Thamarai Kani K
EARTHING CALCULATION
The Earthing Installation Meets The Following Requirements:
a) Earth conductors sized to carry the earth fault current.
b) Earth electrodes selected to effectively dissipate the fault current.
c) Earth resistance will be less than four ohms to facilitate operation of earth fault protections.
d) 415V Switchgear - Solidly Earthed
Calculation:
Reference Documents:
• Plant Earth Resistivity is 20 Ω.m
• System Fault Current (0.415 kV side) is 65 kA
• BS 7430:1998 - Code of Practice Earthing, IEEE 80:2000 - IEEE Guide For Safety in AC
Substation Grounding & IEC60364.
Selection of Earth Conductor Size:
Earth conductor size is governed by the magnitude of fault current and the duration time t in
seconds. In this case the magnitude of fault current is the highest at 65 kA with t=0.5 second in the
415V bus. The cross sectional area is calculated using the formula below furnished under Cl:14 of
BS7430:1998 code of practice for earthing.
𝐼 1
=𝑘
𝑆 √𝑡
Where
S= cross sectional area in Sq.mm.
I= RMS value of fault current in amps.
t=operating time of protective device in amps.
k=factor dependent on material of conductor and initial and final temperature.
For Bare GI Conductor with initial temp of 30 deg.C and final temp of 200 deg.C K=58 from Table
13 of BS 7430:1998.
Operating time, t = 0.5
S = (√𝑡𝑥 𝐼) / 𝑘
S = √0.5𝑥(65𝑥1000)
( )
58
S = 792.45 Sq.mm
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Selected conductor size 2 no's of 8*50mm Bare GI Strip (i.e) = 800 Sq.mm
Breadth = 50 mm
Height = 16 mm
Breath in meter = 0.05 Meter
Height in meter = 0.016 Meter
Selection of Earth Electrode:
Fault current to be dissipated = 65 kA
Duration of fault current = 1 Sec
Maximum permissible current density = 7.57𝑥1000 Amp/Sq.mm
At the surface of the earth electrode = √ (𝑝 𝑥 𝑡 )
Value of earth resistivity (ρ) * = 20 Ohm-m
Maximum permissible current density = 1692.70 Amp/Sq.mm
Maximum value of fault current to be = 65 kA
dissipated
Required earthing material area for the = (Fault current *1000)/(Maximum permissible
dissipation of fault current is current density)
= (65 x 1000)/1693
= 38.400 Sq.mm
Selected electrode diameter = 25 mm
Length = 3000 mm
Considering the perforation of the = 5%
electrode
Actual area of earthing electrode = 3.14 * d *L Sq.mm
= 0.24
Selected area of earthing electrode due = (Actual area of earthing electrode*(100-5))/100
to perforation
= 0.22 Sq.mm
Selected No. of Earth electrode = 40 Nos
(Appx.)
Total area of earth electrodes = 8.948 Sq.mm
Total area of earth strip (Appx.) = 800 Sq.mm
Total area of earth Conductors = 105.60 Sq.mm
Total area of the earthing material = Total area of earth electrodes + total area of earth
(actual) conductors
= 114.55 Sq.mm
Required earthing material area for the = 38.40 Sq.mm
dissipation of fault current is
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Calculation of Earthing Material Resistance:
Earth Resistance of the Earthing Electrode (Re)
Earthing resistance of the earthing electrode (R)
= (ρ / (2 x 3.14 x L)) x (loge ((8 x L) / d) - 1)
Where
L=Length of the rod or pipe, in meters (m)
d=Diameter of rod or pipe, in meters (m)
ρ=Resistivity of soil, in ohm meters (Ω·m)
Earthing resistance of the earthing electrode (R) = 6.228 ohm
Effective earth resistance of all earth electrodes (Re) = (R / No.of Electrodes)
Re = 0.156 ohm
Earth Resistance of the Main Loop (Reg)
Depth of burial electrodes (h) = 3 Meter
Earthing resistance of the main loop (Reg)
Reg = (ρ / (P x 3.14 x L)) (loge ((2 x L2) / wh)) + Q)
Where,
Refer Cl. 10.3 Page 15 of BS 7430: 1998
L = Length of the strip or conductor, in meters (m)
d = Depth of burial electrode, in meters (m)
ρ = Resistivity of soil, in ohm meters (Ω·m)
w = Width (in case of strip) or twice the Diameter (for conductors), in meters (m)
P and Q = Coefficients given in BS7430:1998 Table 5 for different arrangements of electrode (P=2
& Q=-1)
Reg = 0.053 ohm
Combined Total Earth Resistance (R):
1/RCOMBINED = (1/Re) + (1/Reg)
= 25.041
RCOMBINED = 0.039 ohm (Hence Earth resistance will be less than 4 ohms to facilitate
effective operation of EARTH FAULT PROTECTION.)
