Earthing Calculation
Uploaded by
Nipuna Thushara WijesekaraEarthing Calculation
Uploaded by
Nipuna Thushara WijesekaraCETHAR VESSELS LTD.
,
TIRUCHIRAPALLI 620 001
M/s. SHREE CEMENTS LTD.
1X15MW CO-GEN POWER PLANT
EARTHING CALCULATION
10.01.09
NIL
30.12.08
Rev.
Date
Category
Revision as per the
consultant comment
R.V. SUNIL N. BRINTON
Description
Prepared
Verified
J. ANAND
Approved
CUSTOMER
M/s. SHREE CEMENT LIMITED (1 X 13.3 MW)
W. O. No.
MNI005
PROJECT
M/s. SHREE CEMENT LIMITED (CO-GEN ), Unit3, 4 & 5 .
CONSULTANT
M/s. CETHAR CONSULTING ENGINEERS(P) LTD.,
DOC. No.
MNI005-ERC-DOC-1-001
Total No. Of Sheets
4 (Including Front Page)
POWER GROUP
DATE:
10.01.09
EARTHING DESIGN
Page 1 of 4
REV: A
CETHAR VESSELS LTD.,
TIRUCHIRAPALLI 620 001
M/s. SHREE CEMENTS LTD.
1X15MW CO-GEN POWER PLANT
1.0Design Basis :
a) Max. Fault current
50 KA for 1 Sec at 415V.
40 KA for 1 Sec at 11KV.
b) Duration of fault current
1 sec
c) Max. Earth resistivity ()
220 ohm- meter
d) Type of joint
Welded
e) Reference
IS 3043 1987/IEEE 80.
f) Type of conductor
Galvanized Iron
Considering 3 phase fault current = single line ground fault current
Hence 50KA ground fault current is considered for grid design.
2.0Determination of size of Earth Grid conductor
2.1 The size of Earth grid conductor is calculated as per IEEE 80.
1
Amm2 = I x
TCAP x 10-4
Ko + Tm
ln
tc x r x r
Ko + Ta
Tr - Reference temperature for material constants
= 20 C
Amm2 - Minimum conductor cross sectional area in mm2
r Thermal coefficient of Resistivity at reference temp. = 0.00423 1/C
Tm Fusing temperature at conductor for welded joints = 620 C
Ta Ambient temperature
= 50 C
Ko = (1/r)-Tr
= 216.41
r Resistivity of ground conductor at reference temp. = 15
TCAP thermal capacity per unit volume
= 3.749
tc Time duration for sizing of earth mat conductor
=1
I current for sizing earth mat conductor
= 50
50 x 1
Amm2 =
3.749 x 10-4
ln
1x0.00423x15
= 608.2 mm2
Page 2 of 4
216.4 + 620
216.4 + 50
CETHAR VESSELS LTD.,
TIRUCHIRAPALLI 620 001
M/s. SHREE CEMENTS LTD.
1X15MW CO-GEN POWER PLANT
Conductor selected is 75 x 10 mm GI flat whose area is 750 mm2 and it is sufficient
enough to withstand the fault current.
2.2 Design of Ground grid.
Total area covered = 22943.3 m2
Max. Spacing between grid conductor = 3 m
Estimated length of conductor = 598 m
2.3 Earth mat area
L Length of conductor buried in CPP area
= (4 X 32
(4 X 35.6
(2 X 92
(8 X 18
= 128) +
= 142.4)+
= 184 ) +
= 144)
= 598.4
H Depth of buried conductor in CPP area
= 0.6 m
A Area occupied by conductor buried in CPP area
= (35.6 X 32 = 1139.2)+
= (92 X 18 = 1656)
= 2800m2
Rg = ((1 / LT) + (1/Sqrt (20xA) x (1+1/ (1+h sqrt (20/A))
Rg = 220x ((1 / 598.4) + (1/sqrt (20x2800) x (1+1/ (1+0.6x sqrt (20/2800)
Rg = 220 x (0.00167 + 0.00422 x 1.95)
Rg = 2.17 Ohm
2.4 Resistance of earth electrode
Resistance (Re) is given by formula
Re = 1/n ((100 x /2x3.14xL) log (4L/d)
Re Resistance of earth electrode
n Number of earth pits
= 20
Soil resistivity
= 220 ohm-m
L Length of electrode
= 300 cm
d Diameter of electrode
= 5.08 cm
Re = 1/20 ((100 x 220/2x3.14x300) log (4x300/5.08)
Re = 11.67 x 2.373/20
Re = 1.38 ohm
Page 3 of 4
CETHAR VESSELS LTD.,
TIRUCHIRAPALLI 620 001
M/s. SHREE CEMENTS LTD.
1X15MW CO-GEN POWER PLANT
2.5 Determination of grid resistance by below formula
1/R = 1/Rg + 1/Re
Rg = 2.48 ohm
Re = 1.38 ohm
1
--R
1
------2.17
1
-----1.38
1
--R
0.46
0.72
1
--R
1.18
1
----------1.18
R = 0.847 ohm
Result
Since the grid resistance value is less than one ohm. Hence the selected earth electrode,
mat area and quantity of earth pit are sufficient for our application.
Note: We have not considered the TOUCH AND STEP POTENTIAL calculation since
The same calculation is required for Switch yard earthing only as per the IEEE 80.
Page 4 of 4