Siemens Sivacon S8 Planning Information page 96

In the event of a short-circuit to an exposed conductive part
in a TN system, a considerable proportion of the single-pole
short-circuit current is not fed back to the power source
through a connection to earth but through the protective
conductor. The comparatively high single-pole short-circuit
current allows for the use of simple protective devices such
as fuses or miniature circuit-breakers, which clear the fault
within the permissible fault disconnect time.
In building installations, networks with TN-S systems are
preferably used today. When a TN-S system is used in the
entire building, residual currents in the building, and thus
an electromagnetic interference by galvanic coupling, can
be prevented during normal operation because the operat-
ing currents flow back exclusively through the separately
laid insulated N conductor. In case of a central arrangement
of the power sources, the TN-S system can always be
recommended. In that, the system earthing is implemented
at one central earthing point (CEP) for all sources, for
example in the main low-voltage distribution system.
L
1
L
2
L
3
PEN
PE
L
1
L
2
L
3
N
PE
Fig. 10/2: Line diagram for an earthing concept based on a central earthing point (CEP)
Please note that neither the PEN nor the PE must be
switched. If a PEN conductor is used, it is to be insulated
over its entire course – this includes the distribution system
(please refer to the example in Fig. 10/2). The magnitude of
the single-pole short-circuit current directly depends on the
position of the CEP.
Caution: In extensive supply networks with more than one
splitter bridge, stray short-circuit currents may occur.
4-pole switches must be used if two TN-S subsystems are
connected to each other. In TN-S systems, only one earth-
ing bridge may be active at a time. Therefore, it is not
permitted that two earthing bridges be interconnected via
two conductors.
Today, networks with TT systems are still used in rural
supply areas only and in few countries. In this context, the
stipulated independence of the earthing systems must be
L
1
L
2
L
3
Central
Earthing
Point
Main
Equipotential
Bonding conductor
U
L
1
L
2
L
3
L
1
L
2
L
3
N
PE
SIVACON S8 Planning Principles – Technical annex
1
2
3
4
5
6
7
Neutral choke
(no longer required
for modern systems)
8
L
1
L
2
L
3
PEN
PE
9
10
11
93