D-c monitoring system using two-wire transmission lines

Claim 1

A supervisory system comprising: a controlled station; a control station remote from said controlled station; a transmission path including a two-wire line linking said controlled station with said control station; a source of direct current at one of said stations connected across said line; switch means at said controlled station for reversing the polarity of a flow of direct current from said source over said line to discriminate between a normal condition and an offnormal condition thereat; a direct-current monitoring circuit bridged across said line; a signaling circuit inductively coupled to said monitoring circuit and provided with a first output energized in the presence of line current of one polarity and with a second output energized in the presence of line current of the opposite polarity in said monitoring circuit, the inductive coupling between said monitoring and signaling circuits including a pair of magnetic amplifiers each provided with a biasing winding forming part of said monitoring circuit, said magnetic amplifiers further having operating windings in said signaling circuit respectively connected to said outputs, said monitoring circuit including diode means for alternately enabling said magnetic amplifiers in the presence of a current flow of a respective polarity across said line; and three-mode indicator means at said control station connected to said outputs for identifying said normal condition at said controlled station in response to energization of said first output identifying said off-normal condition at said controlled station in response to energization of said second output and identifying an abnormal line condition in response to simultaneous de-energization of both outputs.