Methods and apparatus for measuring the voltage of at least one conductor (121, 122, 123) of an electrical power cable (10) comprise providing a container (22) made from a conductive material around a portion of the cable and at least one electric field sensor (301, 302, 303, 304) between the container and the cable and bringing the container to a constant potential and measuring the electric field with a sensor. The voltage is determined by comparing the measured electric field with electric fields simulated for a plurality of configurations of punctual electric charges.