Page 215 - Electrical Safety of Low Voltage Systems

198 Chapter Eleven

FIGURE 11.10
Neutral and PE
must not be
combined together
to form a second
PEN.

reliability of the connection of the PE to the PEN conductor, which is
considered crucial for the safety against indirect contact.
Once the system has become TN-S, neutral and protective con-
ductors must not be connected together to form a second PEN
(Fig. 11.10).
The reason behind this prohibition is the fact that PE and neutral
wire might not reach the high reliability required for a PEN conductor.
Among other things, both should be labeled as a PEN conductor to
prevent their accidental disconnection, for example, for maintenance
purposes.

FAQs

Q. Why in Fig. 11.1 is the cold water pipe indicated as an EXCP and not as
an earth electrode?
A. A cold water pipe cannot be relied on as an earth electrode, because the
user has no control over its electric continuity to ground. A water utility, in
fact, may interpose an insulating insert across the pipe to protect it against
corrosion.
However, the cold water pipe may be an EXCP, even in the presence of
the insulating insert, as the utility might eliminate it without warning the
customer. For these reasons, we must bond the water pipe for equipotential
reasons at the customer’s side of the meter, but not rely on it as an earth
electrode.
Q. Why is the assumption of adiabatic process during ground faults justified?
A. At the occurrence of a fault-to-ground, the heat developed by the Joule
effect is proportional to the square of the fault current. The heat released by
convention and/or radiation is proportional to the temperature of the con-
ductive material, which increases much more slowly than the fault current
(e.g., the temperature can increase four times when the current increases 50
times). This proportion shows the large disparity between the two energies
and, therefore, the possibility to neglect that released by convention/radiation
during the clearing time.