Calculate Numbers of Plate/Pipe/Strip Earthings (Part2)
March 20, 2013 9 Comments
(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 ΩMeters.
 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 Coefficients
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
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