AC Earthing Design Calculation (HT PANEL & RMU PANEL)
INPUT DATA
1) Design Basis
i) Earth electrode is 80 mm dia copper bonded rod of 3 meter length maintainance free earth pits & main earth loop material
shall be GI strip.
ii) The earth conductor shall be adequately sized to carry the applicable maximum earth fault current.
2) Reference:
i) Code of Practice for Earthing ( IS: 3043: 1987).
ii))METL/190324/161 - ERT Report
ii) GI strip shall be of 1R x 70X10 mm.
3) Assumptions
i) Earthing conductor shall be buried under a depth of 300 mm from the ground level.
ii) Electrode shall be 80 mm dia, 3 M long copper bonded rod.
iii) Fault current clearing time 1 sec.
iv) Electrical Soil Resistivity for the plant area is considered as 428 Ohm-m. (Ref: ERT Report).
v) IDT Grid shall be connected to Inverter earthing Grid.
4) System Parameters
Sr No. Parameters Value Unit
1 Fault Current 25000.00 A From TS
0% Margine 25000.00 A
2 Mean Soil Resistivity, (ρ) 144.54 Ohm-m
3 Earthing Chemical metterial Resitivity(ρc)
4 Diameter of pipe/rod (d) 80 mm
5 Length of pipe/rod (L) 3000 mm
6 Buried Earthing Conductor (GI Flat) 70X10 mm
7 Depth of Buried Conductor, (ω) 300 mm
8 Width of the buried conductor (w) 70 mm
9 Thickness of buried conductor (ti) 10 mm
10 Number of Earthing strip runs 1 Nos
11 Total Length of earthing (lc) 30000 mm
12 Max Fault Current withstand time in seconds (t) 1 sec
Factor dependent on the material of the protective
80 A/mm²
13 conductor (K) for steel
14 Number of Electrodes 16 Nos
5) Calculation
i) To determine minimum size of earth conductor
a) Required cross section of Earthing Conductor
(As per Cl.12.2.2.1 of IS: 3043)
where, 𝐈 𝟏
I = fault current for which fault of negliable impedance can flow =𝐤×
through the device
𝐒 √𝐭
K= factor dependent on the material of the protective conductor,
the insulation and other parts, for 1 Sec A/mm2 : 80 for steel
T = time for fault clearing in sec.
S = Cross sectional area of earthing conductor in sq.mm
b) Required cross section of Earthing S= 312.5 mm² 312.5
10%
c) Corrosion Allowance
343.75 mm²
c) Number of earthing strips run 1 Nos
d) Selected cross sectional of earthing conductor 700 mm²
Hence, 1R x 70 x 10 Sq.mm GI. strips shall be used for main earthing grid.
ii) Calculation for the area required of Earth Electrode from Current density calculation
b) Permissible current density (I) 𝟕. 𝟓𝟕 × 𝟏𝟎³
Where,
𝒊=
I = Permissible current density (I)
√𝝆𝒕
t = Operating time of the disconnecting device in sec IS:3043
ρ = Soil resistivity (Ohm-m) I= 629.65 A/mm²
Fault Current
mm²
Current Density
Earth Electrode Area Required =
39.704 mm²
iii) Total Earth Electrode Area Considered for the plant
iii-a) Area of earth strip =2 x (Strip width + Strip Thickness) x Strip Length x No. of Runs
1 Length of 1R- 70X10 mm strip electrode considered for plant 300 m
2 No. of Run 1 Nos
3 Area of the strip electrode 48 m²
iii-b) Area of earth electrode = πDL
1 No of Earth electrode 16 Nos
2 Length of Electrode 3 m
3 Dia of Electrode( Copper Bonded Rod) in mtr 0.08 m
4 Area of each electrode 0.754 m²
5 Area of total electrode 12.064 m²
Total Combined Area of Strip and Electrode 60.064 m²
Note : The Combine area of electrode should be more than the required area of
39.704 m²
electrode
Hence required number of earth electrode and strip electrode selected is SAFE
iv) Calculation of Earth Resistance
a) Resistance of Total buried earthing conductor, R cond
As per IS: 3043
𝟏𝟎𝟎 𝝆 𝟒 𝒍𝒄
𝑹= 𝒍𝒏
where, 𝟐𝝅𝒍 𝒕
ρ = Soil resistivity (Ohm-m)
Ic = Length of earthing conductor in cm
t = Width of conductor in cm Rcond = 5.710 ohm
v) Calculation of Effective Resistance of Buried Electrode (Relec)
a) Resistance of Total buried earthing conductor, R elec
As per IS: 3043
𝟏𝟎𝟎 𝝆 𝟐 𝒍𝒓
𝑹= 𝒍𝒏
where, 𝟐𝝅𝒍 𝒅
ρ = Soil resistivity (Ohm-m)
Ir = Length of pipe in cm
d = Diameter of pipe in cm R = 33.11 ohm
Resistance considering 16 Nos. of Electrodes Relec = 2.07 ohm
vi) Calculation of Effective Earth Resistance for Solar Field
(Rcond x Relec ) ÷ (Rcond + Relec)
Effective Resistance, Reff
1.519 ohm
The effective resistance of the plant PV array area is less than 3 ohm which meets the effective
resistance of the plant as per clause No: 20.1 of IS:3043. Hence selected earthing conductor is
suitable for withstanding fault current & design is SAFE
AC Earthing Design Calculation (TRANSFOMER)
INPUT DATA
1) Design Basis
i) Earth electrode is 80 mm dia copper bonded rod of 3 meter length maintainance free earth pits & main earth loop material shall
be GI strip.
ii) The earth conductor shall be adequately sized to carry the applicable maximum earth fault current.
