EARTHING CALCULATIONS
Reference : ANSI/IEEE Std 80 2000
Earth grid conductor size
Earth grid material = MS
I = RMS value of fault current = 40 kA
tc = Time of fault current flow = 1 sec
TCAP = Thermal capacity factor = 3.28 J/cm3/0C (Table 1)
a0 = Thermal coefficient of resistivity at reference temperature 0 0C
ar = Thermal coefficient of resistivity at reference temperature 20 0C = 0.0016 (Table 1)
rr = Resistivity of ground conductor at reference temperature 20 0C = 15.9 mo/cm3 (Table 1)
Tm = Maximum allowable temperature = 1510 C
0
Ta = Ambient temperature = 45 C
0
K0 = 1/a0 = 605 (Table 1)
A = Earth grid conductor area
I
= (Equation 40)
TCAP x 10-4 ln K0 + Tm
tc x ar x rr K0 + Ta
= 324.317574 mm2
Corrosion factor considered = 1.15
Hence A = 372.96521 mm2
Earth grid conductor Shape = Rectangle
Earth grid conductor selected = 75 mm X 10 mm = 750 mm2
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Step potential and touch potential
Tolerable step potential and touch potential
r = Soil resistivity = 38.744 om
rs = Resistivity of surface material = 3000 om
hs = Depth of gravel = 0.15 m
Cs = Derating factor for surface gravel resistivity = 0.7722111 (Equation 27)
ts = Fault clearing time = 1 sec
(1000 + 6Csrs) x 0.116
Estep = Tolerable step potential = ts = 2339.2685 V (Equation 29)
(1000 + 1.5Csrs) x 0.116
Etouch = Tolerable touch potential = ts = 702.56713 V (Equation 32)
Actual step potential and touch potential
D = Conductor spacing = 5 m
h = Depth of ground grid conductors = 0.6 m
d = Diameter of equivalent area of grid conductor = 0.0338514 m
Kii = Corrective weighting factor for earth grid placement = 1
(for grids with ground rod conductors along perimeter and throughout grid area)
L = Length of the switchyard = 182.77 m
B = Breadth of the switchyard = 180.00 m
EN = Number of earth electrodes = 71 Nos
EL = Length of earth electrodes = 3 m
2
h0 = Reference depth of grid = 1 m
Kh = Corrective weighting factor for grid depth = 1 + h/ho = 1.26491106 (Equation 83)
hA = Number of parallel conductors lengthwise = B/D +1 = 36
hB = Number of parallel conductors breadthwise = L/D+1 = 39
h1 = Number of parallel conductors in one direction for calculating Em
= hA hB = 37.469988 (Equation 79)
h2 = Number of parallel conductors in one direction for calculating Es
= max(hA , hB) = 39 (Equation 80)
LL = Length of grid conductors along length of switchyard = L x hA = 6579.72 m
LB = Length of grid conductors along breadth of switchyard = B x hB = 7020 m
LA = Length of equipment risers and auxiliary mat in switchyard = 1650 m
LE = Length of earth electrodes in switchyard = EN x EL = 213 m
LT = Total length of earth conductors in switchyard = LL+LB+LA+LE = 15462.72 m
Km = Spacing factor for mesh voltage =
1 ln D2 (D + 2h )2 h Kii ln 8 (Equation 81)
2p 16hd 8Dd 4d Kh p(2h1-1)
= 0.3106
Ks = Spacing factor for step voltage =
1 1 1 1 1-0.5h2-2 (Equation 94)
p 2h D+h D
= 0.386
3
Kim = Corrected factor for grid geometry for calculating Em = 0.644 + 0.148h1 (Equation 89)
= 6.18955822
Kis = Corrected factor for grid geometry for calculating Es = 0.644 + 0.148h2 (Equation 89)
= 6.416
Df = Decrement factor for fault duration time =
( for fault duration time more than 0.5sec) 1
Cp = Corrective projection factor for future growth = 1
(for zero growth in future)
Sf = current division factor = 0.5
IG = Maximum grid current = Cp Df Sf I = 20 kA (Equation 64)
r IG Km Kim
Em = Actual touch potential = LL + LB + LA + 1.15LE = 96.142016 V (Equation 80)
r IG Ks Kis
Es = Actual step potential = LL + LB + LA + 1.15LE = 123.852216 V (Equation 92)
We find that Em is less than Etouch and Es is less than Estep
Ground resistance
As = Area of switchyard = L x B = 32898.6 m2
1 1 1 1
Ground resistance = Rg = LT r 20As 1+h 20/As (Equation 52)
= 0.0999 ohm
We find that Rg is less than 1 ohm
