Introduction:
As it is not possible to use line current directly for measurement or for protective relay due to its high value. Current Transformers are used to,

1.) Reduce the line current to a value (e.g. 1 amp.), which is suitable for standard-measuring instruments, relays, etc.
2.) To isolate the measuring instruments meters, relays, etc. from direct line.
3.) To protect measuring instruments against short circuit current.
4.) To sense the abnormalities in current signals to protective relays to isolate the defective system.

There are two basic types of C.Ts., Bar primary and wound primary C.T. Generally as far as possible C.Ts. of the former type are selected because they are mechanically stronger and cheaper. In the bar primary C.T. the working ampere turns are determined by the primary current and therefore necessarily, the accuracies that can be offered with these C.Ts. become progressively inferior as the rated primary current decreases. In such cases, wound primary C.Ts. are used. A ring type C.T. is essentially a bar primary C.T., without the primary bar. These ring type C.Ts. are very easy to connect in existing switchgear assemblies as these are merely slipped on to the busbars.

The Indian standards specifications 4201 (application guide for current transformers) and also various international specifications recommend the following values of the rated burden of C.Ts. for various types of instruments and the required class of accuracy for various metering applications:

Typical values of VA burden imposed by different meters are given below: