Input Parameters taken for calculation:

Project Name = …………Customer = ………..Document No. = ………..PO No./ LOA No. = ………..

Weight of Person Kg = 50

= Symmetrical fault current, A = 40000

= Fault current division factor = 0.7

= Duration of shock for determining allowable body current, sec = 1

= 3000

= Surface layer thickness, m = 0.175r = 20

h = Depth of ground grid conductors, m = 30

= Number of Electrodes = 71

= Length of Individual Electrode, m = 3d = Diameter of Conductor/Equivalent diameter in case of Flat cond, m = 0.04L = Maximum length of grid conductor in x direction, m = 47

W = Maximum length of grid conductors in y direction, m = 103

= Number of Conductor along the length wise = 17

= Number of Conductor along the width wise = 35D = Average spacing between two conductors = 3

Input Parameters for Ampacity calculation:Type of conductor: mild Steel

20 °C

#REF! mm2

4.50E-03 1/°C

500 °C

45 °C

202.00022

13.8

TCAP= Thermal Capacity per Unit Volume= 3.8 J/cm³ · °C

1 sec

I= Current for sizing of Earthmat conductor = 40 kA

I f

Sf

ts

rs Surface layer resistivity, W-m =

hs Soil resistivity, W-m =

nR

Lr

NL

NW

Tr = Reference Temperature for Material Constants =

Amm2 = Minimum conductor cross section area required =

ar = Thermal Coefficient of Resistivity at Reference Temperature =

Tm= Fusing Temperature of Conductor =

Ta= Ambient Temperature =

K0= (1/ar ) – Tr =

rr =Resistivity of Conductor at Reference Temperature = mW-cm

tc= time duration for sizing the eartmat conductor =

Out Put of Earthing Design Calculation

Minimum Conductor Cross section area Required #REF!

Minimum Diameter of Conductor required 0

Value of Grid Resistance 0

Attainable Tolerable

Step Potential #REF! 0.00

Touch Potential #REF! 0

The Attainable touch as well as Step voltage should be less than tolerable limit.

Reference Drawings:

1. ………………………..

Reference Document :

1. IEEE-80 For Safety in AC Substation Grounding2. Design of Earthing Mat for High Voltage Sub-Station (CBIP - Manual)

CUSTOMER:

………..

Prepared & Checked: Approved: PROJECT:…………

REV DATE PO No/ LOA No : ………..

Title:

EARTHMAT DESIGN CALCULATION

abcd ALSTOM PROJECTS INDIA LTD. Rev

DOC No.

1 Following nomenclature are used in the calculation:

1 r =

2 =3 I f = Symmetrical fault current in substation [A]

4 A =

5 = Surface layer de-rating factor6 d = Diameter of grid conductor [m]7 D = Spacing between parallel conductors [m]

8 = Attainable Touch Voltage [V]

9 = Attainable Step Voltage [V]

10 = Tolerable step voltage for human with 50 kg body weight [V]

11 = Tolerable touch voltage for human with 50 kg body weight [V]

12 = Tolerable step voltage for human with 70 kg body weight [V]

13 = Tolerable touch voltage for human with 70 kg body weight [V]14 h = Depth of Buriel of Earthing conductors [m]

15 = Surface Gravel layer thickness [m]

16 = Maximum grid current that flows between ground grid and surrounding earth [A]

17 = Symmetrical grid current [A]18 K = Reflection factor between different resistivities

19 = Corrective weighting factor that emphasizes the effects of grid depth,

20 = Correction factor for grid geometry

21 = Corrective weighting factor that adjusts for the effects of inner conductors

22 = Spacing factor for mesh voltage

23 = Spacing factor for step voltage

24 = Total length of grid conductor [m]

25 =

26 = Periphery Length of the earthing equivalent area [m]

27 = Total length of ground rods [m]

28 = Length of Individual electrode

29 =

30 = Total effective length of grounding system conductor, including grid and ground rods [m]

31 =L= Maximum length of grid conductor in X direction [m]

32 =W= Maximum length of grid conductors in Y direction [m]

33 n =

34 = Number of Electrodes

35 =

36 = Duration of shock for determining allowable body current [sec]

Soil resistivity [W-m]

rs Surface layer (crushed rock) resistivity [W-m]

Total area enclosed by ground grid [m2]

Cs

Em

Es

Estep50

Etouch50

Estep70

Etouch70

hs

IG

Ig

Kh

Ki

Kii

Km

Ks

Lc

LM Effective length of Lc + LR for mesh voltage [m]

Lp

LR

Lr

LS Effective length of Lc + LR for step voltage [m]

LT

Lx

Ly

Geometric factor composed of factors na , nb , nc , and nd

NR

Rg Resistance of grounding system [W]

ts

2 Input Parameters taken for calculation:

= Symmetrical fault current in substation for conductor sizing, = 40000 A

= Fault current division factor = 0.7

= Duration of shock for determining allowable body current, = 1 sec

= 3000

= Surface layer thickness, = 0.175 mr = 20

h = Depth of ground grid conductors, = 30 m

= Number of Electrodes = 71

= Length of Individual electrode, = 3 md = Diameter of Conductor/Equivalent diameter in case of Flat conductor = 0.04 mL = Maximum length of grid conductor in x direction = 47 m

