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 Earthing
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
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
Earthing rod arrangements
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<S< 35 mm2 | 16 mm2 | 16X(k1/k2) |
| S > 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 |