Page 199 - Electrical Safety of Low Voltage Systems

182 Chapter Eleven

FIGURE 11.1 Elements of the earthing arrangements.

11.2 Earth Electrodes
Earth electrodes must provide a reliable link to ground, primarily for
safety purposes, but also for the proper functioning of equipment.
As already substantiated in Chap. 4, electrodes must be able to carry
ground-fault currents and dissipate them to ground, without causing
hazards caused by thermal effects and/or electric shock. The effec-
tiveness of the earthing system depends upon its ground resistance
R G , which varies with the resistivity of the local soil. Once the charac-
teristic of the soil and the minimum acceptable value for safety of R G
are known, one or more ground electrodes, even of different nature,
must be employed.
Approximate values of the earth resistance at 50/60 Hz of typical-
1
made electrodes may be calculated by using the formulas reported
in Table 11.1.
L is the length of the electrode in contact with the soil, is the soil
average resistivity, and r is the radius of the circle that circumscribes
the grid (Fig. 11.2).
In several countries, cold water pipes are not permitted as earth
electrodes under any circumstances (e.g., Austria, Belgium, Finland,