One easy and quick way to characterize the resonant frequency of cables and wires in a product or system is to inject harmonic energy into the cable and measure the actual resonances. The main impact is that resonances can occur at much lower requencies than when only overhead lines are present. Two illustrative case studies are presented: one for a 275-kV cable, one for a 400 kV cable in combination with a 132-kV capacitor. The resonance condition is when the cable is an integer multiple of half a wavelength in the cable, at the frequency of interest. This has been discussed extensively in the literature on product design for EMI compliance. Electric Power Systems Research, 2021, 200, pp. ⟨hal-03625825⟩ HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific. This work is licensed under the Creative Commons Attribution-Noncommercial-NoDerivs 3. 0 IGO-ported license (CC BY-NC-ND 3.