Iec 61869 2 [2021] -

A merging unit (the device that samples the CT's analog signal and converts it to a digital Ethernet stream) expects a perfect analog input. If the CT's phase error is 1 degree at 10% burden, the merging unit will digitize that error, and the protection relay will calculate the wrong impedance. A fault 10 km away will appear to be 9.8 km away. The zone-1 protection might not trip.

Part One: The Invisible River

A junior engineer, Leo, complains in a design meeting: "Why do we need TPZ class CTs? They cost 40% more. The old 5P20 was fine for 30 years." iec 61869 2