Abstract: A method and system for sending a repetitive permissive guard signal from an electrical power substation to a distributed generation site over existing medium voltage distribution lines to detect an islanding condition and apply transfer trip protection is disclosed. The permissive guard signal causes a receiver at the distributed generation site to control a recloser at the distributed generation site. Loss of the signal at the receiver device causes the tripping of the recloser at the distributed generation site in less than two seconds. The tripping of the recloser physically disconnects the distributed generation site from the electric power grid. The coupling to the medium voltage distribution lines can be implemented via a single phase-to-ground coupling or via a phase-to-phase differential coupling for multi-phase medium voltage distribution lines.