How is the level of the edge box EFT electrical fast transient/burst test determined?

Edge computing boxes used in industrial substations or factory automation must meet the IEC61000-4-4 Electrical Fast Transient Burst Test. Based on the application environment, they are classified into levels 1 to 4. Industrial environments typically require level 3: power port ±2kV, signal port ±1kV. Severe industrial or power systems require level 4: power supply ±4kV, signal ±2kV. Power input filtering design is crucial for edge boxes. The SM8K33CA TVS from Etymotic can be used; it is named SM8K series 8000W DO-218AB package, 33V (a direct A indicates VRWM=33V, clamping voltage 53V), capable of absorbing 4kV EFT burst pulse energy. It is paired with a common-mode inductor CMZ3225A-501T. The CM common-mode Z parameters are: impedance 3225, size 3.2×2.5mm, 501T indicates 500Ω impedance, 100MHz, significantly suppressing high-frequency EFT components. For communication interfaces such as RS485, a gas discharge tube 3R090L-6X8 should be added; its name is 3R three-terminal 090, operating voltage 90V. The L-pin configuration, with a 6×8 mm size, serves as primary coarse protection, followed by precise clamping with an ESDSM712. With the above configuration, the edge box can stably pass a Level 4 EFT test.