The letter ‘P’ indicates it is a protection class CT. The number ’20’ indicates the magnitude of the fault current which can be safely flow through the primary of CT, without saturating the CT. For example if CT ratio is 1000/5 amperes, the current flowing through the primary of CT can be up to 20 x 1000=20 Kilo-Ampere. The number ’20’ is also called Accuracy Limit Factor(ALF) of CT. The Accuracy Limit Factor of 20 indicates that the protection class CT will sense the current with rated accuracy when the 20 times current of the primary rated current flows through the CT. The common Accuracy Limit Factor of the CT is 10,15,20,30.
The Number ‘5’ indicates the accuracy of the CT. When the current flows through the 5P20 protection class CT is 20 times of the rated primary current, the CT reads within the composite error of 5 %.
If the current is above 20 times then the CT will saturate.