The unequal impedance angles of the positive and zero sequence underground-cable impedances, which are in variance with overhead lines, requires a study to evaluate the appropriate relay residual compnesation factor, which is of paramount importance for earth faults. The problem, in many cases, is provoked when the protected zone consists of a mix of underground cables and overhead lines. This paper shows the importance of having an accurate residual vectorial compensation together with a proper selected comparator characteristic.