Page 80 - Electrical Safety of Low Voltage Systems

The Earth 63

4.6.2 Extraneous-Conductive-Part (EXCP)
EXCP is defined as a conductive part not belonging to the electrical
system, which can be touched and is liable to introduce a dangerous
potential into the premises, for example, the earth potential.
The danger introduced by EXCPs is caused by the possibility that
a person may be in simultaneous contact with these and with faulty
ECPs, whose enclosure is energized.
In this case, the feet potential of a person will be immediately
lowered to zero, even if he/she initially stands over an area at nonzero
potential as shown in Fig. 4.20. The EXCP, in fact, will “short circuit”
R BG (Fig. 4.22), and the advantage of having an additional resistance
between the person’s body and the earth will be lost.
The person will face an electric shock driven by V ST , instead of V T ,
and as a result his/her safety will be lowered.
The solution to this problem is the equipotentialization between
ECPs and EXCPs via an equipotential bonding conductor (EBC),
which, by linking together ECPs and EXCPs, keeps them at the same
potential in fault conditions. 15 The person, not insulated from earth,
is still at risk of electrocution, but will at least have “recuperated” his/
her R BG in series to his/her body resistance R B , as shown in Fig. 4.23.
Properly identifying EXCPs is, then, crucial in order to implement
the safe equipotentialization within the installation. Any metalwork,
even if entering the premises not from the earth, should be considered
as an EXCP “candidate.” Metal conduits, metal sheath, or armor of ca-
bles entering the premises from another building may, in fact, have
separate earthing connections and, therefore, introduce the earth po-
tential into the premises, or even nonzero potentials caused by faults
occurring in their building of origin.
In ordinary conditions, we can conventionally assume a R BG of at
least 1000 in series to the body resistance, even in the absence of
a floor. In particular locations (e.g., hospitals, construction sites, and
agricultural buildings), where human beings’ resistance to ground
may be lower, the value of 200 for R BG is used because additional
safety requirements must be met (e.g., the limit for the permissible
touch voltage is reduced to 25 V from 50 V).

FIGURE 4.22
Equivalent fault-
loop for a person
standing in an area
at nonzero potential
and in contact with
an EXCP .