Missing ground in ethernet
The incoming signal TX_D1+/TX_D1- arrives and is connected to an ethernet transceiver through a transformer XFMR1. As there's no GND, an unknown potential difference exists, for instance, between TX_D1- and GND, as represented by Vpot. Well, how one guarantees that a large Vpot won't damage the transformer?


Your system design specification should include a maximum common mode voltage on the ethernet line, which you refer to as Vpot.  With this in hand, the transformer can be specified and designed.  Common mode voltage will dictate the insulation requirements for primary-secondary and for the cable winding to core.  In your comment you mentioned greater than 1.5kV.  Nail this number down before selecting the transformer.  This is how one guarantees that a large Vpot will not damage the transformer.