Page 754 - Industrial Power Engineering and Applications Handbook

periphery of the grid. The ground current discharging
20 -- 50 through a uniformly spaced ground grid is scanty at the
25 -- 40 centre, dense at the edges and a maximum at the corners.
30 I- 30 Accordingly, the worst step and touch voltages would
40 - occur at the outer meshes of the grid, especially at the
t 50-- 2o corners. To make the current density more uniform, a
more non-uniform conductor spacing would therefore
loo-- 10 be necessary with more number of meshes at the centre
4 and fewer towards the periphery. Increasing the number
i..
,o of meshes, i.e. reducing the conductor spacing, would
3 200-- 5
2 tend to reduce the step and touch voltages until a saturation
8 stage is reached, i.e. when, L, approaches Lc. The factor
L 1.15 may now be increased to 1.2 based on field experience
400-- 2.5 and
.3
600-
800 - K, = 0.656 + 0.172 n
1000 - 1 .o
where
n = the number of conductors on each side of a square
grid. If the ground grid is not a square and the number
of conductors lengthwise is IZ, and widthwise nb then
r
-
n = \'ind . n,,
Estimating the maximum touch voltage, E,,,
(actual)
This will largely depend upon the ratio of the current
densities in the far-end conductors, i.e. conductors at the
periphery, and the innermost conductors and can be
expressed as follows, based on extensive research:

Figure 22.18 Nomogram for conductor sizing at 40°C ambient
temperature
K
+ 2 log,
Kh n(2n- 1)
where

where
w=;;-+-+-+ For grids without or with only a few ground rods near
1
1
1
1
L 3 4 n-1 the inner grid (not at the far ends):
K, = 41+$
For n 2 6
WE- ' + log, (n - 1) - 0.423
2(n - 1)
where ho is the reference depth of grid = 1 m
The step voltage falls sharply at higher depths. The values of Ki and L can be determined along similar
L = L, + L, when there are only a few lines to those for the step voltage.
ground rods at the periphery
of the grid Note
In the above equations for maximum step and touch voltages the
or = L, + 1.15L,. when there are proportionately best results will be obtained when the following parameters are
more ground rods at the far achieved:
end of the grid n 5 25
where Lc = total length of the grid conductors and 0.25 2 h 5 2.5 m
L, = total length of the ground rods
d < 0.25 h and
The factor 1.15 represents the higher current density
in the ground rods that are placed at the far ends or D > 2.5 m