Accuracy Class of Current Transformer

Accuracy Class of Current Transformer (CT) is defined in Indian Standard IS 2705 Part-2 and Part-3. This accuracy class of CT is defined so as to provide the permissible error in the current measurement of CT.

As we know that, there are three types of errors defined for a CT: Current Error (or Ratio Error), Phase Angle Error and Composite Error. Thus, the accuracy class of CT must be defined using these errors. The accuracy class of protection class and metering class CTs are defined by Composite Error and Current Error & Phase Angle Error respectively. Let us now discuss the accuracy class of Protection Class and Metering Class CT individually.

Accuracy Class of Metering CT:

As per IS 2705 Part-3, the accuracy class of metering CT is defined by the maximum permissible current error at the rated current. Standard Accuracy Class of Metering CT are 0.1, 0.2, 0.5, 1, 3 and 5. Each of these class have different maximum permissible current error. Let us now have a look at errors specified for these accuracy classes:

Limit of Error for Standard Accuracy Classes 0.1, 0.2, 0.5 and 1:

The limiting error shall not exceed the value tabulated below at rated frequency and burden from 25% to 100% of the rated burden.

Limit of Error for Standard Accuracy Classes 3 and 5:

The limiting error shall not exceed the value tabulated below at rated frequency and burden from 50% to 100% of the rated burden. It shall be noted that phase displacement error is not defined for these accuracy class.

Special Application Metering CT:

Special application metering CTs are a special category in which it is desired that the CT should accurately measure the current from 1% to 120% of the rated current. If the secondary rated current is 5A, this meter shall measure current from 50 mA to 6A accurately. This type of CT is used for revenue meters and in energy meters.

Two accuracy classes 0.2S and 0.5S has been defined in IS 2705. These classes are only applicable for rated secondary current of 5A and for the ratio 25/5, 50/5 and 100/5 and their decimal multiples.

Limit of Error for Standard Accuracy Classes 0.2S and 0.5S:

The limiting error shall not exceed the value tabulated below at rated frequency and burden from 25% to 100% of the rated burden.

Accuracy Class of Protection CT:

Accuracy Class of Protection Current Transformer (CT) is defined by the maximum permissible Composite Error at the rated accuracy limit primary current followed by letter “P” (meaning protection). Here, accuracy limit primary current is the value of primary current corresponding to Accuracy Limit Factor (ALF).

The standard accuracy classes for protection current transformer are 5P, 10P and 15P. 5, 10 and 15 in these classes are the maximum composite error corresponding to accuracy limit primary current.

Limit of Error for Accuracy Classes 5P, 10P and 15P:

The current error (or ratio error), phase displacement error and composite error, at rated frequency and rated burden, shall not exceed the values tabulated below:

On the name plate of Current Transformer, rated accuracy limit factor is indicated following the corresponding output and accuracy class. For example, on name plate of CT, you may find 30 VA, 5P10 or 30/5P10. This marking means that, the burden of CT is 30 VA and the maximum composite error at 10 times of rated current is 5%. Here, accuracy limit factor is 10.