This paper discusses the Δ-networks and other circuits designed to separate the conducted electromagnetic interference (EMI) into its common mode (CM) and differential mode (DM) components. The input impedances of CM/DM separators must be 50 Ω resistive in the measurement frequency band and they must be independent of the values of the noise signals and noise source impedances. The conditions for achieving such input impedances are derived. It is shown that many of the proposed separators, including the Δ-network suggested in the CISPR 16 standard, do not fulfill the input impedance requirement. This leads to unreliable CM and DM measurements, and consequently, to EMI filter oversizing and design by trial-and-error.