Why is it important to consider the Total Harmonic Distortion when installing LED fixtures? What is it and in which way that impacts the installation circuit?

Answer provided by Alfonso, Technical Engineering Manager for EYE Lighting with experience in Europe and Australasia in lighting, electrical systems, energy efficiency and renewable energy.
First of all as a definition, harmonics are voltages or currents at frequencies that are a multiple of the fundamental frequency, 50 Hz in Australia: 100, 150, 200 Hz etc. Total harmonic distortion (THD) refers to the sum of all harmonic components to the power of the fundamental frequency existing in non-linear electrical and electronic devices. It is expressed as a percentage [%].
LED drivers are electronic power supplies in LED luminaires that contain inductive equipment (reactive and capacitive elements). They are non-linear devices because they modify the waveform of the current drawn from the supplied voltage signal, appearing less sinusoidal.
Most LED drivers also include diode bridges that rectify the AC input signal in order to operate the DC LED modules. The switching operation of these diode bridges generates discontinuous currents that eventually distort the sine waves.
So when a LED driver is connected to mains electrical system, it can generate harmonic currents that distort the supply voltage. And the more luminaires (with their non-linear LED drivers) in an electric circuit, greater the disturbance on the distribution system, making it inefficient, affecting the performance of other equipment and overheating the wiring.
This is basically the reason why on new installation projects the electrical specifications for lighting equipment it is generally required a maximum THD below 10% on the fixtures.
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