Many point-to-point communication protocols are designed to have an electrical interface with data/clock/whatever signals along with the ground signal for reference. For example, CAN is composed of +Hi, -Low, and ground. Minimum RS232 usage is TX/RX/Ground, its balanced counter-part RS485 A+/B-/GND. With exception of unbalanced interfaces, in which the GND is obviously needed to reference the signal, I've always seen the GND as needed to equalize the potentials between both ends, such that the potential difference between the equipment could be kept in safe regions. I wonder why such problem doesn't occur in ethernet (or maybe it does), where there's no GND. Here's the problem I see:
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?