CT class - Saturation point of 5P20 10VA CT's

[CraigMW]

Morning All,
Just would like to thank all for the help in the past.

This maybe a silly question,

I have mag.curve results for a multiple ratio CT (200/400/600/5)5P20 10VA.
The Mag. curve was done on the 600/5 ratio - full tap, knee point voltage =98V

On the same class (5P20 10VA), but a single tap CT 600/5 the Mag. curve knee point results =52V

Question:Why does the saturation point differ by almost half between the two CT's on the same ratio and class?


Thanks

 

[DiscoP]

A P class CT doesn't have a rated knee point.

All you know is that for your CT classification is that the error will be less than 5% when 10VA of burden is connected and 20 times nominal current is flowing.

When testing these CTs, the internal CT resistance is needed.

Then you can use ohms law to work out the excitation voltage required to drive 20 times nominal current through 10VA and the CT resistance.

Apply this voltage as a test, and see that he excitation current is less than 5A (5% of 20 times 5 Amps).

 

[CraigMW]

Thanks for the info. Discop,