1 of 12 Earthing Riser

1.0 Notes

1.01

1.02

1.03 Every fifth pole of fence surrounding the switchyard is earthed & 1.5 meter distance is considered in every adjacent pole.

2.0 SELECTION OF CONDUCTOR FOR EARTH RISERS:-

2.01 Earthing conductor Material

Ko + Tm ½

I = A ℓn -------- > Equation 1.01

Ko + Ta

IA =

Ko + Tm ½

ℓn Ko + Ta

Where, ( As per IEEE-80 Page Number 41, Eqn 37)

15

t = Duration of fault current for conductor selection 1

I = is the rms current in KA. 40

Tm = is the maximum allowable temperature for Welded joints in 0 C 419

Ta = is the ambient temperature in 0 C 40

Tr = is the reference temperature for material constants in 0 C 20

∝0 = is the thermal coefficient of resitivity at 0 0 C in 1/

0 C 0.00341

∝r = is the thermal coefficient of resitivity at reference temperature Tr in 1/ 0 C 0.0032

ρr = is the resistivity of the ground conductor at reference temperature Tr in µΩ−cm 20.1

K0= 1 / ∝0 293

TCAP = is the thermal capacity per unit volume in J /(cm3.0C) 3.93

Area of Conductor A mm2

= 587.013907 mm2

Accordingly; the minumum cross section required is = 587.01 Sq.mm

0% (Please refer below table)

Sr. No.

Percentage

Corrosion

1 0% As per CBIP Publication No.223, Page no. 6

2 15%

3 30%

2.02 587.0139067 Sq.mm.

2.03

Width of the condcutor selected 65 mm

Thickness of the conductor selected 10 mm

3.0 Calculation of Quantity of Earth- Risers

a) Height of Main / Lower Bus above gravel 5.5 meters

b) Height of High level Bus above gravel 9.5 meters

Substituting the values in equation 5.01, we get

Allowance for corrosion =

Soil Resitivity

G.I. Flat

TCAP X 10-4

t x ∝r x ρr

TCAP x 10-4

t x ∝r x ρr

I = Actual Fault Current given by Client / Calculated in KA

This document outlines and describes the Technical Parameters for selection of Earthing conductor as per IEEE Std 80-

2000 Revision.

Auxiliary earth mats are placed below the operating mechanism of disconecting equipments like Isolator, CB, NGR,

Earth switches. Generally earth mat of size 1.5 M X 1.5 M is designed with uniform spacing of 300mm.

Selection of Earthing Conductor

Hence area of the conductor required, with allowance for corrosion of 0% = 587.01 x

1.00 =

>25 & < 100 ohms-m

< 25 ohms-m or treated soil

Therefore area of the conductor selected = 65 x 10 = 650 sq.mm, which is equal to the conductor area requirement of

587 ; hence aequate

> 100 ohms-m

2 of 12 Earthing Riser

3.01 Quantity of Earth Risers for Equipments - I

1 CT 5 6 6 P

2 PT/ CVT 3 3 109.80

3 LA 2 0 28.60

4 BPI 1 3 81.20

5 ISOLATOR 1 4 103.50

6 CB 0 0 0.00

7 CC 0 0 0.00

8 WT 0 0 0.00

9 NGR 0 0 0.00

10 Other Eqpt.(If any) 0 0 0.00

11 Other Eqpt.(If any) 0 0 0.00

323.10

16.155

339.26

3.02 Quantity of Earth Risers for Equipments - II

Sr. No.Equipment

DetailsTotal quantity

in Mtrs

1 Transformer 13.20

2 Lighnting Mast 16.50

3 Gantry Column 19.80

4 Tr.Tower 6.60

5 Lighting Pole 33.00

6 Earth Pit 132.00

7 Cable Trench in Mtrs 66.00

8 Fence In Mtrs 44.00

9 Aux. Earth Mats 249.75

10 Other Eqpt.(If any) 0.00

580.85

29.04

609.89

3.03 Quantity of other Earthing material

a) Earthing Material

Sr. No.Equipment

DetailsQuantity /

Equipment

No. of

Equipment

s

Total Qty. in

Mtrs.

