I had a situation where due to ungrounded outlets, holding a server plugged in only by the IEC cable to the ungrounded UPS, and then going to unplug the STP cable connecting it to the cable modem, I got a 70VAC electric shock!
The ATX PSU is an PELV source, and with no ground the chassis is floating, and it happens to be at half the mains supply voltage because of the class X and Y capacitor network in the EMI filtering stage. There is also indicator lamps connecting between neutral and hot to indicate "proper" grounding.
The current path was from hot, through the network of components on the mains side, to the floating ground, through me, through the STP ethernet cable to the modem, through the coax, to the ground bonding of that coax outside, finally to my main breaker where the current returns to the neutral bus bar.
The tranformer is MANDATORY and part of the ethernet spec. If its not its own component on the board visibly its inside the port itself. A lot of ethernet ports are the integrated transformer variety. If there is no transformer its not ethernet, its something else over the same interface.