A device that serves as an input source of currents and voltages from an electric power system to instruments, relays, meters, and control devices. The basic design is that of a transformer with the primary winding connected to the power system, and the secondary winding to the sensing and measuring equipment. Data from these devices are necessary for the operation, control, and protection of the power system. The primary reason for setting up the instrument-transformer interface is to provide analog information at low voltage levels, insulated from the higher system voltages. The range of use is from 480 V through the maxima of the 765–1000-kV power systems.