Abstract: The invention provides a high voltage capacitor suitable for measuring the voltage of an overhead power line. The capacitor dielectric is mounted directly on to the power line conductor, which serves as one electrode of the capacitor. The other electrode of the capacitor is mounted on to the outer surface of the dielectric, and is protected at each side by a guard ring. Stress control is provided between the outer capacitor electrode and the guard rings and also at the outer edges of the guard rings. The capacitor may be enclosed in an earthed housing, which can contain one or more current transformers. The apparatus can be used for fault detection and location, and may be used in combination with a power cable or a switch.