DESIGN CALCULATION FOR EARTHING SYSTEM

1.0 SCOPE

The intent of this document is to define the basis of design, sizing and requirements of safety earthing system for
M/s Cargill

2.0 REFERENCE STANDARDS

The entire earthing system will be designed in accordance with Indian Standard IS-3043.

3.0 BASIC DESIGN DATA AND ASSUMPTIONS

3.1 Highest earth fault current on 0.415 kV system is considered in subsequent

calculations. = 44 kA ref. fault cal.

3.2 Duration of fault current = 1.0 Sec.

3.3 Average Resistivity of soil (Considered) = 60 Ω Meter

3.4 Size and Material of earth electrodes. = 40 mmdia,Cu

3.5 Length of Earth electrode = 3 Meters Long

3.6 Material of main earth grid conductor. = G.I.

3.7 The earthing system shall be designed for life expectancy of = 25 Years

3.8 If resistivity of soil is between 25 - 100 ohm meter, corrosion allowance is = 15 %

to be considered 15 % for G.I. and 30 % for M.S. (as per CBIP manual)

3.9 Length of main earth grid strip, buried under ground at shop (plant) area. = 180 Metres

3.10 Depth of burial of conductor = 100 cm

4.0 DETERMINATION OF SIZE OF EARTH GRID CONDUCTOR

4.1 In accordance with IS 3043 Clause No. 12.2.2.1, the cross-sectional area shall be so calculated
that the current density value determined by the following formula is not exceeded.

I 1
--------------- =K ---------------
S √t

Where:

Cross-sectional area of the grid conductor in sq. mm = S

Value of (ac rms) fault current of negligible impedance, which can = I i.e. 44 kA
flow through the protective device, in amperes

Fault current duration for conductor selection. = t i.e.1 Sec

As per table 6.6A of IS 3043 for G.I. = K 80

Therefore,

44000 1
--------------- = 80 --------------- sq. mm
S √1

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 S = 44000 X √1
------------------------------------- sq. mm
80

44000
S = --------------- sq. mm
80

S = 550 sq. mm

4.1 From above formula, the area of cross-section required for the main grid works out to 550 sq. mm

4.1.1 As specified in above mension assumptions, the earthing system shall be designed for 25 years and minimum rate
of corrosion of steel for selection of earthing conductor considered 15 %

4.1.2 After corrosion factor considered earth grid size worked out to = 633 sq. mm

4.1.3 Therefore, main earth grid conductor in mm have been selected to get =
required surface area for dissipation of fault current. 1 75 8 mm
( Numbers x Width x Thickness)
Area 600 Sqmm

5.0 DETERMINATION OF NUMBER OF EARTH ELECTRODES

In accordance with clause No. 10.3 (b) of IS 3043, maximum current density at the earth electrode surface is given
by the following formula:

7.57 x 10³
I = ----------- Amp / M2
rt

Where:

Current Density at the surface of an earth electrode. = I

Duration of the earth fault (in sec.) = t

Resistivity of the soil (in W m) = r

Therefore,

7.57 x 10³
= --------------------------- Amp / M2
 x 1

= 977.28 Amp / M2

In order to dissipate fault current of 44 kA, total area of electrodes required is worked out as under:

If
Area = ------------- sq. M
I

Where:

Fault current = If

Current density at the surface in Amp / M2 = I

Therefore,

44000
Area = -------------------------- sq. M
977.28

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 = 45.02 sq. M

Say = 45 sq. M

Following electrodes of 40 mm Ø will be provided at the shop (plant) & sub station area

Qty. of
Equipment / Area Name
Electrodes
Main Meter CT Earthing 3
Main Meter PT Earthing 3
Main Meter Kiosk Body Earthing 2
HT Breaker 2
Tranformer Body 2
Tranformer Neutral 2
PCC, Utility Panel, RTCC Panel 4
MLDB, MPDB, PDBs, LDBs, Fire fighting Panel 2
Server 1
UPS 2
Structural 2
Process and Utiity Load 2

Instrument system 1

---------------
Total Nos. of Electrodes 28 Nos

Area of Earth electrode (without perforations) = xDxL

Considering 5% reduction in the gross area of pipe electrode due to perforations and also tapering of electrode
at bottom, net area of each electrode for dissipation of heat is work out as follows

Net area of earth electrode

=  x D x L x 0.95
-6
=  x 40 x 3000 x 0.95 x 10

= 0.358 M²

Therefore, Area of 28 Nos electrodes worked out as follows

= 10.03 M²

In addition to above, there will be mesh of 75 x 8 mm strip around the process area (plant) area & also
a loop of 75 x 8 mm strip connecting all the pipe electrodes in & around the process (Plant) area.
It is estimated that about 180 M of strip will be laid under ground, which will also act as earth electrode.

The total area of 75 x 8mm strip in contact with earth (both sides)

= width of main earth grid strip x Total length of main earth grid x 2

= 0.083 x 180 x 2 M²

= 29.88 M²

Total area for heat dissipation works out to

= Area of total earth electrodes + Area of earth strip

= 39.91 M²

Say 40 M²

Therefore Total area for heat dissipation works out to 40 M², which is more than the required area of
45 M² as work out earlier.

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6.0 CALCULATION FOR EARTH RESISTANCE OF PIPE EARTH ELECTRODE

The earth resistance of pipe earth electrode is calculated based on the following formula is given in
IS -3043 clause No. 9.2.2

100 r 4L
R = ---------------------------- loge ------------------- W
2  L d

Where:

Resistivity of soil in W m = r

Length of pipe in cm = L

Diameter of pipe in cm = d

100 x 60 4 x 300
R = ----------------------------------- loge --------------- W
2  300 4

R = 3.18 loge 300 W

R = 18.16 W

Combined resistance of total numbers of earth electrodes

Resistance of induvisual earth electrode

R = ---------------------------------------------------------- W
Total numbers of earth electrodes

18.156
R = W
-----------------------------
28

R = 0.6484 W

7.0 CALCULATION FOR EARTH RESISTANCE FOR STRIP ELECTRODE

The earth resistance of strip electrode is calculated based on the following formula is given in IS-3043 clause
No. 9.2.3

100 r 2 L²
R = ---------------------------- loge --------------- W
2  L wt

Where

Resistivity of soil in W m = r

Length of strip in cm = L

depth of burial in cm = w

width of strip in cm = t

100 x 60 2 x (180x 10²)²

R = --------------------------------
--------------- loge W
--------------------------------
2  180 x 10² 100 x 7.5

0.053051648 loge 38400000

R =
R = 0.926 W

8.0 COMBINED EARTH RESISTANCE

Resistance of pipe electrodes = 0.6484 W

Resistance of strip electrodes = 0.92647106 W

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Therefore Combined Resistance

0.6484 x 0.926471
R = ---------------------------------------------------------- W
0.6484 + 0.926471

R = 0.38 W

Above figure is less than 1.0 W as stipulated in IS-3043. Design calculation for earthing is suitable from view
point of combined resistance, the same is acceptable.

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