Protective system for mobile electric power apparatus

Claim 1

In a system for supplying three phase electric current through a power circuit including a flexible cable from a fixed source location to a mobile utilization apparatus having a conductive frame, a three phase power transformer at said source location having line and neutral terminals, a ground terminal at said source location conductively connected to said neutral terminal, said cable including line conductors connected to said line terminals and a ground conductor, an electrical load device carried by said conductive frame and having load terminals connected for energization to said line conductors, the electrical neutral of said load device normally floating at a voltage near ground potential under balanced load conditions, a potential transformer mounted on said frame and having wyeconnected primary windings connected to said load terminals and secondary windings, said potential transformer primary windings having a neutral terminal normally floating electrically, a wye connected zig-zag autotransformer mounted on said frame, saId zig-zag autotransformer having phase windings connected to said secondary windings and a neutral terminal conductively connected to said neutral terminal of said potential transformer primary windings, a three phase rectifier energized from intermediate voltage phase winding taps on said zig-zag transformer phase windings and having a common terminal connected to said conductive frame, means connecting one end of said ground conductor to said frame, and means including a low impedance signal relay connecting the other end of said ground conductor to said ground terminal at said source location, whereby a unidirectional monitor signal current supplied by said rectiifier normally traverses said ground conductor and said relay and follows a return path through said power transformer neutral terminal and the neutral terminal and phase windings of said zigzag transformer.