REF CT Class

[SK07]

For restricted earth fault protection relay for transformer, what Class of current transformers are used. Is it 5 P 10 or class X and why?
In the existing system I find Current Transformer Class 5P10 used for REF for Transformer.
Thanks in advance.

 

[RRaghunath]

It depends on the type of principle used for REF protection.
In case of High impedance type, Class-X (or Class-PX) is recommended.
If it is low impedance type (also known as Bias type), Class-P type is acceptable.
More details can be found in the Relay literature.

R Raghunath