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<Etouch
YES
NO
Es<Estep
YES
DETAIL
DESIGN
EARTHING CALCULATION
A GENERAL DESIGN DATA
1 Soil Resistivity, : 100 Ohm-M
2 Gravel Resistivity, s : 2500 Ohm-M
3 Symmetrical Short Circuit Current, Iefs : 13450 A
4 Duration of Earth Fault Current, ts : 0.5 Sec
5 Maximum Allowable Conductor Temp. : 700 ° C
6 Design Ambient Temperature : 40 ° C
7 Thickness of Crushed Gravel, hs : 0.102 mtr.
8 Depth of Earth Grid, h : 0.5 mtr.
9 Reference depth of the Grid, ho 1 mtr.
STANDARDS USED
IEEE Gude for Safety in AC Substation
Grounding IEEE - 80 2000
B SIZE OF EARTHING CONDUCTOR :
I
Amm 2
TCAP 10 4
K 0 Tm Eqn.: 40 Page : 43
ln IEEE Std. 80 - 2000
t K T
c r r 0 a
Where
Material Proposed
r = Resistivity of Conductor Material 0.00378 Ohm - M
r = Thermal co-efficient of resistivity at reference temperature 5.86
Tm = Max. allowable temperature in °C 700 °C
Ta = Ambient temperature in °C 40 °C
Ko = 1/a0 or 1/ar - Tr in °C 245
Iefs = rms current in Ka 13.45 KA
EARTHING CALCULATION
tc = Duration of Current in s 0.5 Sec.
TCAP = thermal capacity per unit volume from Table 1 3.85 J/(cm³°C)
Amm² = Conductor cross section in mm² 65.89 mm²
A kcmil
= 130.07 kcmil
Amm² = 65.89 mm²
THE SIZE OF CONDUCTOR SELECTED = 9.2 mm
Diameter of the Grid Conductor ,d = 0.00916 mtr.
Rounded of = 0.01 mtr.
TOUCH & STEP CRITERIA
s Eqn. 21, Page 21, IEEE 80 2000
K
s
0 .09 1 Eqn. 27, Page 23, IEEE 80 2000
s
Cs
2 hs 0 .09
K = Reflection factor between different material resistivities
= Resistivity of the earth beneath the surface material in W.m
s = Surface material resistivity in W.m
hs = Thickness of the surface material in m
Cs = Surface layer derating factor
K = -0.92
Cs = 0.71
0.157
Estep70 (1000 6Cs s ) Eqn. 30, Page 27, IEEE 80 2000
ts
Where
Estep 70 = Step Voltage for body weight of 70 kg
E step 70 = 2573.75 Volts
EARTHING CALCULATION
E step 70
0.157 Eqn. 33, Page 27, IEEE 80 2000
Etouch70 (10001.5Cs s )
Where ts
Etouch70 = Touch Voltage for body weight of 70 kg
Etouch70 = 809.96 Volts
C INITIAL DESIGN ASSUMPTIONS
Length Breadth
Preliminary Layout of Grid = 70 70
n = Number of parallel conductors = 14
D = Conductor Spacing = 5m
h = Depth of grid burial = 0.5 m
LT 2 Na L
Lp = Length of the conductor across perimeter = 280 m
Nr = No. of Ground Rods = 10
Lr = Length of Ground Rods = 7.5 m
LR = Total length of Ground Rods = 75 m
LT1 = Total length of buried condcutor = 2100 m
LT = Total length of buried conductors & rods = 2175 m
Lx = Maximum length of conductor in X-Axis = 70
Ly = Maximum length of conductor in Y-Axis = 70
D GRID RESISTANCE
1 1 1
Rg 1
LT 20A 1 h 20 / A
Where
A = Area of the Grid = 4900 m²
Rg = Grid Resistance
Rg = 0.67 W
EARTHING CALCULATION
Rg
E MAXIMUM GRID CURRENT
IG D f I g
Where
Ig = Maximum grid current in A 13450 A
Df = Decrement factor for the entire duration of fault, given in s 0.7
IG = 9415 A
F GROUND POTENTIAL RISE
GPR I G Rg
GPR = 6354.8 V
