Earthing Mat Design Calculation Sheet

Prerequisites TECHNICAL DATA SYMBOL

DESCRIPTION

i2t Tm

Energy of max earth fault

Ta

Ambient Temperature

αr

Thermal coefficient of resistivity

ρr

Resistivity of earthing conductor

Max allowable fusing temperature

K0 TCAP

Thermal capacity of conductor per unit volume Soil resistivity Resistivity of the surface layer material Thickness of the surface layer Maximum fault clearing time Total length of buried conductors Depth of the earthing grid Earth fault current

Sf

Current division factor X/R ratio at the fault location

f

System frequency Duration of the fault Total length of horizontal grid conductors Length of grid conductors on the perimeter Total length of earthing electrodes / rods Length of each earthing electrode / rod

A'

Total area of the grid Maximum length of the grids in the x directions Maximum length of the grids in the y directions Maximum distance between any two points on the grid Spacing between parallel grid conductors Cross-sectional diameter of a grid conductor

A is the minimum cross-sectional area of the earthing grid conductor (mm2)

A = 16.630826229 Cs is the surface layer derating factor

Cs= 0.9990025

is the touch voltage limit (V) is the step voltage limit (V)

Etouch,50kg = 17.344084056 Etouch,70kg = 23.474320662

Estep,50kg=

17.499559145

Estep,70kg=

23.684748154

Rg is the earthing grid resistance with respect to remote earth (Ω) Rg = 0.0004898663

Ig is Symmetrical grid current Ig = 20

IG is the maximum grid current IG= 1.7247463558

GPR is the maximum ground potential rise GPR= 0.0008448952

ECHNICAL DATA

CRIPTION

VALUE

UNIT

3

A2s

9

°C

5

°C

tivity

0.006

°C-1

ductor

2

μΩ. cm

ature

146.666667

ctor per unit volume

terial

tors

5 0.003 2 45

Jcm-3°C-1

Ω.m Ω.m m

45

s

10

m

5

m

20

A

1 3 50

Hz

3

s

10

m

the perimeter

5

m

odes / rods

3

m

rode / rod

2

m

25

m2

in the x directions

5

m

in the y directions

6

m

any two points on the grid

4

m

d conductors

7

m

2

m

d conductors

grid conductor

area of the earthing grid conductor (mm2)

pect to remote earth (Ω)

x= 0.02603184

TA is the dc time offset constant

TA= 235.61945 Df is the decrement factor Df = 2.97474999

n is Geometric Factor

na= 4 nb=1, for square grids nc=1 , for square and rectangular grids

nd=1, for square, rectangular and L-shaped grids

NOW for square grids for square and rectangular grids for square, rectangular and L-shaped grids

n= n= n=

2.278474253 1.024295039 2.224431599

Ki is Irregularity factor Ki= 0.8126071

for

Kii is a weighting factor for earth electrodes /rods on the corner mesh Kii=1 for grids with earth electrodes along the grid perimeter or corners Kii = 0.4642175

Kh is a weighting factor for depth of burial Kh = 2.4494897

Km is the geometric spacing factor Km= 0.1506657 Lm is the effective buried length ,for grids with few or no earthing electrodes (and none on corners or along the perimeter) Lm= 13

Em is the mesh voltage Em= 0.0324868

Ks is the geometric spacing factor Ks= 0.0212511

Ls is the effective buried length Ls= 10.05

Es is the maximum allowable step voltage Es= 0.0059272

RESULT

50 kg:

EARTHING GRID DESIGN IS SAFE

RESULT

70 kg:

EARTHING GRID DESIGN IS SAFE

otherwise

nb= 0.5

otherwise

nc= 1.1122158

otherwise

nd= 0.5121475

otherwise

n= 1.1392371

the corner mesh

r or corners

ng electrodes

OR

Lm is the effective buried length , for grids with earthing electrodes on the corners and along the perimeter Lm= 15.58723

ds with earthing he perimeter

or