A constant-current power system using permanent-magnet reference standards and magnetic-amplifier control circuits has been developed for energizing the Anglo-Canadian link (CANTAT). This system provides the necessary high degree of regulation of cable current, continuity of feed and protection for the submarine system. The protection circuits can be tested whilst the system is energized. The design is sufficiently flexible to make the equipment suitable for powering the majority of existing or planned systems using thermionic valve repeaters.