Abstract: Apparatus for monitoring the voltage of electricity supplied on a cable (121) are described. The apparatus includes a sense electrode housed in an electrically-insulative housing to form a capacitive coupling (127) between the sense electrode and the cable. The housing may be of the clamp-on type. A sense capacitor (128) of known capacitance is included, which is electrically coupled to the sense electrode and a local ground. Suitable voltage measurement circuitry (126) is connected to the sense capacitor and is responsive to voltage produced across the sense capacitor to provide an output signal representative of the voltage on the cable. The measured voltage waveform can be filtered to give the fundamental of the supply voltage, which can eliminate or reduce the effects of changes in capacitance within one cycle. A secondary capacitive (130) coupling can be formed with a related neutral wire (122) to improve stability of the voltage waveform measurement.