A current transformer produces a reduced current that is accurately proportional to the current in a circuit, which is too high to be applied directly to metering and protective instruments. It also isolates the measuring instruments from potentially very high voltages in the monitored circuit.

Current transformers are commonly used in metering and protective relays within the electrical power industry. A current transformer can be equipped with one or more independent magnetic cores, each with equal or different characteristics, for measuring, metering, and/or protective purposes.