Apparatus will often utilise power supplies which have a very high input harmonic current content.
Examples of this are switch mode power supplies, electronic speed controllers, and Uninterruptible power supplies.
Harmonic currents may cause distortion of the voltage waveform (notching) and high neutral currents.
The notching effect on the voltage waveform can be particularly troublesome on small distribution systems (i.e. high impedance systems such as standby generators). This is due to instantaneous voltage drop occurring with each pulse of input current as solid state switching devices fire. In turn this can result in equipment malfunction due to out of tolerance line voltage conditions.
It is essential for the equipment designer to have an appreciation of load characteristics so that proper provisions may be designed into the system to accommodate the equipment.
Where possible the electrical engineer should exert influence on those responsible for the purchase of equipment to specify maximum permissible levels of input current harmonics and to require the manufacturer of the equipment to provide suitable filters, input isolation transformers, etc., as needed to maintain those permitted harmonic levels.
Higher than anticipated neutral currents may be present in systems containing high harmonic current due to the fact that odd order harmonic current components in a three phase four wire system (TPN) may become arithmetically additive in the neutral.