Answer / karthik

for metering, protection all will be taken in one CT --only core will be varied in that CT
for measurement -1s1,1s2
for protection - 2s1,2s2
only one CT is used

Answer / biswaindu parida

In a substaion we use different ratio of CTs for different purposes. Such as 200/1 for metering 400/1 for another back up, 800/1 for protection like wise. So suppose if a fault comes say around of 1200A then the secoundary level current for different Cts will not be same.
Lets analyse them indivisually.
1) The 800/1 Ct will read it as 1.5A. For that we can simply use Over current relay and trip the breaker.
2) The 400/1 Ct will read it as 3A . here we can use IDMT relay.
As per its function it will sends trip command to the breaker as soon as possible...because here the time of trip is inversely proportional to the amount of fault current
3) But however in case of 200/1 CT, it will read it as 6A. If it is a metering CT then it should be protected 1st. because it has less saturation point as compared to protection or back up CT. So here we will use instantaneous relay so that it will trip suddenly when the fault current goes above some pre defined value.

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