PS Class CT is also known as PX Class or simply Class X Current Transformer. It is special type of current transformer meant for protection purpose. Various protection class CTs as described in IS 2705 part-3 is only useful for overcurrent protection. You may refer Accuracy Class of CT for detail.
In many protection applications, core saturation will almost occur during the transient phase of a heavy short circuit. The performance of the current transformers during faults is, therefore, an important consideration in providing an effective relaying scheme. In any current transformer the first consideration is the highest secondary winding voltage possible prior to core saturation. This highest secondary voltage is defined by Knee Point Voltage.
Further, for differential protection, Restricted Earth Fault Protection or other unit protection scheme, it is desirable that the characteristics of CTs is same. Deviation in characteristics of CTs may lead to spill current in the secondary and even lead to actuation of protection element even under through fault condition. Also, it is desired that the current transformers should be more accurate as compared to 5P10, 5P20 etc. Thus, while employing CTs for such protection scheme, specification of CTs are given to the manufacturer. Therefore, a CT meant for such protection application shall not only be defined by its accuracy at ALF rather its knee point voltage should also be defined.
PS Class CT is defined by its knee point voltage. This is very important as the CT must not saturate under fault condition.
As per IEC 60044, design requirement for current transformer for general protective purposes are laid out in terms of Rated Primary Current, Turn Ratio, Knee Point Voltage, Exciting Current at knee point voltage (or some other specified point) and secondary winding resistance. Such current transformers are designated Class PS or Class X.