Calculate Numbers of Plate/Pipe/Strip Earthings (Part3)
April 1, 2013 25 Comments
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 OhmMeter
 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, SectionI Page: 570590.

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 
1 
HT switchgear, structures, cable trays & fence, rails, gate and steel column 
55 X 6 mm (GI) 
2 
Lighting Arrestor 
25 X 3 mm (Copper) 
3 
PLC Panel 
25 X 3 mm (Copper) 
4 
DG & Transformer Neutral 
50X6 mm (Copper) 
5 
Transformer Body 
50X6 mm (GI) 
6 
Control & Relay Panel 
25 X 6 mm (GI) 
7 
Lighting Panel & Local Panel 
25 X 6 mm (GI) 
8 
Distribution Board 
25 X 6 mm (GI) 
9 
Motor up to 5.5 kw 
4 mm2 (GI) 
10 
Motor 5.5 kw to 55 kw 
25 X 6 mm (GI) 
11 
Motor 22 kw to 55 kw 
40 X 6 mm (GI) 
12 
Motor Above 55 kw 
55 X 6 mm (GI) 
Selection of Earthing System:
Installations/ Isc Capacity 
IR Value Required 
Soil Type/ Resistivity 
Earth System 
House hold earthing/3kA  8 ohm  Normal Soil/ up to 50 ohmmeter  Single Electrode 
Sandy Soil/ between 50 to 2000 ohm meter  Single Electrode  
Rocky Soil/ More than 2000 ohm meter  Multiple Electrodes  
Commercial premises,Office / 5kA  2 ohm  Normal Soil/ up to 50 ohmmeter  Single Electrode 
Sandy Soil/ between 50 to 2000 ohm meter  Multiple Electrodes  
Rocky Soil/ More than 2000 ohm meter  Multiple Electrodes  
Transformers, substationearthing, LT line equipment/ 15kA  less than 1 ohm  Normal Soil/ up to 50 ohmmeter  Single Electrode 
Sandy Soil/ between 50 to 2000 ohm meter  Multiple Electrodes  
Rocky Soil/ More than 2000 ohm meter  Multiple Electrodes  
LA, High current Equipment./ 50kA  less than 1 ohm  Normal Soil/ up to 50 ohmmeter  Single Electrode 
Sandy Soil/ between 50 to 2000 ohm meter  Multiple Electrodes  
Rocky Soil/ More than 2000 ohm meter  Multiple Electrodes  
PRS, UTS, RTUs, Data processing centre etc./5KA  less than 0.5 ohm  Normal Soil/ up to 50 ohmmeter  Single Electrode 
Sandy Soil/ between 50 to 2000 ohm meter  Multiple Electrodes  
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 Phase Conductor S (mm2)  Area of Earthing conductor (mm2) When It is Same Material as Phase Conductor  Area of Earthing conductor (mm2) When It is Not Same Material as Phase 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 
pls send something on capacitor power factor correction, by varying input voltage to capacitors, their KVAr can be changed, can we design such system, it could work as an APFC without switching ON/off of capacitors
Thank you very much from your help.
Eng.Amiri
Thanks for sending electrical notes.
Dear Sir,
55 x 6 mm GI Strip is a standard size..?
Regards
Rahul
Thanks for the information…..
he is champ
Send all electrical goods of industries rates with specifications
Mukesh
engineer electrical
Yes Sir,
Give me your Address!!!!!!!
Dear sir please Send me all electrical goods of industries latest rates with specifications
sir will you please reply how did you calculate:cross sec area x safety factor x total allowance to 1125 sq.mm=625×1.5×20%=1125 say 1200 sir please reply….
Great Information.
keep it up.
i have also the same doubt
mujey bi send kardey electrical i m student of electrical.mera last year chal raha hey agar notic to is id per send karday,.
It was very useful information, from where I can get the K(materiel constant) value chart for GI Strips
Already given in the explanation…”K=80 (Material Constant, For GI=80, copper K=205, Aluminium K=126).” !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Dear Sir,
Thank you for the valuable information.
I have a query
It seems like the equation for calculating the resistance of plate electrode and rod are different in IS 3043:1987 and BS 7430:1998
Which is the right one..?
Regards
Rahul
First of all a zillion thanks for democratizing electrical knowledge for the world. I have shifted my career from electrical maintenance to designing and finding your articles very helpful. You have really upholded rig veda saying ‘Let knowledge come from every direction’
Sir, regarding a project in Qatar I need the conductor size for COPPER for earthing noncurrent carrying metallic part like steel structures and process equipments like tanks, vessels, pipe rack, pipes, stairways, towers etc.
If you can provide the data according to BS 7430 it will be best because I have to follow that standard in Qatar.
very use ful for estimating cost for quotating rates
I am having two earth pits Is it possible to combine the both earth pits and connect it for MCC Panel, 2*55 KW Motor, 0.37KW Motors. WIll it with stand
sir i required how to mtr to kg any gi and cu earthig size ,pls
sir,
i want to know why cable or wires size are calculated in mm2.
plz sir contact me my name is prashant lawand i am earthing electrode manufacturer in pune 9975977674
Thanks Sir It is Very Much Informative.
you mean Here If=50000 Amp is fault of system if you mean that what is relation between fault of system and earhtig?
Number of Electrode if find out by Equation of R(d) =(1.5/N) x R
how 65.78 value is taken in place of rho/l