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# How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) – part 2

Home / Technical Articles / How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) – part 2

Continued from first part:
How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) – part 1

## 2. Calculate Number of Plate Earthing

The Earth Resistance of Single Plate electrode is calculated as per IS 3040:

R=ρ/A√(3.14/A)

Where:

ρ = Resistivity of Soil (Ω Meter),
A = Area of both side of Plate (m2),

Example: Calculate Number of CI Earthing Plate of 600×600 mm, System has Fault current 65KA for 1 Sec and Soil Resistivity is 100 Ω-Meters.

Current Density At The Surface of Earth Electrode (As per IS 3043):

• Max. Allowable Current Density  I = 7.57×1000/(√ρxt) A/m2
• Max. Allowable Current Density  = 7.57×1000/(√100X1)=757 A/m2
• Surface area of both side of single 600×600 mm Plate= 2 x lxw=2 x 0.06×0.06 = 0.72 m2
• Max. current dissipated by one Earthing Plate = Current Density x Surface area of electrode
• Max. current dissipated by one Earthing Plate =757×0.72= 545.04 Amps
• Resistance of Earthing Plate (Isolated)(R)=ρ/A√(3.14/A)
• Resistance of Earthing Plate (Isolated)(R)=100/0.72x√(3.14/.072)=290.14 Ω
• Number of Earthing Plate required =Fault Current / Max.current dissipated by one Earthing Pipe.
• Number of Earthing Plate required= 65000/545.04 =119 No’s.
• Total Number of Earthing Plate required = 119 No’s.
• Overall resistance of 119 No of Earthing Plate=290.14/119=2.438 Ω.

## 3. Calculating Resistance of Bared Earthing Strip

### 1) Calculation for earth resistance of buried Strip (As per IEEE)

The Earth Resistance of  Single Strip of Rod buried in ground is:

R=ρ/Px3.14xL (loge (2xLxL/Wxh)+Q)

Where:

ρ = Resistivity of Soil (Ω Meter),
h = Depth of Electrode (Meter),
w = Width of Strip or Diameter of Conductor (Meter)
L = Length of Strip or Conductor (Meter)
P and Q are Koefficients

### 2) Calculation for earth resistance of buried Strip (As per IS 3043)

The Earth Resistance of  Single Strip of Rod buried in ground is:

R=100xρ/2×3.14xL (loge (2xLxL/Wxt))

Where:

ρ = Resistivity of Soil (Ω Meter),
L = Length of Strip or Conductor (cm)
w = Width of Strip or Diameter of Conductor (cm)
t = Depth of burial (cm)

Example: Calculate Earthing Resistance of Earthing strip/wire of 36mm Diameter, 262 meter long buried at 500mm depth in ground, soil Resistivity is 65 Ω Meter.

Here:

R = Resistance of earth rod in W.
r = Resistivity of soil(Ω Meter) = 65 Ω Meter
l = length of the rod (cm) = 262m = 26200 cm
d = internal diameter of rod(cm) = 36mm = 3.6cm
h
= Depth of the buried strip/rod (cm)= 500mm = 50cm

• Resistance of Earthing Strip/Conductor (R)=ρ/2×3.14xL (loge (2xLxL/Wt))
• Resistance of Earthing Strip/Conductor (R)=65/2×3.14x26200xln(2x26200x26200/3.6×50)
• Resistance of Earthing Strip/Conductor (R)== 1.7 Ω

## 4. Calculate Min. Cross Section area of Earthing Conductor

Cross Section Area of Earthing Conductor As per IS 3043

(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 = 1 Second
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 equipment

 No Equipment 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 50×6 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 ohm-meter 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 ohm-meter Single Electrode Sandy Soil/ between 50 to 2000 ohm-meter Multiple Electrodes Rocky Soil/ More than 2000 ohm-meter Multiple Electrodes Transformers, substation earthing, LT line equipment/ 15kA less than 1 ohm Normal Soil/ up to 50 ohm-meter 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 ohm-meter 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 ohm-meter 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 and 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 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

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer. ### Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

1. Feleke
Mar 06, 2023

Excuse me sir;
what is 7.57 in Max. allowable current density formula?

2. maher
Jul 26, 2022

hi
it is eng maher from SAK1.COM palestinan state
we work in solar field capacity of 7.5mw
could you help us by making GROUNDING SYSTEM DESIGN

3. Shambhu sharma
May 29, 2022

What is required machine preventive maintenance

4. Amit
Mar 05, 2021

The formula used is old. KIndly Calculate resistance as per new formula re-affirmed IS 3043 (2006)

• Alphonse Nelson
Mar 13, 2021

How do aerial conductors be identified?

5. Sobia Arif
Feb 11, 2020

i need its part 1. can you please tell me where is it?

6. Vallinayagom
Aug 21, 2019

sir
for extending earth connection earth copper strip say (25×3)mm to be used.in strip three types of copper strips available( soft, half hard, hard). which type to be used for extent the earth, why. please kindly explain
thanks sir

7. Subrata Kumar Bera
Jul 15, 2019

Dear Mr. Jignesh

You have mentioned the inter distance of earth electrode of 3/4 mtrs. How do you aririve 3 to 4 mtrs. As per thumb rule, the inter distance shall be maintained twice of the length of pipe electrode. This distance shall be maintained to avoid any back flow of fault current.

Regards
Subrata Kumar Bera

8. edwin
Apr 09, 2019

Can you please provide earthing calculation formula using lattice instead of earth electrode

9. Ajay Kaul
Jun 21, 2018

We want to provide earthing for 10 KW DC solar power plant. Please suggest the weight of the copper plate that should be placed in the earthing pit.
In case you can SMS it will help.
Thanks.

10. Deval
Jan 27, 2018

How we considered costant K in 4. Calculate Min. Cross Section area of Earthing Conductor ?

11. Reza laali
Jan 24, 2018

Dear sir
In oil and gas projects we earth pipes and structures every 25 or 30 meter
Do you have any calculation for this?
Best regard