
9
10 Stainless - clad steel 11 rod 11 Zinc-coated steel 12 rod 12 13 Stanless steel, 304 13
10.8 10
0.0016 10
605 10
1510 10
9.8 11
0.0016 11
605 11
1400 11
8.6 12 2.4 13
0.0032 12 0.0013 13
293 12 749 13
419 12 1400 13
TCAP Thermal Capacity [J/(cm³.°C]
rr 20°C(mW. cm)
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 72 13
3.93 4.03
