Abstract: A ground conductor break in a polyphase A.C. cable having a separate power conductor and an equal number of symmetrically spaced ground conductors for each phase is detected by sensing negative-sequence current components being induced in the ground conductors. In response to two or more ground conductors of a three-phase cable being broken, the break is detected by sensing a current flow in one of the power conductors and an absence of positive sequence current components in the ground conductors. In response to the broken ground conductor being sensed, power conductors are open circuited. The sequence components are sensed with a network including a channel responsive to the current induced in each ground conductor. Each channel includes a current transformer for deriving an A.C. voltage that is a replica of the induced current for the conductor of the particular channel. The network includes operational amplifiers responsive to the voltages derived from the current transformers.