A method for determining individual voltage values of a plurality of power lines of multi-phase power lines, comprising: operatively coupling respective contactless voltage sensing devices to the plurality of power lines of the multi-phase power lines , wherein each of the respective contactless voltage sensing devices , comprises: an electrically conductive housing ; a first impedance element having a first impedance, wherein the first impedance element is configured to be operatively coupled to a respective power line of the plurality of power lines; an antenna operatively coupled to the first impedance element , said antenna including a first portion and a second portion , wherein the first portion of the antenna is insulated from the electrically conductive housing by means of an electrical insulator ; and wherein the second portion of the antenna is disposed outside the electrical insulator , and outside the electrically conductive housing ; a second impedance element having a second impedance, wherein the second impedance element is formed in part by the antenna and a parasitic impedance element , and wherein the parasitic impedance element comprises a parasitic impedance; measurement and communication circuitry operatively coupled to the first impedance element; determining sensed voltage values (V Z1 , V Z2 , V Z3 ) for the plurality of power lines of the multi-phase power lines using the respective contactless voltage sensing devices ; determining a calibration matrix (M) comprising cross-coupling factors, wherein the cross-coupling factors are representative of cross-coupling between the respective contactless voltage sensing devices and other power lines of the multi-phase power lines; and determining the individual voltage values of the respective power lines by deducting at least in part contributions of the cross-coupling from the sensed voltage values using the calibration matrix (M). an electrically conductive housing ; a first impedance element having a first impedance, wherein the first impedance element is configured to be operatively coupled to a respective power line of the plurality of power lines; an antenna operatively coupled to the first impedance element , said antenna including a first portion and a second portion , wherein the first portion of the antenna is insulated from the electrically conductive housing by means of an electrical insulator ; and wherein the second portion of the antenna is disposed outside the electrical insulator , and outside the electrically conductive housing ; a second impedance element having a second impedance, wherein the second impedance element is formed in part by the antenna and a parasitic impedance element , and wherein the parasitic impedance element comprises a parasitic impedance; measurement and communication circuitry operatively coupled to the first impedance element; determining sensed voltage values (V Z1 , V Z2 , V Z3 ) for the plurality of power lines of the multi-phase power lines using the respective contactless voltage sensing devices ; determining a calibration matrix (M) comprising cross-coupling factors, wherein the cross-coupling factors are representative of cross-coupling between the respective contactless voltage sensing devices and other power lines of the multi-phase power lines; and determining the individual voltage values of the respective power lines by deducting at least in part contributions of the cross-coupling from the sensed voltage values using the calibration matrix (M). <EOS>