Recent developments in relay protective schemes, using metallic pilot channels for isolating a transmission line during fault conditions, reduce conditions, considerably the magnitude of the currents which must be handled. Schemes recently developed have many other advantages over the older pilot wire schemes being more economical to install and operate, and having excellent reliability. In addition to the advantages inherent in all pilot schemes, such as high speed tripping and absolute selectivity, the latest schemes provide indication of the condition of the pilot wire circuit at all times.