GCX51 is the type designation for a family of single phase, three-zone phase packaged relays. The first - and second-zone distance measurements are made by a unit having a reactance of ohm type characteristic and the third zone has a directional mho characteristic. GCY51 is the type designation for a family of single phase two-zone and three-zone mho phase distance relays for transmission line protection. A GCY51F is a single phase mho relay with two zones of protection. The M1 and M2 units have a 75° angle of maximum torque and M1 has provision for 0.5 ohm offset phase to neutral. The M2 unit has provision for a forward offset 0-4 ohms phase to neutral, thus giving a conventional two-zone relay or a distinctive ''figure eight'' characteristic as required. The GCXY51 is a single phase, four-zone phase distance relay with two zones or reactance characteristics. A third zone is provided by a unit having a directional mho characteristics, and a fourth zone is provided by a unit with an offset (optional) mho characteristic.