2) Reference:
i) Code of Practice for Earthing ( IS: 3043: 1987).
ii)METL/190324/161-ERT Report
ii) GI strip shall be of 1R x 50x6 mm.
3) Assumptions
i) Earthing conductor shall be buried under a depth of 300 mm from the ground level.
ii) Electrode shall be 80 mm dia, 3 M long copper bonded rod.
iii) Fault current clearing time 1 sec.
iv) Electrical Soil Resistivity for the plant area is considered as 144.54 Ohm-m. (Ref: ERT Report).
4) System Parameters
Sr No. Parameters Value Unit
1 Short Circuit Current 4200.00 A AS PER SC CALCULATION
10% Margine 4620.00 A
2 Mean Soil Resistivity, (ρ) 144.54 Ohm-m
3 Earthing chemcical resistivity((ρc) 0 Ohm-m
4 Diameter of pipe/rod (d) 80 mm
5 Length of pipe/rod (L) 3000 mm
6 Buried Earthing Conductor (GI Flat) 50x6 mm
7 Depth of Buried Conductor, (ω) 300 mm
8 Width of the buried conductor (w) 50 mm
9 Thickness of buried conductor (ti) 6 mm
10 Number of Earthing strip runs 1 Nos
11 Total Length of earthing (lc) 125000 mm
12 Max Fault Current withstand time in seconds (t) 1 sec
Factor dependent on the material of the protective
80 A/mm²
13 conductor (K) for steel
14 Number of Electrodes 6 Nos
5) Calculation
i) To determine minimum size of earth conductor
a) Required cross section of Earthing Conductor
(As per Cl.12.2.2.1 of IS: 3043)
where, 𝐈 𝟏
I = fault current for which fault of negliable impedance can flow =𝐤×
through the device
𝐒 √𝐭
K= factor dependent on the material of the protective conductor,
the insulation and other parts, for 1 Sec A/mm2 : 80 for steel
T = time for fault clearing in sec.
S = Cross sectional area of earthing conductor in sq.mm
b) Required cross section of Earthing S= 57.75 mm²
0%
c) Corrosion Allowance
57.75 mm²
c) Number of earthing strips run 1 Nos
d) Selected cross sectional of earthing conductor 300 mm²
Hence, 50 x 6 Sq.mm GI. strips shall be used for main earthing grid.
ii) Calculation for the area required of Earth Electrode from Current density calculation
b) Permissible current density (I) 𝟕. 𝟓𝟕 × 𝟏𝟎³
Where,
𝒊=
√𝝆𝒕
I = Permissible current density (I)
t = Operating time of the disconnecting device in sec IS:3043
ρ = Soil resistivity (Ohm-m) I= 629.65 A/m²
Fault Current
m²
Current Density
Earth Electrode Area Required =
7.337 m²
iii) Total Earth Electrode Area Considered for the plant
iii-a) Area of earth strip =2 x (Strip width + Strip Thickness) x Strip Length x No. of Runs
1 Length of 1R- 50x6 mm strip electrode considered for plant 130 m
2 No. of Run 1 Nos
3 Area of the strip electrode 14.56 m²
iii-b) Area of earth electrode = πDL
1 No of Earth electrode 6 Nos
2 Length of Electrode 3 m
3 Dia of Electrode( Copper Bonded Rod) in mtr 0.08 m
4 Area of each electrode 0.754 m²
5 Area of total electrode 4.524 m²
Total Combined Area of Strip and Electrode 19.084 m²
Note : The Combine area of electrode should be more than the required area of
7.337 m²
electrode
Hence required number of earth electrode and strip electrode selected is SAFE
iv) Calculation of Earth Resistance
a) Resistance of Total buried earthing conductor, R cond
As per IS: 3043
𝟏𝟎𝟎 𝝆 𝟒 𝒍𝒄
𝑹= 𝒍𝒏
where, 𝟐𝝅𝒍 𝒕
ρ = Soil resistivity (Ohm-m)
ZZ Ic = Length of earthing conductor in cm
t = Width of conductor in cm Rcond = 1.695 ohm
v) Calculation of Effective Resistance of Buried Electrode (Relec)
a) Resistance of Total buried earthing conductor, R elec
As per IS: 3043
𝟏𝟎𝟎 𝝆 𝟐 𝒍𝒓
𝑹= 𝒍𝒏
where, 𝟐𝝅𝒍 𝒅
ρ = Soil resistivity (Ohm-m)
Ir = Length of pipe in cm
d = Diameter of pipe in cm R = 33.11 ohm
Resistance considering 6 Nos. of Electrodes Relec = 5.52 ohm
vi) Calculation of Effective Earth Resistance for tranformer yard
(Rcond x Relec ) ÷ (Rcond + Relec)
Effective Resistance, Reff
1.297 ohm
The effective resistance of the transformer yard area is less than 3 ohm which meets the effective
resistance of the plant as per clause No: 20.1 of IS:3043. Hence selected earthing conductor is suitable
for withstanding fault current & design is SAFE
