Class of CTs

[vinod]

What meant by CLASS of Current Transformers (CT)? C100,C200, CL1.2 etc
What is the defferant between metaring CT, Ground CT and protection CT?
Why we are doing saturation test on CTs?
Is there any relation between Knee point and VA?

Please give me the basic explenation about the above subjects or any web address..

 

[jim dungar]

The class of the CT defines how much burden (resistance of realy/meter and lead length.) The higher the class the more burden a CT can handle and still maintain its accuracy.

There is really no difference in the CTs for different applications but in general:
Metering CTs are built with relatively low accuracies often in the <1% range and for use with small burdens.
Ground CTs are built with very large windows so they go around many conductors.
Protection CTs are built with very tight accuracies (often less than .25%) and they are intended to drive large burdens.

Saturation of Cts is very important, after they saturate a large change in input current yields a small change in ouput current (the CT is no longer linear).

Larger VA ratings usually relates to a larger core which affects the knee point.

 

[vinod]

Your explenation is more than enough for me Mr.Jim... Thanks

 

[kingpb]

There is really no difference in the CTs for different applications but in general:
Metering CTs are built with relatively low accuracies often in the <1% range and for use with small burdens.
Ground CTs are built with very large windows so they go around many conductors.
Protection CTs are built with very tight accuracies (often less than .25%) and they are intended to drive large burdens.

Jim,

The info is helpful but slightly mis-stated in terms of higher and lower accuracy:

Metering CT's have a higher accuracy in the normal operating range, where protection CT's require good linearity with less accuracy over a wide range.