The Ethernet transmission is easy to strike by surge. Generally, the test requirements are 10x700us surge waveform as test standard. But, some countries take different surge waveform.
1) Line-to-Ground: it is Common Mode
2) Line-to Line: it is Differential Mode
1. L-L：TVS suppresses most of surge current to protect transformer from destroyed；Meanwhile, TVS will clamp the differential mode voltage of transformer primary from kV to near 10V to protect the PHY IC on board.
2. L-G：SHN can absorb most of surge current to clamp the common mode voltage of transformer primary from kV to near 30V to protect the PHY IC on board.