Answer / venkat iyer

Selection of CTs depends upon its usage, whether for
metering or protection. Class of accuracy, VA burden and
palatability to instrument safety factors need to be
considered incase of metering CTs. In case of protection
CTs, linearity factor w.r.t its primary current and
accuracy factors need to be considered. Metering CTs are
expressed in terms of Class 1 or Class 3 where as
protection cts are expressed as 5P10 or 5P15 or 5P20 or
10P10 etc. The first digit stands for the class of accuracy
and P stands for protection and the last two digits stands
for linearity.

Answer / mervin

A CT used for Instrumentation is required to be accurate
over a normal working range , whereas CT for protection is
required to five a correct CT ratio several times the rated
current . Cores made using Nicel-Iron alloy gives accuracy
upto 5 times the rated current n suitable for
Instrumentation . Cores made using Cold rolled silicon
steel are accurate upto 15 times the rated current and
suitable for protection .An accuracy of about 2-3% is
desirable for distance and differential relays .CT burden
and CT linearity should also be considered during selection
of a CT .

Answer / ramakrishna m

The selection of CT depends on the current ratings of relays
those are used in the protecting circuitry and also depends
on accuracy that circuitry requires.

Answer / kapil

it depends on the protection circuitary ratings and the
accuracy u desire for the application. it must supply the
rated current to the relays with the accuracy u desire for
ur purpose,and also copper losses in the CTs should be
minimum so it also depends on the laminations and conductor
material u choose.

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