How to Determine Correct Number of Earthing Electrodes

How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) part 2PostedMAY 13 2013byJIGUPARMARinENERGY AND POWER,POWER SUBSTATIONwith8 COMMENTS

Installation of vertical electrodes of earthing

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

2. Calculate Number of Plate EarthingTheEarth Resistanceof Single Plate electrode is calculated as per IS 3040:R=/A(3.14/A)Where:= Resistivity of Soil ( Meter),A= Area of both side of Plate (m2),Example:Calculate Number of CI Earthing Plate of 600600 mm, System has Fault current 65KA for 1 Sec and Soil Resistivity is 100 -Meters.Current Density At The Surface of Earth Electrode (As per IS 3043): Max. Allowable Current Density I = 7.571000/(xt) A/m2 Max. Allowable Current Density = 7.571000/(100X1)=757 A/m2 Surface area of both side of single 600600 mm Plate= 2 x lxw=2 x 0.060.06 = 0.72 m2 Max. current dissipated by one Earthing Plate = Current Density x Surface area of electrode Max. current dissipated by one Earthing Plate =7570.72=545.04Amps Resistance of Earthing Plate (Isolated)(R)=/A(3.14/A) Resistance of Earthing Plate (Isolated)(R)=100/0.72x(3.14/.072)=290.14 Number of Earthing Plate required =Fault Current / Max.current dissipated by one Earthing Pipe. Number of Earthing Plate required= 65000/545.04 =119 Nos. Total Number of Earthing Plate required = 119 Nos. Overall resistance of 119 No of Earthing Plate=290.14/119=2.438 .

3. Calculating Resistance of Bared Earthing Strip1) Calculation for earth resistance of buried Strip (As per IEEE)The Earth Resistance of Single Strip of Rod buried in ground is:R=/Px3.14xL (loge (2xLxL/Wxh)+Q)Where:= Resistivity of Soil ( Meter),h= Depth of Electrode (Meter),w= Width of Strip or Diameter of Conductor (Meter)L= Length of Strip or Conductor (Meter)PandQare 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=100x/23.14xL (loge (2xLxL/Wxt))Where:= Resistivity of Soil ( 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 Meter.Here:R= Resistance of earth rod inW.r= Resistivity of soil( Meter) =65 Meterl= length of the rod (cm) = 262m =26200 cmd= internal diameter of rod(cm) = 36mm =3.6cmh= Depth of the buried strip/rod (cm)= 500mm =50cm Resistance of Earthing Strip/Conductor (R)=/23.14xL (loge (2xLxL/Wt)) Resistance of Earthing Strip/Conductor (R)=65/23.14x26200xln(2x26200x26200/3.650) Resistance of Earthing Strip/Conductor (R)== 1.7

4. Calculate Min. Cross Section area ofEarthing ConductorCross Section Area of Earthing Conductor As per IS 3043(A) =(If xt) / KWhere: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 xt) / KHere:If=50000 AmpT=1 SecondK=80(Material Constant, For GI=80, copper K=205, Aluminium K=126). Cross Section Area of Earthing Conductor (A) =(500001)/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 StripsThumb Rule for Calculate Number of Earthing RodThe approximate earth resistance of the Rod/Pipe electrodes can be calculated by:Earth Resistance of the Rod/Pipe electrodes:R= K x /LWhere:= Resistivity of earth in Ohm-MeterL= 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 < 600K= 1.2 o/L if 600 < L/d < 300Number of Electrode if find out by Equation of R(d) = (1.5/N) x RWhere:R(d)= Desired earth resistanceR= Resistance of single electrodeN= 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 ,Soil Resistivity of =40, Length=2.5 Meter, Diameter of Pipe =38 mm.Here:L/d = 2.5/0.038=65.78 soK = 0.75 The Earth Resistance of the Pipe electrodes R= K x /L= 0.7565.78 =12 One electrode the earth resistance is12 . To get Earth resistance of 1 the total Number of electrodes required = (1.512)/1 =18 No

Calculating Resistance & Number of Earthing RodReference: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 = x[ln (2L/a)-1]/(23.14xL)Earthing Resistance of Parallel Rods:R = x[ln (2L/A]/ (23.14xL)Where:L= length of rod in ground Meter,a= radius of rod Meter= ground resistivity, ohm-MeterA= (axS)S= Rod separation Meter

Earthing rod arrangementsFactor affects on Ground resistanceThe 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 equipmentNoEquipmentEarth strip size

1HT switchgear, structures, cable trays & fence, rails, gate and steel column55 X 6 mm (GI)

2Lighting Arrestor25 X 3 mm (Copper)

3PLC Panel25 X 3 mm (Copper)

4DG & Transformer Neutral50X6 mm (Copper)

5Transformer Body506 mm (GI)

6Control & Relay Panel25 X 6 mm (GI)

7Lighting Panel & Local Panel25 X 6 mm (GI)

8Distribution Board25 X 6 mm (GI)

9Motor up to 5.5 kw4 mm2 (GI)

10Motor 5.5 kw to 55 kw25 X 6 mm (GI)

11Motor 22 kw to 55 kw40 X 6 mm (GI)

12Motor Above 55 kw55 X 6 mm (GI)

Selection of Earthing System:Installations/Isc CapacityIR Value RequiredSoil Type/ResistivityEarth System

House hold earthing/3kA8 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode

Sandy Soil/ between 50 to 2000 ohm-meterSingle Electrode

Rocky Soil/ More than 2000 ohm-meterMultiple Electrodes

Commercial premises,Office / 5kA2 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode

Sandy Soil/ between 50 to 2000 ohm-meterMultiple Electrodes


Transformers, substation earthing, LT line equipment/ 15kAless than 1 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode



LA, High current Equipment./ 50kAless than 1 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode



PRS, UTS, RTUs, Data processing centre etc./5KAless than 0.5 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode



Size of Earthing ConductorRef 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 ConductorArea of Earthing conductor (mm2) When It is Not Same Material as Phase Conductor

S < 16 mm2SSX(k1/k2)

16 mm2 35 mm2S/2SX(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 weightGI Earthing Strip:Size (mm2)Weight

20 x 3500 gm Per meter

25 x 3600 gm Per meter

25 x 61/200 Kg Per meter


40 x 62 Kg Per meter




GI Earthing PlatePlateWeight

600 x 600 x 3 mm10 Kg App.

600 x 600 x 4 mm12 Kg App.

600 x 600 x 5 mm15 Kg App.

600 x 600 x 6 mm18 Kg App.

600 x 600 x 12 mm36 Kg App.

1200 x 1200 x 6 mm70 Kg App.

1200 x 1200 x 12 mm140 Kg App.

GI Earthing PipePipeWeight

3 meter Long BISE5 Kg App.

3 meter r Long BISE9 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 WirePlateWeight

6 Swg5 meter in 1 Kg

8 Swg9 meter in 1 Kg

How to Determine Correct Number of Earthing Electrodes

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