This European Standard details the test methods to determine the attenuation of converted differential-mode signals into common-mode signals, and vice versa, due to balance characteristics of cables used in analogue and digital communication systems by using the transmission measurement method. The unbalance attenuation is measured in, respectively converted to, standard operational conditions. If not otherwise specified, e.g. by product specifications, the standard operational conditions are a differential-mode which is matched with its nominal characteristic impedance (e.g. 100 Ω) and a common-mode which is loaded with 50 Ω. The difference between the (image) unbalance attenuation (matched conditions in the differential and common-mode) to the operational (Betriebs) unbalance attenuation (matched conditions in differential-mode and 50 Ω reference load in the common-mode) is small provided the common-mode impedance Zcom is in the range of 25 Ω to 75 Ω.For cables having a nominal impedance of 100 Ω, the value of the common-mode impedance Zcom is about 75 Ω for up to 25 pair- count unscreened pair cables, 50 Ω for common screened pair cables and more than 25 pair- count unscreened pair cables, and 25 Ω for individually screened pair cables. The impedance of the common-mode circuit Zcom can be measured more precisely either with a time domain reflectometer (TDR) or a network analyser. The two conductors of the pair are connected together at both ends and the impedance is measured between these conductors and the return path.This European Standard is bound to be read in conjunction with EN 50289 1 1, which contains essential provisions for its application.