Earthing Resistance Calculations is-3043

ELECTRICAL

EARTHING REASISTANCE CALCULATION AS PER IS:3043 Document No.

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ELECTRICAL

EARTHING REASISTANCE CALCULATION AS PER IS:3043 Document No.

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Table of Contents Section

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General Notes

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Calcualtion Basis

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Earthing Resistance for Strip and Pipe electroed

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Earthing Resistance for Strip, Cable and Pipe electroed

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GENERAL NOTES 1. This document is an annexure to GS E&C Main document " Grounding Calculations" Document no. 2. The calculations have been done based on GI Strip and Pipe data considered from IS:3043 3 The Calculations are subject to revision as & when the main document gets revised due to change in any Data

PURPOSE This calculation describe the Grounding calculation of the

REFERANCE DOCUMENTS 1 2 3 4 5 6

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EARTHING REASISTANCE CALCULATION AS PER IS:3043 Document No.

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A. EARTHING CALCULATION AS PER IS:3043 The following formulas are used for the spesification IS:3043 for calculations of Main Earthing Loop Resistance for Pipe Electrode and Strip Electrode and theselection of Cross sectional area of Earth Conductor. A.1

Resistance of Pipe Electrode Where, L = Length of of Rod/pipe (cm) Rp = 100 x ρ loge 4L

2 L

A.2

Ohms

d= Diameter of pipe (cm)

d

ρ = Resistivity of Soil (ohm-meter)

Resistance of Strip Electrode Where, L = Length of of Strip (cm) Rs = 100 x ρ loge 2L2 Ohms

2 L A.3

w = Depth of strip laying (cm)

wT

ρ = Resistivity of Soil (ohm-meter) T = Width of strip (cm)

Earth Conductor Size Where, I =RMS Value of Fault Current (Amp) S = Cross section area of conductor (mm²) t = Operating Time of disconnecting device (seconds) K = Factor dependent on material of conductor (taking from IS:3043 Table)

S = I√t K

A.4

Total Earth Electrode Area A = If / Id

Where, If = Fault Current (amp) Id = Electrode Current Dencity (A/m²)

Id = ( 7.57 x 103 ) / √ρ x t

Where, Id = Electrode Current Density (A/m²) ρ = Resistivity of Soil (ohm-meter) t = Operating Time of disconnecting device (seconds)

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EARTHING CALCULATION FOR GI STRIP AND GI PIPE ELECTRODE A. BASIC DATA REQUIRED FOR THE CALCULATION Maximum Fault Level

: : 3000 MVA : 254 MVA : 35 MVA : Second 1 : Ohm-meter 51 : 10% : Galvanised Iron Strip : Galvanised Iron Pipe : Lap welded : °C 50 : Ohm 1 80

132 kV 6.6 kV 415 V System Designed for Fault Duration Soil Resistivity (PS) Voltage Variation Earth Conductor Eartth Electrode Type of joints Max. Ambient temp. below ground Desired resistance of Erath grid K Factor for Steel B. FAULT CURRENT CALCULATION System Volatge (V) 132,000 6,600 415

Fault Level (MVA) 3,000.00 254.00 35.00

Fault Current (kA) 13.12 22.22 48.69

The earth conductor to be selected on the basis of highest fault current, i.e.,

48.69

kA at 415 Bus Voltage

C. CROSS SECTION AREA OF EARTH CONDUCTOR S = I√t = 608.67 K GI Strip Cross sectional ara Height (mm) Width (mm) 40 5 40 10 65 5 65 8 65 10

Sq. mm

Cross Sectional Area (mm²) 200 400 325 520 650

Hence, the GI Strip used for the main earthing mat is

65 x

No. of Conductor 3.04 1.52 1.87 1.17 0.94 10

mm² =

1

Nos.

D. CROSS SECTION AREA OF EARTH ELECTRODE Earth Electroed Current Density Id = ( 7.57 x 103 ) / √ρ x t Earth Electrode Area Taking extra Electrode Area (%) The Final Erath Electrode Area

: : : : :

1060.01 45.94 10 50.53

Amp / m² m² m²

THE TOTAL EARTH ELECTRODE AREA CONDIDERED FOR THE PLANT Perimeter of the unit : 2000 Meter Length of Strip Electrode : 2000 Meter Total surface area of a Strip Electrode : 260.00 Sq. Meter ……………….. (A) Nos. of GI Electrode Pipe Used in the plant Length of the GI Pipe Electrode Effective Buried Length Diameter of the GI Pipe Electride Total Surface area of the GI pipe Electrode

: : : : :

50 3.5 2.5 50 19.64

Nos. Meter Meter mm Sq. Meter …………………..(B)

Total surface area of GI Strip and GI Pipe = (A)+(B) =

:

279.65

Total Number of Earth Electrode

:

5.53

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EARTHING CALCULATION FOR GI STRIP AND GI PIPE ELECTRODE……… Contd. E. EARTH RESISTANCE OE THE STRIP ELECTRODE Resistivity of soil Length of strip Depth of Buried electrode Width of strip

: : : :

51 200000 50 6.5

EARTH RESISTANCE : Resistance for each strip electode Rs = 100 x ρ loge 2L2 Ohms

2 L

Ohm-meter Cm Cm Cm

: :

0.0784

Ohm

: :

1 0.0784

Ohm

wT

Nos. of Strip Electrode in parallel Resistance for all parallel Strip Eelctroed

………………………………. ( C )

F. EARTH RESISTANCE OE THE PIPE ELECTRODE Effective Buried Length Diameter of the GI Pipe Electride No. of Pipes in parallel (N)

: : :

250.00 5.00 50

Cm Cm Nos.

