transfer CT rating from 55 degrees to 30 degrees

[MeinKaun11]

Hi
Few questions please ;

1. Would it be correct to say that as per IEEE C57.13 by default CT thermal rating is rated at 30 degrees if not specified otherwise.

2. If I have a 800: 5 CT rated at 55 degrees with thermal rating factor of 1.33, then what would be the rating at 30 degrees.

Appreciate the input !

 

[scottf]

1. Yes

2. It would be odd for the nameplate of the CT with an RF1.33 @ 55C to not also list the RF @ 30 C. Figure 1 in IEEE C57.13 shows curves based on RF @ 30C that can be calculated to other avg. ambient temps. It doesn't have exact curves to go from 55C to 30C, but an RF1.33 @ 30C is going to be somewhere in the area of RF1.8 @ 30C...RF1.5 @ 30C for sure.

 

[MeinKaun11]

Thanks Scott
Few additions to my earlier post:

1. 2.5L and 10L accuracy CT besides the fact that
2.5L is more accurate than 10L for 20 times the rated
Secondary current, why would somebody ever by a 10L rated or C rates CT(C rated has 10% error as well ).

Just trying to understand why 10L or C was crated was when 2.5L rating was good enough available.

Thanks

 

[scottf]

Currently, 2.5L and 10L exist in CSA standard...IEEE has C (or T) class. And, a good bit of the CSA world still uses the "C" rating.

2.5L is more accurate than 10L, but, depending on the type of CT, could also be more expensive. For most protection applications 10L is more than sufficient. In my opinion, only the most challenging differential protection schemes see any benefit from a 2.5L rating over a 10L rating.