Sound advice, but all Base-T Ethernet ports are isolated by transformers to prevent any nastiness caused by inter-building links.
You'll lose such isolation with some Power-over-Ethernet schemes though.
It is not unknown for data cable screens to be carrying earth currents between PCs where the "earth" potentials at the ends are different and the screen has been"earth" at both ends. Some worse case situations have had cables damaged by those earth currents.
Or in the event of a fault at the distant building, that data cable screen can end up carrying the bulk of the fault current if the building has an independent local earth.