Technical information

8/7
1SDC210033D0202
G3.13 Protection of the neutral
Detection of overcurrents in the neutral conductor so as to break the phase conductors
(neutral protected but not isolated) or to break the neutral conductor itself (neutral protected
and isolated).
G3.14 Distribution systems
The distribution system establishes the status of the neutral in the power supply system and
the method for connecting the conductive part towards earth.
The Italian standard, CEI 64-8/3 (which is aligned with the IEC 60364-3 international Stand-
ard), classifies electrical systems with a combination of two letters. The first indicates the
situation of the power supply system towards earth:
Q
T direct earth connection of an alternating current point, generally the neutral;
Q
I earth insulation, or earth connection of a point, generally the neutral, by means of an
impedance.
The second letter gives the situation of the conductive parts of the electrical installation in
relation to the earth:
Q
T conductive parts directly earthed;
Q
N conductive parts connected to the earthing point of the power supply system.
Other letters may follow which indicate the arrangement of the neutral and protection con-
ductors:
Q
S neutral and protection functions carried out by separate conductors;
Q
C neutral and protection functions carried out by a single conductor (PEN conductor).
The main distribution systems used are illustrated below with reference to these definitions.
G3.15 TT system
In the TT system, the neutral and conductive parts are connected to two electrically inde-
pendent earthing systems.
/

/

/

1
5
%
5
%
G3.16 TN system
In the TN system, the neutral is connected directly to the earth, while the conductive parts
are connected to the same earthing system as the neutral.
The TN system is divided into three different types, depending on whether the neutral and
protection conductors are separate or not:
1. TN-S: the neutral conductor N and the protection conductor PE are separate
/

/

/

1
3(