4.3.12 - Harmonic currents and neutral conductors

A perfectly balanced three-phase system (one with all three phase loads identical in all respects) has no neutral current and thus has no need of a neutral conductor. This is often so with motors, which are fed through three core cables in most cases.

Many three-phase loads are made up of single-phase loads, each connected between one line and neutral. It is not likely in such cases that the loads will be identical, so the neutral will carry the out-of-balance current of the system. The greater the degree of imbalance, the larger the neutral current.

Some three-phase four-core cables have a neutral of reduced cross-section on the assumption that there will be some degree of balance. Such a cable must not he used unless the installer is certain that severe out-of-balance conditions will never occur. Similar action must be taken with a three-phase circuit wired in single-core cables. A reduced neutral conductor may only be used where out-of-balance currents will be very small compared to the line currents.

A problem is likely to occur in systems which generate significant third harmonic currents. Devices such as discharge lamp ballasts and transformers on low load distort the current waveform. Thus, currents at three times normal frequency (third harmonics) are produced, which do not cancel at the star point of a three-phase system as do normal frequency currents, but add up, so that the neutral carries very heavy third harmonic currents. For this reason, it is important not to reduce the cross-sectional area of a neutral used to feed discharge lamps (including fluorescent lamps).

In some cases the neutral current may be considerably larger than the phase currents. Where the load concerned is fed through a multi-core cable, it may be prudent to use five-core (or even six-core) cables, so that two (or three) conductors may be used in parallel for the neutral.

In some cases it may be necessary to insert overload protection in a neutral conductor. Such protection must be arranged to open all phase conductors on operation, but not the neutral. This clearly indicates the use of a special circuit breaker.

It is very important that the neutral of each circuit is kept quite separate from those of other circuits. Good practice suggests that the separate circuit neutrals should be connected in the same order at the neutral block as the corresponding phase conductors at the fuses or circuit breakers.