Page 200 - Electrical Safety of Low Voltage Systems

Earth Electrodes, Protective Conductors 183

Type of Electrode R G
Rod /L
Buried horizontal wire 2 /L
Grid /4r
TABLE 11.1 Approximate Formulas of the
Earth Resistance of Typical-Made Electrodes

France, Germany, Switzerland, and the U.K.). Users, in fact, having
no control over the cold water system, cannot rely on its electric conti-
nuity to ground. However, only with the explicit consent of the water
utility, which guarantees the aforementioned continuity, water pipes
may be relied on as earth electrode (e.g., in Italy).
The same prohibition applies to metal pipes for flammable liquids
or gases. However, the above rule pertaining to earth electrodes does
not preclude the protective bonding of any metalwork entering the
building to the earthing system, for example, by connecting pipes
downstream of their water or gas meters. As we know, this connection
is indispensable in ensuring a safe equipotential area into the user’s
premises and cannot be omitted.

11.2.1 Corrosion Phenomena
Earth electrodes must have a minimum size in order to have adequate
mechanical strength and withstand corrosion.
Corrosion is an electrochemical process that involves two dissimi-
lar metals electrically connected when embedded in electrolytes, such
as earth, concrete, seawater, etc. The two metals, respectively, assume
the role of cathode and anode of a galvanic cell. When the current
leaves the anode, to reclose to the cathode through the electrolyte,
corrosion at the expense of the anode occurs. The phenomenon is
more pronounced when the ratio of the cathode’s surface to the an-
ode’s surface is large. The rule of thumb is to expect appreciable cor-
rosion only when the cathode’s surface is 100 times bigger than the
anode’s.

FIGURE 11.2 Radius
of the circle
circumscribing the
grid.