The invention enables the measurement of the voltage between phases (phase-to-phase voltages) of a multi-phase power cable, e.g. a three-phase power cable, such as may be used in subsea or subterranean environments for electrical power transfer. The invention does not require a power supply at the measurement location, and relies solely on optical fibres (typically present in such cables) to carry light to and from the sensors. Fibre Bragg grating (FBG) based sensors sample the electric field between conductors and convert to a strain on the fibre, as a result of which certain wavelengths of the reflected light are modulated by the instantaneous magnitude of the phase-to-phase voltages to be measured. A sensor module embodying the invention includes a spacer which holds the conductors in a predetermined geometry and locates the FBG sensors between pairs of conductors on which the phase-to-phase voltage measurements are to be performed. At an end of the cable, an interrogation system can extract and interpret the modulation of these wavelengths to infer the measured values of voltage. The invention may be incorporated into a repair splice for retrofitting purposes, and may also incorporate FBG-based current sensors to simultaneously measure current in the conductors.