Calculate Numbers of Plate/Pipe/Strip Earthings
(Part-3)
Calculate Min. Cross Section area of Earthing Conductor:
Cross Section Area of Earthing Conductor As per IS 3043
Cross Section Area of Earthing Conductor (A) =(If x√t) / K
Where t= Fault current Time (Second).
K= Material Constant.
Example:
Calculate Cross Section Area of GI Earthing Conductor for System has 50KA Fault Current
for 1 second. Corrosion will be 1.0 % Per Year and No of Year for Replacement is 20 Years.
Cross Section Area of Earthing Conductor (A) =(If x√t) / K
Here If=50000 Amp
T= 1Second
K=80 (Material Constant, For GI=80, copper K=205, Aluminium K=126).
Cross Section Area of Earthing Conductor (A) =(50000×1)/80
Cross Section Area of GI Earthing Conductor (A)=625 Sq.mm
Allowance for Corrosion = 1.0 % Per Year & Number of Year before replacement say = 20
Years
Total allowance = 20 x 1.0% = 20%
Safety factor = 1.5
Required Earthing Conductor size = Cross sectional area x Total allowance x Safety factor
Required Earthing Conductor size = 1125 Sq.mm say 1200 Sq.mm
Hence, Considered 1Nox12x100 mm GI Strip or 2Nox6 x 100 mm GI Strips
Thumb Rule for Calculate Number of Earthing Rod:
The approximate earth resistance of the Rod/Pipe electrodes can be calculated by
Earth Resistance of the Rod/Pipe electrodes R= K x ρ/L
Where ρ = Resistivity of earth in Ohm-Meter
L= Length of the electrode in Meter.
d= Diameter of the electrode in Meter.
K=0.75 if 25< L/d < 100.
K=1 if 100 < L/d < 600
K=1.2 o/L if 600 < L/d < 300
Number of Electrode if find out by Equation of R(d) =(1.5/N) x R
Where R(d) = Desired earth resistance
R= Resistance of single electrode
N= No. of electrodes installed in parallel at a distance of 3 to 4 Meter interval.
Example:
Calculate Earthing Pipe Resistance and Number of Electrode for getting Earthing
Resistance of 1 Ω ,Soil Resistivity of ρ=40, Length=2.5 Meter, Diameter of Pipe= 38 mm.
Here L/d = 2.5/0.038=65.78 so K=0.75
The Earth Resistance of the Pipe electrodes R= K x ρ/L =0.75×65.78=12 Ω
One electrode the earth resistance is 12 Ω.
To get Earth resistance of 1 Ω the total Number of electrodes required =(1.5×12)/1 =18 No
Calculating Resistance & Number of Earthing Rod:
Reference: As per EHV Transmission Line Reference Book page: 290 and Electrical
Transmission & Distribution Reference Book Westinghouse Electric Corporation, Section-I
Page: 570-590.
Earthing Resistance of Single Rods: R = ρx[ln (2L/a)-1]/(2×3.14xL)
Earthing Resistance of Parallel Rods: R = ρx[ln (2L/A]/ (2×3.14xL)
Where L= length of rod in ground Meter,
a= radius of rod Meter
ρ = ground resistivity, ohm- Meter
A= √(axS)
S= Rod separation Meter
Factor affects on Ground resistance:
The NEC code requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in
contact with the soil. But, there are some factor that affect the ground resistance of a ground
system:
Length / Depth of the ground electrode: double the length, reduce ground resistance by
up to 40%.
Diameter of the ground electrode: double the diameter, lower ground resistance by only
10%.
Number of ground electrodes: for increased effectiveness, space additional electrodes at
least equal to the depth of the ground electrodes.
Ground system design: single ground rod to ground plate.
