In a large building there are two phases of mains supplied.
My understanding is that these will have a minimum separation between sockets of the different phases - so far so good…
If I installed a powerline adaptor from the router at one end of the building it would provide signal to much of the building, but obviously stops where the phase changes.
QUERY: If I put another power line network on the other phase would it be safe to run an ethernet cable between these two closest sockets?
Its not a matter of the phase but of the grounds. Ethernet signals itself are completely isolated but if you’re running a shielded (STP) cable a ground loop will be established. Pretty the same with transformer based DI boxes and the ground lift switch.
If you have to go a long distance I’d probably use a shielded cable with a short unshielded patch cable on one end. But be aware of different ground levels which may be uncomfortable to touch by hand…
I’d not go shielded, there’s no need.