1 Gate 5.00 1 5

2 CT /PT JB 5.00 2 10

3 Bay M. Box 5.00 3 15

4 Other Eqpt.(If any) 5.00 4 20

5 Other Eqpt.(If any) 5.00 5 25

75.00

3.75

78.75

Total Quantity-I in Mtrs

Equipment

Height in

6

40

15

Additional 5 % for Bends

5

Sub-Total Quantity in Mtrs

Sub -Total Quanity in Mtrs

200

Quantity of Equipments

2

10

100

4

Sr NoMain Bus

Equp Qty

High level bus

Equt Qty In

Additional 5 % for Bends

Total Qty. in

Mtrs.

Equipment

Details

Total Quantity- II in Mtrs

Coper Cable / Fexible Braid of size 30 sq.mm.

Sub -Total Quanity in Mtrs

Additional 2 % for Bends

Total Quantity- II in Mtrs

3 of 12 Earthing Riser

4.0

230

M.S. Rod

32 mm. M.S.Rod

5363

G.I. Flat

65 x 10mm. Strip

949.15

47

27

20

32 mm M.S Rod

, 3000 mm.

Coper Cable /

Fexible Braid of

size 30 sq.mm.

78.75

Number of Treated Earth Electrodes

Total Length of conductor in meters for Earth Riser in Mtrs

Number of Earth Electrodes

Type of Earthing conductor for Earth Riser

Earth Riser conductor size in mm

Type of Earthing conductor for Main Earth Mat

Total Length of buried conductors in meters

Size of Earthing Material for Gate, BMB,JB etc.

Quantity of Earthing Material for Gate, BMB,JB etc. in Mtrs

Number of Untreated Earth Electrodes

Size & Length of Earth Electrodes

Buried conductor size in mm

Voltage level of the substation in Kv

SUMMARY

4 of 12 Earthing Riser

1.0 Notes

1.01

1.02

1.03 If Bentonite treatment is considered then 80% reduction in the Measured Resitivity value may be considered

1.04 The term "Mesh Voltage" means the maximum touch voltage within a mesh of a ground grid.

2.0 Legends

A =

I =

IG =

t =

ρ = Soil Resistivity in Ohm- mtr.

ρs = Top Gravel Resistivity in Ohm-mtr.

h = Depth of Conductor buried in meters.

Cs = Surface layer derating factor.

ts = Duration of Shock current in secs

hs = Thickness of the crushed rock surface layer in meters

d = Diameter of earthing conductor in mm or meters

Et = Tolerable Touch Potential in Volts

Es = Tolerable Step Potential in Volts

L = Total length of buried Conductors in meters

Em = Mesh Voltage in Volts

Es = Step Voltage in Volts

D = Spacing between grid conductors in meters

n = Geometric factor.

Km = Spacing factor for mesh voltage

Ks = Spacing factor for step voltage

Ki = Corrected factor for grid geometry

Kii = Corrective weighing factor

Kh = Corrective weighing factor for grid depth

W = Width of Earthing Material

T = Thickness of Earthing Material

Bwf = Body Weight Factor

Operating time of the disconnecting device in secs.

This document outlines and describes the Technical Parameters for Earthing calculation as per IEEE Std 80-

2000 Revision.

Cross-sectional are of the conductor in mm2

RMS value of the AC Ground fault current, which can flow through the conductor in Amps

RMS value of the maximum Grid current in Amps

For the conductor spacing calculations, the rectangular portion of the switchyard is considered however in those

cases, where the switchyard is not essentially rectangular in shape, the value of rectangle having the same area

as that enclosed by irregular switchyard should be taken.

