Instrument transformers (ITs) are specialized transformers whose main role is to step down high currents or voltages in power systems to safe, standardized levels for metering, protection relays, SCADA, and control circuits, while maintaining proportionality and phase. Instrument transformers (ITs) are specialized transformers whose main role is to step down high currents or voltages in power systems to safe, standardized levels for metering, protection relays, SCADA, and control circuits, while maintaining proportionality and phase. The name instrument transformer is a general classification applied to current and voltage devices used to change currents and voltages from one magnitude to another or to perform an isolating function, that is, to isolate the utilization current or voltage from the supply voltage for safety to. Instrument transformers (ITs) are designed to transform voltage or current from the high values in the transmission and distribu-tion systems to the low values that can be utilized by low voltage metering devices. Rockefeller worked for Westinghouse Electric Corporation for twenty-one years in application and system design of protective relaying systems. He has also served as a private consultant since 1982. This guide contains. The purpose of this document was to get to know the theory behind instrument transformers in substations and to create a calculation template in Microsoft Excel which would simplify the current transformer calculation process. Instrument transformer calculations and dimensioning for substation. Instrument transformers are used both to protect personnel and apparatus from high voltage and to allow reasonable insulation levels and current-carrying capacity in relays, meters, and instruments.