current transformer (CT) is a device that measures the current in a circuit by producing a proportional current in its secondary winding.
A CT can have different accuracy classes depending on its purpose. A 5P20 CT means that it is a protection class CT with an accuracy of 5% and an accuracy limit factor (ALF) of 20.
This means that the CT can handle up to 20 times the rated primary current without saturating and with an error of less than 5%.
For example, if the CT has a ratio of 1000/5 A, it can measure up to 20,000 A in the primary with an accuracy of 5%.
Protection class CTs are used for applications such as overcurrent, earth fault, differential, and distance protection.
They are designed to operate reliably under high fault currents and to isolate the secondary circuit from the primary circuit.