Page 274 - Electrical Safety of Low Voltage Systems

Applications of Electrical Safety 257

FIGURE 15.14 Hazardous condition in the presence of a sound PE.

The reduction of such voltage drop can be obtained by connecting
together all the ECPs and the EXCPs to a local equipotential earthing
bus located within the patient vicinity (Fig. 15.15).
This supplementary equipotential bonding connection lowers the
resistance of the protective conductor serving the faulty ECP (i.e.,
R PEA1 < R PEA ), thereby decreasing the touch voltage (i.e., V A1 < V A ).
It is important to note that due to the patient’s enhanced sensitivity
to electric currents, EXCPs in medical locations need to be redefined,
with respect to the standard definition that we gave in ordinary loca-
tions. In medical locations, in fact, IEC standards assume the threshold
of 25 V as the maximum permissible touch voltage, and a lethal current
for catheterized patients of 50 A. In these assumptions, the resulting
resistance-to-ground of any metal part in the patient vicinity must ex-
ceed 500 k in order not to be an EXCP, and therefore, not bonded to
the local earthing bus.

15.8.4 Electrical Separation
The local supplementary bonding connection, even though within the
patient vicinity, cannot always sufficiently decrease the resistance of
the PE and therefore limit the touch voltage to safe values. This is
true especially in TN systems, where the ground-fault current may be
rather high and so may the voltage drop on the PEs. In nonfault con-
ditions, instead, currents leaking from pieces of equipment connected
to the same local earthing bus are virtually identical and in phase, and