FIELD DATA A,r
CONDUCTOR SIZE ts,d,Io
TOUCH & STEP CRITERIA Etouch70, Estep70 INITIAL DESIGN D,n,LT,h GRID RESISTANCE Rg,LT GRID CURRENT IG GPR < Etouch
MESH & STEP VOLTAGES Em,Es,Km,Ks,Ki,Kii Es<Estep Em<Etouch DETAIL DESIGN MODIFY DESIGN D,n,LT,LR YES YES YES NO NO NO
A GENERAL DESIGN DATA
1 Soil Resistivity, : 600 Ohm-M
2 Gravel Resistivity, : 2000 Ohm-M
3 Symmetrical Short Circuit Current, Iefs : 50000 A
4 Duration of Earth Fault Current, ts : 0.3 Sec
5 Design Ambient Temperature : 50 ° C
6 Thickness of Crushed Gravel, : 0.3 m
7 Depth of Earth Grid, h : 0.8 m
8 Reference depth of the Grid,
h
o 1 m9 Standard Used IEEE - 80 : 2000
B SIZE OF EARTHING CONDUCTOR :
Eqn.: 40 Page : 43 IEEE 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 = 1/a0 or 1/ar - Tr in °C 293
Iefs = rms current in Ka 50 KA
tc = 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 Diameter of equivalent Grid Conductor ,d = 0.03 m
=
a m r r cK
T
T
K
t
TCAP
I
Amm
0 0 4 2ln
10
r
a
r r r a s rr
hsEqn. 21, Page 21, IEEE 80 2000
Eqn. 27, Page 23, IEEE 80 2000
= Reflection factor between different material resistivities = Resistivity of the earth beneath the surface material in W.m = Surface material resistivity in W.m
= 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 TOUCH & STEP CRITERIA
s s
K
r
r
r
r
=
Kr
s r K09
.
0
2
1
09
.
0
=
hs
C
s sr
r
s C hs s C s s s stept
C
E
70=
(
1000
6
r
)
0
.
157
70 stepE
70 stepE
s s s toucht
C
E
70=
(
1000
1
.
5
r
)
0
.
157
70 touchE
70 touchE
C INITIAL DESIGN ASSUMPTIONS
Length Breadth
Preliminary Layout of Grid = 450 250
n = Number of parallel conductors = 15
D = Conductor Spacing = 30 m
h = Depth of grid burial = 0.8 m
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
Where
A = Area of the Grid = 112500 m²
= Grid Resistance
= 0.87W
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
L
Na
L
T=
2
=
A
h
A
L
R
T g/
20
1
1
1
20
1
1
r
g R g R g I g f GD
I
I
=
f D G I g GR
I
GPR
=
GPRThe 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)
A Mesh Voltage
Eqn. 80, Page 91, IEEE 80, 2000
= Corrective factor for current 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 93 IEEE 80, 2000
Where
= Corrective wieghting factor that adjusts the effect of inner conductors on the corner mesh CALCULATION FOR ACTUAL DERIVED STEP & MESH VOLTAGE
VERIFICATION FOR HUMAN SAFETY
R Ly x r C i m G
L
l
L
L
L
K
K
I
Design
Emesh
=
2 222
.
1
55
.
1
)
(
r
Ki
Km
= 1 2 8 ln 4 8 2 16 ln 2 1 2 2 n Kh Kii d h Dd h D hd D KmKii
GPR n iin
K
2)
2
(
1
=
n K i = 0.644 0.148 d c b an
n
n
n
n
=
P T a L L n = 2 a n b n c n d nn
Ki
= 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 94 IEEE 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
