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
Rocky Soil/ More than 2000 ohm-meterMultiple Electrodes
Transformers, substation earthing, LT line equipment/ 15kAless than 1 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode
Sandy Soil/ between 50 to 2000 ohm-meterMultiple Electrodes
Rocky Soil/ More than 2000 ohm-meterMultiple Electrodes
LA, High current Equipment./ 50kAless than 1 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode
Sandy Soil/ between 50 to 2000 ohm-meterMultiple Electrodes
Rocky Soil/ More than 2000 ohm-meterMultiple Electrodes
PRS, UTS, RTUs, Data processing centre etc./5KAless than 0.5 ohmNormal Soil/ up to 50 ohm-meterSingle Electrode
Sandy Soil/ between 50 to 2000 ohm-meterMultiple Electrodes
Rocky Soil/ More than 2000 ohm-meterMultiple Electrodes
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
32 x 61/600 Kg Per meter
40 x 62 Kg Per meter
50 x 62/400 Kg Per meter
65 x 105/200 Kg Per meter
75 x 127/200 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
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