5 of 12 Earthing Calculation

3.0 DATA

a) = 15,061 6,000 MVA

b) = 40,000

c) 230

d) = 1

e)=

0.5As per IEEE-80 Page No.87

f)

= 110

= 67

(Consider 1 m extra length outside fence on both sides of switchyard i.e L + 2, B + 2. )= 110 x 67

g) = 176

h) = 3000 As per IEEE-80 Page No.51 & 52

i) = 0.6 As per IEEE-80 Page No.88

j) Thickness of the crushed rock surface layer in meters = 0.15

k) = M.S. Rod

l) = 500 As per IEEE-80 Page No.44

4.0 Number of earth electrode

4.01 32 mm M.S Rod is considered for the following equipment

4.02 Length of Earth Electrodes 3 meters

Per Eqpt qty Eqpt Qty Total qty

1 15 15

1 4 4

1 7 7

1 4 4

1 4 4

1 3 3

1 0 0 By Others

1 0 0 By Others

1 10 10

47

Switchyard Size in Mtrs

Length of Switchyard in meters

Standard Fault Current (A) considered for Conductor Sizing

Fault Duration for selection of conductor in seconds (tc)Fault Duration for calculation of Step & Touch Potentials

(ts) in seconds

Therefore Area of swichyard considered for calc.

VTs / CVTs

Lightning Arrester (treated earth pits)

Equipment

Depth of Conductor buried (h) in meters

Maximum allowable temperature for welded joint 0 C

Earthing conductor Material

Top Gravel Resistivity (ρs) in Ohm-meter

Current Transformer

Lightning Mast (treated earth pits)

Breadth of Switchyard in meters

Driven earth electrodes of Outer Dia.

Soil Resistivity (ρ) in Ohm-meter

Power Transformer (treated earth pits)

Total

Corners of the buries grid and distributed

earth rods on the periphery of grid

Actual Fault Current arrived from TNEB data (A)

Voltage Level of Sub-Station in KV

Tower / Equipment

Control Room

PLCC (treated earth pits)

ΩΩ

ΩΩ

6 of 12 Earthing Calculation

5.0 CONDUCTOR SELECTION

5.01

Ko + Tm ½

I = A ℓn -------- > Equation 5.01

Ko + Ta

IA =

Ko + Tm ½

ℓn Ko + Ta

Where, ( As per IEEE-80 Page No 41 & 42, Eqn 37)

I = is the rms current in KA. 40

Tm = is the maximum allowable temperature in 0 C 500

Ta = is the ambient temperature in 0 C 40

Tr = is the reference temperature for material constants in 0 C 20

∝0 = is the thermal coefficient of resitivity at 0 0 C in 1/

0 C 0.00165

∝r = is the thermal coefficient of resitivity at reference temperature Tr in 1/ 0 C 0.0016

ρr = is the resistivity of the ground conductor at reference temperature Tr in µΩ−cm 15.9

K0= 1 / ∝0 605

TCAP = is the thermal capacity per unit volume in J /(cm3.0C) 3.28

Area of Conductor A mm2

= 480.1192781 mm2

Accordingly; the minumum cross section required is = 480.12 Sq.mm

15% (Please refer below table)

Sr. No. Corrosion

1 0%

2 15% As per IS:3043

3 30%

5.02 552.1371698 Sq.mm.

5.03

Width of the condcutor selected 75 mm

Thickness of the conductor selected 10 mm

5.04 = NA meter

5.05 If M.S. Rod is to be selected then,

Required diameter of circular conductor (M.S. Rod) can be calculated as follows.

π d2

4d = 26.52092 mm

Therefore Selected size of Main Earthing Conductor is 32 mm

5.06 M.S. Rod Diameter(d) = = 0.032 mm

Selection of Earthing Conductor

Substituting the values in equation 5.01, we get

< 25 ohms-m or treated soil

Allowance for corrosion =

Soil Resitivity

TCAP x 10-4

t x ∝r x ρr

t x ∝r x ρr

TCAP X 10-4

Hence area of the conductor required, with allowance for corrosion of

15% = 480.12 x 1.15 =

> 100 ohms-m

>25 & < 100 ohms-m

Not Applicable, as Earthing Conductor is M.S.Rod.

