Chapter A General rules of electrical installation design

G44
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G - Sizing and protection of conductors
7 The neutral conductor
Examples
Consider a three-phase circuit with a design load of 37 A to be installed using fourcore
PVC insulated cable clipped to a wall, installation method C. From Figure G24,
a 6 mm 2 cable with copper conductors has a current-carrying capacity of 40 A and
hence is suitable if harmonics are not present in the circuit.
b If 20 % third harmonic is present, then a reduction factor of 0,86 is applied and the
design load becomes: 37/0.86 = 43 A.
For this load a 10 mm 2 cable is necessary.
b If 40 % third harmonic is present, the cable size selection is based on the neutral
current which is: 37 x 0,4 x 3 = 44,4 A and a reduction factor of 0,86 is applied,
leading to a design load of: 44.4/0.86 = 51.6 A.
For this load a 10 mm 2 cable is suitable.
b If 50 % third harmonic is present, the cable size is again selected on the basis of
the neutral current, which is: 37 x 0,5 x 3 = 55,5 A .In this case the rating factor is
1 and a 16 mm 2 cable is required.
7.2 Protection of the neutral conductor
(see Fig. G64 next page)
Protection against overload
If the neutral conductor is correctly sized (including harmonics), no specific
protection of the neutral conductor is required because it is protected by the phase
protection.
However, in practice, if the c.s.a. of the neutral conductor is lower than the phase
c.s.a, a neutral overload protection must be installed.
Protection against short-circuit
If the c.s.a. of the neutral conductor is lower than the c.s.a. of the phase conductor,
the neutral conductor must be protected against short-circuit.
If the c.s.a. of the neutral conductor is equal or greater than the c.s.a. of the phase
conductor, no specific protection of the neutral conductor is required because it is
protected by the phase protection.
7.3 Breaking of the neutral conductor
(see Fig. G64 next page)
The need to break or not the neutral conductor is related to the protection against
indirect contact.
In TN-C scheme
The neutral conductor must not be open-circuited under any circumstances since it
constitutes a PE as well as a neutral conductor.
In TT, TN-S and IT schemes
In the event of a fault, the circuit-breaker will open all poles, including the neutral
pole, i.e. the circuit-breaker is omnipolar.
The action can only be achieved with fuses in an indirect way, in which the operation
of one or more fuses triggers a mechanical trip-out of all poles of an associated
series-connected load-break switch.
7.4 Isolation of the neutral conductor
(see Fig. G64 next page)
It is considered to be the good practice that every circuit be provided with the means
for its isolation.
Schneider Electric - Electrical installation guide 2008