Archive for February, 2017

Understanding Current Transformer Accuracy Classes

February 21, 2017 Leave a comment

IEC 61869-2 defines the new current transformer accuracy classes intended to replace the old standard, IEC60044-1 (note that IEC 61869-1 is designed for instrument transformers) .  The new and old standards are essentially identical, but IEC 61869-2 consolidated two parts of the older standard:

  • IEC 60044-1 : Instrument transformers – Part 1: Current transformers
  • IEC 60044-6 : Instrument transformers – Part 6: Requirements for protective current transformers for transient performance

While IEEE/ANSI C57.13 remains active, most current manufacturers prefer the IEC standard because it is not specific to instrument transformers with a 5A output.  Therefore, instead of dealing strictly with 5A output devices, the IEC standard is more general, covering devices with a mA output or voltage output.

The next thing to be aware of is that the 0.5 Class (of IEC 61869-2) and the 0.5s Class (of IEC 61869-2) are different.  The 0.5s version is a more rigorous standard when it comes to performance on lower current ratios.  For example, at 5% of the rated current, the percentage current ratio error for the 0.5 Class is 1.5% whereas 0.5s requires 0.75% or better.

The biggest single mistake I see people make when comparing accuracy of CTs is to assume that 0.5% accuracy = IEC 61869-5 0.5 Class (or some other standard).  THIS IS NOT NECESSARILY THE CASE!

I recently tested several CTs from a manufacturer (that I’ll leave unnamed).  The website specified the CTs were 0.5% accurate from 10% to 120% of the rated current.  I tested the CTs and I found they were correct.  However, they were not 0.5 Class!  Here’s why:

IEC 61869-2 0.5 Accuracy

This particular unit, which was rated at 20A, has an error at 20A that is below 0.5%.   In addition, I tested the part at 10%, 25%, 50%, 75% and 120% of the rated current, and in each case the linearity was below 0.5%.  Therefore, the manufacturer’s claim is true; however, the unit’s phase shift is too large to meet the 0.5 Class.  This is because it would have to be less than 30 minutes (0.5 degrees) at 100% of the rated current.

In conclusion, 0.5% accuracy ≠ 0.5 Class ≠ 0.5s Class.  Be sure to look at the standard the CT complies with when comparing products.  And, if it doesn’t state any standard, be wary!