Page 151 - Electrical Safety of Low Voltage Systems

134 Chapter Seven

FIGURE 7.19 Interruption of the PEN conductor.

In sum, the TN-C system within a facility must be used only in
areas where none of the aforementioned safety issues can occur. A sen-
sible rating of the zones within any facility, with regard to the presence
of dust and/or explosive atmospheres, is, therefore, an important pre-
requisite to evaluate if the PEN conductor may be employed.

7.9 The Neutral Conductor in TN Systems
As already mentioned, in TN-C systems, the neutral conductor must
never be switched off. In TT systems, instead, the neutral must be
interrupted with the phase wires, as, upon faults caused by the utility,
it may become energized; therefore, in TT systems, the neutral wire
must be treated as a “live” conductor.
In TN-S systems, where the user owns and maintains the supply
source, the risk of energizing the neutral conductor can and must be
analyzed, thereby, allowing an “educated” decision about its possible
simultaneous interruption with the line wires.
A hazardous situation in the case of “passing-through” neutral is
exemplified in Fig. 7.20.

FIGURE 7.20 Hazardous situation in the case of nonswitched neutral.