PS class is used for protection purposes, where we need to be very careful about the saturation of CT. PS class provides a higher saturation current which helps us to get the maximum fault current that occurs in our electrical system. The accuracy of the PS class is not defined. generally used for unit protection differentials.
A 5P20 CT means it will give a +_5% error when it reads 20 times the rated current. which means when a fault happens more than the CT rating up to 20 times it can measure with 5% errors with plus minus.
The PS class and 5P10 class of accuracy refer to the accuracy of a current transformer. A PS class current transformer has a primary current rating of 1A and a secondary current rating of 5A. The accuracy of a PS class transformer is typically within 1% of the rated primary current. A 5P10 class current transformer has a primary current rating of 5A and a secondary current rating of 1A. The accuracy of a 5P10 class transformer is typically within 0.2% of the rated primary current.
In other words, a 5P10 class current transformer is more accurate than a PS class current transformer. The 5P10 class transformer has a higher accuracy rating, which means that it will have a smaller margin of error when measuring current. This makes it a better choice for applications where very precise current measurements are required.