Page 764 - Industrial Power Engineering and Applications Handbook

Grounding practices 22/721
Tolerable touch voltage n, = no. of conductors lengthwise
b = width of the grid in m
0.116 nb = no. of conductors widthwise
Ji a = length of the grid in m and
E,,50, 5 (1.5 c, p, + 1000) x ~
for a 50 kg body (22.7) n = no of grounding rods
a = depth of grounding rods in m
0.157
Et(,,), 5 (I .5 c, p, + 1000) x __ (ii) For grid depths > 250 mm
zit
for a 70 kg body (22.8)

Design parameters
where A = a . b
I, = lg . Dt (22.9)
I, = maximum ground fault current
Zg = symmetrical ground fault current Measuring average resistivity of soil
D, = decrement factor
p=2mR, (22.15)
Body resistance
f??t$h = 6 . c, . p, + Rh Minimum size of grid conductors
= 6. C, . p, + 1000 (22.10) 1;
A= (22 161
f??fsh = total touch resistance through the body
Rh = body resistance
and.
A = cross-sectional area of ground conductor
Rzf,,\ = 1.5 x c\ ' p& + Rh in mm2
1; = ground fault current in kA (r.m.s.)
= 1.5 x c, ' p\ + 1000 (22.11)
= it may be substituted with I, (equation
R2fp5 = total step resistance through the body (22.9))
C, = reduction factor for derating the nominal value Tcap = thermal capacity factor in Jlcm'/"C
of surface layer resistivity, corresponding to t = duration of fault in seconds
a crushed rock layer of thickness, h, and a = thermal coefficient of resistivity at a
reflection factor k reference temperature of 20°C
where pz0 = resistivity of ground conductor at a
k=-- '-" reference temperature of 20°C in p Q/cm
and
P + P\ &)(at 0°C) = reciprocal of
= maximum allowable temperature in "C
I,,,,
p = ground resistivity in Qm tamb. = ambient temperature in "C
p, = crushed rock (gravel) resistivity in Rm
Ground resistance
Actual maximum touch and step voltages of a
(i) For grid depths up to 250 mm grounding station

(22.12) Max. step voltage, E,(actual) = P' K, ' K, ' IC (22.17)
L
R, = station ground resistance in R and mesh or maximum touch voltage,
p = average resistivity of soil in Rm
p. K,, . K, .I,
- 15 ground resistance at the E, (actual) = L (22.1 X)
4 ,;A=
surface of the ground For details refer to the text
__ = ground resistance of the total buried
length (L) of conductors Estimating the value of ground conductor length
to design the grounding grid
Total length of the buried conductors p' K, . K, .I, 4
L> (22.19)
L = IZ., . b + ilk a + II.. . h metres (22.13) ( I .5 C, . p5 + 1 000) X 0.1 57
h