Protection of the neutral conductor
From Electrical Installation Guide
(see Fig. G64)
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.
