What is meant by "useful current range" of a CT?

[electrickiwi]

I have a note from a supplier about a CT supplied with a generator AVR that its ratio is 130:1 and its "useful current range" is 325-650 A. What is meant by this in terms of the 0-325 A range? Is the response non-linear in that region so it loses accuracy, or am I missing something?
Thanks,
EK

 

[cranky108]

At low currents (primary) a CT will not produce accurate output because there is to little primary amps to completly excite the CT core. This is why metering CT's generally have a smaller core to lower the range the CT works in. In relaying CT's there is generally a larger core to extent the range to include larger current values.

I'm guessing you are not in the US, so I can't speak for the wording, but I do sumise those are the accurcy limits (for some given accurcy).

 

[itsmoked]

And continuing cranky's response, too high a current causes the core to saturate and that destroys the information you're seeking from the CT. The measured current will no longer change with greater still current flowing.

The "range" is the region where enough current is flowing to produce a "true" output but less current then will saturate the core which would stop the CT from producing the "true" output again.

Keith Cress
kcress -

http://www.flaminsystems.com