Chapter A General rules of electrical installation design

G42
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G - Sizing and protection of conductors
(1) Harmonics of order 3 and multiple of 3
I 1 H1
Fig. G63a : Triplen harmonics are in phase and cumulate in the Neutral
7 The neutral conductor
The c.s.a. and the protection of the neutral conductor, apart from its current-carrying
requirement, depend on several factors, namely:
b The type of earthing system, TT, TN, etc.
b The harmonic currents
b The method of protection against indirect contact hazards according to the
methods described below
The color of the neutral conductor is statutorily blue. PEN conductor, when insulated,
shall be marked by one of the following methods :
b Green-and-yellow throughout its length with, in addition, light blue markings at the
terminations, or
b Light blue throughout its length with, in addition, green-and-yellow markings at the
terminations
7.1 Sizing the neutral conductor
Influence of the type of earthing system
TT and TN-S schemes
b Single-phase circuits or those of c.s.a. y 16 mm 2 (copper) 25 mm 2 (aluminium): the
c.s.a. of the neutral conductor must be equal to that of the phases
b Three-phase circuits of c.s.a. > 16 mm 2 copper or 25 mm 2 aluminium: the c.s.a. of
the neutral may be chosen to be:
v Equal to that of the phase conductors, or
v Smaller, on condition that:
- The current likely to flow through the neutral in normal conditions is less than the
permitted value Iz. The influence of triplen (1) harmonics must be given particular
consideration or
- The neutral conductor is protected against short-circuit, in accordance with the
following Sub-clause G-7.2
- The size of the neutral conductor is at least equal to 16 mm 2 in copper or 25 mm 2 in
aluminium
TN-C scheme
The same conditions apply in theory as those mentioned above, but in practice,
the neutral conductor must not be open-circuited under any circumstances since
it constitutes a PE as well as a neutral conductor (see Figure G58 “c.s.a. of PEN
conductor” column).
IT scheme
In general, it is not recommended to distribute the neutral conductor, i.e. a 3-phase
3-wire scheme is preferred. When a 3-phase 4-wire installation is necessary,
however, the conditions described above for TT and TN-S schemes are applicable.
Influence of harmonic currents
Effects of triplen harmonics
Harmonics are generated by the non-linear loads of the installation (computers,
fluorescent lighting, rectifiers, power electronic choppers) and can produce high
currents in the Neutral. In particular triplen harmonics of the three Phases have a
tendency to cumulate in the Neutral as:
b Fundamental currents are out-of-phase by 2π/3 so that their sum is zero
b On the other hand, triplen harmonics of the three Phases are always positioned in
the same manner with respect to their own fundamental, and are in phase with each
other (see Fig. G63a).
+
I 1 H3
I 2 H1 + I 2 H3
I 3 H1
I N =
3
1
I k H1
+
+
I 3 H3
I k H3
0 + 3
IH3 Schneider Electric - Electrical installation guide 2008
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