Current transformers are mainly used where currents cannot be measured directly. They are special types of transformers that convert the primary current into a (usually) smaller, standardized secondary current of a certain accuracy (class) and galvanically separate the primary and secondary circuits from each other. The physically induced saturation phenomenon of the core material also ensures that the secondary circuit is protected against excessive currents.

A basic distinction can be made between single-core current transformers and wound-core current transformers. The most common representative of single-core current transformers is the plug-in current transformer, which is plugged into the live conductor and thus forms a transformer with a primary winding (and secondary windings corresponding to the ratio).