OMNAMO ELECTRICAL ENGINEERING: Calculate Numbers of Plate/Pipe/Strip Earthings (Part-3)

Calculate Min. Cross Section area of Earthing Conductor:

Cross Section Area of Earthing Conductor As per IS 3043
Cross Section Area of Earthing Conductor (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= 1Second
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

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

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

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.

No	Equipments	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	50X6 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, substationearthing, 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

Ref IS 3043 &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

GI Earthing Strip: