The definition of emission limits for harmonic, interharmonic, and supraharmonic (2-9 kHz) currents of customer installations in the German technical rules VDE AR-N 4100 and the D-A-CH-CZ rules for assessing network disturbances requires accurate information on the frequency-dependent network impedance. In the absence of precise data, the documents provide assumptions for its approximation. This paper evaluates the validity of these assumptions using data from extensive measurement campaigns recently conducted in low voltage networks by comparing measured and approximated results. Additionally, it introduces a novel approach for approximating the frequency-dependent network impedance in the 9-150 kHz frequency range, providing a foundation for the credible definition of current emission limits of customer installations in this frequency range to advance the normative framework.