Isolation current transformer

[sky72]

Why we need to have an isolation current transformer for metering if we using the 600/300/5A, class 5P20, 15VA ring type current transformer for overcurrent and earthfault protection?

 

[SooryaShrestha]

I think it is not isolation, but the quality of measurement accuracy, that makes the measuring CT and the Protection CT having different cores.
In 5P20 CT the limits of the current error at rated primary current will be +/- 1%, the phase angle error at rated primary current will be +/- 60 minutes and the composite error will be +/- 5%. This shows there is a very high tolerable value for protection CT. Of course Class 5 accuracy is also available in measuring CT. But generally for measuring CT class 1 or better is used. So the cores will be different for measuring CT and the protection CT. For protection you may even need two separate cores because of different use of the protective relays.

 

[jbartos]

Comment. Also there are other aspects such as burdens. The metering CTs tend to have burdens from about 2.5VA to 45VA and relaying CTs tend to have burdens from about 25VA to 200VA.

 

[230842]

5P20 CT for protection must withstand an overcurrent 20 times the nominal with a 5% accuracy. Such an overcurrent is dangerous for metering instrumentation so that CT for metering purposes need to be saturated much before. These two different behaviours need two different core materials and designs. That is the answer to your question. for