What causes harmonic currents/voltages in LV systems?

• A. Nonlinear loads
• B. Balanced loads
• C. Grounding issues
• D. Lightning surges

Answer: (A)

Harmonics come from non-sinusoidal current draw. In low-voltage systems, many devices are nonlinear—switch-mode power supplies, rectifiers, and variable-speed drives are common examples. These loads don’t draw current smoothly with the voltage; they pull current in short pulses to charge capacitors or operate electronics. That pulsed current contains components at multiples of the fundamental frequency (2nd, 3rd, 5th, etc.), so the current waveform is distorted and the supply impedance then converts those harmonics into distorted voltages as well.

Balanced three-phase systems help keep harmonics in check by cancelling some of the harmonics, but the root cause of the distortion is the nonlinear loads themselves. Grounding issues mainly affect safety and fault currents, not the creation of harmonics in steady operation. Lightning surges cause transient spikes, not steady harmonic content. So the primary cause of harmonic currents and voltages in LV networks is nonlinear loads.