Answer / ramesh kumar k

Mr. RK is partially right.... I will tell you clearly...
CTs are of 3 types depending on their usage... they are
1. Metering 2.Protection & 3. Special Protection(PS)

Metering & Protection CTs name itself self explanatory.

In metering CTs our main aim is to measure the currents
without damaging the instruments (i.e. Ammeters) connected
to it... so at fault levels (i.e. high currents) the
secondary also indicates corresponding heavy currents... so
the meter connected to it will gets damaged... (A 5A
ameeter can't measure 50A)... inorder to make the meter
safe... we will prepare the core of the meter in such a way
that at faults currents the metering CT core gets
saturated... and it will not induce any more current in the
CT secondary... so the current in secondary ckt. is ZERO.
So the meter is safe... THIS METERING CT WILL BE WORKING
WITHIN NORMAL RANGE OF CURRENT AND WILLNOT WORK IN FAULT
CONDITIONS (IT WILL NOT GIVE OUTPUT).

Where as Protection CT will have to work only in fault
condition. Because the line has to be protected in fault
condition only. The core of this CT is made in reverse to
that of metering CT. It will not saturate in fault
condition and still give the output to the relay...

Thats why Metering CT can't be used for protection
purpose... and protection CT can be used for metering but
it will not meet the accuracy requirements (If accuracy
requiremets are compulsory... prot CT can't be used for
metering)


Where as

Answer / rk

metering cts are desined not to record fault current i.e.
if ctr is Ex. 200/5 at a fault current of 2000amp the
secondary current would be still 5 amps not 50amp. reverse
is in prot cts it will serve 50amps if its ALF permits .
Metring CT are like acc class 0.5 etc Protn CTs have 5P10,
5P20etc i.e. it will maintain its ratio up to 20times the
the primary current (200amp)of ct.

Answer / siddharth tayade

mainly the class is diffrence in the protection and
metering c.t.and knee point voltage is conciderd in
protection core

Answer / asit sarkar

Generally CT have two error 1.Phase angle error,2.Ratio error.
Now,in Metering CT we have to minimize the ratio error as
well as in case of Protection CT we have to reduce the phase
angle error.

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