Instrument Current Transformer:

The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the similar categories of equipment as a potential transformer.  This is completed through constructing the secondary coil consisting of several turns of wire, around the primary coil, that holds only a few turns of wire. Within this manner, measurements of high values of current could be obtained.

A current transformer should always be short-circuited whenever not linked to an external load. Since the magnetic circuit of a current transformer is designed for low magnetizing current whenever under load, this huge increase in magnetizing current will build up a large flux in the magnetic  circuit  and  cause  the  transformer  to  act  as  a  step-up  transformer,  inducing  an excessively high voltage within the secondary whenever under no load.