Class X CTs are current transformers which are used for special applications. The "X" indicates that they do not belong to any standardized class. They are customized transformers therefore it is necessary to specify the individual characteristics of the current transformers such as the turns ratio, rated primary and secondary current, Voltage knee point, the maximum magnetizing current at the knee point and the Maximum resistance of the secondary winding.
Class X CTs are generally used where high knee points are require to prevent operation at higher currents without saturation.
Class X CTs are further divided into Class A and Class B CTs.
Class A CTs are more expensive and are designed to operate even at maximum fault current without saturating. Class B CTs cost less and are used in high-impedance applications. They tend to saturate during transient conditions.