Page 209 - Electrical Safety of Low Voltage Systems

192 Chapter Eleven

11.3.2 Metallic Layers of Cables as Protective Conductor
As already anticipated, metallic sheaths, or armors, of cables may
be used to carry the ground-fault current, or a portion of it, when
both their ends, supply side and load side, are linked to the earthing
system. In this configuration, a multiple return path may be available
for fault currents: sheaths/armors and protective conductors are in
parallel. The majority of the ground current will circulate through
the protective conductor, as they offer a much lesser impedance than
metallic sheaths/armors. It is assumed as a general rule of thumb that
no more than 15% of the fault current will circulate through metallic
sheaths/armors. However, the current-carrying capability of sheaths
and armors within the operational time frame of the protective device
must be assessed. There may be cases when metal layers of cables
cannot sustain any part of the fault current for the duration of the
fault, and therefore, should not be included in the fault-loop. This can
be achieved by lifting one end of metallic layers of cables.
However, by not bonding both ends of a cable’s metallic layers,
safety issues may arise. To this regard, let us consider Fig. 11.4 in
which a cable is shown whose metallic sheath is bonded only at one
end.
A medium-voltage ground fault simultaneously energizes the
ground-grid and the metallic sheath of a cable linked to it. The earth
potential is, therefore, transferred from the bonded end of the sheath
to the floating one. Along the ground grid, whose behavior can be
approximated by the combination of buried spherical electrodes, the
superficial earth potential is not constant, but decreases and assumes
its lowest value almost in correspondence with its edge (Fig. 11.5).
If the sheath is accessible (e.g., at its termination at the equipment
box) between the person’s hand (at fault potential V T ) and feet (at grid
potential), a dangerous potential difference V ST may exist.
Bonding connections between both ends of suitable metallic
sheath (i.e., capable to carry the fault current) of medium-voltage

FIGURE 11.4 Metallic sheath bonded only at one end.