552.14

Equivalent conductor diameter (d) =

=

Therefore area of the conductor selected = 804 sq.mm, which is equal to the conductor area requirement of 552

; hence aequate.

w X T

π4

7 of 12 Earthing Calculation

6.00 Calculation of tolerable touch (et) & step (es) potential

6.01 Surface layer derating factor (Cs). Refer equation 27 of IEEE 80, page no. 23

Cs = -------- > Equation 6.01

Cs = 0.7827692

Weight of the Person = 50 Kg As per IEEE-80 Page No. 27

Body weight Factor for 50 Kg = 0.116

Body weight Factor for 70 Kg = 0.157

6.02 Tolerable Touch Potential Et (Refer equation 32 of IEEE 80, page no. 27)

Et -------- > Equation 6.02

6.03 Tolerable Step Potential Es (Refer equation 29 of IEEE 80, page no. 27)

Es -------- > Equation 6.03

Es

Substituting values in equation 6.02 , we get

Substituting values in equation 6.03 , we get

= (1000 + 6x0.783x3000)0.116

(Sqr. Root 0.5)

(Sqr. Root 0.5)

(1000 +1.5x0.783x3000)0.116

2475.4707

741.90427 Et = Volts

Substituting values in equation 6.01 , we get

Volts

09.02

109.0

1+×

−×

−s

s

h

ρ

ρ

( )ts

BwfsxCsxρ5.11000+

( )ts

BwfsxCsx ρ61000 +

8 of 12 Earthing Calculation

7.0 CALCULATION OF LENGTH OF GROUND CONDUCTOR

7.01 = 3 Meters

7.02 = 23 Nos

7.03 = 37 Nos

7.04= 5009 Meters

7.05 = 3 Meters

= 47 Nos

= 141 Meters

7.06 = 354 Meters

8.0

8.01 Spacing factor for Mesh voltage Km (formula number 81 of IEEE 80, page number 93)

------ > Eqn 8.1

Where Kii = 1

Kh = = = 1.2649111

8.02 n = na X nb X nc X nd As per IEEE 80 page 93 formula number 84 ------ > Eqn 8.2

2 x 5009

354

0.5

nb = 1.02

nc = 1 for rectangular grids

nd = 1 for rectangular grids

8.03 Substituting values in equation 8.2, we get n = 28.30 x 1.02 x 1 x 128.73

8.04 Substituting values in equation 8.1, we get Km = 0.225

8.05 Correction factor for grid geometry (Ki) (formula number 89, page 94 of IEEE 80)

Ki = 0.644 + 0.148 n = 0.644 + 0.148 x 28.73 4.90

8.06 = 1 As per IEEE-80 Page No. 152 to 162

Hence IG = 15061 x 1.0 = 15061.31137 Amperes

8.07 Effective buried conductor length ( LM) for Mesh voltage

5009 + 141 = 5150 Meters

8.08 Mesh Voltage (Em) (formula number 80 of IEEE 80, page number 91)

176 x 15061 x 0.225 x 4.90

Em = 566.56 Volts

8.09

47 x 3

Length of buried Conductors in horizontal

grid Lc23 x 110 + 37 x 67

Spacing (3-15m) As per IEEE-80 Page No.88

Length of each ground rod Lr

Number of ground rods

Total length of ground rods LR

The peripheral length of grid Lp 2 (110 + 67 )

Volts

------ > Eqn 8.8

Substituting values in equation 8.8,we get Em =

The calculated mesh / touch voltage is = 566.56 volts. Which is less than the tolerable touch voltage of

741.90 Volts.Hence Safe.

5150

28.30

Em =

CALCULATION OF MESH / TOUCH VOLTAGE

=

Conductor spacing assumed (D)

No. of Conductors parallel to 110 meters

na = =

The switchyard fault level is 15061 Ampers. As per IEEE 80, page 153, a split factor of 1.00 can be considered.

