Abstract: In a utility power distribution system having at least one primary electric power source and at least one secondary electric power source, each of said primary and secondary sources being connected to the distribution system through respective controllable circuit breakers, the improvement comprising communication signal generating apparatus arranged for introducing a communication signal into the power distribution system from the connection of the primary power source, communication signal receiving apparatus arranged for receiving the communication signal via the power distribution system at the secondary power source, and apparatus responsive to the interruption of receipt of the communication signal at the receiving apparatus for operating the circuit breakers at the secondary power source for disconnecting the secondary power source from the power distribution system.