CT's are categorized as Protection CT, Special Protection
CT and Measuring CT. Based on this, the CT's are
classified. Here is the meaning of the CT classes:
The letter 'P' indicates it is a protection CT.
The number 5 indicates the accuracy of the CT. Most common
accuracy numbers are 5 and 10.
The number 20 (called accuracy limit factor) indicates that
the CT will sense the current with the specified accuracy
even with 20 times of its secondary current flows in the
secondary. This is required for protection CT, because the
fault current is high and the CT should be able to sense
the high fault current accurately to protect the system.
The common numbers are 10, 15, 20 and 30.
PS is for 'Protection Special'. This class of CT's are used
for special protection such as differential protection,
distance protection etc.
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.
Measuring CT's are intended to work in normal and it
doesn't require working with accuracy beyond the CT
secondary current rating, so the accuracy limit factor is
not mentioned here.