Earth Pit Calculation

What national code is applicable? Where is the earth pit based?

Location is India only.

Dear Sir

you have to first check the surface where you are planning to make earth pit by making cross pit in that ground .then you have to check which type of earthing you have to done either CU earthing or GI earthing or chemical earting acc to your soil situvation .then after this you have to design pit so that electrical equpiment earthing seprete and elctronics equpiment seprete.after this you have to calculate indivisble earthing and after this you have to calculate grid earhing

Eg:- Supply voltage – 11 kV, fault level at 11 kV side at substation – 350 MVA, lengthof 11 kV feeder from substation to factory – 3 km, 11 kV conductor size – 95 Sq. mm,spacing of conductor – 1 m, resistance of line – 0.5 Ω/ km. Rating of transformer atfactory – 900 kVA, 11 kV/433 V, % impedance – 6 Ω (2 Nos in parallel). Soil

resistivity, ρ = 200 Ω-m

Take base values: 100 MVA, 11 kV
Base MVA x 100
Short Circuit MVA
= 100 x 100
350
% Source impedance =
= 28.57 %
95 = 5.5 mm
π
11 kV cond. radius =
Spacing between cond. = 1000mm
-7
e
-7
e
2 log 1000 +0.5 x 10 x 3000
5.5
Inductance of line for eqvt. spacing = (2 log d + 0.5) x 10 H/m
r
= ⎛ ⎛ ⎞ ⎞
⎜ ⎜ ⎟ ⎟ ⎝ ⎝ ⎠ ⎠
= 0.0033 H
XL = 2π F L = 1.03 Ω
R = 0.5 x 3 = 1.5 Ω
2 2
L L Z = x + R = 1.82 Ω
2
2
1.82 x 100 x 100 = 150.4%
11
% line impedance = line impedance (Ω) x Base MVA x 100
Base kV
=
⎛ ⎞
⎜ ⎟
⎝ ⎠
Total % impedance up to factory = 150.4 + 28.5 = 178.97%
S/C MVA at 11 kV side at factory = (100 x 100)/ 178.97 = 55.87%
Considering the future expansion (say new substation) in the source side , S/C
MVA is taken as 250 MVA.
[Note: unless otherwise specified, a minimum fault level at 11 kV shall be
taken as 250 MVA]
250 x 103 Fault level at 11kV side = = 13.122 kA
3 x 11
% impedance corresponding to 250 MVA fault level = 100 x 100 = 40%
250
14
6 x 100 = 666.67%
0.9
% impedance of transformer at new base MVA ( ie,100 MVA) =
666.67
= 333.33%
2
since two transformers are in parallel the effective impedance =
Total % impedance up to 433 V bus = 40 + 333.33 = 373.33%
S/C MVA at 433 kV bus
100 x 100 = 26.78 MVA
373.33
26.78 x 103 = 35.71 kA
3 x 433
Corresponding fault current =
Earthing design:
Current density of copper – 118 A/mm2 (for 3 sec)
3
Size of conuctor at 11 kV side = 13.122 x 10 = 111.2 mm2
118
Nearest standard size = 25 x 6 mm cu strip
3
Size of conductor at MV side = 35.71 x 10 = 302.32 mm2
118
Nearest standard size = 63 x 6 mm cu strip
3
7.57 x 10 = 7570 = 309 A/ m2
ρ t 200 x 3
Permissible current density at electrode =
Plate electrode of 1.2 x 1.2 x 0.012m is used for earthing
Total area of both sides of plate electrode = 1.2 x 1.2 x 2 = 2.88 m2
3
Area required to dissipate fault at 11 kV side = 13.122 x 10 = 42.46 m2
309
Number of plate required = 42.46 = 14.74
2.88
Therefore 15 plate electrodes are to be provided.