Difference between Restricted Earth Fault & Unrestricted Earth Fault protection?

Restrited earth fault is normally given to on star connected end of power equipment like generators,transformers etc.mostly on low voltage side. for REF protection 4 nos CTs are using one each on phase and one in nuetral. It is working on the principle of balanced currents betwwen phases and nuetral.Unrestricted E/F protection working on the principle of comparing the unbalance on the phases only.For REF protection PX class cts are using but for UREF 5P20 Cts using.

For Differential Protection CTs using on both side HT & LV side each phase, and comparing the unbalance current for this protection also PX class CTs are using.

Thank yoy guest...

But why REF protection is used? Either it is used for %age winding protection? If yes can anybody tell what is the calculation of how much portion of winding is protected by using REF protection?

thnx

Kindly advise how to determine Vk value of Class X CT using for REF.

My Tx 5000MVA. distance between Tx CT to Relay is about 20m.

Thank you