Current Transformer (CT):
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 ofthe CT classes:
Class 5P20:
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.
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.