Protection of the neutral conductor in three (3) phase systems needs to be given importance during design, particularly on reduced neutral conductor size and when the presence of harmonics is expected from the system. IEC provides some guidelines to protect the neutral conductor from overloads and short circuits.
- Where the cross-sectional area of the neutral conductor is less than that of the line conductors. The overcurrent detection for the neutral conductor shall be appropriate to the cross-sectional area of the neutral conductor. This protection shall disconnect the line conductors, but not necessarily of the neutral conductor. In this case, the neutral conductor shall be protected against short-circuit current.
- Where the harmonic content of the line currents is such that the current in the neutral conductor is expected to exceed the current-carrying capacity of the neutral conductor. The protection may be achieved by the overcurrent protective devices in the line conductors. In that case, it is not necessary to provide overcurrent protection for the neutral conductor or a disconnecting device for that conductor.
The overload detection shall be compatible with the nature of the current through the neutral and shall cause the disconnection of the line conductors but not necessarily the neutral conductor. Where disconnection of the neutral conductor is required, disconnection and reconnection shall be such that the neutral conductor shall not be disconnected before the line conductors and shall be reconnected at the same time as or before the line conductors.
Further requirements regarding protection of neutral conductors are given in IEC 60364-5-52.
Source: IEC 60364-4-43, IEC 60364-5-52