PLANT BELOW GROUND EARTHING CALCULATION
1.0 INTRODUCTION:
Earthing system is installed in such a manner that it will limit the effect of ground potential gradients
to such a voltage & current level that it will not endanger the safety of people or equipment under normal &
fault conditions as well as assure the continuity of service.
2.0 DESIGN BASIS:
CODES & STANDARDS
IS: 3043 : Code of Practice for earthing
IS: 2309 : Code of Practice for the Protection of building & Allied Structures against Lightning.
IEEE: 665 : IEEE guide for safety in Generating Station grounding.
IEEE: 80 : IEEE guide for safety in AC substation grounding.
CBIP manual on substation
System Earthing adopted at various voltage levels is as follows:-
11KV System : Non-effectively grounded
415V System : Effectively grounded
230V AC System : Effectively grounded
110V DC System : Ungrounded
As solidly /effectively grounded system produce higher fault current thus 415V system are considered for
earthing design.
System fault levels shall be considered as
11KV System : 40KA for 3Sec
415V System : 50KA for 1Sec
230V AC System : 25KA for 1Sec
110V DC System : 25KA for 1Sec
3.0 CALCUALTION:
3.1 Size of Earth Conductor
The following factors are considered in earthing design:
(i) Duration of fault current for earth-mat sizing (t): 1 sec
(ii) Conductor type for buried earth mat & risers: Zinc Coated Steel flat
Earthing conductor size is calculated using following equation as per IS:3043
Ix√t
Minimum size, A =
K
Where,
A : Area Cross Section area in mm
I : Value (ac, rms) of fault current for a fault of negligible impedance which can flow
through the prospective device, in Amp – 50KA
t : Operation time of the disconnecting device in sec. – 1 sec.
K : factor dependent on the material of the protective conductor, theinsulation and other
parts and the intial & final temperature.Value of K for protective, steel Conductor.
Qc : Voltmetric heat capacity of Conductor material (j/°C mm3 ) – 3.8x10-3 for steel Table
5 of IS : 3043
B : reciprocal of temperature coefficient of resistivity at 0°C forconductor (°C) 202°C for
Steel Table 5 of IS:3043
δ20 : Electricity resistivity of conductor Material at 20C (Ω mm)–138 x 10-6 (Ω mm) for
steel Table 5 of IS: 3043
θ1 : initial temperature of conductor (°C) - 50°C
θt : Final temperature of conductor (°C) - 620°C (for Welded joint)
I = 50000
Ki = Sq.Root ((QC (B+20) / S20) = 78
Qc = 0.0038
B = 202
δ20 = 0.00014
78.19
K = Ki * ln (1 + (θt-θ1)
(B+θ1)
Where, K = 78 ln (1 + (620-50)
(202+50)
θt = 620
θ1 = 50
K = 85.0148
A = (Ix√t) / K
= 588.133 mm2
1
= 588 mm2
As per CBIP manual no corrosion allowance is required for steel having corrosion resistance coating.
Required conductor Size = 588 mm2
= 75 x 10 mm
Hence, 75 X 10 mm Galvanized Steel flat is selected for buried earthmat.
3.2 Resistance of Earth Grid
The value of the grid resistance to remote earth (Rg) can be calculated using the following equation:
Rg = p * ( (1/L)+ (1/20A)1/2 x ((1+1/(1+ h X (20/A)1/2)))
Where,
p , Soil resistivity in Ωm = 100
L, Length of buried conductor (M) = 694.713
2
A, Area occupied by conductor in m = 4492.058
H, Depth of buried conductor (M) = 0.6
Rg = 100 * ( (1/552.748)+ (1/20x4130.229)^(1/2) x ((1+1/(1+ 0.6 X (20/4130.229A)^(1/2))))
= 0.79836 Ω
3.3 Number of Earth Electrodes
3.3.1 Resistance of each pipe Electrode
The value of the resistance of each Pipe Electrode (Rp) can be calculated using the following equation
as per IS: 3043
100ρ x ln 4L
Rp =
2πL d
Where,
p , Soil resistivity in Ωm = 100
L, Length of electrode in cm = 275
d, Diameter of electrode in cm = 10
100x100 x ln 4x275
Rp =
2πx275 10
= 27.22 ohms
3.3.2 Resistance of each strip Electrode
Rs
= 100 x ρ ln 2 x Ls2
2 x 3.14 x Ls wt
Where,
p , Soil resistivity in Ωm = 100
L, Length of strip in cm = 69471.3
t, Width of the strip in cm = 7.5
W, Depth of burial in cm = 60
100x100 x ln 2x55274.82
Rs =
2πx55274.8 60x7.5
= 0.39 ohms
3.3.3 Total number of Pipe Electrode required for Grid resistance
N = (Rp X Rs)-(Rg X Rp)
Rs - Rg
Net Earth resistance R in ohms = 27.218
= 28 Nos
3.4 Resistance of Earth Electrode (Re)
= 1 (100 x E x ln (4L/d)
Re
n 2πL
Where,
n, No of earth pits = 28
L, Length of strip in cm = 275
d, Dia of electrode in cm = 10
E , Soil resistivity in Ωm 100
= 1 (100 x 100 x ln (4x275/10)
Re
28 2πx275
= 0.97 ohms
3.5 Combined Resistance
Re X Rg
Net Earth resistance R in ohms =
Re + Rg
0.86 x 0.97
Net Earth resistance R in ohms =
0.86+0.97
= 0.438 ohms
This is less than the CEIG stipulated value of 1 Ohm
Remarks:
1 Material constant used here is as per IS 3043- 1987 only (considering Non hazardous area)
In this revision only resistance vaules of the electrodes will slightly vary from Rev:01. because of using log formula
2 instead of ln formula.
3 For fault level calculation here we are attaching short circuit analysis and reference.
