Page 241 - Electrical Safety of Low Voltage Systems

224 Chapter Fourteen

FIGURE 14.1 The
Wenner method.

earthing systems, the actual site of its installation should be tested in
order to obtain this very important parameter.
The Wenner method is probably the most widely used technique
to create soil models. With this method, four small rods are driven
into the earth in a straight line and uniformly spaced by the distance
a. The rods are driven to a depth much smaller than a, so that we can
consider them as “point” electrodes.
With reference to Fig. 14.1, a known constant current I is then
injected between the outer rods A and D, and the resulting potential
difference V BC between the two inners pins B and C is measured.
Let us now consider the four rods as identical hemispherical elec-
trodes separated by the distance a in a uniform soil of resistivity .We
will assume that the length of their radii is very small compared to a,
so that the soil can be considered undisturbed by the presence of the
hemispheres (Fig. 14.2).

FIGURE 14.2 Earth potential distributions.