P103-E0020-001-R0- Earthing Calculation-20080216

VA TECH WABAG No. 11 Murray's Gate Road, Alwarpet Chennai 600 018 India

CLIENT

:

Public Health & Engineering Department

PLANT

:

195 MLD Water Treatment Plant

PROJECT

:

Narmada Water Scheme

DOCUMENT TITLE:

EARTHING CALCULATION

Master DOC. NO.

: KKV/NWS/VAT/P103-E0020-001

VATECH DOC. NO.

REV. NO

:

0

: P103-E0020-001

STAMPING AREA

0

16.02.08

ISSUED FOR INFORMATION

AG

PVO

BT

Rev No

Date

Description

Prepared

Checked

Approved

Discipline

:

ELECTRICAL

Total No of Sheets : 8 (Including cover sheet)

Contents

Sl No.

Title

Sheet No.

01

Steps for Earthing system Design Calculations

03 & 04

02

Calculation of Size of Earth Strip

05

03

Calculation of Resistance of Earth Electrode

06

04

Calculation of Resistance of Earth Strip

07

05

Verifying the Total Grid Resistance

08

Notes : 01 02 03 04

Soil Electrical Resistivity is Considered = 50 ohm-m System Fault Level Considered for 1 second = 25 KA Contribution of Earthing Strip is Added for Calculating Quantity of Electrodes Total Length of Buried Conductor is Minimum Required for the Earthing and the same will be shown in the Earthing Layout-Overall plant area Drg. No. P103-E0021-01

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SHEET 2 of 8

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Steps for Earthing System Design Calculations 1. Reference Standard IS : 3043 – Code of Practice for Earthing 2. Design Data a) Soil resistivity - 50 Ohm-m b) Power distribution system nominal fault level @415V - 25kA (as assumed) c) Duration of fault level for the purpose of conductor sizing – 1.0 sec. (as assumed) 3. Steps for design calculations STEP 1 -

To determine minimum size of earth conductor (Sheet 5 of 8) According to Cl. 12.2.2.1, IS:3043 S = I t x k

Where , I S k t

STEP 2 -

Value of earth fault current of negligible impedance which can flow through the protective device, in ampres Cross sectional area in Square millimeters Constant depends on material of earth strip Operating time of the disconnecting device in seconds

Calculate Resistance of Earth Electrode (Sheet 6 of 8) According to (Cl. 9.2.2, IS:3043) 100ρ Re

=

X

4L Log e d

2πL Where , Rs ρ L

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Resistance of the earth strip Resistivity of the Soil Length of the strip in CM

SHEET 3 of 8

Ohms Ohm-m

PREPD BY

DATE

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16.02.08

Steps for Earthing System Design Calculations STEP 3 - Calculate Net Resistance of earth electrode (Sheet 6 of 8)

Rearth

Re =

N Where , Re

Resistance of the pipe electrode

N Rearth

No. of earth electrodes (earth pits) considered Net resistance of earth pits.

Ohms Nos. Ohms

STEP 4 - Calculate resistance of earthing strip (Sheet 7 of 8) According to Cl. 9.2.3, IS:3043 Rs = 2 L2 100 ρ X Log e 2πL ω t Where , Rs Resistance of the earth strip Ohms Resistivity of the Soil Ohm-m ρ L Length of the strip in CM Depth of the burial in CM ω cm t Strip Width

STEP 4 - Calculate Total Grid Resistance (Sheet 8 of 8) Rearth

x Rs

Rearth

+ Rs

Total grid resistance =

Ohms

Where , Rearth Rs

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P103-E0020-001

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Net resistance of earth pits. Resistance of the earth strip

SHEET 4 of 8

Ohms Ohms

PREPD BY

DATE

AG

16.02.08

Calculation of Size of Earth Strip Formula Used For Calculation (Cl. 12.2.2.1, IS:3043)

I

1 =

k

S

t

I

Value of (ac rms) earth fault current of negligible impedance which can flow through the protective device, in amperes

25

kA

S

Cross sectional area in Square millimeters

?

Sq.mm

k

Constant depends on material of earth strip (Table 6A, IS:3043)

80

(for steel)

t

Operating time of the disconnecting device in seconds

1

Sec.

S =

I

x

t

k =

25

x

x

1000

1

80 25 x 1000

x

1

=

=

312.50

Sq.mm

80

Minimum cross section of Earthing Conductor (Calculated) = Size of selected Earthing Conductor = Cross sectional area of 50 x 10mm G.I Earthing conductor =

312.50

Sq.mm

50 x 10

mm

500

Sq.mm

Thus, selected size of Earthing Conductor meets the requirements

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SHEET 5 of 8

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Calculation of Resistance of Earth Electrode Resistance of Pipe (or) Rod Electrode (Cl. 9.2.2, IS:3043) 100ρ Re = 2πL Re ρ L d Rearth

4L

X

log e

d

Resistance of the pipe electrode, ohms Resistivity of the Soil, in ohm-m Length of the pipe, in cm Diameter of the Rod(or)Pipe, in cm Net resistance of earth pits.

100 Re

50

x x

3.14

5000

X

=

4

X

=

2

Re

? 50 300 4

1884

Re

=

2.654

X

Re

=

2.654

X

Re

=

15.137

x 300

x

300

4

1200 log e

log e

X

log e

(SPECIFIED) (SPECIFIED)

4

300.000

5.704

Ohms

No. of Electrical earth electrodes (earth pits) considered (N) =

4

(additional 4 nos of Lightning Rod Earth Electrodes & 2 nos of Test Pipe Earth Electrodes inside the grid not considered in calculation) (as per Earthing Layout Dwg. Overall plant area Drg. No. P103-E0021-01)

Rearth

Re =

N 15.137 Rearth

=

4

Rearth

3.78 Ohms

=

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Calculation of Resistance of Earth Strip According to Cl. 9.2.3, IS:3043 Formula Rs = 100 ρ

2 L2 X log e

2πL Rs ρ L ω t

ω t

Resistance of the earth strip, in ohms Resistivity of the Soil, in ohm-m Length of the strip, in cm Depth of the burial, in cm Strip Width, in cm Rs

100

=

2x

3.14

? 50 37500 60 5

x

50

x

37500

5000 Rs

X log e

2

x

1406250000

60

x

5

2812500000 X log e

=

235500

300

Rs

=

0.02

X log e

9375000

Rs

=

0.02

x

16.05

Rs

(SPECIFIED)

=

0.34

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Ohms

SHEET 7 of 8

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Verifying The Total Grid Resistance We know, Rearth = Rs

=

3.78 Ohms (Pl. Refer Sheet No. 6 of 8) 0.341 Ohms (Pl. Refer Sheet No. 7 of 8)

Rearth

x

Rs

Rearth

+

Rs

3.78

x

0.341

+

0.341

Ohms

Total grid resistance =

Ohms

Total grid resistance = 3.78

1.2903 Ohms

Total grid resistance = 4.1250

0.31

Total grid resistance =

Ohms

Conclusion : 1. Main Earthing grid conductor size - 50 x 10 mm G.I Flat. 2. Overall earthing system resistance - 0.31 Ohms

VA TECH Doc. No.

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P092-E0020-01

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SHEET 8 of 8

< 1 ohm

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DATE

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16.02.08