Page 69 - Electrical Safety of Low Voltage Systems

52 Chapter Four

FIGURE 4.8 Identical hemispherical electrodes connected in parallel and
displaced by distance d.

current and the uniform resistivity of the soil. Each electrode will
1
leak current / 2I, as per the symmetry of the system.
If the electrodes are too close to each other, they interfere, caus-
ing a change in the shape of the total potential with respect to their
own single potential curve. The curve of the total potential can be
determined by means of the superposition principle, considering the
contributions of each electrode separately, and, then, adding them up.
In Fig. 4.9, a possible result of this procedure is shown.
Because of their metal connection, whose resistance we consider
negligible, the two electrodes will attain the same earth potential
V GTOT , which we can calculate as follows. If we consider two different
systems of reference, originating at the electrodes A and B (Fig. 4.8 or
4.9), and define R GTOT as the total earth resistance of the two-electrode

FIGURE 4.9
Equivalent potential
due to two
interfering
electrodes.