How to design the EMC of the power FTU current transformer interface?

The current transformer (CT/PT) interface EMC design prevents secondary-side interference from entering. A 7D390K (390V) varistor and an SMBJ24CA TVS diode are connected in parallel on the CT secondary side to absorb open-circuit overvoltage. A PBZ1608E600Z0T ferrite bead (600Ω@100MHz) is connected in series on the PT secondary side. Shielded twisted-pair cable is used for the sampling line, with the shield grounded at a single point on the FTU terminal. A CMZ3225A-501T common-mode inductor is used for filtering at the PCB interface. The transformer casing is grounded with a 4mm² grounding wire. With this design, the transformer interface can withstand IEC 61000-4-5 4kV surge and IEC 61000-4-4 4kV EFT tests, maintaining a sampling accuracy of 0.5 class.