Active In-feed Converters (AIC) generate harmonics with frequencies up to 100 kHz resulting in power losses over the distribution lines, transformers and consumer loads, and interferences with control signals. The value of the harmonic voltage at an In-plant Point of Coupling (IPC) depends on the grid impedance at the harmonic frequency. As the spectral impedance of a distribution grid section cannot be estimated by calculation, it has to be measured. This paper presents the “on-line” grid impedance meter designed for spectral impedance measurements of low voltage distribution grids over the frequency range between 2 and 150 kHz. A useful application of spectral grid impedance measurement is presented at the end of this paper, with the analysis of interferences between two photovoltaic inverters.