Current Transformer and Potential Transformer are Instrument transformers.
Current Transformer (CT) is a step-up transformer which steps down the current to a known ratio. The primary of this transformer consists of one or more turns of thick wire connected in series with the line. The secondary consists of a large number of turns of fine wire and provides for the measuring instruments and relays a current which is a constant fraction of current in the line
Potential Transformer (PT) is a step down transformer and steps down the voltage to a known ratio. The primary of this transformer consists of a large number of fine wire connected across the line. The secondary winding consists of a few turns and provides for measuring instruments and relays a voltage which is a known fraction of the line voltage.