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 System Calculation
IEEE Std 80
A GENERAL DESIGN DATA
1 Soil Resistivity, ρ : 100 Ohm-M
2 Symmetrical Short Circuit Current, Iefs : 60000 A
3 Duration of Earth Fault Current, ts : 0.5 Sec
4 Design Ambient Temperature : 40 ° C
5 Depth of Earth Grid, h : 0.6 mtr.
STANDARDS USED
IEEE Gude for Safety in AC Substation
Grounding IEEE - 80 2000
B SIZE OF EARTHING CONDUCTOR :
I
Amm2 =
√(
Eqn.: 40 Page : 43
) ( )
TCAP×10−4 K 0 +T m
ln IEEE Std. 80 - 2000
tc αr ρr K 0 +T a
Where
Material Proposed Copper annealed soft - drawn
αr = Resistivity of Conductor Material 0.00393 Ohm - M
ρr = Thermal co-efficient of resistivity at reference temperature Tr 1.72
Tm = Max. allowable temperature in °C 1083 °C
Ta = Ambient temperature in °C 40 °C
Ko = 1/a0 or 1/ar - Tr in °C 234
Iefs = rms current in Ka 60 KA
tc = Duration of Current in s 0.5 Sec.
TCAP = thermal capacity per unit volume from Table 1 3.42 J/(cm³°C)
Amm² = Conductor cross section in mm² 150.53 mm²
A kcmil = 297.16 kcmil
Earthing System Calculation
IEEE Std 80
A kcmil
Amm² = 150.53 mm²
THE SIZE OF CONDUCTOR SELECTED = 13.8 mm
Diameter of the Grid Conductor ,d = 0.01385 mtr.
Rounded of = 0.01 mtr.
C INITIAL DESIGN ASSUMPTIONS
Length Breadth
Preliminary Layout of Grid = 12 3
D = Conductor Spacing = 3 m
h = Depth of grid burial = 0.6 m
LT =2× Na× L
Lp = Length of the conductor across perimeter = 48 m
Nr = No. of Ground Rods = 1
Lr = Length of Ground Rods = 3 m
LR = Total length of Ground Rods = 3 m
LT1 = Total length of buried condcutor = 39 m
LT = Total length of buried conductors & rods = 42 m
Lx = Maximum length of conductor in X-Axis = 12
Ly = Maximum length of conductor in Y-Axis = 3
D GRID RESISTANCE
R g =ρ
[ 1
+
1
LT √ 20 A
1+
( 1
1+h √ 20/ A )]
Where
A = Area of the Grid = 36 m²
Rg = Grid Resistance
Rg = 8.68 W
Earthing System Calculation
IEEE Std 80
EARTHING CALCULATION
SUMMARY
A EARTH GRID CONDUCTOR
Type of Conductor Copper annealed soft - drawn
Size of Conductor 151 mm
Length of Conductor 39 mtr
Depth of Conductor 0.6 m below GL
B GROUND RODS
Total Length of Ground Rods 3 mtr
Length of Individual Ground Rods 3 mtr
No. of Ground Rods 1 No.
C HUMAN SAFETY
UNIT Designed Value Permissible Value
Step Voltage Volt #REF! #REF!
Mesh Voltage Volt #REF! #REF!
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
2 hard - drawn 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
6 Grade 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
12 rod 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
