Choosing a proper CT

[zientek92]

Hello everyone, 

I have this question in my protection of transmisson and distribution course that I can't find answer for. 

The question is:

It is required to specify a CT to be used in conjunction with a time graded overcurrent relay. The CT secondary and the relay are both rated at 1 A. The relay is of electronic type and the relay maximum burden is 0.25 VA at rated current. The total loop resistance between the CT and the relay including the lead wires, CT internal resistance, and relay resistance is 0.5 Ω. The maximum fault current, referred to the CT secondary is 25 times the relay rating.Determine the CT complete specification assuming the CT 
burden is 5 VA; i.e. the CT will deliver 5 V at 1 A.

Is enyone who could tell me how to figure out the windings ratio of this CT? Will that be protection CT with accuracy class 5P25? 

Thanks for any answer.

 

[kerching]

There are very few people on here who can help you with this.

the best plan of attack ( as will be repeated by others) is that you post your method and calculations, then you can be guided through the process

 

[Rob.]

You have everything you need there. 

Ignore the protection class. They haven't given enough information for you to summat that.