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How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) – part 2

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How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) – part 2
How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) – part 2

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


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 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

The approximate earth resistance of the Rod/Pipe electrodes can be calculated by:

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
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

NoEquipmentEarth 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 Body50×6 mm (GI)
6Control & Relay Panel25 X 6 mm (GI)
7Lighting Panel & Local Panel25 X 6 mm (GI)
8Distribution Board25 X 6 mm (GI)

9

Motor 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 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 ConductorArea of Earthing conductor (mm2) When It is Not Same Material as Phase Conductor
S < 16 mm2SSX(k1/k2)
16 mm2<S< 35 mm216 mm216X(k1/k2)
S > 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 weight

GI 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 Plate

PlateWeight
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 Pipe

PipeWeight
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 Wire

PlateWeight
6 Swg5 meter in 1 Kg
8 Swg9 meter in 1 Kg

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Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.