CT Class Differences

These are the CT accuracy class. The class 0.2s is more accurate than 0.2 class, especially in the higher and lower ranges of CT's rated current. See these links for detail:

http://www.northern-design.co.uk/docs/other/ND%20Instrument%20Transformer%20Accuracy.pdf

http://w3.energy.siemens.com/cms/00000013/aune/Documents/CT%20Dimensioning.pdf

- MS

Accuracy class of 0.2 means +/- 0.2 % error. But, the declared accuracy is guaranteed only between 100% & 120% rating. And, with some increased error, the CT performance can be guaranteed from 5% loading. Below this load, the error is not guaranteed. It can be anything.

Normally these classes of accuracy are used for Tariff Metering by the Utility Company.

In large industrial projects, the project would be commissioned in steps only. In such a case, initially the loading will be very low. If the tariff metering CT errors are haphazard, it may not be acceptable either to the supplier (in case of negative error) or to the consumer (in the case of positive error). And the magnitude can be quite large, say, with a sanctioned demand of 10MVA, even 1% error means 100kVA.

To cater to such situations, Standards have introduced two new accuracy classes, 0.2S & 0.5S for tariff metering purposes. These special CTs guarantee the declared accuracy, even with 20% loading. And some definite error can be defined even with a load as low as 1%. Thus they are suitable for industries where loads are commissioned in steps or stages.