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34 Symmetra MW 800 kW 3 × 400/230 V Installation - 990-1510D
TN Systems
Characteristics
TN systems have one point connected directly to ground. All exposed conductive parts must be
connected to that point by protective conductors.
Depending on the way the neutral and protective conductors are fed, there are three types of TN
systems:
• TN-S system: a separate protective conductor is used in the system
• TN-C-S system: the neutral and protective conductors are combined to one single conductor in
a part of the system
• TN-C system: the neutral and protective conductors are combined to one single conductor in the
whole system
Reference to IEC 60364-4-41 413.1.3
All exposed conductive parts of the installation must be connected to the earthed point of the power
system by protective conductors which must be earthed at or near to each relevant transformer or
generator.
Exposed conductive parts that are accessible at the same time must be connected to the same earthing
system, either individually, in groups or collectively.
Normally the earthed point of the power system is the neutral point. If a neutral point is not available
or accessible, a phase conductor must be earthed. The phase conductor must not serve as a PEN
conductor.
In fixed installations a single conductor may serve both as a protective conductor and a neutral
conductor (PEN conductor).
Reference to IEC 60364-5-54 546.2.3
If from any point in the installation the neutral and protective functions are provided by separate
conductors, it is inadmissible to connect these conductors to each other from that point. At the point
of separation, separate terminals or bars must be provided for the protective and neutral conductors.
The PEN conductor must be connected to the terminal or bar intended for the protective conductor.
If there are other effective earth connections, the protective conductors must be connected to such
points when it is possible. It may be necessary to earth at additional points to ensure that the
potentials of protective conductors remain as close as possible to that of earth in case of a fault.