In addition to the accuracy class, current transformers for protection service are distinguished by their class, relative to the impedance of their secondary winding. The classes are:

– Class A – are transformers whose secondary winding has a reactance which can not be neglected. In this class, all transformers that do not fit into class B are included.

– Class B – are those whose secondary winding has reactance that can be forsaken. In this class, the transformers with toroidal core, or simply those of bushing, are framed.

The current transformers in series with the system are dependent on the same overcurrent requests as the current, such as the current resulting from a three-phase fault.

A current transformer is said to have an accuracy class 10, for example, when the percentage ratio error during the measurements made from its secondary nominal current up to 20 times the value of said current is 10%.

According to NBR 6856, the ratio error of the current transformer must be concentrated to the current value with more than one shunt in the secondary winding has its class of accuracy included with its operation in the position that takes the largest number of turns.