Calculate Numbers of Plate/Pipe/Strip Earthings (Part-2)

(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 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