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
IG K m K i
Emesh ( Design ) Eqn. 80, Page 91,
Lr
L C 1 . 55 1 . 22 L
IEEE 80, 2000
L2x l Ly
2 R
Ki = Corrective factor for current irregu-
larity
K i 0 . 644 0 . 148 n
Where
n n a n b n c n d
2 LT
n a
LP
EARTHING CALCULATION
2 LT
n a
LP
na = 15.0
nb = 1 for square grids = 1
nc = 1 for square and rectangular grids = 1
nd = 1 for square, rectangular and L-shaped g = 1
n = 15.00
Ki = 2.86
Km = Spacing factor for Mesh Voltage Eqn. 68 Page 113 IEEE 80
Eqn. 81, Page 93
Km
1 D2
ln
D 2 h 2 h Kii ln 8
IEEE 80, 2000
2 16 hd 8 Dd 4 d Kh 2 n 1
Where
Kii = Corrective wieghting factor that adjusts the effect of inner conductors on the corner mesh
1
K ii 2
(2 n) n
Kii = 0.62
Kii = 1.00 With Rods
Kh = Corrective weighting factor that empasising the grid depth
h
= 1
ho
Where
ho = Reference depth of grid = 1
h = Depth of the ground grid conductor = 0.5
Kh = 1.22
Km = 0.65
Emesh (Design ) = 786.11 Volts
EARTHING CALCULATION
Emesh (Design )
Calculated Mesh Voltage is Lower than the Tolerable Touch Voltage.HENCE SAFE
B Step Voltage
Voltage developed for step as per the earthing system proposed during full Earth fault current
Estep ( Design )
Ks Ki I G Eqn. 92, Page 94
0 . 75 L C 0 . 85 L R IEEE 80, 2000
Where
Ks = Spacing factor for Step voltage
1 1 1 1 0 .5 n 2
Ks Eqn. 94, Page 94, IEEE 80, 2000
2 h D h D
Ks = 0.440
K i 0 . 644 0 . 148 n
Ki = 2.864
Estep (Design ) = 724.08 Volts
Estep (Design ) = 724 Volts
Calculated Step Voltage is Lower than the Tolerable Step Voltage.HENCE SAFE
EARTHING CALCULATION
SUMMARY
A EARTH GRID CONDUCTOR
Type of Conductor Copper-clad steel wire
Size of Conductor 9 mm
Length of Conductor 2100 mtr
Depth of Conductor 0.5 m below GL
B GROUND RODS
Total Length of Ground Rods 75 mtr
Length of Individual Ground Rods 7.5 mtr
No. of Ground Rods 10 No.
C HUMAN SAFETY
UNIT Designed Value Permissible Value
Step Voltage
Volt 2574 724
Mesh Voltage
Volt 809.96 786
TABLE 1 - MATERIAL CONSTANTS
Material Fusing
Description Conductivity ar factor K0 at Temperature
(%) at 20°C (0°C) Tm (°C)
Copper annealed
1 soft - drawn 1 100 1 0.00393 1 234 1 1083 1
Copper, commercial
hard - drawn
2 2 97 2 0.00381 2 242 2 1084 2
Copper-clad steel
3 wire 3 40 3 0.00378 3 245 3 1084 3
Copper-clad steel
4 wire 4 30 4 0.00378 4 245 4 700 4
Copper-clad steel
5 rod 5 20 5 0.00378 5 245 5 1084 5
Aluminium EC Grade
6 6 61 6 0.00403 6 228 6 657 6
Aluminium 5005
7 alloy 7 53.5 7 0.00353 7 263 7 652 7
Aluminium 6201
8 alloy 8 52.5 8 0.00347 8 268 8 654 8
Aluminium-clad steel
9 wire 9 20.3 9 0.0036 9 258 9 657 9
Steel, 1020
10 10 10.8 10 0.0016 10 605 10 1510 10
Stainless - clad steel
11 rod 11 9.8 11 0.0016 11 605 11 1400 11
Zinc-coated steel rod
12 12 8.6 12 0.0032 12 293 12 419 12
13 Stanless steel, 304 13 2.4 13 0.0013 13 749 13 1400 13
TCAP
rr Thermal
20°C(mW. Capacity
cm) [J/(cm³.°C]
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 3.93
72 13 4.03
