Current transformer accuracy class

[nraju]

As per VDE0410, the measuring class of measuring instruments has to be Class 1. Does it mean that the CT secondary which feeds such instruments has to be Class 1? The CT secondaries of all measuring CTs which I have seen are all of 1 ampere. Do we also have measuring CTs of Class 1 but with 5 amperes secondary?

 

[Bung]

I don't know of any physical reason why you couldn't have a Class 1 metering CT with a 5A secondary. What does VDE0410 have to say about it? Presumably if it is defining metering class CTs it will also define allowable nominal secondary ratings?


Bung
Life is non-linear...

 

[RalphChristie]

Is the VDE the German standard? I'm taking a wild shot, but I think it would be very similar to the IEC-standards.

Most CTs have either an 1A secondary or a 5A secondary. That means you have to use an instrument with an input designed for that current. (thus an instrument with an 1A input for an 1A secondary and an instrument with a 5A input for a 5A secondary)

If the standard says you have to use a Class 1 CT, you have to use it or something better (like a Class 0.1, Class 0.2 or Class 0.5)

The specification for a Class 1 CT is 1% current (ratio) error and a phase displacement error of 60minutes between 100% and 120% of the rated current.
The specification for a Class 0.1 CT is 0.1% current (ratio) error and a phase displacement error of 5minutes between 100% and 120% of the rated current.