Page 202 - Electrical Safety of Low Voltage Systems

Earth Electrodes, Protective Conductors 185

The concrete, in fact, by absorbing and retaining moisture provides
around the conductive re-bars an even lower resistivity than that of
the local soil. As a consequence, materials suitable to be embedded
in concrete (e.g., hot-dip galvanized/stainless steel) should not be
coated with insulating materials, if they are to be used as electrodes.
The concrete-encased electrode makes a very effective earth electrode
and at no extra cost for the user. In the presence of more than one
concrete-encased electrode in a structure, it is sufficient to bond only
one to the main earthing bus, as the entire foundation network is
interconnected due to metal re-bars.
However, some may have concerns about the connection of the
steel foundation re-bars to other made-electrodes with higher elec-
trochemical potential (e.g., copper rods) eventually employed in the
earthing system. This bond, which creates a single electrode system
necessary to have an equipotential area, is feared to generate corro-
sion of the re-bars. Steel re-bars in concrete, in fact, may result anodic
to copper rods, and, therefore, undergo corrosion.
In reality, the electrochemical potential of steel, when embedded
in concrete, increases and reaches a value close to that of copper. In ad-
dition, the surface of earthing rods (i.e., the cathode) is much smaller
than the equivalent surface of the network of re-bars embedded in
foundations (i.e., the anode). Therefore, only negligible corrosion will
occur, especially in residential and commercial power systems, whose
earthing electrodes are usually limited in number. However, to com-
pletely eliminate the risk of corrosion of elements of foundations, it
would be best to use tin-coated copper rods in lieu of bare copper
ones, or employ hot-dip galvanized steel rods.

11.3 Protective Conductors
Protective conductors (PEs) provide safety against indirect contact by
linking ECPs to the main earthing terminal, thereby creating a clear
path for the fault currents. Cross-sectional areas of protective con-
ductors must be adequately large, so that fault currents can promptly
activate the protective device and automatically disconnect the sup-
ply.
Additionally, protective conductors must be able to withstand the
flow of the ground-fault current without reaching dangerous tem-
peratures to the surrounding environment or shorten the life of, or
damage, their insulation.
Minimumstandardcross-sectionalareasdeemedadequateforPEs
are shown in Table 11.4, when the protective conductor is of the same
material as the line conductor.
If a protective conductor is common to more than one circuit, it
must be selected in correspondence with the phase conductor’s largest