[willglynn]

I'd like to add a few asterisks:

Magnetic coupling removes issues with potentials _up to the dielectric breakdown voltage_. Ethernet magnetics are considered high potential components, and even the entry level options will isolate at least 1.5kV, but fault events often exceed that figure.

Magnetic coupling removes issues with _common mode_ potentials. If the + and - side of a pair are both a thousand volts away from the pair on the secondary side, no problem. If a wire pair suddenly measures a thousand volts across… well, Ethernet transformers are typically wound 1:1.

_Ideal_ magnetic coupling removes issues with potentials. Ideal transformers behave as above, but real transformers have parasitic effects, particularly winding capacitance. Fast transients (including ESD) can and do capacitively couple across the transformer from primary to secondary.

Magnetics are important but do not solve the problem on their own. It is possible to design and manufacture electrically robust copper Ethernet systems – for a given definition of robustness (typically defined as passing some specific EMC test) – but even then real world electrical faults can and do destroy robust Ethernet systems. Fiber has none of these concerns.

[aidenn0]

All very true; good clarifications. In context my point was that it would be totally safe to run a copper Ethernet cable between two houses on separate electrical grids (or even an un-earthed battery powered computer).

[Aloha]

Inductive pickup (foreign voltage) from even fairly long runs (20kf +) of well maintained copper is usually much less less than half a volt, measuring either T-R, or T-G/R-G. It's supposed to be floating to ground - nothing on a twisted pair should be ground referenced, if you do have voltage to ground, you have a short.

If it was not floating, you'd get atmospherics, hum, and other issues that you saw in old fashioned ground return systems.

Indeed, measurement of voltage and continuity of T-G and R-G is a standard way to check for faulty cable

[bcrl]

Unless the telco cut the outer shielding on the twisted pair cables everywhere a terminal was installed. That was one of the remediations the incumbent had to undertake here when they started deploying 50Mbps VDSL2 FTTN service. Oops.