Home > Current Transformers/Transducers > More on CT Accuracy Classes

More on CT Accuracy Classes

A couple of days ago I posted an article called Understanding Current Transformer Accuracy Classes.  I recommend reading that blog first. Today I want to elaborate a little more.

First, I would like to share the exact tables found in the IEC 61869-2 standard.

IEC 61869-2 Table 201

Accuracy Class ± Current ratio error permissible at rated current shown below ± Phase displacement (in mins) at rated current shown below
5% 20% 100% 120% 5% 20% 100% 120%
0.1 0.4% 0.2% 0.1% 0.1% 15 (0.25°) 8 (0.133°) 5 (0.083°) 5 (0.083°)
0.2 0.75% 0.35% 0.2% 0.2% 30 (0.5°) 15 (0.25°) 10 (0.167°) 10 (0.167°)
0.5 1.5% 0.75% 0.5% 0.5% 90  (1.5°) 45 (0.75°) 30 (0.5°) 30 (0.5°)
1.0 3.0% 1.5% 1.0% 1.0% 180 (2.0°) 90 (1.5°) 60 (1.0°) 60 (1.0°)

IEC 61869-2 Table 202

Accuracy Class ± Current ratio error permissible at rated current shown below ± Phase displacement (in mins) at rated current shown below
1% 5% 20% 100% 120% 1% 5% 20% 100% 120%
0.2S 0.75% 0.35% 0.2% 0.2% 0.2% 30 (0.5°) 15 (0.25°) 10 (0.167°) 10 (0.167°) 10 (0.167°)
0.5S 1.5% 0.75% 0.5% 0.5% 0.5% 90 (1.5°) 45 (0.75°) 30 (0.5°) 30 (0.5°) 30 (0.5°)

Note that Table 202 has two differences compared to 201:
1. It specifies behavior at 1% of the rated current.
2. It is more rigorous (difficult to achieve) at lower current percentages.

Of course we can visualize the data in the tables as well. In this graph I’m depicting the 0.5 and 1.0 classes.

0.5 Class versus 1.0 Class

0.5 Class versus 1.0 Class

Perhaps even more interesting, here’s the difference between 0.5 and 0.5S Classes.  Note that the standard doesn’t specify how a 0.5 Class CT should behave below 5%, and that the 0.5S class is more rigorous below 100% of the rated current.

0.5 Class versus 0.5S Class

0-5s-versus-0-5-class

I’m too lazy to draw similar graphs for the allowed phase shift, but hopefully you get the idea.  It is important however to realize that it is the combination of compliance with the accuracy AND the phase displacement tables that allows a unit to be IEC 61869-2 compliant.

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