Earth resistance for each GI Pipe Electrode : Rp = 100 x ρ loge 4L

2 L

Ohms

=

17.19

Ohm

d

= R2

Resistance for all parallel GI Pipe Electroed

= (Rp/N) =

0.344

Ohm

…………………………………(D)

Combined Resistance of Earth due to Pipe Electrode and Strip Electrode =

R1 x R2 (R1 +R2)

=

0.0638

Ohm

Grounding system is valid for the combined resistance less than 1.0 Ohm GROUNDING SYSTEM IS

=

ACCEPTABLE

SUMMARY A. GI STRIP ELECTRODE SIZE OF ELECTRODE WIDTH (mm) HEIGHT (mm) 65 10

NO. OF ELECTRODE 1

B. GI PIPE ELECTRODE SIZE OF ELECTRODE LENGTH (mm) DIA (mm) 3500 50

NO. OF ELECTRODE 50

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EARTHING CALCULATION FOR GI STRIP ELECTRODE AND CABLE ELECTRONICS EARTH STATION A. BASIC DATA REQUIRED FOR THE CALCULATION Maximum Fault Level

: : 3000 MVA : 254 MVA : 35 MVA : Second 1 : Ohm-meter 51 : 10% : Galvanised Iron Strip : Galvanised Iron Pipe : Lap welded : °C 50 : Ohm 1 80

132 kV 6.6 kV 415 V System Designed for Fault Duration Soil Resistivity (PS) Voltage Variation Earth Conductor Eartth Electrode Type of joints Max. Ambient temp. below ground Desired resistance of Erath grid K Factor for Steel B. FAULT CURRENT CALCULATION System Volatge (V) 132,000 6,600 415

Fault Level (MVA) 3,000.00 254.00 35.00

Fault Current (kA) 13.12 22.22 48.69

The earth conductor to be selected on the basis of highest fault current, i.e.,

48.69

kA at 415 Bus Voltage

C. CROSS SECTION AREA OF EARTH CONDUCTOR S = I√t = 608.67 K GI Strip Cross sectional ara Height (mm) Width (mm) 40 5 40 10 65 5 65 8 65 10

Sq. mm

Cross Sectional Area (mm²) 200 400 325 520 650

Hence, the GI Strip used for the main earthing mat is

65 x

No. of Conductor 3.04 1.52 1.87 1.17 0.94 10

mm² =

1

Nos.

D. EARTH RESISTANCE OE THE STRIP ELECTRODE + CABLE Length of strip Depth of Buried electrode Width of strip

: : :

50000 50 6.5

Cm Cm Cm

EARTH RESISTANCE FOR STRIP ELECTRODE : Resistance for each strip electode : Rs = 100 x ρ log 2L2 Ohms e

:

0.2685

Ohm

Nos. of Strip Electrode in parallel Resistance for all parallel Strip Eelctroed

: :

1 0.2685

Ohm

EARTH RESISTANCE FOR CABLE Cable Cross Section Area Length of Cable Resistance of Cable Total Resistance of the Cable

: : : :

70 500 0.321 0.1605

Sq. mm Meter Ohm / Km Ohm …………………………………( B )

2 L

wT ………………………………. ( A )

:

TOTAL EARTH RESISTANCE FOR STRIP AND CABLE R1 = (A) + (B) =

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EARTHING REASISTANCE CALCULATION AS PER IS:3043 Document No.

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EARTHING CALCULATION FOR GI STRIP ELECTRODE AND CABLE ELECTRONICS EARTH STATION E. EARTH RESISTANCE OE THE PIPE ELECTRODE Resistivity of soil Effective Buried Length Diameter of the GI Pipe Electrode No. of Pipes in parallel (N)

: : : :

51 250.00 5.00 8

Ohm-meter Cm Cm Nos.

Earth resistance for each GI Pipe Electrode : Rp = 100 x ρ loge 4L

2 L

Ohms

=

17.19

Ohm

d

Resistance for all parallel GI Pipe Electroed

= R2

= (Rp/N) =

Combined Resistance of Earth due to Pipe Electrode and Strip Electrode =

R1 x R2 (R1 +R2)

=

0.3576

Ohm

2.1493

Ohm R1 R2

= =

0.4290 2.1493

Ohm Ohm

Grounding system is valid for the combined resistance less than 1.0 Ohm GROUNDING SYSTEM IS

=

ACCEPTABLE

SUMMARY A. GI STRIP ELECTRODE SIZE OF ELECTRODE WIDTH (mm) HEIGHT (mm) 65 10

NO. OF ELECTRODE 1

B. EARTING CABLE CABLE TYPE CONDUCTOR DIA (mm²) Cu 70

LENGTH OF CABLE (METER) 500

C. GI PIPE ELECTRODE SIZE OF ELECTRODE LENGTH (mm) DIA (mm) 2500 50

NO. OF ELECTRODE 8

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