CT Differences

The major difference between the measuring CTs and Protection CTs are:

(1) The measuring CTs are more accurate than the protection CTs. The most common composite error levels of measuring CTs are 0.5 and 1, while the most common composite error levels for protection CTs are 5 and 10.

(2) The saturation levels of protection CTs are much higher than the CT's current ratings. For protection CTs, it is 10, 15, 20 or 30 times higher, while for measuring CT it is at best 2 times.

(3) Protection CT can be used for the course measuring purpose (not accurate measurement), while measuring CT shall not be used for protection purpose because of low saturation level.

(4) The CT class (IEC 60044) for protection CT is denoted by 'P' (example 5P20), while the measuring CT is by M (example 1M)

See the following links for additional help:

http://cr4.globalspec.com/thread/43440

http://www.itl-uk.com/introduction-protection-transformers.html

http://community.myelectrical.com/wikis/myelectricalwiki/ct-dimensioning.aspx

http://cr4.globalspec.com/thread/51403

- MS