CT, PT

A current transformer is a type of transformer that is used to measure alternating current. It produces a current in its secondary which is proportional to the current in its primary. Current transformers, along with voltage or potential transformers, are instrument transformers.

CT refers to Current transformer and PT refers to Potential Transformer. They are used to measure current and voltage in high voltage circuits respectively where an ordinary ammeter and voltmeter cannot be used. Also they are used for protective purposes as input to relays in a power system.