The GI Earthing Conductor sizes for various Equipments:
No Equipments Earth Strip Size
HT switchgear, structures, cable trays & fence, rails, gate and steel
column
1 55 X 6 mm (GI)
Lighting Arrestor
2 25 X 3 mm (Copper)
PLC Panel
3 25 X 3 mm (Copper)
DG & Transformer Neutral
4 50X6 mm (Copper)
Transformer Body
5 50X6 mm (GI)
Control & Relay Panel
6 25 X 6 mm (GI)
Lighting Panel & Local Panel
7 25 X 6 mm (GI)
Distribution Board
8 25 X 6 mm (GI)
Motor up to 5.5 kw
9 4 mm2 (GI)
Motor 5.5 kw to 55 kw
10 25 X 6 mm (GI)
Motor 22 kw to 55 kw
11 40 X 6 mm (GI)
Motor Above 55 kw
12 55 X 6 mm (GI)
Selection of Earthing System:
Installations/ Isc IR Value
Capacity Required Soil Type/ Resistivity Earth System
Normal Soil/ up to 50 ohm-meter Single Electrode
Sandy Soil/ between 50 to 2000 ohm- meter Single Electrode
House hold
earthing/3kA 8 ohm Rocky Soil/ More than 2000 ohm- meter Multiple Electrodes
Normal Soil/ up to 50 ohm-meter Single Electrode
Sandy Soil/ between 50 to 2000 ohm- meter Multiple Electrodes
Commercial
premises,Office / 5kA 2 ohm Rocky Soil/ More than 2000 ohm- meter Multiple Electrodes
Transformers, less than 1 Normal Soil/ up to 50 ohm-meter Single Electrode
substationearthing, LT ohm
Sandy Soil/ between 50 to 2000 ohm- meter Multiple Electrodes
Rocky Soil/ More than 2000 ohm- meter Multiple Electrodes
Normal Soil/ up to 50 ohm-meter Single Electrode
Sandy Soil/ between 50 to 2000 ohm- meter Multiple Electrodes
LA, High current less than 1
Equipment./ 50kA ohm Rocky Soil/ More than 2000 ohm- meter Multiple Electrodes
Normal Soil/ up to 50 ohm-meter Single Electrode
PRS, UTS, RTUs, Data
Sandy Soil/ between 50 to 2000 ohm- meter Multiple Electrodes
processing centre less than 0.5
etc./5KA ohm Rocky Soil/ More than 2000 ohm- meter Multiple Electrodes
Size of Earthing Conductor:
Ref IS 3043 &Handbook on BS 7671: The Lee Wiring Regulations by Trevor E. Marks.
Size of Earthing Conductor
Area of Earthing conductor (mm2) When
Area of Earthing conductor (mm2) When It is Not Same Material as Phase
Area of Phase Conductor S (mm2) It is Same Material as Phase Conductor Conductor
S < 16 mm2 S SX(k1/k2)
16 mm2<S< 35 mm2 16 mm2 16X(k1/k2)
S > 35 mm2 S/2 SX(k1/2k2)
K1 is value of Phase conductor,k2 is value of earthing conductor
Value of K for GI=80, Alu=126,Cu=205 for 1 Sec
Standard Earthing Strip/Plate/Pipe/wire Weight:
GI Earthing Strip:
Size (mm2) Weight
20 x 3 500 gm Per meter
25 x 3 600 gm Per meter
25 x 6 1/200 Kg Per meter
32 x 6 1/600 Kg Per meter
40 x 6 2 Kg Per meter
50 x 6 2/400 Kg Per meter
65 x 10 5/200 Kg Per meter
75 x 12 7/200 Kg Per meter
GI Earthing Plate:
Plate Weight
600 x 600 x 3 mm 10 Kg App.
600 x 600 x 4 mm 12 Kg App.
600 x 600 x 5 mm 15 Kg App.
600 x 600 x 6 mm 18 Kg App.
600 x 600 x 12 mm 36 Kg App.
1200 x 1200 x 6 mm 70 Kg App.
1200 x 1200 x 12 mm 140 Kg App.
GI Earthing Pipe:
Pipe Weight
3 meter Long BISE 5 Kg App.
3 meter r Long BISE 9 Kg App.
4.5 meter (15′ Long BISE) 5 Kg App.
4.5 meter (15′ Long BISE) 9 Kg App.
4.5 meter (15′ Long BISE) 14 Kg App
GI Earthing Wire:
Plate Weight
6 Swg 5 meter in 1 Kg
8 Swg 9 meter in 1 Kg
