Current Transformer (CT) has two windings called primary and secondary windings like other transformer.  It is widely used in Power System for measurement of actual current in the bus and  for protection of bus due to over currents. CT can be mounted in any side high voltage or low voltage leads of Power Transformer.

CT represented as 5P20  means, the ratio error is less than 5% at 20 times of rated current (In).

It is not recommended not to evaluate which one is better, 5P20 or 5P10, as both are giving less than 5% of error while measurement, 5P10 gets saturated over and above 10 times of rated current where as other one at 20 times.

It is the application which differentiate the use of 5P10 and 5P20.

If the breaker is used for backup protection say incomer etc you can use 5P20, but if it is say an incomer ACB/ MCCB of PMCC, 5P10 is used.

Class PS

PS is for ‘Protection Special’. This class of CT’s are used
for special protection such as differential protection,
distance protection etc.

Class 1M

The letter ‘M’ indicates it is a measuring CT.

The number 1 indicated the accuracy of the CT. The
measuring CT’s should be more accurate than the protection
CT. The most common accuracy numbers are 0.5 and 1.

Over current protection

ANSI                      IEC                                                                   Protection functions

50                          I>>                                                                    Instantaneous overcurrent protection

50, 51                   I>t, Ip                                                                Overcurrent protection (phase)

50N, 51N             IE>t, IEp                                                           Overcurrent protection (ground)