For anyone today who's got copper wire stretched between any buildings, you are in danger and your configuration is quite unwise.
For every standalone building that has a ground, that building also has an electrical potential. This potential is not necessarily the same between two neighboring buildings. When you stretch a copper line between them, you are tempting fate, electrically, not to mention the dangers of lightning strikes and the elements attacking that connection.
It is generally recognized that fiber optics are not only very efficient and durable, but also impervious to electrical troubles of the type that may plague inter-building connections. Simply grab switches or routers that support at least one fiber-optic connection, and link up!
Ethernet is electrically isolated at every port (unless you really go out of your way in to design it otherwise in an internal design). Your advice is still correct but it's more about lightning or something gone wrong, it's not an immediate risk.
I don't remember how it happened, but I have seen faulty Ethernet devices suffering a power surge that burned the switch port they were connected to, so be careful.
Inductance coils (usually inside the RJ45 jack itself) are what provides isolation. They are made from very thin wires so if something goes wrong this is the first and hopefully only thing that will break. If you apply enough voltage of course you can break things beyond this.
I did work for a small community college back in the 90s that had expanded their building multiple times since the 70s. The electrical potential between different parts of the building was significant enough to make wired Ethernet unreliable. (I never did understand why this was-- multiple electric service entrances with separate grounds, I assume.)
Because of this they were an early adopter of fiber-based networking. I remember being wowed by ATM over fiber running between closets at a blazing 155Mbps (with tons of 10BASE-T clients connected to FORE chassis switches).
I think it's generally OK if you leave the shielding unconnected at one end (good practice anyway to avoid ground loops). There is an electrically-insulating transformer in the connector (nothing that would resist a lightning strike, of course, though some surge protectors have 8P8C connectors).
+1 for fiber though. Long fibers are cheaper, SFP connectors and modules are commonplace and have become quite cheap. Just need to learn a few new things, such as simplex/BiDi vs duplex fiber, and multimode vs mono mode.
Besides you worry too much. Entire cities were wired with cat5 hanged off steel cable between 9 to 15 story buildings in early 2000s, before fiber. Nothing burned down.
If your cable is shielded it's liable to spark and trip breakers. Happened to me when one building was horribly miswired (someone swapped live and ground somewhere) and fried part of my computer before the breaker tripped. In the worst case there isn't actually any breaker in the path (they are only on live wires and the worry here is ground loops) so your cable just melts. That didn't happen to me.
Unshielded cable is theoretically isolated at both ends but who actually knows with some chinesium equipment?
Before the world standardized on Ethernet/IP, Microsoft had an abstraction layer called DirectPlay. ("Direct" was that era's "Copilot" word for Microsoft)
FYI, EtherNet/IP is an industrial control protocol, not TCP/IP over Ethernet. Yes, that is the real name of the protocol, a more confusing name could not have been chosen...
When I did LAN parties we had 10base2 over thin coax. Terminators at each end. Of course it was unstable because people kept disconnecting and everyone lost connection.
