CBCT

That "limitation" is amusingly, if not amazingly, high. How many volts?

6600 volts

I think we need a little more information.

What are you doing that necessitates an allowance of 400A leakage?

I know one thing, I wouldn't want to be paying your electricity bill.

it is industrial application, I want to make REF (Restricted earth fault) scheme effective, because at present the CT ratio is 3000/1 and NGR value is such that earth fault current is limited to 100 Amps so setting of REF to 10% makes current sensing at 300Amps! so if I increase the value of earth fault current to 400Amps then 10% setting would make scheme effective.

I think there is no need to have REF protection and 100A NER should be good enough even if you have large distribution system connected to your switchboard. That is my view.

Coming to your question - yes, 50/1A CBCT in the outgoing feeders can be retained. I don't think the CT will have any saturation issues when the current is 400A, i.e. 8-times its primary rating. However, you can verify with the CT name plate details.

Even if it is saturating, it may not be a big deal as residually connected earth fault protection can be set in the feeder protection to supplement the sensitive earthfault protection at higher fault currents.

Thnks for rply, it is quite helpful,

The transformer size is 25MVA, still you would suggest not to implement REF?

And what if (regarding CBCT) current is 800A instead of 400A?

I must be missing something if you are using 50:1 CTs to measure up to 100A and now want to use the same 50:1 CTs to measure up to 400A.

Yes, replace the CTs.

YUP

Bar types are available with higher insulation levels, here you can replace them with higher ratio. It operates on the same principal of windows CT.