It is well known that when a 3-phase load current includes third or other triple harmonic currents these currents are additive in the neutral.
This neutral current will give rise to additional heating of the cables and other equipment in the circuit. Hence appropriate cable sizes and equipment ratings have to be selected to take account of this additional heating.
When the load current includes other harmonics, that are not additive in the neutral, they will have a heating effect on the line conductors. This heating effect has to be taken into account, in addition to the effect of the triplen harmonics, when selecting cables and other equipment.
The heating effect of the non triplen harmonic currents can be taken into account by calculating a correction factor, Cf, to be applied to the fundamental design current. For cable sizes less than 50 mm² this factor can be calculated from the following equation:
This factor is applied to the fundamental design current to determine the actual design current, taking account of the effect of harmonics.
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By Mr M Coates ERA Technology Ltd
Originally published in Wiring Matters 23 Summer