A method and an apparatus for detecting a phase-to-earth fault on a three-phase electric line of an electric network, the apparatus determining a neutral admittance based on a ratio of a difference between zero sequence currents on the three-phase electric line and a difference between zero sequence voltages in the electric network before an earth fault and during the earth fault. The apparatus also compares the determined neutral admittance, to a predetermined operation characteristic to detect a phase-to-earth fault on the three-phase electric line. The predetermined operation characteristic, when presented on an admittance plane, defines a closed area such that the center of the closed area is offset from the origin of the admittance plane towards a negative susceptance direction and/or towards a negative conductance direction.