A current transformer is an application-specific transformer designed to measure alternating current.
As an instrument transformer, a current transformer scales a large current value into a smaller current value that is easier for instruments and relays to handle. This isolates the instrument circuit from the main power circuit, which helps protect equipment and improve performance.
Current transformers are built like any other transformer, with primary and secondary windings and a core. Current that passes through the primary winding is proportionally transformed through induction into the secondary winding before it supplies the instrument circuit. This secondary current value is more useful for low-voltage applications.