A) SYSTEM DATA & DESIGN INPUTS
Sr. No. Description Input /
Output Data
1 Lx: Length of subsoil ground grid : meter 640
2 L Y : Breadth of subsoil ground grid : meter 500
3 A : Total area enclosed by subsoil ground grid :meter² 320000
4 ρ: Soil resistivity : Ohm. Meter 125
If =I : Symmetrical single line to ground fault current for
5 39.6
calculating conductor size of subsoil ground grid : KA
tC : Duration of fault current for calculating conductor size
6 0.5
of subsoil ground grid : Sec
7 αr : Thermal co-efficient of resistivity at reference 0.00393
temperature of 20OC of subsoil round grid conductor
material : OC
8 ρr : Resistivity of subsoil ground grid conductor material 1.72
at reference temperature of 20 C : Micro Ohm.Cm
O
9 TCAP : Thermal capacity per unit volume : J/(cm 3.OC) 3.42
10 Tm : Maximum allowable temperature for subsoil ground 1084
grid conductor material suitable for exothermic joints : OC
11 Ta : Design ground temperature : OC 40
K0 : At αr = 0.00393 for copper annealed soft-drown at
12 234
reference temperature of 20oC
13 3000
ρs : Surface layer wet concrete resistivity : Ohm. Meter
14 hS : Surface layer thickness : meter 0.1
tS : Duration of shock for determining allowable body
15 0.5
current : sec
Df : Decrement factor for en re dura on of fault of 30
16 1.065
Cycles (0.5 Sec) & X/R ratio equal 10
17 Sf : Fault current division factor 0.6
18 DX : Spacing between parallel conductors of subsoil 10
ground Grid on longer side : meter
19 DY : Spacing between parallel conductors of subsoil 10
ground Grid on shorter side : meter
24 h : depth of subsoil ground grid conductors: meter 0.5
B) CALCULATION
B.1 Calculation of Conductor Size of Subsoil Ground Grid
99.33
Ac : calculated cross section area : mm2
1
Ac : calculated cross section area : mm2
1
2 Ac : Selected cross section area : mm2 70
3 d : Diameter of subsoil ground grid conductor : meter 0.0094
B.2 Calculation for Allowable Step and Touch Voltages
1 K : the reflection factor between different material -0.9200
resistivities
2 CS : the surface layer derating factor 0.7026
3 Estep50 : allowable step voltage : V 2238.700
4 Etouch50 : allowable touch voltage : V 682.712
B.3 Initial Subsoil Ground Grid Design
NX : No. of subsoil ground grid conductor on longer
1 1
side Length of subsoil ground grid
NY : No. of subsoil ground grid conductor on shorter side
2 51
Breadth of subsoil ground grid
3 LC : Total length of subsoil grounding Conductor : meter 33140
4 No of rods 5
5 Length of each rod: meter 3
6 LR : Length of Total grounding rods: meter 15
7 LT : Effective length of LC + L R for mesh 33155
voltage, meter
B.4 Resistance of Subsoil Ground Grid System
1 Rg : Grounding grid resistance: Ohm 0.10
B.5 Maximum Grid Current (IG)
1 25304.4
IG : Grounding fault current: Amps
B.6 Ground Potential Rise (GPR)
1 GPR : grounding potential rise : V 2591.09
B.7 Actual Step & Mesh Voltage to the tolerable step and touch voltages
Es(%) = Actual step voltage (% of tolerable step voltage) 81.1
Em(%) = Actual touch voltage (% of tolerable touch 24.7
voltage)
References
A= L X. L Y
As per IEEE 80 2000 Clause 11.3.3
As per Std. IEEE 80-2000 Table – 1
As per Std. IEEE 80-2000 Table – 1
As per Std. IEEE 80-2000 Table – 1
As per Std. IEEE 80-2000 Table – 1
As per Std. IEEE 80-2000 Page 44
& Note.3 Page 48
Also Refer to Example.1 given in Annex B
of IEEE- 80 - 2000
As per Std. IEEE 80-2000 Table – 1
As per Std. IEEE 80-2000 Table – 7
As per Std. IEEE 80-2000 Clause 16.2.2
Refer sample calculation given in Annex B of IEEE-
80 - 2000
As per Std. IEEE 80-2000 Table – 10
As per Std. IEEE 80-2000 Clause 16.7
As per Std. IEEE 80-2000 Clause 16.7
Eq. 40 of Std. IEEE 80-2000, Page 43
Ac = I x tc.αr. ρr x 104/TCAP
ln ( K0 + Tm)/(K0 + Ta)
49.664392289
(2 x SQRT (Ac /pi ) )/1000
≈ (LY/DY)+1
LC = (N X . L y )+ (N y . L x )
= No of rods x Length of each rod
Eq. 52 of Std. IEEE 80-2000
Rg <= 1 Ohm
Eq. 64 & 65 of Std. IEEE 80-2000
IG = Df . If . Sf
GPR > E touch50
GPR = I G . Rg
Eq. (B.12) of Std. IEEE 80-2000
