it means accuracy class of CT. 5 p means 5 % of composite error will happen till 20 times of current is applied . So CT will follow its rated characterstics upto 20 times currentRead more: What_does_mean_by_5p20_CT
Two types of CTs. one is for metering another one is for ProtectionThe metering CT has very less error,the protection CT has more error but thathas more saturation level.5 - 5% error. P for protection and 10 indicates 10 times the rated current.