Page 150 - Electrical Safety of Low Voltage Systems

TN Grounding System 133

FIGURE 7.18 TN-C system and PEN conductor.
this practice may cause several technical issues. Let us now examine
Fig. 7.18.
If the electrical three-phase system is not balanced, the neutral
current is nonzero and will not only legitimately circulate through the
PEN conductor, but also, inevitably, through the ECPs and the EXCPs
that are part of the main and supplementary equipotential bonding
system. Neutral currents, which in the ordinary functioning of the
installation return to the source via paths not specifically designed
to carry them, may be defined as “stray currents.” In addition, in
ground-faultconditions,bothEXCPsandhealthyECPswillcarryfault
currents.
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Nonlinear loads, which draw currents that are no longer sine
waves, also cause the existence of nonzero neutral currents; therefore,
theneutralcurrentisnolongerzero,eveniftheloadisbalancedamong
the phases.
The presence of stray currents on metal parts, by increasing their
superficial temperature or even by causing sparks, worsens the fire
and explosion hazards. This is crucial in locations where accumulation
of dust, both on enclosures and EXCPs, and explosive atmospheres
are expected.
Additionally, the neutral currents flowing through ECPs and
EXCPs may induce disturbances to sensitive electronic equipment,
such as computers, control systems, and similar, preventing their
regular operation.
The accidental interruption of the PEN conductor creates a haz-
ardous situation even in the absence of any fault situation (Fig. 7.19).
The enclosures of the single-phase equipment in Fig. 7.19, in fact,
become energized, even if healthy, by acquiring the line-to-ground
potential V ph . The continuity of the PEN conductor is crucial and,
therefore, must be guaranteed.