W = Maximum length of grid conductors in y direction = 103 m

= Number of Conductor along the length wise = 17

= Number of Conductor along the width wise = 35

Design Steps :

3 Ampacity calculation

Type of conductor: mild Steel

20 °C

#REF! mm2

4.50E-03 1/°C

500 °C

45 °C

202.0002

13.8

TCAP= Thermal Capacity per Unit Volume= 3.8 J/cm³ · °C

1 sec

I= Current for sizing of Earthmat conductor = 40 kA

As specified in IEEE std-80-2000

Equation no: (37)

=

I f

Sf

ts

rs Surface layer resistivity, W-m =

hs Soil resistivity, W-m =

nR

Lr

NL

NW

Tr= Reference Temperature for Material Constants =

Amm2= Minimum conductor cross section area required =

ar =Thermal Coefficient of Resistivity at Reference Temperature =

Tm= Fusing Temperature of Conductor =

Ta= Ambient Temperature =

K0= (1/ar ) – Tr =

rr =Resistivity of Conductor at Reference Temperature = mW-cm

tc= time duration for sizing the eartmat conductor =

Amm2

Minimum conductor diameter required dmin =

As per "Design of Steel earthing Grids in India" IEEE Transactions on power apparatus andsystems, volume PAS- 98 no.6, Nov/Dec-1979. PP 2126-2134, Corrosion allowance is recommended as per following Rules

15 mills = 0.381 mm

91.5 mills = 2.324 mm

180 mills = 4.572 mm

= + 2 x 4.572

= 9.14

Minimum conductor diameter with corrosion allowance

Minimum Cross section required with corrosion allowance = 1000.000 mm2

Cross section of selected conductor = 1256.637 mm2

Selected Conductor Cross section =

Calculation for Tolerable Touch and step Potential for Person of 50Kg

As specified in IEEE std 80-2000Equation no: (27)

=

Following equation can be used to compute the tolerable touch and step voltages respectively

As specified in IEEE std 80-2000Equation no: (29)

= V

As specified in IEEE std 80-2000Equation no: (32)

= V

_ In case of conductor to laid in soil having soil resistivity greater than 100 ohm-meter -

_ In case of conductor to laid in soil having soil resistivity from 25 to 100 ohm-meter -

_ In case of conductor to laid in soil having soil resistivity Lower than 25 ohm-meter -

dmin with corrosion = dmin + Corrosion Allowancedmin

The reduction factor (Derating Factor) Cs can be approximated by the equation

Cs

Estep50

Etouch50

Calculation for Tolerable Touch and step Potential for Person of 70Kg

=

Following equation can be used to compute the tolerable touch and step voltages respectively

As specified in IEEE std 80-2000Equation no: (30)

= VAs specified in IEEE std 80-2000Equation no: (33)

= V

The reduction factor (Derating Factor) Cs can be approximated by the equation

Cs

Estep70

Etouch70

4 Grid Configuration

Refer Annexure -I for Grid Configuration

Maximum Length in X- Direction L = 47

Maximum Length in Y- Direction W = 103

17

35

71

3

= 3590.333 m

Total Area of Earth Mat, A = 4841 m2

5 Determination of Grid ResistanceAs specified in IEEE std 80-2000

Equation no: ( 52)

= Ohm

6 Calculation of Attainable step and touch voltage

= 1 For grid with ground rods in perimeter

D = Average spacing between two conductors = 3

Geometric factor

As specified in IEEE std 80-2000

Equation no: (85)

Lc = Total Length of Grid Conductor

Lc = 3377.333

Lp = Periphery Length of earthing equivalent Area 2 x ( L + W )

Lp = 300

= Total length of all ground rods(electrodes) in m Nr x Lr

= 213

= 22.52

As specified in IEEE std 80-2000

Equation no: (86)

= 1.038238

Number of Conductor along the lengthwise NL =

Number of Conductor along the widthwise NW =

Number of Electrodes Nr =

length of Individual Electrode Lr =

LT

Rg

Kii

( L x Nw ) + ( W x NL )

LRLR

na

nb

= 1 for square and rectangular grid

= 1 for square and rectangular grid and L-shaped grid

n = Equation no: (84)

n = 23.3765

As specified in IEEE std 80-2000Equation no: (83)

5.567764

Now,As specified in IEEE std 80-2000Equation no: (80)

As specified in IEEE std 80-2000Equation no: (92)

Equation no: (81)

= #REF!

As specified in IEEE std 80-2000Equation no: (89)

= 4.103722

As specified in IEEE std 80-2000Equation no: (91)

= #REF!

=

= #REF! A

= #REF! V

As specified in IEEE std 80-2000

Equation no: (94)

nc

nd

na x nb x nc x nd

Kh

Km

Ki

Lm

Ig Sf x If

Ig

Em

=

= #REF!Ks

As specified in IEEE std 80-2000

Equation no: (93)

= #REF!

= #REF! V

7 Safety check

For the safe earthing design , Attainable step & Touch voltages should be less than Tolerable values respectively.

Attainable Tolerable

#REF! 0.000

#REF! 0.00

Ls

Es

Touch voltage in V

Step voltage in V