CT overloading

[Yaya]

Can I 20/1 CT(probably 2.5VA, or maybe 5VA) be used for measuring current on a 380V motor with a FLC of 23A. I am not worried about accuracy, but rather whether the CT might fail in service. The only connected load consists of 2 ammeters, one situated about 100m away via 2,5mm^2 cable.
The CT was previously used on a motor with a FLC of 16A.

 

[EEIre]

The primary of the CT (20A) you have is too small for this application as the flc of the motor is 23A. The VA rating looks fine for the application.

Suggest that you get a new CT of a similar spec but with a primary rating of at least 30A. i.e. 30/1A

 

[scottf]

Yaya-

What is the rating factor of the CT you have. It is common (in fact preferred) to run CTs over the rated current, especially for metering applications.

 

[busbar]

In ANSI regions, it is typical to find relay-class CTs labeled with rating factors of 1.33 @ 30°C and 1.0 @ 55°C. Id est, a so-applied 1.33RF, 1-ampere-secondary CT is intended to run at 1.33 amperes continuously at ambient temperatures of 30°C or less. Reference IEEE C57.13-1993.

 

[DanDel]

Yaya, your problem may be that a 20:1A CT is designed for an analog ammeter of max input of 1A and 20A full scale reading. The meter and CT must match for any kind of basic accuracy(again, assuming an analog meter). I don't believe this is what you expected when you said that you are not worried about accuracy.
A digital instrument can be set up for this application.

While using a CT for a load slightly above its primary rating usually does not damage the CT, I do not agree that it is common or preferred to run metering CTs above their ratings.

The only way to be sure about whether or not you will be overloading the CT will be to run a burden analysis.

 

[scottf]

DanDel-

I must disagree with your assessment. Most meter classes are defined such that the rated accuracy is applicable from the rated current up to the rating factor.

For example, according to IEEEC57.13, for a 0.3% class CT, the 0.3% accuracy is to be applicable from the rated current to the maximum continuous current (rating factor times rated current) and then the accuracy is 0.6% from rated current to 10% of rated current.

IEC, BS, CSA, etc... all work in a similar manner.

 

[DanDel]

scottf, my concern was not with the basic accuracy of the CT above its rating, but its match to an analog meter. If indeed analog ammeters are being used, the CT rating must match the FS of the meter.

 

[jbartos]

Suggestion: The current waveform should not contain an excessive harmonic content with the 20:1 CT being considered and assumed to have 1.33RF.

 

[scottf]

jbartos-

Please expand on your comment. I'm not sure I follow you on the harmonics comment. Also, I believe it is safe to assuem an RF of higher than 1.33. For ring-type CTs, where the only limiting factor for RF is the secondary coil/leads, RF in excess of 2.0 are common.

DanDel-

Is it common to have analog meters with the full scale current to be less than 2A?

 

[Yaya]

DanDel,
The analog ammeters that are connected to the CT's are used for indication only, so the accuracy of the reading is not that important.

seanmx, A new CT will be installed after a month or two. I was wondering if it is ok to use the existing CT in the interim.

 

[DanDel]

Yaya, you will not damage the CT by running it marginally over its rating.
scottf, analog metering is typically 5A or 1A (mostly 5A) input, FS can be almost any value, since it relates to the CT primary.