Air Core and Iron Core Current Transformers

The cores of Current Transformers are usually constructed of Iron.  Some CTs have an air core.  Iron Core CTs have higher power output. However, they have low accuracy.  They are vulnerable to both static and transient errors.

Air core CTs have high accuracy and exhibit linear characteristics.  Hence, they are also known as linear couplers. They also cannot get saturated.

However, the downside of Air core Current Transformers is that they have low capacity.  Hence, the cannot be used to power electromagnetic relays.  They can only be used for static relays.