whose accuracy is more measuring or protection CT

Sounds like a homework question. I'd suggest doing some Google research first, then asking specific questions.

Definitely the measuring CT has to be more accurate.

- MS

Measuring ct is more accurate.

Class 0.5 ...... 0.5% accuracy

Class 1........ 1% accuracy.

Protaction cts need not be that accurate, but should not saturate at the short circuit levels.

So,

Class 5P10 ....... 5% accuracy, but 10*Ir fault levels can be sensed without saturation

Class 5P15 ........ 5% accuracy, but can handle 15* Ir fault level.

Class 5P20 ...... 5% accuracy at 20*Ir fault leves, (and 1% at rated current).

Class 10P20..... 10% accurracy at 20*Ir fault leves, (and 3% at rated current).