LM = LC + LR =

No. of Conductors parallel to 67 meters

nb =354

4 x sqr.root (110x67 )

Split factor

( )

( )

22

21 8ln ln

2 16 8 4 2 1

D hD h KiiKm

hd Dd d Kh nπ π

+ = + − +

−

hoh+1

P

C

L

L.2

A

LP

×4

G m i

M

I K K

L

ρ

9 of 12 Earthing Calculation

9.0

9.01

9.02 Effective buried conductor length(Ls) for step voltage (formula number 93, page number 94)

LS = 0.75LC + 0.85LR ------ > Eqn 9.2

Substituting values in equation 9.2,we get Ls = 0.75 x 5009 + 0.85 x 141 = 3877

9.03 Step Voltage (Es) (formula number 92 of IEEE 80, page number 94)

176 x 15061 x 0.460 x 4.90

Em = 1539.43 Volts

9.04

10.0

10.1 As per IEEE 80, formula number 52, P.No. 65, the Grid resistance RG is given by:

Where

L = 5504

h = Depth of Conductor buried in meters = 0.6 Mtrs

A = Area of Grid = 110 x 67 Sq. mtrs 7370

ρ = 176

The calculated step voltage is = 1539.43 volts. Which is less than the tolerable Step voltage of 2475.47

Volts.Hence Safe.

0.460

3877

Substituting values in equation 9.1,we get Ks =

Substituting values in equation 9.2,we get Es = Volts

CALCULATION OF STEP VOLTAGE

Ks =

Es = ------ > Eqn 9.3

Soil Resistivity in Ohm-mtr

Spacing factor for Step Voltage (Ks). (formula number 94, page number 94)

------ > Eqn 9.1

Length of buried conductors in m. = 5009 + 141 + 354 Meters =

CALCULATION OF GRID RESISTANCE

------ > Eqn 10.1

( )

−+

++ − 2

5.0111

2

11 n

DhDhπ

G s i

S

I K K

L

ρ

1 1 11

2 0 2 01G

RL A h

A

ρ

= × + + × +

10 of 12 Earthing Calculation

0.9349 Ohms

10.2

11.0

Substituting values in equation 10.1,we get RG =

According to IEEE 80-2000 clause no. 14.1 (Usual requirements of ground resistance). For most large

substations, the ground resistance is usually less than 1Ω or less.

CONCLUSIONS

The calculated buried grid resistance is =0.935 Ohms., which is less than 1.0 Ohms as specified in IEEE

80. Hence the spacing of buried conductors at 3.0 meters is Safe.

11 of 12 Earthing Calculation

12.0

176

230

15061.31137

110

67

47

27

20

M.S. Rod

3

3

5363

23 Nos

37 Nos

566.563 Volts

1539.434 Volts

741.904 Volts

2475.471 Volts

0.935 Ohms

32 mm. M.S.Rod

Number of Earth Electrodes (considered in calculations)

Combined Resistance of Grid Electrode with earth electrode as per IEEE 80 in Ohms

Total Length of buried conductors in meters considered in calculations

Number of conductors parallel to Length of Substation

Number of Grid conductors parallel to Breadth of Substation

Calculated maximum step potential in Volts

Breadth of Substation in meters considered for calculations

Tolerable touch potential in Volts

Calculated maximum touch potential in Volts

Number of Earth Electrodes (Treated)

Type of Earthing conductor

Buried conductor size in mm

Tolerable step potential in Volts

Spacing of conductors parallel to Length of Substation in meters

Spacing of conductors parallel to Breadth of substation in meters

SUMMARY

Number of Earth Electrodes (untreated)

Length of Substation in meters considered for calculations

RMS value of the AC Ground fault current, which can flow through the conductor in Amps

Soil Resistivity in Ohm meter

Voltage level of the substation in Kv

12 of 12 Earthing Calculation