What is current transformer?

A current transformer is used for measurement of electric current . Current transformers together with potential transformers are known as Instrument transformers. When current in a circuit is very high we cannot directly apply the measuring instruments. A current transformer reduced current accurately proportional to the current in the circuit . It can be conveniently connected to the measuring instruments.
         Current transformer is actually a step up transformer as regaring to voltage but its current will be stapped down. If the current transformer has primary to secondary current ratio of 100:5 then it stepup the voltage 20 times,whereas step down the current by 1/20 th of its original value.Hence if we know the current ratio of the current transformer and the ammeter we can calculate line current ratio. (current ratio * ammeter reading ).
   Current transformer also isolate the measuring instruments from the very high voltage circuit. Current transformers are commonly used in metering and protecting relays in electrical industry .