Instrument Transformer Construction

Potential (or voltage) transformers consist of two separate windings on a common magnetic steel core. One winding consists of fewer turns of heavier copper wire on the core.  It is called the secondary winding. The other winding consists of a high number of turns of smaller copper wire wound on top of the secondary. It is known as the primary winding.

Current transformers are constructed in several different ways. One method is like the potential transformer where there are two separate copper windings on a magnetic steel core. It differs in that the primary winding consists of a few turns of heavy copper wire capable of carrying the full current while the secondary winding consists of many turns of thinner copper wire with a current rating of 5-20 amperes, depending on the specifications. This is called a wound type due to the wound primary coil.

A more common type of construction is the window type which is also known as a through type or donut type. These current transformers have an opening where the conductor carrying the primary load is passed. The primary conductor constitutes the primary winding of the CT (one pass through the window is one turn on the primary).

A variant of the window type is the bar type.  The bar type has a fixed primary conductor mounted in the window which has standard terminal bar connections to attach cables.